core/fmt/mod.rs
1//! Utilities for formatting and printing strings.
2
3#![stable(feature = "rust1", since = "1.0.0")]
4
5use crate::cell::{Cell, Ref, RefCell, RefMut, SyncUnsafeCell, UnsafeCell};
6use crate::char::{EscapeDebugExtArgs, MAX_LEN_UTF8};
7use crate::marker::PhantomData;
8use crate::num::fmt as numfmt;
9use crate::ops::Deref;
10use crate::{iter, mem, result, str};
11
12mod builders;
13#[cfg(not(no_fp_fmt_parse))]
14mod float;
15#[cfg(no_fp_fmt_parse)]
16mod nofloat;
17mod num;
18mod rt;
19
20#[stable(feature = "fmt_flags_align", since = "1.28.0")]
21#[rustc_diagnostic_item = "Alignment"]
22/// Possible alignments returned by `Formatter::align`
23#[derive(Copy, Clone, Debug, PartialEq, Eq)]
24pub enum Alignment {
25 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
26 /// Indication that contents should be left-aligned.
27 Left,
28 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
29 /// Indication that contents should be right-aligned.
30 Right,
31 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
32 /// Indication that contents should be center-aligned.
33 Center,
34}
35
36#[stable(feature = "debug_builders", since = "1.2.0")]
37pub use self::builders::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
38#[unstable(feature = "debug_closure_helpers", issue = "117729")]
39pub use self::builders::{FromFn, from_fn};
40
41/// The type returned by formatter methods.
42///
43/// # Examples
44///
45/// ```
46/// use std::fmt;
47///
48/// #[derive(Debug)]
49/// struct Triangle {
50/// a: f32,
51/// b: f32,
52/// c: f32
53/// }
54///
55/// impl fmt::Display for Triangle {
56/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
57/// write!(f, "({}, {}, {})", self.a, self.b, self.c)
58/// }
59/// }
60///
61/// let pythagorean_triple = Triangle { a: 3.0, b: 4.0, c: 5.0 };
62///
63/// assert_eq!(format!("{pythagorean_triple}"), "(3, 4, 5)");
64/// ```
65#[stable(feature = "rust1", since = "1.0.0")]
66pub type Result = result::Result<(), Error>;
67
68/// The error type which is returned from formatting a message into a stream.
69///
70/// This type does not support transmission of an error other than that an error
71/// occurred. This is because, despite the existence of this error,
72/// string formatting is considered an infallible operation.
73/// `fmt()` implementors should not return this `Error` unless they received it from their
74/// [`Formatter`]. The only time your code should create a new instance of this
75/// error is when implementing `fmt::Write`, in order to cancel the formatting operation when
76/// writing to the underlying stream fails.
77///
78/// Any extra information must be arranged to be transmitted through some other means,
79/// such as storing it in a field to be consulted after the formatting operation has been
80/// cancelled. (For example, this is how [`std::io::Write::write_fmt()`] propagates IO errors
81/// during writing.)
82///
83/// This type, `fmt::Error`, should not be
84/// confused with [`std::io::Error`] or [`std::error::Error`], which you may also
85/// have in scope.
86///
87/// [`std::io::Error`]: ../../std/io/struct.Error.html
88/// [`std::io::Write::write_fmt()`]: ../../std/io/trait.Write.html#method.write_fmt
89/// [`std::error::Error`]: ../../std/error/trait.Error.html
90///
91/// # Examples
92///
93/// ```rust
94/// use std::fmt::{self, write};
95///
96/// let mut output = String::new();
97/// if let Err(fmt::Error) = write(&mut output, format_args!("Hello {}!", "world")) {
98/// panic!("An error occurred");
99/// }
100/// ```
101#[stable(feature = "rust1", since = "1.0.0")]
102#[derive(Copy, Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
103pub struct Error;
104
105/// A trait for writing or formatting into Unicode-accepting buffers or streams.
106///
107/// This trait only accepts UTF-8–encoded data and is not [flushable]. If you only
108/// want to accept Unicode and you don't need flushing, you should implement this trait;
109/// otherwise you should implement [`std::io::Write`].
110///
111/// [`std::io::Write`]: ../../std/io/trait.Write.html
112/// [flushable]: ../../std/io/trait.Write.html#tymethod.flush
113#[stable(feature = "rust1", since = "1.0.0")]
114pub trait Write {
115 /// Writes a string slice into this writer, returning whether the write
116 /// succeeded.
117 ///
118 /// This method can only succeed if the entire string slice was successfully
119 /// written, and this method will not return until all data has been
120 /// written or an error occurs.
121 ///
122 /// # Errors
123 ///
124 /// This function will return an instance of [`std::fmt::Error`][Error] on error.
125 ///
126 /// The purpose of that error is to abort the formatting operation when the underlying
127 /// destination encounters some error preventing it from accepting more text;
128 /// in particular, it does not communicate any information about *what* error occurred.
129 /// It should generally be propagated rather than handled, at least when implementing
130 /// formatting traits.
131 ///
132 /// # Examples
133 ///
134 /// ```
135 /// use std::fmt::{Error, Write};
136 ///
137 /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
138 /// f.write_str(s)
139 /// }
140 ///
141 /// let mut buf = String::new();
142 /// writer(&mut buf, "hola")?;
143 /// assert_eq!(&buf, "hola");
144 /// # std::fmt::Result::Ok(())
145 /// ```
146 #[stable(feature = "rust1", since = "1.0.0")]
147 fn write_str(&mut self, s: &str) -> Result;
148
149 /// Writes a [`char`] into this writer, returning whether the write succeeded.
150 ///
151 /// A single [`char`] may be encoded as more than one byte.
152 /// This method can only succeed if the entire byte sequence was successfully
153 /// written, and this method will not return until all data has been
154 /// written or an error occurs.
155 ///
156 /// # Errors
157 ///
158 /// This function will return an instance of [`Error`] on error.
159 ///
160 /// # Examples
161 ///
162 /// ```
163 /// use std::fmt::{Error, Write};
164 ///
165 /// fn writer<W: Write>(f: &mut W, c: char) -> Result<(), Error> {
166 /// f.write_char(c)
167 /// }
168 ///
169 /// let mut buf = String::new();
170 /// writer(&mut buf, 'a')?;
171 /// writer(&mut buf, 'b')?;
172 /// assert_eq!(&buf, "ab");
173 /// # std::fmt::Result::Ok(())
174 /// ```
175 #[stable(feature = "fmt_write_char", since = "1.1.0")]
176 fn write_char(&mut self, c: char) -> Result {
177 self.write_str(c.encode_utf8(&mut [0; MAX_LEN_UTF8]))
178 }
179
180 /// Glue for usage of the [`write!`] macro with implementors of this trait.
181 ///
182 /// This method should generally not be invoked manually, but rather through
183 /// the [`write!`] macro itself.
184 ///
185 /// # Errors
186 ///
187 /// This function will return an instance of [`Error`] on error. Please see
188 /// [write_str](Write::write_str) for details.
189 ///
190 /// # Examples
191 ///
192 /// ```
193 /// use std::fmt::{Error, Write};
194 ///
195 /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
196 /// f.write_fmt(format_args!("{s}"))
197 /// }
198 ///
199 /// let mut buf = String::new();
200 /// writer(&mut buf, "world")?;
201 /// assert_eq!(&buf, "world");
202 /// # std::fmt::Result::Ok(())
203 /// ```
204 #[stable(feature = "rust1", since = "1.0.0")]
205 fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
206 // We use a specialization for `Sized` types to avoid an indirection
207 // through `&mut self`
208 trait SpecWriteFmt {
209 fn spec_write_fmt(self, args: Arguments<'_>) -> Result;
210 }
211
212 impl<W: Write + ?Sized> SpecWriteFmt for &mut W {
213 #[inline]
214 default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result {
215 if let Some(s) = args.as_statically_known_str() {
216 self.write_str(s)
217 } else {
218 write(&mut self, args)
219 }
220 }
221 }
222
223 impl<W: Write> SpecWriteFmt for &mut W {
224 #[inline]
225 fn spec_write_fmt(self, args: Arguments<'_>) -> Result {
226 if let Some(s) = args.as_statically_known_str() {
227 self.write_str(s)
228 } else {
229 write(self, args)
230 }
231 }
232 }
233
234 self.spec_write_fmt(args)
235 }
236}
237
238#[stable(feature = "fmt_write_blanket_impl", since = "1.4.0")]
239impl<W: Write + ?Sized> Write for &mut W {
240 fn write_str(&mut self, s: &str) -> Result {
241 (**self).write_str(s)
242 }
243
244 fn write_char(&mut self, c: char) -> Result {
245 (**self).write_char(c)
246 }
247
248 fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
249 (**self).write_fmt(args)
250 }
251}
252
253/// The signedness of a [`Formatter`] (or of a [`FormattingOptions`]).
254#[derive(Copy, Clone, Debug, PartialEq, Eq)]
255#[unstable(feature = "formatting_options", issue = "118117")]
256pub enum Sign {
257 /// Represents the `+` flag.
258 Plus,
259 /// Represents the `-` flag.
260 Minus,
261}
262
263/// Specifies whether the [`Debug`] trait should use lower-/upper-case
264/// hexadecimal or normal integers.
265#[derive(Copy, Clone, Debug, PartialEq, Eq)]
266#[unstable(feature = "formatting_options", issue = "118117")]
267pub enum DebugAsHex {
268 /// Use lower-case hexadecimal integers for the `Debug` trait (like [the `x?` type](../../std/fmt/index.html#formatting-traits)).
269 Lower,
270 /// Use upper-case hexadecimal integers for the `Debug` trait (like [the `X?` type](../../std/fmt/index.html#formatting-traits)).
271 Upper,
272}
273
274/// Options for formatting.
275///
276/// `FormattingOptions` is a [`Formatter`] without an attached [`Write`] trait.
277/// It is mainly used to construct `Formatter` instances.
278#[derive(Copy, Clone, Debug, PartialEq, Eq)]
279#[unstable(feature = "formatting_options", issue = "118117")]
280pub struct FormattingOptions {
281 /// Flags, with the following bit fields:
282 ///
283 /// ```text
284 /// 31 30 29 28 27 26 25 24 23 22 21 20 0
285 /// ┌───┬───────┬───┬───┬───┬───┬───┬───┬───┬───┬──────────────────────────────────┐
286 /// │ 1 │ align │ p │ w │ X?│ x?│'0'│ # │ - │ + │ fill │
287 /// └───┴───────┴───┴───┴───┴───┴───┴───┴───┴───┴──────────────────────────────────┘
288 /// │ │ │ │ └─┬───────────────────┘ └─┬──────────────────────────────┘
289 /// │ │ │ │ │ └─ The fill character (21 bits char).
290 /// │ │ │ │ └─ The debug upper/lower hex, zero pad, alternate, and plus/minus flags.
291 /// │ │ │ └─ Whether a width is set. (The value is stored separately.)
292 /// │ │ └─ Whether a precision is set. (The value is stored separately.)
293 /// │ ├─ 0: Align left. (<)
294 /// │ ├─ 1: Align right. (>)
295 /// │ ├─ 2: Align center. (^)
296 /// │ └─ 3: Alignment not set. (default)
297 /// └─ Always set.
298 /// This makes it possible to distinguish formatting flags from
299 /// a &str size when stored in (the upper bits of) the same field.
300 /// (fmt::Arguments will make use of this property in the future.)
301 /// ```
302 // Note: This could use a special niche type with range 0x8000_0000..=0xfdd0ffff.
303 // It's unclear if that's useful, though.
304 flags: u32,
305 /// Width if width flag (bit 27) above is set. Otherwise, always 0.
306 width: u16,
307 /// Precision if precision flag (bit 28) above is set. Otherwise, always 0.
308 precision: u16,
309}
310
311// This needs to match with compiler/rustc_ast_lowering/src/format.rs.
312mod flags {
313 pub(super) const SIGN_PLUS_FLAG: u32 = 1 << 21;
314 pub(super) const SIGN_MINUS_FLAG: u32 = 1 << 22;
315 pub(super) const ALTERNATE_FLAG: u32 = 1 << 23;
316 pub(super) const SIGN_AWARE_ZERO_PAD_FLAG: u32 = 1 << 24;
317 pub(super) const DEBUG_LOWER_HEX_FLAG: u32 = 1 << 25;
318 pub(super) const DEBUG_UPPER_HEX_FLAG: u32 = 1 << 26;
319 pub(super) const WIDTH_FLAG: u32 = 1 << 27;
320 pub(super) const PRECISION_FLAG: u32 = 1 << 28;
321 pub(super) const ALIGN_BITS: u32 = 0b11 << 29;
322 pub(super) const ALIGN_LEFT: u32 = 0 << 29;
323 pub(super) const ALIGN_RIGHT: u32 = 1 << 29;
324 pub(super) const ALIGN_CENTER: u32 = 2 << 29;
325 pub(super) const ALIGN_UNKNOWN: u32 = 3 << 29;
326 pub(super) const ALWAYS_SET: u32 = 1 << 31;
327}
328
329impl FormattingOptions {
330 /// Construct a new `FormatterBuilder` with the supplied `Write` trait
331 /// object for output that is equivalent to the `{}` formatting
332 /// specifier:
333 ///
334 /// - no flags,
335 /// - filled with spaces,
336 /// - no alignment,
337 /// - no width,
338 /// - no precision, and
339 /// - no [`DebugAsHex`] output mode.
340 #[unstable(feature = "formatting_options", issue = "118117")]
341 pub const fn new() -> Self {
342 Self {
343 flags: ' ' as u32 | flags::ALIGN_UNKNOWN | flags::ALWAYS_SET,
344 width: 0,
345 precision: 0,
346 }
347 }
348
349 /// Sets or removes the sign (the `+` or the `-` flag).
350 ///
351 /// - `+`: This is intended for numeric types and indicates that the sign
352 /// should always be printed. By default only the negative sign of signed
353 /// values is printed, and the sign of positive or unsigned values is
354 /// omitted. This flag indicates that the correct sign (+ or -) should
355 /// always be printed.
356 /// - `-`: Currently not used
357 #[unstable(feature = "formatting_options", issue = "118117")]
358 pub fn sign(&mut self, sign: Option<Sign>) -> &mut Self {
359 let sign = match sign {
360 None => 0,
361 Some(Sign::Plus) => flags::SIGN_PLUS_FLAG,
362 Some(Sign::Minus) => flags::SIGN_MINUS_FLAG,
363 };
364 self.flags = self.flags & !(flags::SIGN_PLUS_FLAG | flags::SIGN_MINUS_FLAG) | sign;
365 self
366 }
367 /// Sets or unsets the `0` flag.
368 ///
369 /// This is used to indicate for integer formats that the padding to width should both be done with a 0 character as well as be sign-aware
370 #[unstable(feature = "formatting_options", issue = "118117")]
371 pub fn sign_aware_zero_pad(&mut self, sign_aware_zero_pad: bool) -> &mut Self {
372 if sign_aware_zero_pad {
373 self.flags |= flags::SIGN_AWARE_ZERO_PAD_FLAG;
374 } else {
375 self.flags &= !flags::SIGN_AWARE_ZERO_PAD_FLAG;
376 }
377 self
378 }
379 /// Sets or unsets the `#` flag.
380 ///
381 /// This flag indicates that the "alternate" form of printing should be
382 /// used. The alternate forms are:
383 /// - [`Debug`] : pretty-print the [`Debug`] formatting (adds linebreaks and indentation)
384 /// - [`LowerHex`] as well as [`UpperHex`] - precedes the argument with a `0x`
385 /// - [`Octal`] - precedes the argument with a `0b`
386 /// - [`Binary`] - precedes the argument with a `0o`
387 #[unstable(feature = "formatting_options", issue = "118117")]
388 pub fn alternate(&mut self, alternate: bool) -> &mut Self {
389 if alternate {
390 self.flags |= flags::ALTERNATE_FLAG;
391 } else {
392 self.flags &= !flags::ALTERNATE_FLAG;
393 }
394 self
395 }
396 /// Sets the fill character.
397 ///
398 /// The optional fill character and alignment is provided normally in
399 /// conjunction with the width parameter. This indicates that if the value
400 /// being formatted is smaller than width some extra characters will be
401 /// printed around it.
402 #[unstable(feature = "formatting_options", issue = "118117")]
403 pub fn fill(&mut self, fill: char) -> &mut Self {
404 self.flags = self.flags & (u32::MAX << 21) | fill as u32;
405 self
406 }
407 /// Sets or removes the alignment.
408 ///
409 /// The alignment specifies how the value being formatted should be
410 /// positioned if it is smaller than the width of the formatter.
411 #[unstable(feature = "formatting_options", issue = "118117")]
412 pub fn align(&mut self, align: Option<Alignment>) -> &mut Self {
413 let align: u32 = match align {
414 Some(Alignment::Left) => flags::ALIGN_LEFT,
415 Some(Alignment::Right) => flags::ALIGN_RIGHT,
416 Some(Alignment::Center) => flags::ALIGN_CENTER,
417 None => flags::ALIGN_UNKNOWN,
418 };
419 self.flags = self.flags & !flags::ALIGN_BITS | align;
420 self
421 }
422 /// Sets or removes the width.
423 ///
424 /// This is a parameter for the “minimum width” that the format should take
425 /// up. If the value’s string does not fill up this many characters, then
426 /// the padding specified by [`FormattingOptions::fill`]/[`FormattingOptions::align`]
427 /// will be used to take up the required space.
428 #[unstable(feature = "formatting_options", issue = "118117")]
429 pub fn width(&mut self, width: Option<u16>) -> &mut Self {
430 if let Some(width) = width {
431 self.flags |= flags::WIDTH_FLAG;
432 self.width = width;
433 } else {
434 self.flags &= !flags::WIDTH_FLAG;
435 self.width = 0;
436 }
437 self
438 }
439 /// Sets or removes the precision.
440 ///
441 /// - For non-numeric types, this can be considered a “maximum width”. If
442 /// the resulting string is longer than this width, then it is truncated
443 /// down to this many characters and that truncated value is emitted with
444 /// proper fill, alignment and width if those parameters are set.
445 /// - For integral types, this is ignored.
446 /// - For floating-point types, this indicates how many digits after the
447 /// decimal point should be printed.
448 #[unstable(feature = "formatting_options", issue = "118117")]
449 pub fn precision(&mut self, precision: Option<u16>) -> &mut Self {
450 if let Some(precision) = precision {
451 self.flags |= flags::PRECISION_FLAG;
452 self.precision = precision;
453 } else {
454 self.flags &= !flags::PRECISION_FLAG;
455 self.precision = 0;
456 }
457 self
458 }
459 /// Specifies whether the [`Debug`] trait should use lower-/upper-case
460 /// hexadecimal or normal integers
461 #[unstable(feature = "formatting_options", issue = "118117")]
462 pub fn debug_as_hex(&mut self, debug_as_hex: Option<DebugAsHex>) -> &mut Self {
463 let debug_as_hex = match debug_as_hex {
464 None => 0,
465 Some(DebugAsHex::Lower) => flags::DEBUG_LOWER_HEX_FLAG,
466 Some(DebugAsHex::Upper) => flags::DEBUG_UPPER_HEX_FLAG,
467 };
468 self.flags = self.flags & !(flags::DEBUG_LOWER_HEX_FLAG | flags::DEBUG_UPPER_HEX_FLAG)
469 | debug_as_hex;
470 self
471 }
472
473 /// Returns the current sign (the `+` or the `-` flag).
474 #[unstable(feature = "formatting_options", issue = "118117")]
475 pub const fn get_sign(&self) -> Option<Sign> {
476 if self.flags & flags::SIGN_PLUS_FLAG != 0 {
477 Some(Sign::Plus)
478 } else if self.flags & flags::SIGN_MINUS_FLAG != 0 {
479 Some(Sign::Minus)
480 } else {
481 None
482 }
483 }
484 /// Returns the current `0` flag.
485 #[unstable(feature = "formatting_options", issue = "118117")]
486 pub const fn get_sign_aware_zero_pad(&self) -> bool {
487 self.flags & flags::SIGN_AWARE_ZERO_PAD_FLAG != 0
488 }
489 /// Returns the current `#` flag.
490 #[unstable(feature = "formatting_options", issue = "118117")]
491 pub const fn get_alternate(&self) -> bool {
492 self.flags & flags::ALTERNATE_FLAG != 0
493 }
494 /// Returns the current fill character.
495 #[unstable(feature = "formatting_options", issue = "118117")]
496 pub const fn get_fill(&self) -> char {
497 // SAFETY: We only ever put a valid `char` in the lower 21 bits of the flags field.
498 unsafe { char::from_u32_unchecked(self.flags & 0x1FFFFF) }
499 }
500 /// Returns the current alignment.
501 #[unstable(feature = "formatting_options", issue = "118117")]
502 pub const fn get_align(&self) -> Option<Alignment> {
503 match self.flags & flags::ALIGN_BITS {
504 flags::ALIGN_LEFT => Some(Alignment::Left),
505 flags::ALIGN_RIGHT => Some(Alignment::Right),
506 flags::ALIGN_CENTER => Some(Alignment::Center),
507 _ => None,
508 }
509 }
510 /// Returns the current width.
511 #[unstable(feature = "formatting_options", issue = "118117")]
512 pub const fn get_width(&self) -> Option<u16> {
513 if self.flags & flags::WIDTH_FLAG != 0 { Some(self.width) } else { None }
514 }
515 /// Returns the current precision.
516 #[unstable(feature = "formatting_options", issue = "118117")]
517 pub const fn get_precision(&self) -> Option<u16> {
518 if self.flags & flags::PRECISION_FLAG != 0 { Some(self.precision) } else { None }
519 }
520 /// Returns the current precision.
521 #[unstable(feature = "formatting_options", issue = "118117")]
522 pub const fn get_debug_as_hex(&self) -> Option<DebugAsHex> {
523 if self.flags & flags::DEBUG_LOWER_HEX_FLAG != 0 {
524 Some(DebugAsHex::Lower)
525 } else if self.flags & flags::DEBUG_UPPER_HEX_FLAG != 0 {
526 Some(DebugAsHex::Upper)
527 } else {
528 None
529 }
530 }
531
532 /// Creates a [`Formatter`] that writes its output to the given [`Write`] trait.
533 ///
534 /// You may alternatively use [`Formatter::new()`].
535 #[unstable(feature = "formatting_options", issue = "118117")]
536 pub fn create_formatter<'a>(self, write: &'a mut (dyn Write + 'a)) -> Formatter<'a> {
537 Formatter { options: self, buf: write }
538 }
539}
540
541#[unstable(feature = "formatting_options", issue = "118117")]
542impl Default for FormattingOptions {
543 /// Same as [`FormattingOptions::new()`].
544 fn default() -> Self {
545 // The `#[derive(Default)]` implementation would set `fill` to `\0` instead of space.
546 Self::new()
547 }
548}
549
550/// Configuration for formatting.
551///
552/// A `Formatter` represents various options related to formatting. Users do not
553/// construct `Formatter`s directly; a mutable reference to one is passed to
554/// the `fmt` method of all formatting traits, like [`Debug`] and [`Display`].
555///
556/// To interact with a `Formatter`, you'll call various methods to change the
557/// various options related to formatting. For examples, please see the
558/// documentation of the methods defined on `Formatter` below.
559#[allow(missing_debug_implementations)]
560#[stable(feature = "rust1", since = "1.0.0")]
561#[rustc_diagnostic_item = "Formatter"]
562pub struct Formatter<'a> {
563 options: FormattingOptions,
564
565 buf: &'a mut (dyn Write + 'a),
566}
567
568impl<'a> Formatter<'a> {
569 /// Creates a new formatter with given [`FormattingOptions`].
570 ///
571 /// If `write` is a reference to a formatter, it is recommended to use
572 /// [`Formatter::with_options`] instead as this can borrow the underlying
573 /// `write`, thereby bypassing one layer of indirection.
574 ///
575 /// You may alternatively use [`FormattingOptions::create_formatter()`].
576 #[unstable(feature = "formatting_options", issue = "118117")]
577 pub fn new(write: &'a mut (dyn Write + 'a), options: FormattingOptions) -> Self {
578 Formatter { options, buf: write }
579 }
580
581 /// Creates a new formatter based on this one with given [`FormattingOptions`].
582 #[unstable(feature = "formatting_options", issue = "118117")]
583 pub fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b> {
584 Formatter { options, buf: self.buf }
585 }
586}
587
588/// This structure represents a safely precompiled version of a format string
589/// and its arguments. This cannot be generated at runtime because it cannot
590/// safely be done, so no constructors are given and the fields are private
591/// to prevent modification.
592///
593/// The [`format_args!`] macro will safely create an instance of this structure.
594/// The macro validates the format string at compile-time so usage of the
595/// [`write()`] and [`format()`] functions can be safely performed.
596///
597/// You can use the `Arguments<'a>` that [`format_args!`] returns in `Debug`
598/// and `Display` contexts as seen below. The example also shows that `Debug`
599/// and `Display` format to the same thing: the interpolated format string
600/// in `format_args!`.
601///
602/// ```rust
603/// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
604/// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
605/// assert_eq!("1 foo 2", display);
606/// assert_eq!(display, debug);
607/// ```
608///
609/// [`format()`]: ../../std/fmt/fn.format.html
610#[lang = "format_arguments"]
611#[stable(feature = "rust1", since = "1.0.0")]
612#[derive(Copy, Clone)]
613pub struct Arguments<'a> {
614 // Format string pieces to print.
615 pieces: &'a [&'static str],
616
617 // Placeholder specs, or `None` if all specs are default (as in "{}{}").
618 fmt: Option<&'a [rt::Placeholder]>,
619
620 // Dynamic arguments for interpolation, to be interleaved with string
621 // pieces. (Every argument is preceded by a string piece.)
622 args: &'a [rt::Argument<'a>],
623}
624
625/// Used by the format_args!() macro to create a fmt::Arguments object.
626#[doc(hidden)]
627#[unstable(feature = "fmt_internals", issue = "none")]
628impl<'a> Arguments<'a> {
629 #[inline]
630 pub const fn new_const<const N: usize>(pieces: &'a [&'static str; N]) -> Self {
631 const { assert!(N <= 1) };
632 Arguments { pieces, fmt: None, args: &[] }
633 }
634
635 /// When using the format_args!() macro, this function is used to generate the
636 /// Arguments structure.
637 #[inline]
638 pub const fn new_v1<const P: usize, const A: usize>(
639 pieces: &'a [&'static str; P],
640 args: &'a [rt::Argument<'a>; A],
641 ) -> Arguments<'a> {
642 const { assert!(P >= A && P <= A + 1, "invalid args") }
643 Arguments { pieces, fmt: None, args }
644 }
645
646 /// Specifies nonstandard formatting parameters.
647 ///
648 /// An `rt::UnsafeArg` is required because the following invariants must be held
649 /// in order for this function to be safe:
650 /// 1. The `pieces` slice must be at least as long as `fmt`.
651 /// 2. Every `rt::Placeholder::position` value within `fmt` must be a valid index of `args`.
652 /// 3. Every `rt::Count::Param` within `fmt` must contain a valid index of `args`.
653 #[inline]
654 pub const fn new_v1_formatted(
655 pieces: &'a [&'static str],
656 args: &'a [rt::Argument<'a>],
657 fmt: &'a [rt::Placeholder],
658 _unsafe_arg: rt::UnsafeArg,
659 ) -> Arguments<'a> {
660 Arguments { pieces, fmt: Some(fmt), args }
661 }
662
663 /// Estimates the length of the formatted text.
664 ///
665 /// This is intended to be used for setting initial `String` capacity
666 /// when using `format!`. Note: this is neither the lower nor upper bound.
667 #[inline]
668 pub fn estimated_capacity(&self) -> usize {
669 let pieces_length: usize = self.pieces.iter().map(|x| x.len()).sum();
670
671 if self.args.is_empty() {
672 pieces_length
673 } else if !self.pieces.is_empty() && self.pieces[0].is_empty() && pieces_length < 16 {
674 // If the format string starts with an argument,
675 // don't preallocate anything, unless length
676 // of pieces is significant.
677 0
678 } else {
679 // There are some arguments, so any additional push
680 // will reallocate the string. To avoid that,
681 // we're "pre-doubling" the capacity here.
682 pieces_length.checked_mul(2).unwrap_or(0)
683 }
684 }
685}
686
687impl<'a> Arguments<'a> {
688 /// Gets the formatted string, if it has no arguments to be formatted at runtime.
689 ///
690 /// This can be used to avoid allocations in some cases.
691 ///
692 /// # Guarantees
693 ///
694 /// For `format_args!("just a literal")`, this function is guaranteed to
695 /// return `Some("just a literal")`.
696 ///
697 /// For most cases with placeholders, this function will return `None`.
698 ///
699 /// However, the compiler may perform optimizations that can cause this
700 /// function to return `Some(_)` even if the format string contains
701 /// placeholders. For example, `format_args!("Hello, {}!", "world")` may be
702 /// optimized to `format_args!("Hello, world!")`, such that `as_str()`
703 /// returns `Some("Hello, world!")`.
704 ///
705 /// The behavior for anything but the trivial case (without placeholders)
706 /// is not guaranteed, and should not be relied upon for anything other
707 /// than optimization.
708 ///
709 /// # Examples
710 ///
711 /// ```rust
712 /// use std::fmt::Arguments;
713 ///
714 /// fn write_str(_: &str) { /* ... */ }
715 ///
716 /// fn write_fmt(args: &Arguments<'_>) {
717 /// if let Some(s) = args.as_str() {
718 /// write_str(s)
719 /// } else {
720 /// write_str(&args.to_string());
721 /// }
722 /// }
723 /// ```
724 ///
725 /// ```rust
726 /// assert_eq!(format_args!("hello").as_str(), Some("hello"));
727 /// assert_eq!(format_args!("").as_str(), Some(""));
728 /// assert_eq!(format_args!("{:?}", std::env::current_dir()).as_str(), None);
729 /// ```
730 #[stable(feature = "fmt_as_str", since = "1.52.0")]
731 #[rustc_const_stable(feature = "const_arguments_as_str", since = "1.84.0")]
732 #[must_use]
733 #[inline]
734 pub const fn as_str(&self) -> Option<&'static str> {
735 match (self.pieces, self.args) {
736 ([], []) => Some(""),
737 ([s], []) => Some(s),
738 _ => None,
739 }
740 }
741
742 /// Same as [`Arguments::as_str`], but will only return `Some(s)` if it can be determined at compile time.
743 #[unstable(feature = "fmt_internals", reason = "internal to standard library", issue = "none")]
744 #[must_use]
745 #[inline]
746 #[doc(hidden)]
747 pub fn as_statically_known_str(&self) -> Option<&'static str> {
748 let s = self.as_str();
749 if core::intrinsics::is_val_statically_known(s.is_some()) { s } else { None }
750 }
751}
752
753// Manually implementing these results in better error messages.
754#[stable(feature = "rust1", since = "1.0.0")]
755impl !Send for Arguments<'_> {}
756#[stable(feature = "rust1", since = "1.0.0")]
757impl !Sync for Arguments<'_> {}
758
759#[stable(feature = "rust1", since = "1.0.0")]
760impl Debug for Arguments<'_> {
761 fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
762 Display::fmt(self, fmt)
763 }
764}
765
766#[stable(feature = "rust1", since = "1.0.0")]
767impl Display for Arguments<'_> {
768 fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
769 write(fmt.buf, *self)
770 }
771}
772
773/// `?` formatting.
774///
775/// `Debug` should format the output in a programmer-facing, debugging context.
776///
777/// Generally speaking, you should just `derive` a `Debug` implementation.
778///
779/// When used with the alternate format specifier `#?`, the output is pretty-printed.
780///
781/// For more information on formatters, see [the module-level documentation][module].
782///
783/// [module]: ../../std/fmt/index.html
784///
785/// This trait can be used with `#[derive]` if all fields implement `Debug`. When
786/// `derive`d for structs, it will use the name of the `struct`, then `{`, then a
787/// comma-separated list of each field's name and `Debug` value, then `}`. For
788/// `enum`s, it will use the name of the variant and, if applicable, `(`, then the
789/// `Debug` values of the fields, then `)`.
790///
791/// # Stability
792///
793/// Derived `Debug` formats are not stable, and so may change with future Rust
794/// versions. Additionally, `Debug` implementations of types provided by the
795/// standard library (`std`, `core`, `alloc`, etc.) are not stable, and
796/// may also change with future Rust versions.
797///
798/// # Examples
799///
800/// Deriving an implementation:
801///
802/// ```
803/// #[derive(Debug)]
804/// struct Point {
805/// x: i32,
806/// y: i32,
807/// }
808///
809/// let origin = Point { x: 0, y: 0 };
810///
811/// assert_eq!(
812/// format!("The origin is: {origin:?}"),
813/// "The origin is: Point { x: 0, y: 0 }",
814/// );
815/// ```
816///
817/// Manually implementing:
818///
819/// ```
820/// use std::fmt;
821///
822/// struct Point {
823/// x: i32,
824/// y: i32,
825/// }
826///
827/// impl fmt::Debug for Point {
828/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
829/// f.debug_struct("Point")
830/// .field("x", &self.x)
831/// .field("y", &self.y)
832/// .finish()
833/// }
834/// }
835///
836/// let origin = Point { x: 0, y: 0 };
837///
838/// assert_eq!(
839/// format!("The origin is: {origin:?}"),
840/// "The origin is: Point { x: 0, y: 0 }",
841/// );
842/// ```
843///
844/// There are a number of helper methods on the [`Formatter`] struct to help you with manual
845/// implementations, such as [`debug_struct`].
846///
847/// [`debug_struct`]: Formatter::debug_struct
848///
849/// Types that do not wish to use the standard suite of debug representations
850/// provided by the `Formatter` trait (`debug_struct`, `debug_tuple`,
851/// `debug_list`, `debug_set`, `debug_map`) can do something totally custom by
852/// manually writing an arbitrary representation to the `Formatter`.
853///
854/// ```
855/// # use std::fmt;
856/// # struct Point {
857/// # x: i32,
858/// # y: i32,
859/// # }
860/// #
861/// impl fmt::Debug for Point {
862/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
863/// write!(f, "Point [{} {}]", self.x, self.y)
864/// }
865/// }
866/// ```
867///
868/// `Debug` implementations using either `derive` or the debug builder API
869/// on [`Formatter`] support pretty-printing using the alternate flag: `{:#?}`.
870///
871/// Pretty-printing with `#?`:
872///
873/// ```
874/// #[derive(Debug)]
875/// struct Point {
876/// x: i32,
877/// y: i32,
878/// }
879///
880/// let origin = Point { x: 0, y: 0 };
881///
882/// let expected = "The origin is: Point {
883/// x: 0,
884/// y: 0,
885/// }";
886/// assert_eq!(format!("The origin is: {origin:#?}"), expected);
887/// ```
888
889#[stable(feature = "rust1", since = "1.0.0")]
890#[rustc_on_unimplemented(
891 on(
892 crate_local,
893 label = "`{Self}` cannot be formatted using `{{:?}}`",
894 note = "add `#[derive(Debug)]` to `{Self}` or manually `impl {Debug} for {Self}`"
895 ),
896 message = "`{Self}` doesn't implement `{Debug}`",
897 label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{Debug}`"
898)]
899#[doc(alias = "{:?}")]
900#[rustc_diagnostic_item = "Debug"]
901#[rustc_trivial_field_reads]
902pub trait Debug {
903 #[doc = include_str!("fmt_trait_method_doc.md")]
904 ///
905 /// # Examples
906 ///
907 /// ```
908 /// use std::fmt;
909 ///
910 /// struct Position {
911 /// longitude: f32,
912 /// latitude: f32,
913 /// }
914 ///
915 /// impl fmt::Debug for Position {
916 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
917 /// f.debug_tuple("")
918 /// .field(&self.longitude)
919 /// .field(&self.latitude)
920 /// .finish()
921 /// }
922 /// }
923 ///
924 /// let position = Position { longitude: 1.987, latitude: 2.983 };
925 /// assert_eq!(format!("{position:?}"), "(1.987, 2.983)");
926 ///
927 /// assert_eq!(format!("{position:#?}"), "(
928 /// 1.987,
929 /// 2.983,
930 /// )");
931 /// ```
932 #[stable(feature = "rust1", since = "1.0.0")]
933 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
934}
935
936// Separate module to reexport the macro `Debug` from prelude without the trait `Debug`.
937pub(crate) mod macros {
938 /// Derive macro generating an impl of the trait `Debug`.
939 #[rustc_builtin_macro]
940 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
941 #[allow_internal_unstable(core_intrinsics, fmt_helpers_for_derive)]
942 pub macro Debug($item:item) {
943 /* compiler built-in */
944 }
945}
946#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
947#[doc(inline)]
948pub use macros::Debug;
949
950/// Format trait for an empty format, `{}`.
951///
952/// Implementing this trait for a type will automatically implement the
953/// [`ToString`][tostring] trait for the type, allowing the usage
954/// of the [`.to_string()`][tostring_function] method. Prefer implementing
955/// the `Display` trait for a type, rather than [`ToString`][tostring].
956///
957/// `Display` is similar to [`Debug`], but `Display` is for user-facing
958/// output, and so cannot be derived.
959///
960/// For more information on formatters, see [the module-level documentation][module].
961///
962/// [module]: ../../std/fmt/index.html
963/// [tostring]: ../../std/string/trait.ToString.html
964/// [tostring_function]: ../../std/string/trait.ToString.html#tymethod.to_string
965///
966/// # Internationalization
967///
968/// Because a type can only have one `Display` implementation, it is often preferable
969/// to only implement `Display` when there is a single most "obvious" way that
970/// values can be formatted as text. This could mean formatting according to the
971/// "invariant" culture and "undefined" locale, or it could mean that the type
972/// display is designed for a specific culture/locale, such as developer logs.
973///
974/// If not all values have a justifiably canonical textual format or if you want
975/// to support alternative formats not covered by the standard set of possible
976/// [formatting traits], the most flexible approach is display adapters: methods
977/// like [`str::escape_default`] or [`Path::display`] which create a wrapper
978/// implementing `Display` to output the specific display format.
979///
980/// [formatting traits]: ../../std/fmt/index.html#formatting-traits
981/// [`Path::display`]: ../../std/path/struct.Path.html#method.display
982///
983/// # Examples
984///
985/// Implementing `Display` on a type:
986///
987/// ```
988/// use std::fmt;
989///
990/// struct Point {
991/// x: i32,
992/// y: i32,
993/// }
994///
995/// impl fmt::Display for Point {
996/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
997/// write!(f, "({}, {})", self.x, self.y)
998/// }
999/// }
1000///
1001/// let origin = Point { x: 0, y: 0 };
1002///
1003/// assert_eq!(format!("The origin is: {origin}"), "The origin is: (0, 0)");
1004/// ```
1005#[rustc_on_unimplemented(
1006 on(
1007 any(_Self = "std::path::Path", _Self = "std::path::PathBuf"),
1008 label = "`{Self}` cannot be formatted with the default formatter; call `.display()` on it",
1009 note = "call `.display()` or `.to_string_lossy()` to safely print paths, \
1010 as they may contain non-Unicode data"
1011 ),
1012 message = "`{Self}` doesn't implement `{Display}`",
1013 label = "`{Self}` cannot be formatted with the default formatter",
1014 note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead"
1015)]
1016#[doc(alias = "{}")]
1017#[rustc_diagnostic_item = "Display"]
1018#[stable(feature = "rust1", since = "1.0.0")]
1019pub trait Display {
1020 #[doc = include_str!("fmt_trait_method_doc.md")]
1021 ///
1022 /// # Examples
1023 ///
1024 /// ```
1025 /// use std::fmt;
1026 ///
1027 /// struct Position {
1028 /// longitude: f32,
1029 /// latitude: f32,
1030 /// }
1031 ///
1032 /// impl fmt::Display for Position {
1033 /// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1034 /// write!(f, "({}, {})", self.longitude, self.latitude)
1035 /// }
1036 /// }
1037 ///
1038 /// assert_eq!(
1039 /// "(1.987, 2.983)",
1040 /// format!("{}", Position { longitude: 1.987, latitude: 2.983, }),
1041 /// );
1042 /// ```
1043 #[stable(feature = "rust1", since = "1.0.0")]
1044 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1045}
1046
1047/// `o` formatting.
1048///
1049/// The `Octal` trait should format its output as a number in base-8.
1050///
1051/// For primitive signed integers (`i8` to `i128`, and `isize`),
1052/// negative values are formatted as the two’s complement representation.
1053///
1054/// The alternate flag, `#`, adds a `0o` in front of the output.
1055///
1056/// For more information on formatters, see [the module-level documentation][module].
1057///
1058/// [module]: ../../std/fmt/index.html
1059///
1060/// # Examples
1061///
1062/// Basic usage with `i32`:
1063///
1064/// ```
1065/// let x = 42; // 42 is '52' in octal
1066///
1067/// assert_eq!(format!("{x:o}"), "52");
1068/// assert_eq!(format!("{x:#o}"), "0o52");
1069///
1070/// assert_eq!(format!("{:o}", -16), "37777777760");
1071/// ```
1072///
1073/// Implementing `Octal` on a type:
1074///
1075/// ```
1076/// use std::fmt;
1077///
1078/// struct Length(i32);
1079///
1080/// impl fmt::Octal for Length {
1081/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1082/// let val = self.0;
1083///
1084/// fmt::Octal::fmt(&val, f) // delegate to i32's implementation
1085/// }
1086/// }
1087///
1088/// let l = Length(9);
1089///
1090/// assert_eq!(format!("l as octal is: {l:o}"), "l as octal is: 11");
1091///
1092/// assert_eq!(format!("l as octal is: {l:#06o}"), "l as octal is: 0o0011");
1093/// ```
1094#[stable(feature = "rust1", since = "1.0.0")]
1095pub trait Octal {
1096 #[doc = include_str!("fmt_trait_method_doc.md")]
1097 #[stable(feature = "rust1", since = "1.0.0")]
1098 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1099}
1100
1101/// `b` formatting.
1102///
1103/// The `Binary` trait should format its output as a number in binary.
1104///
1105/// For primitive signed integers ([`i8`] to [`i128`], and [`isize`]),
1106/// negative values are formatted as the two’s complement representation.
1107///
1108/// The alternate flag, `#`, adds a `0b` in front of the output.
1109///
1110/// For more information on formatters, see [the module-level documentation][module].
1111///
1112/// [module]: ../../std/fmt/index.html
1113///
1114/// # Examples
1115///
1116/// Basic usage with [`i32`]:
1117///
1118/// ```
1119/// let x = 42; // 42 is '101010' in binary
1120///
1121/// assert_eq!(format!("{x:b}"), "101010");
1122/// assert_eq!(format!("{x:#b}"), "0b101010");
1123///
1124/// assert_eq!(format!("{:b}", -16), "11111111111111111111111111110000");
1125/// ```
1126///
1127/// Implementing `Binary` on a type:
1128///
1129/// ```
1130/// use std::fmt;
1131///
1132/// struct Length(i32);
1133///
1134/// impl fmt::Binary for Length {
1135/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1136/// let val = self.0;
1137///
1138/// fmt::Binary::fmt(&val, f) // delegate to i32's implementation
1139/// }
1140/// }
1141///
1142/// let l = Length(107);
1143///
1144/// assert_eq!(format!("l as binary is: {l:b}"), "l as binary is: 1101011");
1145///
1146/// assert_eq!(
1147/// // Note that the `0b` prefix added by `#` is included in the total width, so we
1148/// // need to add two to correctly display all 32 bits.
1149/// format!("l as binary is: {l:#034b}"),
1150/// "l as binary is: 0b00000000000000000000000001101011"
1151/// );
1152/// ```
1153#[stable(feature = "rust1", since = "1.0.0")]
1154pub trait Binary {
1155 #[doc = include_str!("fmt_trait_method_doc.md")]
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1158}
1159
1160/// `x` formatting.
1161///
1162/// The `LowerHex` trait should format its output as a number in hexadecimal, with `a` through `f`
1163/// in lower case.
1164///
1165/// For primitive signed integers (`i8` to `i128`, and `isize`),
1166/// negative values are formatted as the two’s complement representation.
1167///
1168/// The alternate flag, `#`, adds a `0x` in front of the output.
1169///
1170/// For more information on formatters, see [the module-level documentation][module].
1171///
1172/// [module]: ../../std/fmt/index.html
1173///
1174/// # Examples
1175///
1176/// Basic usage with `i32`:
1177///
1178/// ```
1179/// let y = 42; // 42 is '2a' in hex
1180///
1181/// assert_eq!(format!("{y:x}"), "2a");
1182/// assert_eq!(format!("{y:#x}"), "0x2a");
1183///
1184/// assert_eq!(format!("{:x}", -16), "fffffff0");
1185/// ```
1186///
1187/// Implementing `LowerHex` on a type:
1188///
1189/// ```
1190/// use std::fmt;
1191///
1192/// struct Length(i32);
1193///
1194/// impl fmt::LowerHex for Length {
1195/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1196/// let val = self.0;
1197///
1198/// fmt::LowerHex::fmt(&val, f) // delegate to i32's implementation
1199/// }
1200/// }
1201///
1202/// let l = Length(9);
1203///
1204/// assert_eq!(format!("l as hex is: {l:x}"), "l as hex is: 9");
1205///
1206/// assert_eq!(format!("l as hex is: {l:#010x}"), "l as hex is: 0x00000009");
1207/// ```
1208#[stable(feature = "rust1", since = "1.0.0")]
1209pub trait LowerHex {
1210 #[doc = include_str!("fmt_trait_method_doc.md")]
1211 #[stable(feature = "rust1", since = "1.0.0")]
1212 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1213}
1214
1215/// `X` formatting.
1216///
1217/// The `UpperHex` trait should format its output as a number in hexadecimal, with `A` through `F`
1218/// in upper case.
1219///
1220/// For primitive signed integers (`i8` to `i128`, and `isize`),
1221/// negative values are formatted as the two’s complement representation.
1222///
1223/// The alternate flag, `#`, adds a `0x` in front of the output.
1224///
1225/// For more information on formatters, see [the module-level documentation][module].
1226///
1227/// [module]: ../../std/fmt/index.html
1228///
1229/// # Examples
1230///
1231/// Basic usage with `i32`:
1232///
1233/// ```
1234/// let y = 42; // 42 is '2A' in hex
1235///
1236/// assert_eq!(format!("{y:X}"), "2A");
1237/// assert_eq!(format!("{y:#X}"), "0x2A");
1238///
1239/// assert_eq!(format!("{:X}", -16), "FFFFFFF0");
1240/// ```
1241///
1242/// Implementing `UpperHex` on a type:
1243///
1244/// ```
1245/// use std::fmt;
1246///
1247/// struct Length(i32);
1248///
1249/// impl fmt::UpperHex for Length {
1250/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1251/// let val = self.0;
1252///
1253/// fmt::UpperHex::fmt(&val, f) // delegate to i32's implementation
1254/// }
1255/// }
1256///
1257/// let l = Length(i32::MAX);
1258///
1259/// assert_eq!(format!("l as hex is: {l:X}"), "l as hex is: 7FFFFFFF");
1260///
1261/// assert_eq!(format!("l as hex is: {l:#010X}"), "l as hex is: 0x7FFFFFFF");
1262/// ```
1263#[stable(feature = "rust1", since = "1.0.0")]
1264pub trait UpperHex {
1265 #[doc = include_str!("fmt_trait_method_doc.md")]
1266 #[stable(feature = "rust1", since = "1.0.0")]
1267 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1268}
1269
1270/// `p` formatting.
1271///
1272/// The `Pointer` trait should format its output as a memory location. This is commonly presented
1273/// as hexadecimal. For more information on formatters, see [the module-level documentation][module].
1274///
1275/// Printing of pointers is not a reliable way to discover how Rust programs are implemented.
1276/// The act of reading an address changes the program itself, and may change how the data is represented
1277/// in memory, and may affect which optimizations are applied to the code.
1278///
1279/// The printed pointer values are not guaranteed to be stable nor unique identifiers of objects.
1280/// Rust allows moving values to different memory locations, and may reuse the same memory locations
1281/// for different purposes.
1282///
1283/// There is no guarantee that the printed value can be converted back to a pointer.
1284///
1285/// [module]: ../../std/fmt/index.html
1286///
1287/// # Examples
1288///
1289/// Basic usage with `&i32`:
1290///
1291/// ```
1292/// let x = &42;
1293///
1294/// let address = format!("{x:p}"); // this produces something like '0x7f06092ac6d0'
1295/// ```
1296///
1297/// Implementing `Pointer` on a type:
1298///
1299/// ```
1300/// use std::fmt;
1301///
1302/// struct Length(i32);
1303///
1304/// impl fmt::Pointer for Length {
1305/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1306/// // use `as` to convert to a `*const T`, which implements Pointer, which we can use
1307///
1308/// let ptr = self as *const Self;
1309/// fmt::Pointer::fmt(&ptr, f)
1310/// }
1311/// }
1312///
1313/// let l = Length(42);
1314///
1315/// println!("l is in memory here: {l:p}");
1316///
1317/// let l_ptr = format!("{l:018p}");
1318/// assert_eq!(l_ptr.len(), 18);
1319/// assert_eq!(&l_ptr[..2], "0x");
1320/// ```
1321#[stable(feature = "rust1", since = "1.0.0")]
1322#[rustc_diagnostic_item = "Pointer"]
1323pub trait Pointer {
1324 #[doc = include_str!("fmt_trait_method_doc.md")]
1325 #[stable(feature = "rust1", since = "1.0.0")]
1326 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1327}
1328
1329/// `e` formatting.
1330///
1331/// The `LowerExp` trait should format its output in scientific notation with a lower-case `e`.
1332///
1333/// For more information on formatters, see [the module-level documentation][module].
1334///
1335/// [module]: ../../std/fmt/index.html
1336///
1337/// # Examples
1338///
1339/// Basic usage with `f64`:
1340///
1341/// ```
1342/// let x = 42.0; // 42.0 is '4.2e1' in scientific notation
1343///
1344/// assert_eq!(format!("{x:e}"), "4.2e1");
1345/// ```
1346///
1347/// Implementing `LowerExp` on a type:
1348///
1349/// ```
1350/// use std::fmt;
1351///
1352/// struct Length(i32);
1353///
1354/// impl fmt::LowerExp for Length {
1355/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1356/// let val = f64::from(self.0);
1357/// fmt::LowerExp::fmt(&val, f) // delegate to f64's implementation
1358/// }
1359/// }
1360///
1361/// let l = Length(100);
1362///
1363/// assert_eq!(
1364/// format!("l in scientific notation is: {l:e}"),
1365/// "l in scientific notation is: 1e2"
1366/// );
1367///
1368/// assert_eq!(
1369/// format!("l in scientific notation is: {l:05e}"),
1370/// "l in scientific notation is: 001e2"
1371/// );
1372/// ```
1373#[stable(feature = "rust1", since = "1.0.0")]
1374pub trait LowerExp {
1375 #[doc = include_str!("fmt_trait_method_doc.md")]
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1378}
1379
1380/// `E` formatting.
1381///
1382/// The `UpperExp` trait should format its output in scientific notation with an upper-case `E`.
1383///
1384/// For more information on formatters, see [the module-level documentation][module].
1385///
1386/// [module]: ../../std/fmt/index.html
1387///
1388/// # Examples
1389///
1390/// Basic usage with `f64`:
1391///
1392/// ```
1393/// let x = 42.0; // 42.0 is '4.2E1' in scientific notation
1394///
1395/// assert_eq!(format!("{x:E}"), "4.2E1");
1396/// ```
1397///
1398/// Implementing `UpperExp` on a type:
1399///
1400/// ```
1401/// use std::fmt;
1402///
1403/// struct Length(i32);
1404///
1405/// impl fmt::UpperExp for Length {
1406/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1407/// let val = f64::from(self.0);
1408/// fmt::UpperExp::fmt(&val, f) // delegate to f64's implementation
1409/// }
1410/// }
1411///
1412/// let l = Length(100);
1413///
1414/// assert_eq!(
1415/// format!("l in scientific notation is: {l:E}"),
1416/// "l in scientific notation is: 1E2"
1417/// );
1418///
1419/// assert_eq!(
1420/// format!("l in scientific notation is: {l:05E}"),
1421/// "l in scientific notation is: 001E2"
1422/// );
1423/// ```
1424#[stable(feature = "rust1", since = "1.0.0")]
1425pub trait UpperExp {
1426 #[doc = include_str!("fmt_trait_method_doc.md")]
1427 #[stable(feature = "rust1", since = "1.0.0")]
1428 fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1429}
1430
1431/// Takes an output stream and an `Arguments` struct that can be precompiled with
1432/// the `format_args!` macro.
1433///
1434/// The arguments will be formatted according to the specified format string
1435/// into the output stream provided.
1436///
1437/// # Examples
1438///
1439/// Basic usage:
1440///
1441/// ```
1442/// use std::fmt;
1443///
1444/// let mut output = String::new();
1445/// fmt::write(&mut output, format_args!("Hello {}!", "world"))
1446/// .expect("Error occurred while trying to write in String");
1447/// assert_eq!(output, "Hello world!");
1448/// ```
1449///
1450/// Please note that using [`write!`] might be preferable. Example:
1451///
1452/// ```
1453/// use std::fmt::Write;
1454///
1455/// let mut output = String::new();
1456/// write!(&mut output, "Hello {}!", "world")
1457/// .expect("Error occurred while trying to write in String");
1458/// assert_eq!(output, "Hello world!");
1459/// ```
1460///
1461/// [`write!`]: crate::write!
1462#[stable(feature = "rust1", since = "1.0.0")]
1463pub fn write(output: &mut dyn Write, args: Arguments<'_>) -> Result {
1464 let mut formatter = Formatter::new(output, FormattingOptions::new());
1465 let mut idx = 0;
1466
1467 match args.fmt {
1468 None => {
1469 // We can use default formatting parameters for all arguments.
1470 for (i, arg) in args.args.iter().enumerate() {
1471 // SAFETY: args.args and args.pieces come from the same Arguments,
1472 // which guarantees the indexes are always within bounds.
1473 let piece = unsafe { args.pieces.get_unchecked(i) };
1474 if !piece.is_empty() {
1475 formatter.buf.write_str(*piece)?;
1476 }
1477
1478 // SAFETY: There are no formatting parameters and hence no
1479 // count arguments.
1480 unsafe {
1481 arg.fmt(&mut formatter)?;
1482 }
1483 idx += 1;
1484 }
1485 }
1486 Some(fmt) => {
1487 // Every spec has a corresponding argument that is preceded by
1488 // a string piece.
1489 for (i, arg) in fmt.iter().enumerate() {
1490 // SAFETY: fmt and args.pieces come from the same Arguments,
1491 // which guarantees the indexes are always within bounds.
1492 let piece = unsafe { args.pieces.get_unchecked(i) };
1493 if !piece.is_empty() {
1494 formatter.buf.write_str(*piece)?;
1495 }
1496 // SAFETY: arg and args.args come from the same Arguments,
1497 // which guarantees the indexes are always within bounds.
1498 unsafe { run(&mut formatter, arg, args.args) }?;
1499 idx += 1;
1500 }
1501 }
1502 }
1503
1504 // There can be only one trailing string piece left.
1505 if let Some(piece) = args.pieces.get(idx) {
1506 formatter.buf.write_str(*piece)?;
1507 }
1508
1509 Ok(())
1510}
1511
1512unsafe fn run(fmt: &mut Formatter<'_>, arg: &rt::Placeholder, args: &[rt::Argument<'_>]) -> Result {
1513 let (width, precision) =
1514 // SAFETY: arg and args come from the same Arguments,
1515 // which guarantees the indexes are always within bounds.
1516 unsafe { (getcount(args, &arg.width), getcount(args, &arg.precision)) };
1517
1518 let options = FormattingOptions { flags: arg.flags, width, precision };
1519
1520 // Extract the correct argument
1521 debug_assert!(arg.position < args.len());
1522 // SAFETY: arg and args come from the same Arguments,
1523 // which guarantees its index is always within bounds.
1524 let value = unsafe { args.get_unchecked(arg.position) };
1525
1526 // Set all the formatting options.
1527 fmt.options = options;
1528
1529 // Then actually do some printing
1530 // SAFETY: this is a placeholder argument.
1531 unsafe { value.fmt(fmt) }
1532}
1533
1534unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> u16 {
1535 match *cnt {
1536 rt::Count::Is(n) => n,
1537 rt::Count::Implied => 0,
1538 rt::Count::Param(i) => {
1539 debug_assert!(i < args.len());
1540 // SAFETY: cnt and args come from the same Arguments,
1541 // which guarantees this index is always within bounds.
1542 unsafe { args.get_unchecked(i).as_u16().unwrap_unchecked() }
1543 }
1544 }
1545}
1546
1547/// Padding after the end of something. Returned by `Formatter::padding`.
1548#[must_use = "don't forget to write the post padding"]
1549pub(crate) struct PostPadding {
1550 fill: char,
1551 padding: u16,
1552}
1553
1554impl PostPadding {
1555 fn new(fill: char, padding: u16) -> PostPadding {
1556 PostPadding { fill, padding }
1557 }
1558
1559 /// Writes this post padding.
1560 pub(crate) fn write(self, f: &mut Formatter<'_>) -> Result {
1561 for _ in 0..self.padding {
1562 f.buf.write_char(self.fill)?;
1563 }
1564 Ok(())
1565 }
1566}
1567
1568impl<'a> Formatter<'a> {
1569 fn wrap_buf<'b, 'c, F>(&'b mut self, wrap: F) -> Formatter<'c>
1570 where
1571 'b: 'c,
1572 F: FnOnce(&'b mut (dyn Write + 'b)) -> &'c mut (dyn Write + 'c),
1573 {
1574 Formatter {
1575 // We want to change this
1576 buf: wrap(self.buf),
1577
1578 // And preserve these
1579 options: self.options,
1580 }
1581 }
1582
1583 // Helper methods used for padding and processing formatting arguments that
1584 // all formatting traits can use.
1585
1586 /// Performs the correct padding for an integer which has already been
1587 /// emitted into a str. The str should *not* contain the sign for the
1588 /// integer, that will be added by this method.
1589 ///
1590 /// # Arguments
1591 ///
1592 /// * is_nonnegative - whether the original integer was either positive or zero.
1593 /// * prefix - if the '#' character (Alternate) is provided, this
1594 /// is the prefix to put in front of the number.
1595 /// * buf - the byte array that the number has been formatted into
1596 ///
1597 /// This function will correctly account for the flags provided as well as
1598 /// the minimum width. It will not take precision into account.
1599 ///
1600 /// # Examples
1601 ///
1602 /// ```
1603 /// use std::fmt;
1604 ///
1605 /// struct Foo { nb: i32 }
1606 ///
1607 /// impl Foo {
1608 /// fn new(nb: i32) -> Foo {
1609 /// Foo {
1610 /// nb,
1611 /// }
1612 /// }
1613 /// }
1614 ///
1615 /// impl fmt::Display for Foo {
1616 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1617 /// // We need to remove "-" from the number output.
1618 /// let tmp = self.nb.abs().to_string();
1619 ///
1620 /// formatter.pad_integral(self.nb >= 0, "Foo ", &tmp)
1621 /// }
1622 /// }
1623 ///
1624 /// assert_eq!(format!("{}", Foo::new(2)), "2");
1625 /// assert_eq!(format!("{}", Foo::new(-1)), "-1");
1626 /// assert_eq!(format!("{}", Foo::new(0)), "0");
1627 /// assert_eq!(format!("{:#}", Foo::new(-1)), "-Foo 1");
1628 /// assert_eq!(format!("{:0>#8}", Foo::new(-1)), "00-Foo 1");
1629 /// ```
1630 #[stable(feature = "rust1", since = "1.0.0")]
1631 pub fn pad_integral(&mut self, is_nonnegative: bool, prefix: &str, buf: &str) -> Result {
1632 let mut width = buf.len();
1633
1634 let mut sign = None;
1635 if !is_nonnegative {
1636 sign = Some('-');
1637 width += 1;
1638 } else if self.sign_plus() {
1639 sign = Some('+');
1640 width += 1;
1641 }
1642
1643 let prefix = if self.alternate() {
1644 width += prefix.chars().count();
1645 Some(prefix)
1646 } else {
1647 None
1648 };
1649
1650 // Writes the sign if it exists, and then the prefix if it was requested
1651 #[inline(never)]
1652 fn write_prefix(f: &mut Formatter<'_>, sign: Option<char>, prefix: Option<&str>) -> Result {
1653 if let Some(c) = sign {
1654 f.buf.write_char(c)?;
1655 }
1656 if let Some(prefix) = prefix { f.buf.write_str(prefix) } else { Ok(()) }
1657 }
1658
1659 // The `width` field is more of a `min-width` parameter at this point.
1660 let min = self.options.width;
1661 if width >= usize::from(min) {
1662 // We're over the minimum width, so then we can just write the bytes.
1663 write_prefix(self, sign, prefix)?;
1664 self.buf.write_str(buf)
1665 } else if self.sign_aware_zero_pad() {
1666 // The sign and prefix goes before the padding if the fill character
1667 // is zero
1668 let old_options = self.options;
1669 self.options.fill('0').align(Some(Alignment::Right));
1670 write_prefix(self, sign, prefix)?;
1671 let post_padding = self.padding(min - width as u16, Alignment::Right)?;
1672 self.buf.write_str(buf)?;
1673 post_padding.write(self)?;
1674 self.options = old_options;
1675 Ok(())
1676 } else {
1677 // Otherwise, the sign and prefix goes after the padding
1678 let post_padding = self.padding(min - width as u16, Alignment::Right)?;
1679 write_prefix(self, sign, prefix)?;
1680 self.buf.write_str(buf)?;
1681 post_padding.write(self)
1682 }
1683 }
1684
1685 /// Takes a string slice and emits it to the internal buffer after applying
1686 /// the relevant formatting flags specified.
1687 ///
1688 /// The flags recognized for generic strings are:
1689 ///
1690 /// * width - the minimum width of what to emit
1691 /// * fill/align - what to emit and where to emit it if the string
1692 /// provided needs to be padded
1693 /// * precision - the maximum length to emit, the string is truncated if it
1694 /// is longer than this length
1695 ///
1696 /// Notably this function ignores the `flag` parameters.
1697 ///
1698 /// # Examples
1699 ///
1700 /// ```
1701 /// use std::fmt;
1702 ///
1703 /// struct Foo;
1704 ///
1705 /// impl fmt::Display for Foo {
1706 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1707 /// formatter.pad("Foo")
1708 /// }
1709 /// }
1710 ///
1711 /// assert_eq!(format!("{Foo:<4}"), "Foo ");
1712 /// assert_eq!(format!("{Foo:0>4}"), "0Foo");
1713 /// ```
1714 #[stable(feature = "rust1", since = "1.0.0")]
1715 pub fn pad(&mut self, s: &str) -> Result {
1716 // Make sure there's a fast path up front.
1717 if self.options.flags & (flags::WIDTH_FLAG | flags::PRECISION_FLAG) == 0 {
1718 return self.buf.write_str(s);
1719 }
1720
1721 // The `precision` field can be interpreted as a maximum width for the
1722 // string being formatted.
1723 let (s, char_count) = if let Some(max_char_count) = self.options.get_precision() {
1724 let mut iter = s.char_indices();
1725 let remaining = match iter.advance_by(usize::from(max_char_count)) {
1726 Ok(()) => 0,
1727 Err(remaining) => remaining.get(),
1728 };
1729 // SAFETY: The offset of `.char_indices()` is guaranteed to be
1730 // in-bounds and between character boundaries.
1731 let truncated = unsafe { s.get_unchecked(..iter.offset()) };
1732 (truncated, usize::from(max_char_count) - remaining)
1733 } else {
1734 // Use the optimized char counting algorithm for the full string.
1735 (s, s.chars().count())
1736 };
1737
1738 // The `width` field is more of a minimum width parameter at this point.
1739 if char_count < usize::from(self.options.width) {
1740 // If we're under the minimum width, then fill up the minimum width
1741 // with the specified string + some alignment.
1742 let post_padding =
1743 self.padding(self.options.width - char_count as u16, Alignment::Left)?;
1744 self.buf.write_str(s)?;
1745 post_padding.write(self)
1746 } else {
1747 // If we're over the minimum width or there is no minimum width, we
1748 // can just emit the string.
1749 self.buf.write_str(s)
1750 }
1751 }
1752
1753 /// Writes the pre-padding and returns the unwritten post-padding.
1754 ///
1755 /// Callers are responsible for ensuring post-padding is written after the
1756 /// thing that is being padded.
1757 pub(crate) fn padding(
1758 &mut self,
1759 padding: u16,
1760 default: Alignment,
1761 ) -> result::Result<PostPadding, Error> {
1762 let align = self.options.get_align().unwrap_or(default);
1763 let fill = self.options.get_fill();
1764
1765 let padding_left = match align {
1766 Alignment::Left => 0,
1767 Alignment::Right => padding,
1768 Alignment::Center => padding / 2,
1769 };
1770
1771 for _ in 0..padding_left {
1772 self.buf.write_char(fill)?;
1773 }
1774
1775 Ok(PostPadding::new(fill, padding - padding_left))
1776 }
1777
1778 /// Takes the formatted parts and applies the padding.
1779 ///
1780 /// Assumes that the caller already has rendered the parts with required precision,
1781 /// so that `self.precision` can be ignored.
1782 ///
1783 /// # Safety
1784 ///
1785 /// Any `numfmt::Part::Copy` parts in `formatted` must contain valid UTF-8.
1786 unsafe fn pad_formatted_parts(&mut self, formatted: &numfmt::Formatted<'_>) -> Result {
1787 if self.options.width == 0 {
1788 // this is the common case and we take a shortcut
1789 // SAFETY: Per the precondition.
1790 unsafe { self.write_formatted_parts(formatted) }
1791 } else {
1792 // for the sign-aware zero padding, we render the sign first and
1793 // behave as if we had no sign from the beginning.
1794 let mut formatted = formatted.clone();
1795 let mut width = self.options.width;
1796 let old_options = self.options;
1797 if self.sign_aware_zero_pad() {
1798 // a sign always goes first
1799 let sign = formatted.sign;
1800 self.buf.write_str(sign)?;
1801
1802 // remove the sign from the formatted parts
1803 formatted.sign = "";
1804 width = width.saturating_sub(sign.len() as u16);
1805 self.options.fill('0').align(Some(Alignment::Right));
1806 }
1807
1808 // remaining parts go through the ordinary padding process.
1809 let len = formatted.len();
1810 let ret = if usize::from(width) <= len {
1811 // no padding
1812 // SAFETY: Per the precondition.
1813 unsafe { self.write_formatted_parts(&formatted) }
1814 } else {
1815 let post_padding = self.padding(width - len as u16, Alignment::Right)?;
1816 // SAFETY: Per the precondition.
1817 unsafe {
1818 self.write_formatted_parts(&formatted)?;
1819 }
1820 post_padding.write(self)
1821 };
1822 self.options = old_options;
1823 ret
1824 }
1825 }
1826
1827 /// # Safety
1828 ///
1829 /// Any `numfmt::Part::Copy` parts in `formatted` must contain valid UTF-8.
1830 unsafe fn write_formatted_parts(&mut self, formatted: &numfmt::Formatted<'_>) -> Result {
1831 unsafe fn write_bytes(buf: &mut dyn Write, s: &[u8]) -> Result {
1832 // SAFETY: This is used for `numfmt::Part::Num` and `numfmt::Part::Copy`.
1833 // It's safe to use for `numfmt::Part::Num` since every char `c` is between
1834 // `b'0'` and `b'9'`, which means `s` is valid UTF-8. It's safe to use for
1835 // `numfmt::Part::Copy` due to this function's precondition.
1836 buf.write_str(unsafe { str::from_utf8_unchecked(s) })
1837 }
1838
1839 if !formatted.sign.is_empty() {
1840 self.buf.write_str(formatted.sign)?;
1841 }
1842 for part in formatted.parts {
1843 match *part {
1844 numfmt::Part::Zero(mut nzeroes) => {
1845 const ZEROES: &str = // 64 zeroes
1846 "0000000000000000000000000000000000000000000000000000000000000000";
1847 while nzeroes > ZEROES.len() {
1848 self.buf.write_str(ZEROES)?;
1849 nzeroes -= ZEROES.len();
1850 }
1851 if nzeroes > 0 {
1852 self.buf.write_str(&ZEROES[..nzeroes])?;
1853 }
1854 }
1855 numfmt::Part::Num(mut v) => {
1856 let mut s = [0; 5];
1857 let len = part.len();
1858 for c in s[..len].iter_mut().rev() {
1859 *c = b'0' + (v % 10) as u8;
1860 v /= 10;
1861 }
1862 // SAFETY: Per the precondition.
1863 unsafe {
1864 write_bytes(self.buf, &s[..len])?;
1865 }
1866 }
1867 // SAFETY: Per the precondition.
1868 numfmt::Part::Copy(buf) => unsafe {
1869 write_bytes(self.buf, buf)?;
1870 },
1871 }
1872 }
1873 Ok(())
1874 }
1875
1876 /// Writes some data to the underlying buffer contained within this
1877 /// formatter.
1878 ///
1879 /// # Examples
1880 ///
1881 /// ```
1882 /// use std::fmt;
1883 ///
1884 /// struct Foo;
1885 ///
1886 /// impl fmt::Display for Foo {
1887 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1888 /// formatter.write_str("Foo")
1889 /// // This is equivalent to:
1890 /// // write!(formatter, "Foo")
1891 /// }
1892 /// }
1893 ///
1894 /// assert_eq!(format!("{Foo}"), "Foo");
1895 /// assert_eq!(format!("{Foo:0>8}"), "Foo");
1896 /// ```
1897 #[stable(feature = "rust1", since = "1.0.0")]
1898 pub fn write_str(&mut self, data: &str) -> Result {
1899 self.buf.write_str(data)
1900 }
1901
1902 /// Glue for usage of the [`write!`] macro with implementors of this trait.
1903 ///
1904 /// This method should generally not be invoked manually, but rather through
1905 /// the [`write!`] macro itself.
1906 ///
1907 /// Writes some formatted information into this instance.
1908 ///
1909 /// # Examples
1910 ///
1911 /// ```
1912 /// use std::fmt;
1913 ///
1914 /// struct Foo(i32);
1915 ///
1916 /// impl fmt::Display for Foo {
1917 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1918 /// formatter.write_fmt(format_args!("Foo {}", self.0))
1919 /// }
1920 /// }
1921 ///
1922 /// assert_eq!(format!("{}", Foo(-1)), "Foo -1");
1923 /// assert_eq!(format!("{:0>8}", Foo(2)), "Foo 2");
1924 /// ```
1925 #[stable(feature = "rust1", since = "1.0.0")]
1926 #[inline]
1927 pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result {
1928 if let Some(s) = fmt.as_statically_known_str() {
1929 self.buf.write_str(s)
1930 } else {
1931 write(self.buf, fmt)
1932 }
1933 }
1934
1935 /// Returns flags for formatting.
1936 #[must_use]
1937 #[stable(feature = "rust1", since = "1.0.0")]
1938 #[deprecated(
1939 since = "1.24.0",
1940 note = "use the `sign_plus`, `sign_minus`, `alternate`, \
1941 or `sign_aware_zero_pad` methods instead"
1942 )]
1943 pub fn flags(&self) -> u32 {
1944 // Extract the debug upper/lower hex, zero pad, alternate, and plus/minus flags
1945 // to stay compatible with older versions of Rust.
1946 self.options.flags >> 21 & 0x3F
1947 }
1948
1949 /// Returns the character used as 'fill' whenever there is alignment.
1950 ///
1951 /// # Examples
1952 ///
1953 /// ```
1954 /// use std::fmt;
1955 ///
1956 /// struct Foo;
1957 ///
1958 /// impl fmt::Display for Foo {
1959 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1960 /// let c = formatter.fill();
1961 /// if let Some(width) = formatter.width() {
1962 /// for _ in 0..width {
1963 /// write!(formatter, "{c}")?;
1964 /// }
1965 /// Ok(())
1966 /// } else {
1967 /// write!(formatter, "{c}")
1968 /// }
1969 /// }
1970 /// }
1971 ///
1972 /// // We set alignment to the right with ">".
1973 /// assert_eq!(format!("{Foo:G>3}"), "GGG");
1974 /// assert_eq!(format!("{Foo:t>6}"), "tttttt");
1975 /// ```
1976 #[must_use]
1977 #[stable(feature = "fmt_flags", since = "1.5.0")]
1978 pub fn fill(&self) -> char {
1979 self.options.get_fill()
1980 }
1981
1982 /// Returns a flag indicating what form of alignment was requested.
1983 ///
1984 /// # Examples
1985 ///
1986 /// ```
1987 /// use std::fmt::{self, Alignment};
1988 ///
1989 /// struct Foo;
1990 ///
1991 /// impl fmt::Display for Foo {
1992 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1993 /// let s = if let Some(s) = formatter.align() {
1994 /// match s {
1995 /// Alignment::Left => "left",
1996 /// Alignment::Right => "right",
1997 /// Alignment::Center => "center",
1998 /// }
1999 /// } else {
2000 /// "into the void"
2001 /// };
2002 /// write!(formatter, "{s}")
2003 /// }
2004 /// }
2005 ///
2006 /// assert_eq!(format!("{Foo:<}"), "left");
2007 /// assert_eq!(format!("{Foo:>}"), "right");
2008 /// assert_eq!(format!("{Foo:^}"), "center");
2009 /// assert_eq!(format!("{Foo}"), "into the void");
2010 /// ```
2011 #[must_use]
2012 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
2013 pub fn align(&self) -> Option<Alignment> {
2014 self.options.get_align()
2015 }
2016
2017 /// Returns the optionally specified integer width that the output should be.
2018 ///
2019 /// # Examples
2020 ///
2021 /// ```
2022 /// use std::fmt;
2023 ///
2024 /// struct Foo(i32);
2025 ///
2026 /// impl fmt::Display for Foo {
2027 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2028 /// if let Some(width) = formatter.width() {
2029 /// // If we received a width, we use it
2030 /// write!(formatter, "{:width$}", format!("Foo({})", self.0), width = width)
2031 /// } else {
2032 /// // Otherwise we do nothing special
2033 /// write!(formatter, "Foo({})", self.0)
2034 /// }
2035 /// }
2036 /// }
2037 ///
2038 /// assert_eq!(format!("{:10}", Foo(23)), "Foo(23) ");
2039 /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2040 /// ```
2041 #[must_use]
2042 #[stable(feature = "fmt_flags", since = "1.5.0")]
2043 pub fn width(&self) -> Option<usize> {
2044 if self.options.flags & flags::WIDTH_FLAG == 0 {
2045 None
2046 } else {
2047 Some(self.options.width as usize)
2048 }
2049 }
2050
2051 /// Returns the optionally specified precision for numeric types.
2052 /// Alternatively, the maximum width for string types.
2053 ///
2054 /// # Examples
2055 ///
2056 /// ```
2057 /// use std::fmt;
2058 ///
2059 /// struct Foo(f32);
2060 ///
2061 /// impl fmt::Display for Foo {
2062 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2063 /// if let Some(precision) = formatter.precision() {
2064 /// // If we received a precision, we use it.
2065 /// write!(formatter, "Foo({1:.*})", precision, self.0)
2066 /// } else {
2067 /// // Otherwise we default to 2.
2068 /// write!(formatter, "Foo({:.2})", self.0)
2069 /// }
2070 /// }
2071 /// }
2072 ///
2073 /// assert_eq!(format!("{:.4}", Foo(23.2)), "Foo(23.2000)");
2074 /// assert_eq!(format!("{}", Foo(23.2)), "Foo(23.20)");
2075 /// ```
2076 #[must_use]
2077 #[stable(feature = "fmt_flags", since = "1.5.0")]
2078 pub fn precision(&self) -> Option<usize> {
2079 if self.options.flags & flags::PRECISION_FLAG == 0 {
2080 None
2081 } else {
2082 Some(self.options.precision as usize)
2083 }
2084 }
2085
2086 /// Determines if the `+` flag was specified.
2087 ///
2088 /// # Examples
2089 ///
2090 /// ```
2091 /// use std::fmt;
2092 ///
2093 /// struct Foo(i32);
2094 ///
2095 /// impl fmt::Display for Foo {
2096 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2097 /// if formatter.sign_plus() {
2098 /// write!(formatter,
2099 /// "Foo({}{})",
2100 /// if self.0 < 0 { '-' } else { '+' },
2101 /// self.0.abs())
2102 /// } else {
2103 /// write!(formatter, "Foo({})", self.0)
2104 /// }
2105 /// }
2106 /// }
2107 ///
2108 /// assert_eq!(format!("{:+}", Foo(23)), "Foo(+23)");
2109 /// assert_eq!(format!("{:+}", Foo(-23)), "Foo(-23)");
2110 /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2111 /// ```
2112 #[must_use]
2113 #[stable(feature = "fmt_flags", since = "1.5.0")]
2114 pub fn sign_plus(&self) -> bool {
2115 self.options.flags & flags::SIGN_PLUS_FLAG != 0
2116 }
2117
2118 /// Determines if the `-` flag was specified.
2119 ///
2120 /// # Examples
2121 ///
2122 /// ```
2123 /// use std::fmt;
2124 ///
2125 /// struct Foo(i32);
2126 ///
2127 /// impl fmt::Display for Foo {
2128 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2129 /// if formatter.sign_minus() {
2130 /// // You want a minus sign? Have one!
2131 /// write!(formatter, "-Foo({})", self.0)
2132 /// } else {
2133 /// write!(formatter, "Foo({})", self.0)
2134 /// }
2135 /// }
2136 /// }
2137 ///
2138 /// assert_eq!(format!("{:-}", Foo(23)), "-Foo(23)");
2139 /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2140 /// ```
2141 #[must_use]
2142 #[stable(feature = "fmt_flags", since = "1.5.0")]
2143 pub fn sign_minus(&self) -> bool {
2144 self.options.flags & flags::SIGN_MINUS_FLAG != 0
2145 }
2146
2147 /// Determines if the `#` flag was specified.
2148 ///
2149 /// # Examples
2150 ///
2151 /// ```
2152 /// use std::fmt;
2153 ///
2154 /// struct Foo(i32);
2155 ///
2156 /// impl fmt::Display for Foo {
2157 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2158 /// if formatter.alternate() {
2159 /// write!(formatter, "Foo({})", self.0)
2160 /// } else {
2161 /// write!(formatter, "{}", self.0)
2162 /// }
2163 /// }
2164 /// }
2165 ///
2166 /// assert_eq!(format!("{:#}", Foo(23)), "Foo(23)");
2167 /// assert_eq!(format!("{}", Foo(23)), "23");
2168 /// ```
2169 #[must_use]
2170 #[stable(feature = "fmt_flags", since = "1.5.0")]
2171 pub fn alternate(&self) -> bool {
2172 self.options.flags & flags::ALTERNATE_FLAG != 0
2173 }
2174
2175 /// Determines if the `0` flag was specified.
2176 ///
2177 /// # Examples
2178 ///
2179 /// ```
2180 /// use std::fmt;
2181 ///
2182 /// struct Foo(i32);
2183 ///
2184 /// impl fmt::Display for Foo {
2185 /// fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2186 /// assert!(formatter.sign_aware_zero_pad());
2187 /// assert_eq!(formatter.width(), Some(4));
2188 /// // We ignore the formatter's options.
2189 /// write!(formatter, "{}", self.0)
2190 /// }
2191 /// }
2192 ///
2193 /// assert_eq!(format!("{:04}", Foo(23)), "23");
2194 /// ```
2195 #[must_use]
2196 #[stable(feature = "fmt_flags", since = "1.5.0")]
2197 pub fn sign_aware_zero_pad(&self) -> bool {
2198 self.options.flags & flags::SIGN_AWARE_ZERO_PAD_FLAG != 0
2199 }
2200
2201 // FIXME: Decide what public API we want for these two flags.
2202 // https://github.com/rust-lang/rust/issues/48584
2203 fn debug_lower_hex(&self) -> bool {
2204 self.options.flags & flags::DEBUG_LOWER_HEX_FLAG != 0
2205 }
2206 fn debug_upper_hex(&self) -> bool {
2207 self.options.flags & flags::DEBUG_UPPER_HEX_FLAG != 0
2208 }
2209
2210 /// Creates a [`DebugStruct`] builder designed to assist with creation of
2211 /// [`fmt::Debug`] implementations for structs.
2212 ///
2213 /// [`fmt::Debug`]: self::Debug
2214 ///
2215 /// # Examples
2216 ///
2217 /// ```rust
2218 /// use std::fmt;
2219 /// use std::net::Ipv4Addr;
2220 ///
2221 /// struct Foo {
2222 /// bar: i32,
2223 /// baz: String,
2224 /// addr: Ipv4Addr,
2225 /// }
2226 ///
2227 /// impl fmt::Debug for Foo {
2228 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2229 /// fmt.debug_struct("Foo")
2230 /// .field("bar", &self.bar)
2231 /// .field("baz", &self.baz)
2232 /// .field("addr", &format_args!("{}", self.addr))
2233 /// .finish()
2234 /// }
2235 /// }
2236 ///
2237 /// assert_eq!(
2238 /// "Foo { bar: 10, baz: \"Hello World\", addr: 127.0.0.1 }",
2239 /// format!("{:?}", Foo {
2240 /// bar: 10,
2241 /// baz: "Hello World".to_string(),
2242 /// addr: Ipv4Addr::new(127, 0, 0, 1),
2243 /// })
2244 /// );
2245 /// ```
2246 #[stable(feature = "debug_builders", since = "1.2.0")]
2247 pub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a> {
2248 builders::debug_struct_new(self, name)
2249 }
2250
2251 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2252 /// binaries. `debug_struct_fields_finish` is more general, but this is
2253 /// faster for 1 field.
2254 #[doc(hidden)]
2255 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2256 pub fn debug_struct_field1_finish<'b>(
2257 &'b mut self,
2258 name: &str,
2259 name1: &str,
2260 value1: &dyn Debug,
2261 ) -> Result {
2262 let mut builder = builders::debug_struct_new(self, name);
2263 builder.field(name1, value1);
2264 builder.finish()
2265 }
2266
2267 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2268 /// binaries. `debug_struct_fields_finish` is more general, but this is
2269 /// faster for 2 fields.
2270 #[doc(hidden)]
2271 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2272 pub fn debug_struct_field2_finish<'b>(
2273 &'b mut self,
2274 name: &str,
2275 name1: &str,
2276 value1: &dyn Debug,
2277 name2: &str,
2278 value2: &dyn Debug,
2279 ) -> Result {
2280 let mut builder = builders::debug_struct_new(self, name);
2281 builder.field(name1, value1);
2282 builder.field(name2, value2);
2283 builder.finish()
2284 }
2285
2286 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2287 /// binaries. `debug_struct_fields_finish` is more general, but this is
2288 /// faster for 3 fields.
2289 #[doc(hidden)]
2290 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2291 pub fn debug_struct_field3_finish<'b>(
2292 &'b mut self,
2293 name: &str,
2294 name1: &str,
2295 value1: &dyn Debug,
2296 name2: &str,
2297 value2: &dyn Debug,
2298 name3: &str,
2299 value3: &dyn Debug,
2300 ) -> Result {
2301 let mut builder = builders::debug_struct_new(self, name);
2302 builder.field(name1, value1);
2303 builder.field(name2, value2);
2304 builder.field(name3, value3);
2305 builder.finish()
2306 }
2307
2308 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2309 /// binaries. `debug_struct_fields_finish` is more general, but this is
2310 /// faster for 4 fields.
2311 #[doc(hidden)]
2312 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2313 pub fn debug_struct_field4_finish<'b>(
2314 &'b mut self,
2315 name: &str,
2316 name1: &str,
2317 value1: &dyn Debug,
2318 name2: &str,
2319 value2: &dyn Debug,
2320 name3: &str,
2321 value3: &dyn Debug,
2322 name4: &str,
2323 value4: &dyn Debug,
2324 ) -> Result {
2325 let mut builder = builders::debug_struct_new(self, name);
2326 builder.field(name1, value1);
2327 builder.field(name2, value2);
2328 builder.field(name3, value3);
2329 builder.field(name4, value4);
2330 builder.finish()
2331 }
2332
2333 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2334 /// binaries. `debug_struct_fields_finish` is more general, but this is
2335 /// faster for 5 fields.
2336 #[doc(hidden)]
2337 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2338 pub fn debug_struct_field5_finish<'b>(
2339 &'b mut self,
2340 name: &str,
2341 name1: &str,
2342 value1: &dyn Debug,
2343 name2: &str,
2344 value2: &dyn Debug,
2345 name3: &str,
2346 value3: &dyn Debug,
2347 name4: &str,
2348 value4: &dyn Debug,
2349 name5: &str,
2350 value5: &dyn Debug,
2351 ) -> Result {
2352 let mut builder = builders::debug_struct_new(self, name);
2353 builder.field(name1, value1);
2354 builder.field(name2, value2);
2355 builder.field(name3, value3);
2356 builder.field(name4, value4);
2357 builder.field(name5, value5);
2358 builder.finish()
2359 }
2360
2361 /// Shrinks `derive(Debug)` code, for faster compilation and smaller binaries.
2362 /// For the cases not covered by `debug_struct_field[12345]_finish`.
2363 #[doc(hidden)]
2364 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2365 pub fn debug_struct_fields_finish<'b>(
2366 &'b mut self,
2367 name: &str,
2368 names: &[&str],
2369 values: &[&dyn Debug],
2370 ) -> Result {
2371 assert_eq!(names.len(), values.len());
2372 let mut builder = builders::debug_struct_new(self, name);
2373 for (name, value) in iter::zip(names, values) {
2374 builder.field(name, value);
2375 }
2376 builder.finish()
2377 }
2378
2379 /// Creates a `DebugTuple` builder designed to assist with creation of
2380 /// `fmt::Debug` implementations for tuple structs.
2381 ///
2382 /// # Examples
2383 ///
2384 /// ```rust
2385 /// use std::fmt;
2386 /// use std::marker::PhantomData;
2387 ///
2388 /// struct Foo<T>(i32, String, PhantomData<T>);
2389 ///
2390 /// impl<T> fmt::Debug for Foo<T> {
2391 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2392 /// fmt.debug_tuple("Foo")
2393 /// .field(&self.0)
2394 /// .field(&self.1)
2395 /// .field(&format_args!("_"))
2396 /// .finish()
2397 /// }
2398 /// }
2399 ///
2400 /// assert_eq!(
2401 /// "Foo(10, \"Hello\", _)",
2402 /// format!("{:?}", Foo(10, "Hello".to_string(), PhantomData::<u8>))
2403 /// );
2404 /// ```
2405 #[stable(feature = "debug_builders", since = "1.2.0")]
2406 pub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a> {
2407 builders::debug_tuple_new(self, name)
2408 }
2409
2410 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2411 /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2412 /// for 1 field.
2413 #[doc(hidden)]
2414 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2415 pub fn debug_tuple_field1_finish<'b>(&'b mut self, name: &str, value1: &dyn Debug) -> Result {
2416 let mut builder = builders::debug_tuple_new(self, name);
2417 builder.field(value1);
2418 builder.finish()
2419 }
2420
2421 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2422 /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2423 /// for 2 fields.
2424 #[doc(hidden)]
2425 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2426 pub fn debug_tuple_field2_finish<'b>(
2427 &'b mut self,
2428 name: &str,
2429 value1: &dyn Debug,
2430 value2: &dyn Debug,
2431 ) -> Result {
2432 let mut builder = builders::debug_tuple_new(self, name);
2433 builder.field(value1);
2434 builder.field(value2);
2435 builder.finish()
2436 }
2437
2438 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2439 /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2440 /// for 3 fields.
2441 #[doc(hidden)]
2442 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2443 pub fn debug_tuple_field3_finish<'b>(
2444 &'b mut self,
2445 name: &str,
2446 value1: &dyn Debug,
2447 value2: &dyn Debug,
2448 value3: &dyn Debug,
2449 ) -> Result {
2450 let mut builder = builders::debug_tuple_new(self, name);
2451 builder.field(value1);
2452 builder.field(value2);
2453 builder.field(value3);
2454 builder.finish()
2455 }
2456
2457 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2458 /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2459 /// for 4 fields.
2460 #[doc(hidden)]
2461 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2462 pub fn debug_tuple_field4_finish<'b>(
2463 &'b mut self,
2464 name: &str,
2465 value1: &dyn Debug,
2466 value2: &dyn Debug,
2467 value3: &dyn Debug,
2468 value4: &dyn Debug,
2469 ) -> Result {
2470 let mut builder = builders::debug_tuple_new(self, name);
2471 builder.field(value1);
2472 builder.field(value2);
2473 builder.field(value3);
2474 builder.field(value4);
2475 builder.finish()
2476 }
2477
2478 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2479 /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2480 /// for 5 fields.
2481 #[doc(hidden)]
2482 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2483 pub fn debug_tuple_field5_finish<'b>(
2484 &'b mut self,
2485 name: &str,
2486 value1: &dyn Debug,
2487 value2: &dyn Debug,
2488 value3: &dyn Debug,
2489 value4: &dyn Debug,
2490 value5: &dyn Debug,
2491 ) -> Result {
2492 let mut builder = builders::debug_tuple_new(self, name);
2493 builder.field(value1);
2494 builder.field(value2);
2495 builder.field(value3);
2496 builder.field(value4);
2497 builder.field(value5);
2498 builder.finish()
2499 }
2500
2501 /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2502 /// binaries. For the cases not covered by `debug_tuple_field[12345]_finish`.
2503 #[doc(hidden)]
2504 #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2505 pub fn debug_tuple_fields_finish<'b>(
2506 &'b mut self,
2507 name: &str,
2508 values: &[&dyn Debug],
2509 ) -> Result {
2510 let mut builder = builders::debug_tuple_new(self, name);
2511 for value in values {
2512 builder.field(value);
2513 }
2514 builder.finish()
2515 }
2516
2517 /// Creates a `DebugList` builder designed to assist with creation of
2518 /// `fmt::Debug` implementations for list-like structures.
2519 ///
2520 /// # Examples
2521 ///
2522 /// ```rust
2523 /// use std::fmt;
2524 ///
2525 /// struct Foo(Vec<i32>);
2526 ///
2527 /// impl fmt::Debug for Foo {
2528 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2529 /// fmt.debug_list().entries(self.0.iter()).finish()
2530 /// }
2531 /// }
2532 ///
2533 /// assert_eq!(format!("{:?}", Foo(vec![10, 11])), "[10, 11]");
2534 /// ```
2535 #[stable(feature = "debug_builders", since = "1.2.0")]
2536 pub fn debug_list<'b>(&'b mut self) -> DebugList<'b, 'a> {
2537 builders::debug_list_new(self)
2538 }
2539
2540 /// Creates a `DebugSet` builder designed to assist with creation of
2541 /// `fmt::Debug` implementations for set-like structures.
2542 ///
2543 /// # Examples
2544 ///
2545 /// ```rust
2546 /// use std::fmt;
2547 ///
2548 /// struct Foo(Vec<i32>);
2549 ///
2550 /// impl fmt::Debug for Foo {
2551 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2552 /// fmt.debug_set().entries(self.0.iter()).finish()
2553 /// }
2554 /// }
2555 ///
2556 /// assert_eq!(format!("{:?}", Foo(vec![10, 11])), "{10, 11}");
2557 /// ```
2558 ///
2559 /// [`format_args!`]: crate::format_args
2560 ///
2561 /// In this more complex example, we use [`format_args!`] and `.debug_set()`
2562 /// to build a list of match arms:
2563 ///
2564 /// ```rust
2565 /// use std::fmt;
2566 ///
2567 /// struct Arm<'a, L, R>(&'a (L, R));
2568 /// struct Table<'a, K, V>(&'a [(K, V)], V);
2569 ///
2570 /// impl<'a, L, R> fmt::Debug for Arm<'a, L, R>
2571 /// where
2572 /// L: 'a + fmt::Debug, R: 'a + fmt::Debug
2573 /// {
2574 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2575 /// L::fmt(&(self.0).0, fmt)?;
2576 /// fmt.write_str(" => ")?;
2577 /// R::fmt(&(self.0).1, fmt)
2578 /// }
2579 /// }
2580 ///
2581 /// impl<'a, K, V> fmt::Debug for Table<'a, K, V>
2582 /// where
2583 /// K: 'a + fmt::Debug, V: 'a + fmt::Debug
2584 /// {
2585 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2586 /// fmt.debug_set()
2587 /// .entries(self.0.iter().map(Arm))
2588 /// .entry(&Arm(&(format_args!("_"), &self.1)))
2589 /// .finish()
2590 /// }
2591 /// }
2592 /// ```
2593 #[stable(feature = "debug_builders", since = "1.2.0")]
2594 pub fn debug_set<'b>(&'b mut self) -> DebugSet<'b, 'a> {
2595 builders::debug_set_new(self)
2596 }
2597
2598 /// Creates a `DebugMap` builder designed to assist with creation of
2599 /// `fmt::Debug` implementations for map-like structures.
2600 ///
2601 /// # Examples
2602 ///
2603 /// ```rust
2604 /// use std::fmt;
2605 ///
2606 /// struct Foo(Vec<(String, i32)>);
2607 ///
2608 /// impl fmt::Debug for Foo {
2609 /// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2610 /// fmt.debug_map().entries(self.0.iter().map(|&(ref k, ref v)| (k, v))).finish()
2611 /// }
2612 /// }
2613 ///
2614 /// assert_eq!(
2615 /// format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
2616 /// r#"{"A": 10, "B": 11}"#
2617 /// );
2618 /// ```
2619 #[stable(feature = "debug_builders", since = "1.2.0")]
2620 pub fn debug_map<'b>(&'b mut self) -> DebugMap<'b, 'a> {
2621 builders::debug_map_new(self)
2622 }
2623
2624 /// Returns the sign of this formatter (`+` or `-`).
2625 #[unstable(feature = "formatting_options", issue = "118117")]
2626 pub const fn sign(&self) -> Option<Sign> {
2627 self.options.get_sign()
2628 }
2629
2630 /// Returns the formatting options this formatter corresponds to.
2631 #[unstable(feature = "formatting_options", issue = "118117")]
2632 pub const fn options(&self) -> FormattingOptions {
2633 self.options
2634 }
2635}
2636
2637#[stable(since = "1.2.0", feature = "formatter_write")]
2638impl Write for Formatter<'_> {
2639 fn write_str(&mut self, s: &str) -> Result {
2640 self.buf.write_str(s)
2641 }
2642
2643 fn write_char(&mut self, c: char) -> Result {
2644 self.buf.write_char(c)
2645 }
2646
2647 #[inline]
2648 fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
2649 if let Some(s) = args.as_statically_known_str() {
2650 self.buf.write_str(s)
2651 } else {
2652 write(self.buf, args)
2653 }
2654 }
2655}
2656
2657#[stable(feature = "rust1", since = "1.0.0")]
2658impl Display for Error {
2659 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2660 Display::fmt("an error occurred when formatting an argument", f)
2661 }
2662}
2663
2664// Implementations of the core formatting traits
2665
2666macro_rules! fmt_refs {
2667 ($($tr:ident),*) => {
2668 $(
2669 #[stable(feature = "rust1", since = "1.0.0")]
2670 impl<T: ?Sized + $tr> $tr for &T {
2671 fn fmt(&self, f: &mut Formatter<'_>) -> Result { $tr::fmt(&**self, f) }
2672 }
2673 #[stable(feature = "rust1", since = "1.0.0")]
2674 impl<T: ?Sized + $tr> $tr for &mut T {
2675 fn fmt(&self, f: &mut Formatter<'_>) -> Result { $tr::fmt(&**self, f) }
2676 }
2677 )*
2678 }
2679}
2680
2681fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
2682
2683#[unstable(feature = "never_type", issue = "35121")]
2684impl Debug for ! {
2685 #[inline]
2686 fn fmt(&self, _: &mut Formatter<'_>) -> Result {
2687 *self
2688 }
2689}
2690
2691#[unstable(feature = "never_type", issue = "35121")]
2692impl Display for ! {
2693 #[inline]
2694 fn fmt(&self, _: &mut Formatter<'_>) -> Result {
2695 *self
2696 }
2697}
2698
2699#[stable(feature = "rust1", since = "1.0.0")]
2700impl Debug for bool {
2701 #[inline]
2702 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2703 Display::fmt(self, f)
2704 }
2705}
2706
2707#[stable(feature = "rust1", since = "1.0.0")]
2708impl Display for bool {
2709 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2710 Display::fmt(if *self { "true" } else { "false" }, f)
2711 }
2712}
2713
2714#[stable(feature = "rust1", since = "1.0.0")]
2715impl Debug for str {
2716 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2717 f.write_char('"')?;
2718
2719 // substring we know is printable
2720 let mut printable_range = 0..0;
2721
2722 fn needs_escape(b: u8) -> bool {
2723 b > 0x7E || b < 0x20 || b == b'\\' || b == b'"'
2724 }
2725
2726 // the loop here first skips over runs of printable ASCII as a fast path.
2727 // other chars (unicode, or ASCII that needs escaping) are then handled per-`char`.
2728 let mut rest = self;
2729 while rest.len() > 0 {
2730 let Some(non_printable_start) = rest.as_bytes().iter().position(|&b| needs_escape(b))
2731 else {
2732 printable_range.end += rest.len();
2733 break;
2734 };
2735
2736 printable_range.end += non_printable_start;
2737 // SAFETY: the position was derived from an iterator, so is known to be within bounds, and at a char boundary
2738 rest = unsafe { rest.get_unchecked(non_printable_start..) };
2739
2740 let mut chars = rest.chars();
2741 if let Some(c) = chars.next() {
2742 let esc = c.escape_debug_ext(EscapeDebugExtArgs {
2743 escape_grapheme_extended: true,
2744 escape_single_quote: false,
2745 escape_double_quote: true,
2746 });
2747 if esc.len() != 1 {
2748 f.write_str(&self[printable_range.clone()])?;
2749 Display::fmt(&esc, f)?;
2750 printable_range.start = printable_range.end + c.len_utf8();
2751 }
2752 printable_range.end += c.len_utf8();
2753 }
2754 rest = chars.as_str();
2755 }
2756
2757 f.write_str(&self[printable_range])?;
2758
2759 f.write_char('"')
2760 }
2761}
2762
2763#[stable(feature = "rust1", since = "1.0.0")]
2764impl Display for str {
2765 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2766 f.pad(self)
2767 }
2768}
2769
2770#[stable(feature = "rust1", since = "1.0.0")]
2771impl Debug for char {
2772 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2773 f.write_char('\'')?;
2774 let esc = self.escape_debug_ext(EscapeDebugExtArgs {
2775 escape_grapheme_extended: true,
2776 escape_single_quote: true,
2777 escape_double_quote: false,
2778 });
2779 Display::fmt(&esc, f)?;
2780 f.write_char('\'')
2781 }
2782}
2783
2784#[stable(feature = "rust1", since = "1.0.0")]
2785impl Display for char {
2786 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2787 if f.options.flags & (flags::WIDTH_FLAG | flags::PRECISION_FLAG) == 0 {
2788 f.write_char(*self)
2789 } else {
2790 f.pad(self.encode_utf8(&mut [0; MAX_LEN_UTF8]))
2791 }
2792 }
2793}
2794
2795#[stable(feature = "rust1", since = "1.0.0")]
2796impl<T: ?Sized> Pointer for *const T {
2797 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2798 if <<T as core::ptr::Pointee>::Metadata as core::unit::IsUnit>::is_unit() {
2799 pointer_fmt_inner(self.expose_provenance(), f)
2800 } else {
2801 f.debug_struct("Pointer")
2802 .field_with("addr", |f| pointer_fmt_inner(self.expose_provenance(), f))
2803 .field("metadata", &core::ptr::metadata(*self))
2804 .finish()
2805 }
2806 }
2807}
2808
2809/// Since the formatting will be identical for all pointer types, uses a
2810/// non-monomorphized implementation for the actual formatting to reduce the
2811/// amount of codegen work needed.
2812///
2813/// This uses `ptr_addr: usize` and not `ptr: *const ()` to be able to use this for
2814/// `fn(...) -> ...` without using [problematic] "Oxford Casts".
2815///
2816/// [problematic]: https://github.com/rust-lang/rust/issues/95489
2817pub(crate) fn pointer_fmt_inner(ptr_addr: usize, f: &mut Formatter<'_>) -> Result {
2818 let old_options = f.options;
2819
2820 // The alternate flag is already treated by LowerHex as being special-
2821 // it denotes whether to prefix with 0x. We use it to work out whether
2822 // or not to zero extend, and then unconditionally set it to get the
2823 // prefix.
2824 if f.options.get_alternate() {
2825 f.options.sign_aware_zero_pad(true);
2826
2827 if f.options.get_width().is_none() {
2828 f.options.width(Some((usize::BITS / 4) as u16 + 2));
2829 }
2830 }
2831 f.options.alternate(true);
2832
2833 let ret = LowerHex::fmt(&ptr_addr, f);
2834
2835 f.options = old_options;
2836
2837 ret
2838}
2839
2840#[stable(feature = "rust1", since = "1.0.0")]
2841impl<T: ?Sized> Pointer for *mut T {
2842 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2843 Pointer::fmt(&(*self as *const T), f)
2844 }
2845}
2846
2847#[stable(feature = "rust1", since = "1.0.0")]
2848impl<T: ?Sized> Pointer for &T {
2849 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2850 Pointer::fmt(&(*self as *const T), f)
2851 }
2852}
2853
2854#[stable(feature = "rust1", since = "1.0.0")]
2855impl<T: ?Sized> Pointer for &mut T {
2856 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2857 Pointer::fmt(&(&**self as *const T), f)
2858 }
2859}
2860
2861// Implementation of Display/Debug for various core types
2862
2863#[stable(feature = "rust1", since = "1.0.0")]
2864impl<T: ?Sized> Debug for *const T {
2865 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2866 Pointer::fmt(self, f)
2867 }
2868}
2869#[stable(feature = "rust1", since = "1.0.0")]
2870impl<T: ?Sized> Debug for *mut T {
2871 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2872 Pointer::fmt(self, f)
2873 }
2874}
2875
2876macro_rules! peel {
2877 ($name:ident, $($other:ident,)*) => (tuple! { $($other,)* })
2878}
2879
2880macro_rules! tuple {
2881 () => ();
2882 ( $($name:ident,)+ ) => (
2883 maybe_tuple_doc! {
2884 $($name)+ @
2885 #[stable(feature = "rust1", since = "1.0.0")]
2886 impl<$($name:Debug),+> Debug for ($($name,)+) where last_type!($($name,)+): ?Sized {
2887 #[allow(non_snake_case, unused_assignments)]
2888 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2889 let mut builder = f.debug_tuple("");
2890 let ($(ref $name,)+) = *self;
2891 $(
2892 builder.field(&$name);
2893 )+
2894
2895 builder.finish()
2896 }
2897 }
2898 }
2899 peel! { $($name,)+ }
2900 )
2901}
2902
2903macro_rules! maybe_tuple_doc {
2904 ($a:ident @ #[$meta:meta] $item:item) => {
2905 #[doc(fake_variadic)]
2906 #[doc = "This trait is implemented for tuples up to twelve items long."]
2907 #[$meta]
2908 $item
2909 };
2910 ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
2911 #[doc(hidden)]
2912 #[$meta]
2913 $item
2914 };
2915}
2916
2917macro_rules! last_type {
2918 ($a:ident,) => { $a };
2919 ($a:ident, $($rest_a:ident,)+) => { last_type!($($rest_a,)+) };
2920}
2921
2922tuple! { E, D, C, B, A, Z, Y, X, W, V, U, T, }
2923
2924#[stable(feature = "rust1", since = "1.0.0")]
2925impl<T: Debug> Debug for [T] {
2926 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2927 f.debug_list().entries(self.iter()).finish()
2928 }
2929}
2930
2931#[stable(feature = "rust1", since = "1.0.0")]
2932impl Debug for () {
2933 #[inline]
2934 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2935 f.pad("()")
2936 }
2937}
2938#[stable(feature = "rust1", since = "1.0.0")]
2939impl<T: ?Sized> Debug for PhantomData<T> {
2940 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2941 write!(f, "PhantomData<{}>", crate::any::type_name::<T>())
2942 }
2943}
2944
2945#[stable(feature = "rust1", since = "1.0.0")]
2946impl<T: Copy + Debug> Debug for Cell<T> {
2947 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2948 f.debug_struct("Cell").field("value", &self.get()).finish()
2949 }
2950}
2951
2952#[stable(feature = "rust1", since = "1.0.0")]
2953impl<T: ?Sized + Debug> Debug for RefCell<T> {
2954 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2955 let mut d = f.debug_struct("RefCell");
2956 match self.try_borrow() {
2957 Ok(borrow) => d.field("value", &borrow),
2958 Err(_) => d.field("value", &format_args!("<borrowed>")),
2959 };
2960 d.finish()
2961 }
2962}
2963
2964#[stable(feature = "rust1", since = "1.0.0")]
2965impl<T: ?Sized + Debug> Debug for Ref<'_, T> {
2966 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2967 Debug::fmt(&**self, f)
2968 }
2969}
2970
2971#[stable(feature = "rust1", since = "1.0.0")]
2972impl<T: ?Sized + Debug> Debug for RefMut<'_, T> {
2973 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2974 Debug::fmt(&*(self.deref()), f)
2975 }
2976}
2977
2978#[stable(feature = "core_impl_debug", since = "1.9.0")]
2979impl<T: ?Sized> Debug for UnsafeCell<T> {
2980 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2981 f.debug_struct("UnsafeCell").finish_non_exhaustive()
2982 }
2983}
2984
2985#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2986impl<T: ?Sized> Debug for SyncUnsafeCell<T> {
2987 fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2988 f.debug_struct("SyncUnsafeCell").finish_non_exhaustive()
2989 }
2990}
2991
2992// If you expected tests to be here, look instead at coretests/tests/fmt/;
2993// it's a lot easier than creating all of the rt::Piece structures here.
2994// There are also tests in alloctests/tests/fmt.rs, for those that need allocations.