1//! Utilities for formatting and printing `String`s.
2//!
3//! This module contains the runtime support for the [`format!`] syntax extension.
4//! This macro is implemented in the compiler to emit calls to this module in
5//! order to format arguments at runtime into strings.
6//!
7//! # Usage
8//!
9//! The [`format!`] macro is intended to be familiar to those coming from C's
10//! `printf`/`fprintf` functions or Python's `str.format` function.
11//!
12//! Some examples of the [`format!`] extension are:
13//!
14//! ```
15//! format!("Hello"); // => "Hello"
16//! format!("Hello, {}!", "world"); // => "Hello, world!"
17//! format!("The number is {}", 1); // => "The number is 1"
18//! format!("{:?}", (3, 4)); // => "(3, 4)"
19//! format!("{value}", value=4); // => "4"
20//! let people = "Rustaceans";
21//! format!("Hello {people}!"); // => "Hello Rustaceans!"
22//! format!("{} {}", 1, 2); // => "1 2"
23//! format!("{:04}", 42); // => "0042" with leading zeros
24//! format!("{:#?}", (100, 200)); // => "(
25//! // 100,
26//! // 200,
27//! // )"
28//! ```
29//!
30//! From these, you can see that the first argument is a format string. It is
31//! required by the compiler for this to be a string literal; it cannot be a
32//! variable passed in (in order to perform validity checking). The compiler
33//! will then parse the format string and determine if the list of arguments
34//! provided is suitable to pass to this format string.
35//!
36//! To convert a single value to a string, use the [`to_string`] method. This
37//! will use the [`Display`] formatting trait.
38//!
39//! ## Positional parameters
40//!
41//! Each formatting argument is allowed to specify which value argument it's
42//! referencing, and if omitted it is assumed to be "the next argument". For
43//! example, the format string `{} {} {}` would take three parameters, and they
44//! would be formatted in the same order as they're given. The format string
45//! `{2} {1} {0}`, however, would format arguments in reverse order.
46//!
47//! Things can get a little tricky once you start intermingling the two types of
48//! positional specifiers. The "next argument" specifier can be thought of as an
49//! iterator over the argument. Each time a "next argument" specifier is seen,
50//! the iterator advances. This leads to behavior like this:
51//!
52//! ```
53//! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"
54//! ```
55//!
56//! The internal iterator over the argument has not been advanced by the time
57//! the first `{}` is seen, so it prints the first argument. Then upon reaching
58//! the second `{}`, the iterator has advanced forward to the second argument.
59//! Essentially, parameters that explicitly name their argument do not affect
60//! parameters that do not name an argument in terms of positional specifiers.
61//!
62//! A format string is required to use all of its arguments, otherwise it is a
63//! compile-time error. You may refer to the same argument more than once in the
64//! format string.
65//!
66//! ## Named parameters
67//!
68//! Rust itself does not have a Python-like equivalent of named parameters to a
69//! function, but the [`format!`] macro is a syntax extension that allows it to
70//! leverage named parameters. Named parameters are listed at the end of the
71//! argument list and have the syntax:
72//!
73//! ```text
74//! identifier '=' expression
75//! ```
76//!
77//! For example, the following [`format!`] expressions all use named arguments:
78//!
79//! ```
80//! format!("{argument}", argument = "test"); // => "test"
81//! format!("{name} {}", 1, name = 2); // => "2 1"
82//! format!("{a} {c} {b}", a="a", b='b', c=3); // => "a 3 b"
83//! ```
84//!
85//! If a named parameter does not appear in the argument list, `format!` will
86//! reference a variable with that name in the current scope.
87//!
88//! ```
89//! let argument = 2 + 2;
90//! format!("{argument}"); // => "4"
91//!
92//! fn make_string(a: u32, b: &str) -> String {
93//! format!("{b} {a}")
94//! }
95//! make_string(927, "label"); // => "label 927"
96//! ```
97//!
98//! It is not valid to put positional parameters (those without names) after
99//! arguments that have names. Like with positional parameters, it is not
100//! valid to provide named parameters that are unused by the format string.
101//!
102//! # Formatting Parameters
103//!
104//! Each argument being formatted can be transformed by a number of formatting
105//! parameters (corresponding to `format_spec` in [the syntax](#syntax)). These
106//! parameters affect the string representation of what's being formatted.
107//!
108//! ## Width
109//!
110//! ```
111//! // All of these print "Hello x !"
112//! println!("Hello {:5}!", "x");
113//! println!("Hello {:1$}!", "x", 5);
114//! println!("Hello {1:0$}!", 5, "x");
115//! println!("Hello {:width$}!", "x", width = 5);
116//! let width = 5;
117//! println!("Hello {:width$}!", "x");
118//! ```
119//!
120//! This is a parameter for the "minimum width" that the format should take up.
121//! If the value's string does not fill up this many characters, then the
122//! padding specified by fill/alignment will be used to take up the required
123//! space (see below).
124//!
125//! The value for the width can also be provided as a [`usize`] in the list of
126//! parameters by adding a postfix `$`, indicating that the second argument is
127//! a [`usize`] specifying the width.
128//!
129//! Referring to an argument with the dollar syntax does not affect the "next
130//! argument" counter, so it's usually a good idea to refer to arguments by
131//! position, or use named arguments.
132//!
133//! ## Fill/Alignment
134//!
135//! ```
136//! assert_eq!(format!("Hello {:<5}!", "x"), "Hello x !");
137//! assert_eq!(format!("Hello {:-<5}!", "x"), "Hello x----!");
138//! assert_eq!(format!("Hello {:^5}!", "x"), "Hello x !");
139//! assert_eq!(format!("Hello {:>5}!", "x"), "Hello x!");
140//! ```
141//!
142//! The optional fill character and alignment is provided normally in conjunction with the
143//! [`width`](#width) parameter. It must be defined before `width`, right after the `:`.
144//! This indicates that if the value being formatted is smaller than
145//! `width` some extra characters will be printed around it.
146//! Filling comes in the following variants for different alignments:
147//!
148//! * `[fill]<` - the argument is left-aligned in `width` columns
149//! * `[fill]^` - the argument is center-aligned in `width` columns
150//! * `[fill]>` - the argument is right-aligned in `width` columns
151//!
152//! The default [fill/alignment](#fillalignment) for non-numerics is a space and
153//! left-aligned. The
154//! default for numeric formatters is also a space character but with right-alignment. If
155//! the `0` flag (see below) is specified for numerics, then the implicit fill character is
156//! `0`.
157//!
158//! Note that alignment might not be implemented by some types. In particular, it
159//! is not generally implemented for the `Debug` trait. A good way to ensure
160//! padding is applied is to format your input, then pad this resulting string
161//! to obtain your output:
162//!
163//! ```
164//! println!("Hello {:^15}!", format!("{:?}", Some("hi"))); // => "Hello Some("hi") !"
165//! ```
166//!
167//! ## Sign/`#`/`0`
168//!
169//! ```
170//! assert_eq!(format!("Hello {:+}!", 5), "Hello +5!");
171//! assert_eq!(format!("{:#x}!", 27), "0x1b!");
172//! assert_eq!(format!("Hello {:05}!", 5), "Hello 00005!");
173//! assert_eq!(format!("Hello {:05}!", -5), "Hello -0005!");
174//! assert_eq!(format!("{:#010x}!", 27), "0x0000001b!");
175//! ```
176//!
177//! These are all flags altering the behavior of the formatter.
178//!
179//! * `+` - This is intended for numeric types and indicates that the sign
180//! should always be printed. By default only the negative sign of signed values
181//! is printed, and the sign of positive or unsigned values is omitted.
182//! This flag indicates that the correct sign (`+` or `-`) should always be printed.
183//! * `-` - Currently not used
184//! * `#` - This flag indicates that the "alternate" form of printing should
185//! be used. The alternate forms are:
186//! * `#?` - pretty-print the [`Debug`] formatting (adds linebreaks and indentation)
187//! * `#x` - precedes the argument with a `0x`
188//! * `#X` - precedes the argument with a `0x`
189//! * `#b` - precedes the argument with a `0b`
190//! * `#o` - precedes the argument with a `0o`
191//! * `0` - This is used to indicate for integer formats that the padding to `width` should
192//! both be done with a `0` character as well as be sign-aware. A format
193//! like `{:08}` would yield `00000001` for the integer `1`, while the
194//! same format would yield `-0000001` for the integer `-1`. Notice that
195//! the negative version has one fewer zero than the positive version.
196//! Note that padding zeros are always placed after the sign (if any)
197//! and before the digits. When used together with the `#` flag, a similar
198//! rule applies: padding zeros are inserted after the prefix but before
199//! the digits. The prefix is included in the total width.
200//!
201//! ## Precision
202//!
203//! For non-numeric types, this can be considered a "maximum width". If the resulting string is
204//! longer than this width, then it is truncated down to this many characters and that truncated
205//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
206//!
207//! For integral types, this is ignored.
208//!
209//! For floating-point types, this indicates how many digits after the decimal point should be
210//! printed.
211//!
212//! There are three possible ways to specify the desired `precision`:
213//!
214//! 1. An integer `.N`:
215//!
216//! the integer `N` itself is the precision.
217//!
218//! 2. An integer or name followed by dollar sign `.N$`:
219//!
220//! use format *argument* `N` (which must be a `usize`) as the precision.
221//!
222//! 3. An asterisk `.*`:
223//!
224//! `.*` means that this `{...}` is associated with *two* format inputs rather than one:
225//! - If a format string in the fashion of `{:<spec>.*}` is used, then the first input holds
226//! the `usize` precision, and the second holds the value to print.
227//! - If a format string in the fashion of `{<arg>:<spec>.*}` is used, then the `<arg>` part
228//! refers to the value to print, and the `precision` is taken like it was specified with an
229//! omitted positional parameter (`{}` instead of `{<arg>:}`).
230//!
231//! For example, the following calls all print the same thing `Hello x is 0.01000`:
232//!
233//! ```
234//! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
235//! println!("Hello {0} is {1:.5}", "x", 0.01);
236//!
237//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
238//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
239//!
240//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
241//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
242//!
243//! // Hello {next arg -> arg 0 ("x")} is {second of next two args -> arg 2 (0.01) with precision
244//! // specified in first of next two args -> arg 1 (5)}
245//! println!("Hello {} is {:.*}", "x", 5, 0.01);
246//!
247//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision
248//! // specified in next arg -> arg 0 (5)}
249//! println!("Hello {1} is {2:.*}", 5, "x", 0.01);
250//!
251//! // Hello {next arg -> arg 0 ("x")} is {arg 2 (0.01) with precision
252//! // specified in next arg -> arg 1 (5)}
253//! println!("Hello {} is {2:.*}", "x", 5, 0.01);
254//!
255//! // Hello {next arg -> arg 0 ("x")} is {arg "number" (0.01) with precision specified
256//! // in arg "prec" (5)}
257//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
258//! ```
259//!
260//! While these:
261//!
262//! ```
263//! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
264//! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
265//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
266//! ```
267//!
268//! print three significantly different things:
269//!
270//! ```text
271//! Hello, `1234.560` has 3 fractional digits
272//! Hello, `123` has 3 characters
273//! Hello, ` 123` has 3 right-aligned characters
274//! ```
275//!
276//! ## Localization
277//!
278//! In some programming languages, the behavior of string formatting functions
279//! depends on the operating system's locale setting. The format functions
280//! provided by Rust's standard library do not have any concept of locale and
281//! will produce the same results on all systems regardless of user
282//! configuration.
283//!
284//! For example, the following code will always print `1.5` even if the system
285//! locale uses a decimal separator other than a dot.
286//!
287//! ```
288//! println!("The value is {}", 1.5);
289//! ```
290//!
291//! # Escaping
292//!
293//! The literal characters `{` and `}` may be included in a string by preceding
294//! them with the same character. For example, the `{` character is escaped with
295//! `{{` and the `}` character is escaped with `}}`.
296//!
297//! ```
298//! assert_eq!(format!("Hello {{}}"), "Hello {}");
299//! assert_eq!(format!("{{ Hello"), "{ Hello");
300//! ```
301//!
302//! # Syntax
303//!
304//! To summarize, here you can find the full grammar of format strings.
305//! The syntax for the formatting language used is drawn from other languages,
306//! so it should not be too alien. Arguments are formatted with Python-like
307//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
308//! `%`. The actual grammar for the formatting syntax is:
309//!
310//! ```text
311//! format_string := text [ maybe_format text ] *
312//! maybe_format := '{' '{' | '}' '}' | format
313//! format := '{' [ argument ] [ ':' format_spec ] [ ws ] * '}'
314//! argument := integer | identifier
315//!
316//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision]type
317//! fill := character
318//! align := '<' | '^' | '>'
319//! sign := '+' | '-'
320//! width := count
321//! precision := count | '*'
322//! type := '' | '?' | 'x?' | 'X?' | identifier
323//! count := parameter | integer
324//! parameter := argument '$'
325//! ```
326//! In the above grammar,
327//! - `text` must not contain any `'{'` or `'}'` characters,
328//! - `ws` is any character for which [`char::is_whitespace`] returns `true`, has no semantic
329//! meaning and is completely optional,
330//! - `integer` is a decimal integer that may contain leading zeroes and must fit into an `usize` and
331//! - `identifier` is an `IDENTIFIER_OR_KEYWORD` (not an `IDENTIFIER`) as defined by the [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html).
332//!
333//! # Formatting traits
334//!
335//! When requesting that an argument be formatted with a particular type, you
336//! are actually requesting that an argument ascribes to a particular trait.
337//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
338//! well as [`isize`]). The current mapping of types to traits is:
339//!
340//! * *nothing* ⇒ [`Display`]
341//! * `?` ⇒ [`Debug`]
342//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
343//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
344//! * `o` ⇒ [`Octal`]
345//! * `x` ⇒ [`LowerHex`]
346//! * `X` ⇒ [`UpperHex`]
347//! * `p` ⇒ [`Pointer`]
348//! * `b` ⇒ [`Binary`]
349//! * `e` ⇒ [`LowerExp`]
350//! * `E` ⇒ [`UpperExp`]
351//!
352//! What this means is that any type of argument which implements the
353//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
354//! are provided for these traits for a number of primitive types by the
355//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
356//! then the format trait used is the [`Display`] trait.
357//!
358//! When implementing a format trait for your own type, you will have to
359//! implement a method of the signature:
360//!
361//! ```
362//! # #![allow(dead_code)]
363//! # use std::fmt;
364//! # struct Foo; // our custom type
365//! # impl fmt::Display for Foo {
366//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
367//! # write!(f, "testing, testing")
368//! # } }
369//! ```
370//!
371//! Your type will be passed as `self` by-reference, and then the function
372//! should emit output into the Formatter `f` which implements `fmt::Write`. It is up to each
373//! format trait implementation to correctly adhere to the requested formatting parameters.
374//! The values of these parameters can be accessed with methods of the
375//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
376//! provides some helper methods.
377//!
378//! Additionally, the return value of this function is [`fmt::Result`] which is a
379//! type alias of <code>[Result]<(), [std::fmt::Error]></code>. Formatting implementations
380//! should ensure that they propagate errors from the [`Formatter`] (e.g., when
381//! calling [`write!`]). However, they should never return errors spuriously. That
382//! is, a formatting implementation must and may only return an error if the
383//! passed-in [`Formatter`] returns an error. This is because, contrary to what
384//! the function signature might suggest, string formatting is an infallible
385//! operation. This function only returns a result because writing to the
386//! underlying stream might fail and it must provide a way to propagate the fact
387//! that an error has occurred back up the stack.
388//!
389//! An example of implementing the formatting traits would look
390//! like:
391//!
392//! ```
393//! use std::fmt;
394//!
395//! #[derive(Debug)]
396//! struct Vector2D {
397//! x: isize,
398//! y: isize,
399//! }
400//!
401//! impl fmt::Display for Vector2D {
402//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
403//! // The `f` value implements the `Write` trait, which is what the
404//! // write! macro is expecting. Note that this formatting ignores the
405//! // various flags provided to format strings.
406//! write!(f, "({}, {})", self.x, self.y)
407//! }
408//! }
409//!
410//! // Different traits allow different forms of output of a type. The meaning
411//! // of this format is to print the magnitude of a vector.
412//! impl fmt::Binary for Vector2D {
413//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
414//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
415//! let magnitude = magnitude.sqrt();
416//!
417//! // Respect the formatting flags by using the helper method
418//! // `pad_integral` on the Formatter object. See the method
419//! // documentation for details, and the function `pad` can be used
420//! // to pad strings.
421//! let decimals = f.precision().unwrap_or(3);
422//! let string = format!("{magnitude:.decimals$}");
423//! f.pad_integral(true, "", &string)
424//! }
425//! }
426//!
427//! fn main() {
428//! let myvector = Vector2D { x: 3, y: 4 };
429//!
430//! println!("{myvector}"); // => "(3, 4)"
431//! println!("{myvector:?}"); // => "Vector2D {x: 3, y:4}"
432//! println!("{myvector:10.3b}"); // => " 5.000"
433//! }
434//! ```
435//!
436//! ### `fmt::Display` vs `fmt::Debug`
437//!
438//! These two formatting traits have distinct purposes:
439//!
440//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
441//! represented as a UTF-8 string at all times. It is **not** expected that
442//! all types implement the [`Display`] trait.
443//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
444//! Output will typically represent the internal state as faithfully as possible.
445//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
446//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
447//!
448//! Some examples of the output from both traits:
449//!
450//! ```
451//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
452//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
453//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
454//! ```
455//!
456//! # Related macros
457//!
458//! There are a number of related macros in the [`format!`] family. The ones that
459//! are currently implemented are:
460//!
461//! ```ignore (only-for-syntax-highlight)
462//! format! // described above
463//! write! // first argument is either a &mut io::Write or a &mut fmt::Write, the destination
464//! writeln! // same as write but appends a newline
465//! print! // the format string is printed to the standard output
466//! println! // same as print but appends a newline
467//! eprint! // the format string is printed to the standard error
468//! eprintln! // same as eprint but appends a newline
469//! format_args! // described below.
470//! ```
471//!
472//! ### `write!`
473//!
474//! [`write!`] and [`writeln!`] are two macros which are used to emit the format string
475//! to a specified stream. This is used to prevent intermediate allocations of
476//! format strings and instead directly write the output. Under the hood, this
477//! function is actually invoking the [`write_fmt`] function defined on the
478//! [`std::io::Write`] and the [`std::fmt::Write`] trait. Example usage is:
479//!
480//! ```
481//! # #![allow(unused_must_use)]
482//! use std::io::Write;
483//! let mut w = Vec::new();
484//! write!(&mut w, "Hello {}!", "world");
485//! ```
486//!
487//! ### `print!`
488//!
489//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
490//! macro, the goal of these macros is to avoid intermediate allocations when
491//! printing output. Example usage is:
492//!
493//! ```
494//! print!("Hello {}!", "world");
495//! println!("I have a newline {}", "character at the end");
496//! ```
497//! ### `eprint!`
498//!
499//! The [`eprint!`] and [`eprintln!`] macros are identical to
500//! [`print!`] and [`println!`], respectively, except they emit their
501//! output to stderr.
502//!
503//! ### `format_args!`
504//!
505//! [`format_args!`] is a curious macro used to safely pass around
506//! an opaque object describing the format string. This object
507//! does not require any heap allocations to create, and it only
508//! references information on the stack. Under the hood, all of
509//! the related macros are implemented in terms of this. First
510//! off, some example usage is:
511//!
512//! ```
513//! # #![allow(unused_must_use)]
514//! use std::fmt;
515//! use std::io::{self, Write};
516//!
517//! let mut some_writer = io::stdout();
518//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
519//!
520//! fn my_fmt_fn(args: fmt::Arguments<'_>) {
521//! write!(&mut io::stdout(), "{args}");
522//! }
523//! my_fmt_fn(format_args!(", or a {} too", "function"));
524//! ```
525//!
526//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
527//! This structure can then be passed to the [`write`] and [`format`] functions
528//! inside this module in order to process the format string.
529//! The goal of this macro is to even further prevent intermediate allocations
530//! when dealing with formatting strings.
531//!
532//! For example, a logging library could use the standard formatting syntax, but
533//! it would internally pass around this structure until it has been determined
534//! where output should go to.
535//!
536//! [`fmt::Result`]: Result "fmt::Result"
537//! [Result]: core::result::Result "std::result::Result"
538//! [std::fmt::Error]: Error "fmt::Error"
539//! [`write`]: write() "fmt::write"
540//! [`to_string`]: crate::string::ToString::to_string "ToString::to_string"
541//! [`write_fmt`]: ../../std/io/trait.Write.html#method.write_fmt
542//! [`std::io::Write`]: ../../std/io/trait.Write.html
543//! [`std::fmt::Write`]: ../../std/fmt/trait.Write.html
544//! [`print!`]: ../../std/macro.print.html "print!"
545//! [`println!`]: ../../std/macro.println.html "println!"
546//! [`eprint!`]: ../../std/macro.eprint.html "eprint!"
547//! [`eprintln!`]: ../../std/macro.eprintln.html "eprintln!"
548//! [`format_args!`]: ../../std/macro.format_args.html "format_args!"
549//! [`fmt::Arguments`]: Arguments "fmt::Arguments"
550//! [`format`]: format() "fmt::format"
551
552#![stable(feature = "rust1", since = "1.0.0")]
553
554#[stable(feature = "fmt_flags_align", since = "1.28.0")]
555pub use core::fmt::Alignment;
556#[stable(feature = "rust1", since = "1.0.0")]
557pub use core::fmt::Error;
558#[unstable(feature = "debug_closure_helpers", issue = "117729")]
559pub use core::fmt::FormatterFn;
560#[stable(feature = "rust1", since = "1.0.0")]
561pub use core::fmt::{write, Arguments};
562#[stable(feature = "rust1", since = "1.0.0")]
563pub use core::fmt::{Binary, Octal};
564#[stable(feature = "rust1", since = "1.0.0")]
565pub use core::fmt::{Debug, Display};
566#[stable(feature = "rust1", since = "1.0.0")]
567pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
568#[stable(feature = "rust1", since = "1.0.0")]
569pub use core::fmt::{Formatter, Result, Write};
570#[stable(feature = "rust1", since = "1.0.0")]
571pub use core::fmt::{LowerExp, UpperExp};
572#[stable(feature = "rust1", since = "1.0.0")]
573pub use core::fmt::{LowerHex, Pointer, UpperHex};
574
575#[cfg(not(no_global_oom_handling))]
576use crate::string;
577
578/// The `format` function takes an [`Arguments`] struct and returns the resulting
579/// formatted string.
580///
581/// The [`Arguments`] instance can be created with the [`format_args!`] macro.
582///
583/// # Examples
584///
585/// Basic usage:
586///
587/// ```
588/// use std::fmt;
589///
590/// let s = fmt::format(format_args!("Hello, {}!", "world"));
591/// assert_eq!(s, "Hello, world!");
592/// ```
593///
594/// Please note that using [`format!`] might be preferable.
595/// Example:
596///
597/// ```
598/// let s = format!("Hello, {}!", "world");
599/// assert_eq!(s, "Hello, world!");
600/// ```
601///
602/// [`format_args!`]: core::format_args
603/// [`format!`]: crate::format
604#[cfg(not(no_global_oom_handling))]
605#[must_use]
606#[stable(feature = "rust1", since = "1.0.0")]
607#[inline]
608pub fn format(args: Arguments<'_>) -> string::String {
609 fn format_inner(args: Arguments<'_>) -> string::String {
610 let capacity: usize = args.estimated_capacity();
611 let mut output: String = string::String::with_capacity(capacity);
612 output.write_fmt(args).expect(msg:"a formatting trait implementation returned an error");
613 output
614 }
615
616 args.as_str().map_or_else(|| format_inner(args), f:crate::borrow::ToOwned::to_owned)
617}
618