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Rollup merge of #65332 - RalfJung:fmt, r=cramertj

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std::fmt: reorder docs

This moves the "Formatting Parameters" section up above right after the discussion of named and positional arguments. Then comes the "Syntax" section, summarizing the discussion of format string syntax.
And only *then* we get to "Formatting Traits" -- that section has some *huge* code examples, so it really should not interrupt the discussion of the grammar. Also users are much more likely to come here to learn about the format string grammar than to come here to learn about the `Binary` trait.
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Tyler Mandry 2019-10-14 17:52:37 -07:00 committed by GitHub
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src/liballoc/fmt.rs
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@ -80,250 +80,6 @@
//! arguments which have names. Like with positional parameters, it is not
//! valid to provide named parameters that are unused by the format string.
//!
//! ## Argument types
//!
//! Each argument's type is dictated by the format string.
//! There are various parameters which require a particular type, however.
//! An example is the `{:.*}` syntax, which sets the number of decimal places
//! in floating-point types:
//!
//! ```
//! let formatted_number = format!("{:.*}", 2, 1.234567);
//!
//! assert_eq!("1.23", formatted_number)
//! ```
//!
//! If this syntax is used, then the number of characters to print precedes the
//! actual object being formatted, and the number of characters must have the
//! type [`usize`].
//!
//! ## Formatting traits
//!
//! When requesting that an argument be formatted with a particular type, you
//! are actually requesting that an argument ascribes to a particular trait.
//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
//! well as [`isize`]). The current mapping of types to traits is:
//!
//! * *nothing* ⇒ [`Display`]
//! * `?` ⇒ [`Debug`]
//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
//! * `o` ⇒ [`Octal`](trait.Octal.html)
//! * `x` ⇒ [`LowerHex`](trait.LowerHex.html)
//! * `X` ⇒ [`UpperHex`](trait.UpperHex.html)
//! * `p` ⇒ [`Pointer`](trait.Pointer.html)
//! * `b` ⇒ [`Binary`]
//! * `e` ⇒ [`LowerExp`](trait.LowerExp.html)
//! * `E` ⇒ [`UpperExp`](trait.UpperExp.html)
//!
//! What this means is that any type of argument which implements the
//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
//! are provided for these traits for a number of primitive types by the
//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
//! then the format trait used is the [`Display`] trait.
//!
//! When implementing a format trait for your own type, you will have to
//! implement a method of the signature:
//!
//! ```
//! # #![allow(dead_code)]
//! # use std::fmt;
//! # struct Foo; // our custom type
//! # impl fmt::Display for Foo {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! # write!(f, "testing, testing")
//! # } }
//! ```
//!
//! Your type will be passed as `self` by-reference, and then the function
//! should emit output into the `f.buf` stream. It is up to each format trait
//! implementation to correctly adhere to the requested formatting parameters.
//! The values of these parameters will be listed in the fields of the
//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
//! provides some helper methods.
//!
//! Additionally, the return value of this function is [`fmt::Result`] which is a
//! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations
//! should ensure that they propagate errors from the [`Formatter`][`Formatter`] (e.g., when
//! calling [`write!`]). However, they should never return errors spuriously. That
//! is, a formatting implementation must and may only return an error if the
//! passed-in [`Formatter`] returns an error. This is because, contrary to what
//! the function signature might suggest, string formatting is an infallible
//! operation. This function only returns a result because writing to the
//! underlying stream might fail and it must provide a way to propagate the fact
//! that an error has occurred back up the stack.
//!
//! An example of implementing the formatting traits would look
//! like:
//!
//! ```
//! use std::fmt;
//!
//! #[derive(Debug)]
//! struct Vector2D {
//! x: isize,
//! y: isize,
//! }
//!
//! impl fmt::Display for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! // The `f` value implements the `Write` trait, which is what the
//! // write! macro is expecting. Note that this formatting ignores the
//! // various flags provided to format strings.
//! write!(f, "({}, {})", self.x, self.y)
//! }
//! }
//!
//! // Different traits allow different forms of output of a type. The meaning
//! // of this format is to print the magnitude of a vector.
//! impl fmt::Binary for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
//! let magnitude = magnitude.sqrt();
//!
//! // Respect the formatting flags by using the helper method
//! // `pad_integral` on the Formatter object. See the method
//! // documentation for details, and the function `pad` can be used
//! // to pad strings.
//! let decimals = f.precision().unwrap_or(3);
//! let string = format!("{:.*}", decimals, magnitude);
//! f.pad_integral(true, "", &string)
//! }
//! }
//!
//! fn main() {
//! let myvector = Vector2D { x: 3, y: 4 };
//!
//! println!("{}", myvector); // => "(3, 4)"
//! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
//! println!("{:10.3b}", myvector); // => " 5.000"
//! }
//! ```
//!
//! ### `fmt::Display` vs `fmt::Debug`
//!
//! These two formatting traits have distinct purposes:
//!
//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
//! represented as a UTF-8 string at all times. It is **not** expected that
//! all types implement the [`Display`] trait.
//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
//! Output will typically represent the internal state as faithfully as possible.
//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
//!
//! Some examples of the output from both traits:
//!
//! ```
//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
//! ```
//!
//! ## Related macros
//!
//! There are a number of related macros in the [`format!`] family. The ones that
//! are currently implemented are:
//!
//! ```ignore (only-for-syntax-highlight)
//! format! // described above
//! write! // first argument is a &mut io::Write, the destination
//! writeln! // same as write but appends a newline
//! print! // the format string is printed to the standard output
//! println! // same as print but appends a newline
//! eprint! // the format string is printed to the standard error
//! eprintln! // same as eprint but appends a newline
//! format_args! // described below.
//! ```
//!
//! ### `write!`
//!
//! This and [`writeln!`] are two macros which are used to emit the format string
//! to a specified stream. This is used to prevent intermediate allocations of
//! format strings and instead directly write the output. Under the hood, this
//! function is actually invoking the [`write_fmt`] function defined on the
//! [`std::io::Write`] trait. Example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::io::Write;
//! let mut w = Vec::new();
//! write!(&mut w, "Hello {}!", "world");
//! ```
//!
//! ### `print!`
//!
//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
//! macro, the goal of these macros is to avoid intermediate allocations when
//! printing output. Example usage is:
//!
//! ```
//! print!("Hello {}!", "world");
//! println!("I have a newline {}", "character at the end");
//! ```
//! ### `eprint!`
//!
//! The [`eprint!`] and [`eprintln!`] macros are identical to
//! [`print!`] and [`println!`], respectively, except they emit their
//! output to stderr.
//!
//! ### `format_args!`
//!
//! This is a curious macro which is used to safely pass around
//! an opaque object describing the format string. This object
//! does not require any heap allocations to create, and it only
//! references information on the stack. Under the hood, all of
//! the related macros are implemented in terms of this. First
//! off, some example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::fmt;
//! use std::io::{self, Write};
//!
//! let mut some_writer = io::stdout();
//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
//!
//! fn my_fmt_fn(args: fmt::Arguments) {
//! write!(&mut io::stdout(), "{}", args);
//! }
//! my_fmt_fn(format_args!(", or a {} too", "function"));
//! ```
//!
//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
//! This structure can then be passed to the [`write`] and [`format`] functions
//! inside this module in order to process the format string.
//! The goal of this macro is to even further prevent intermediate allocations
//! when dealing formatting strings.
//!
//! For example, a logging library could use the standard formatting syntax, but
//! it would internally pass around this structure until it has been determined
//! where output should go to.
//!
//! # Syntax
//!
//! The syntax for the formatting language used is drawn from other languages,
//! so it should not be too alien. Arguments are formatted with Python-like
//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
//! `%`. The actual grammar for the formatting syntax is:
//!
//! ```text
//! format_string := <text> [ maybe-format <text> ] *
//! maybe-format := '{' '{' | '}' '}' | <format>
//! format := '{' [ argument ] [ ':' format_spec ] '}'
//! argument := integer | identifier
//!
//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
//! fill := character
//! align := '<' | '^' | '>'
//! sign := '+' | '-'
//! width := count
//! precision := count | '*'
//! type := identifier | '?' | ''
//! count := parameter | integer
//! parameter := argument '$'
//! ```
//!
//! # Formatting Parameters
//!
//! Each argument being formatted can be transformed by a number of formatting
@ -479,6 +235,234 @@
//! them with the same character. For example, the `{` character is escaped with
//! `{{` and the `}` character is escaped with `}}`.
//!
//! # Syntax
//!
//! To summarize, you can find the full grammar of format strings.
//! The syntax for the formatting language used is drawn from other languages,
//! so it should not be too alien. Arguments are formatted with Python-like
//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
//! `%`. The actual grammar for the formatting syntax is:
//!
//! ```text
//! format_string := <text> [ maybe-format <text> ] *
//! maybe-format := '{' '{' | '}' '}' | <format>
//! format := '{' [ argument ] [ ':' format_spec ] '}'
//! argument := integer | identifier
//!
//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
//! fill := character
//! align := '<' | '^' | '>'
//! sign := '+' | '-'
//! width := count
//! precision := count | '*'
//! type := identifier | '?' | ''
//! count := parameter | integer
//! parameter := argument '$'
//! ```
//!
//! # Formatting traits
//!
//! When requesting that an argument be formatted with a particular type, you
//! are actually requesting that an argument ascribes to a particular trait.
//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
//! well as [`isize`]). The current mapping of types to traits is:
//!
//! * *nothing* ⇒ [`Display`]
//! * `?` ⇒ [`Debug`]
//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
//! * `o` ⇒ [`Octal`](trait.Octal.html)
//! * `x` ⇒ [`LowerHex`](trait.LowerHex.html)
//! * `X` ⇒ [`UpperHex`](trait.UpperHex.html)
//! * `p` ⇒ [`Pointer`](trait.Pointer.html)
//! * `b` ⇒ [`Binary`]
//! * `e` ⇒ [`LowerExp`](trait.LowerExp.html)
//! * `E` ⇒ [`UpperExp`](trait.UpperExp.html)
//!
//! What this means is that any type of argument which implements the
//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
//! are provided for these traits for a number of primitive types by the
//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
//! then the format trait used is the [`Display`] trait.
//!
//! When implementing a format trait for your own type, you will have to
//! implement a method of the signature:
//!
//! ```
//! # #![allow(dead_code)]
//! # use std::fmt;
//! # struct Foo; // our custom type
//! # impl fmt::Display for Foo {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! # write!(f, "testing, testing")
//! # } }
//! ```
//!
//! Your type will be passed as `self` by-reference, and then the function
//! should emit output into the `f.buf` stream. It is up to each format trait
//! implementation to correctly adhere to the requested formatting parameters.
//! The values of these parameters will be listed in the fields of the
//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
//! provides some helper methods.
//!
//! Additionally, the return value of this function is [`fmt::Result`] which is a
//! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations
//! should ensure that they propagate errors from the [`Formatter`][`Formatter`] (e.g., when
//! calling [`write!`]). However, they should never return errors spuriously. That
//! is, a formatting implementation must and may only return an error if the
//! passed-in [`Formatter`] returns an error. This is because, contrary to what
//! the function signature might suggest, string formatting is an infallible
//! operation. This function only returns a result because writing to the
//! underlying stream might fail and it must provide a way to propagate the fact
//! that an error has occurred back up the stack.
//!
//! An example of implementing the formatting traits would look
//! like:
//!
//! ```
//! use std::fmt;
//!
//! #[derive(Debug)]
//! struct Vector2D {
//! x: isize,
//! y: isize,
//! }
//!
//! impl fmt::Display for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! // The `f` value implements the `Write` trait, which is what the
//! // write! macro is expecting. Note that this formatting ignores the
//! // various flags provided to format strings.
//! write!(f, "({}, {})", self.x, self.y)
//! }
//! }
//!
//! // Different traits allow different forms of output of a type. The meaning
//! // of this format is to print the magnitude of a vector.
//! impl fmt::Binary for Vector2D {
//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
//! let magnitude = magnitude.sqrt();
//!
//! // Respect the formatting flags by using the helper method
//! // `pad_integral` on the Formatter object. See the method
//! // documentation for details, and the function `pad` can be used
//! // to pad strings.
//! let decimals = f.precision().unwrap_or(3);
//! let string = format!("{:.*}", decimals, magnitude);
//! f.pad_integral(true, "", &string)
//! }
//! }
//!
//! fn main() {
//! let myvector = Vector2D { x: 3, y: 4 };
//!
//! println!("{}", myvector); // => "(3, 4)"
//! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
//! println!("{:10.3b}", myvector); // => " 5.000"
//! }
//! ```
//!
//! ### `fmt::Display` vs `fmt::Debug`
//!
//! These two formatting traits have distinct purposes:
//!
//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
//! represented as a UTF-8 string at all times. It is **not** expected that
//! all types implement the [`Display`] trait.
//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
//! Output will typically represent the internal state as faithfully as possible.
//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
//!
//! Some examples of the output from both traits:
//!
//! ```
//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
//! ```
//!
//! # Related macros
//!
//! There are a number of related macros in the [`format!`] family. The ones that
//! are currently implemented are:
//!
//! ```ignore (only-for-syntax-highlight)
//! format! // described above
//! write! // first argument is a &mut io::Write, the destination
//! writeln! // same as write but appends a newline
//! print! // the format string is printed to the standard output
//! println! // same as print but appends a newline
//! eprint! // the format string is printed to the standard error
//! eprintln! // same as eprint but appends a newline
//! format_args! // described below.
//! ```
//!
//! ### `write!`
//!
//! This and [`writeln!`] are two macros which are used to emit the format string
//! to a specified stream. This is used to prevent intermediate allocations of
//! format strings and instead directly write the output. Under the hood, this
//! function is actually invoking the [`write_fmt`] function defined on the
//! [`std::io::Write`] trait. Example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::io::Write;
//! let mut w = Vec::new();
//! write!(&mut w, "Hello {}!", "world");
//! ```
//!
//! ### `print!`
//!
//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
//! macro, the goal of these macros is to avoid intermediate allocations when
//! printing output. Example usage is:
//!
//! ```
//! print!("Hello {}!", "world");
//! println!("I have a newline {}", "character at the end");
//! ```
//! ### `eprint!`
//!
//! The [`eprint!`] and [`eprintln!`] macros are identical to
//! [`print!`] and [`println!`], respectively, except they emit their
//! output to stderr.
//!
//! ### `format_args!`
//!
//! This is a curious macro which is used to safely pass around
//! an opaque object describing the format string. This object
//! does not require any heap allocations to create, and it only
//! references information on the stack. Under the hood, all of
//! the related macros are implemented in terms of this. First
//! off, some example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
//! use std::fmt;
//! use std::io::{self, Write};
//!
//! let mut some_writer = io::stdout();
//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
//!
//! fn my_fmt_fn(args: fmt::Arguments) {
//! write!(&mut io::stdout(), "{}", args);
//! }
//! my_fmt_fn(format_args!(", or a {} too", "function"));
//! ```
//!
//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
//! This structure can then be passed to the [`write`] and [`format`] functions
//! inside this module in order to process the format string.
//! The goal of this macro is to even further prevent intermediate allocations
//! when dealing formatting strings.
//!
//! For example, a logging library could use the standard formatting syntax, but
//! it would internally pass around this structure until it has been determined
//! where output should go to.
//!
//! [`usize`]: ../../std/primitive.usize.html
//! [`isize`]: ../../std/primitive.isize.html
//! [`i8`]: ../../std/primitive.i8.html