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std::fmt: Various fixes and improvements to documentation #96568

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merged 5 commits into from
May 2, 2022
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std::fmt: Various fixes and improvements to documentation #96568

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1288883
std::fmt: Fixed documentation for specifying precision via `.*`
EliasHolzmann Apr 30, 2022
afd80a2
std::fmt: Added argument index comments to examples for specifying pr…
EliasHolzmann Apr 30, 2022
79d9afd
std::fmt: Fix the grammar documentation
EliasHolzmann Apr 30, 2022
c70f3ab
std::fmt: Removed reference to Formatter::buf and other private fields
EliasHolzmann Apr 30, 2022
f3b86c3
std::fmt: Improved list of formatting macros
EliasHolzmann Apr 30, 2022
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49 changes: 31 additions & 18 deletions library/alloc/src/fmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -221,10 +221,12 @@
//!
//! 3. An asterisk `.*`:
//!
//! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the
//! first input holds the `usize` precision, and the second holds the value to print. Note that
//! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers
//! to the *value* to print, and the `precision` must come in the input preceding `<arg>`.
//! `.*` means that this `{...}` is associated with *two* format inputs rather than one:
//! - If a format string in the fashion of `{:<spec>.*}` is used, then the first input holds
//! the `usize` precision, and the second holds the value to print.
//! - If a format string in the fashion of `{<arg>:<spec>.*}` is used, then the `<arg>` part
//! refers to the value to print, and the `precision` is taken like it was specified with an
//! omitted positional parameter (`{}` instead of `{<arg>:}`).
//!
//! For example, the following calls all print the same thing `Hello x is 0.01000`:
//!
Expand All @@ -238,15 +240,19 @@
//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {second of next two args (0.01) with precision
//! // specified in first of next two args (5)}
//! // Hello {next arg -> arg 0 ("x")} is {second of next two args -> arg 2 (0.01) with precision
//! // specified in first of next two args -> arg 1 (5)}
//! println!("Hello {} is {:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg 2 (0.01) with precision
//! // specified in its predecessor (5)}
//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision
//! // specified in next arg -> arg 0 (5)}
//! println!("Hello {1} is {2:.*}", 5, "x", 0.01);
//!
//! // Hello {next arg -> arg 0 ("x")} is {arg 2 (0.01) with precision
//! // specified in next arg -> arg 1 (5)}
//! println!("Hello {} is {2:.*}", "x", 5, 0.01);
//!
//! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
//! // Hello {next arg -> arg 0 ("x")} is {arg "number" (0.01) with precision specified
//! // in arg "prec" (5)}
//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
//! ```
Expand Down Expand Up @@ -304,7 +310,7 @@
//! ```text
//! format_string := text [ maybe_format text ] *
//! maybe_format := '{' '{' | '}' '}' | format
//! format := '{' [ argument ] [ ':' format_spec ] '}'
//! format := '{' [ argument ] [ ':' format_spec ] [ ws ] * '}'
//! argument := integer | identifier
//!
//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision]type
Expand All @@ -317,7 +323,12 @@
//! count := parameter | integer
//! parameter := argument '$'
//! ```
//! In the above grammar, `text` must not contain any `'{'` or `'}'` characters.
//! In the above grammar,
//! - `text` must not contain any `'{'` or `'}'` characters,
//! - `ws` is any character for which [`char::is_whitespace`] returns `true`, has no semantic
//! meaning and is completely optional,
//! - `integer` is a decimal integer that may contain leading zeroes and
//! - `identifier` is an `IDENTIFIER_OR_KEYWORD` (not an `IDENTIFIER`) as defined by the [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html).
//!
//! # Formatting traits
//!
Expand Down Expand Up @@ -358,9 +369,9 @@
//! ```
//!
//! 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
//! should emit output into the Formatter `f` which implements `fmt::Write`. It is up to each
//! format trait implementation to correctly adhere to the requested formatting parameters.
//! The values of these parameters can be accessed with methods of the
//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
//! provides some helper methods.
//!
Expand Down Expand Up @@ -449,7 +460,7 @@
//!
//! ```ignore (only-for-syntax-highlight)
//! format! // described above
//! write! // first argument is a &mut io::Write, the destination
//! write! // first argument is either a &mut io::Write or a &mut fmt::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
Expand All @@ -460,11 +471,11 @@
//!
//! ### `write!`
//!
//! This and [`writeln!`] are two macros which are used to emit the format string
//! [`write!`] 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:
//! [`std::io::Write`] and the [`std::fmt::Write`] trait. Example usage is:
//!
//! ```
//! # #![allow(unused_must_use)]
Expand All @@ -491,7 +502,7 @@
//!
//! ### `format_args!`
//!
//! This is a curious macro used to safely pass around
//! [`format_args!`] is a curious macro 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
Expand Down Expand Up @@ -529,10 +540,12 @@
//! [`to_string`]: crate::string::ToString::to_string "ToString::to_string"
//! [`write_fmt`]: ../../std/io/trait.Write.html#method.write_fmt
//! [`std::io::Write`]: ../../std/io/trait.Write.html
//! [`std::fmt::Write`]: ../../std/fmt/trait.Write.html
//! [`print!`]: ../../std/macro.print.html "print!"
//! [`println!`]: ../../std/macro.println.html "println!"
//! [`eprint!`]: ../../std/macro.eprint.html "eprint!"
//! [`eprintln!`]: ../../std/macro.eprintln.html "eprintln!"
//! [`format_args!`]: ../../std/macro.format_args.html "format_args!"
//! [`fmt::Arguments`]: Arguments "fmt::Arguments"
//! [`format`]: format() "fmt::format"

Expand Down