Macro `format_args`

macro_rules! format_args {
    ($fmt:expr) => { ... };
    ($fmt:expr, $($args:tt)*) => { ... };
}

Constructs parameters for the other string-formatting macros.

This macro functions by taking a formatting string literal containing {} for each additional argument passed. format_args! prepares the additional parameters to ensure the output can be interpreted as a string and canonicalizes the arguments into a single type. Any value that implements the Display trait can be passed to format_args!, as can any Debug implementation be passed to a {:?} within the formatting string.

This macro produces a value of type fmt::Arguments. This value can be passed to the macros within std::fmt for performing useful redirection. All other formatting macros (format!, [write!], println!, etc) are proxied through this one. format_args!, unlike its derived macros, avoids heap allocations.

You can use the fmt::Arguments value that format_args! returns in Debug and Display contexts as seen below. The example also shows that Debug and Display format to the same thing: the interpolated format string in format_args!.

let args = format_args!("{} foo {:?}", 1, 2);
let debug = format!("{args:?}");
let display = format!("{args}");
assert_eq!("1 foo 2", display);
assert_eq!(display, debug);

See the formatting documentation in std::fmt for details of the macro argument syntax, and further information.

Examples

use std::fmt;

let s = fmt::format(format_args!("hello {}", "world"));
assert_eq!(s, format!("hello {}", "world"));

Argument lifetimes

Except when no formatting arguments are used, the produced fmt::Arguments value borrows temporary values. To allow it to be stored for later use, the arguments' lifetimes, as well as those of temporaries they borrow, may be extended when format_args! appears in the initializer expression of a let statement. The syntactic rules used to determine when temporaries' lifetimes are extended are documented in the Reference.