format in alloc - Rust
Description: Creates a `String` using interpolation of runtime expressions.

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## format

# Macro format

1.36.0 · Source

    macro_rules! format {
        ($($arg:tt)*) => { ... };
    }

Expand description

Creates a `String` using interpolation of runtime expressions.

The first argument `format!` receives is a format string. This must be a string literal. The power of the formatting string is in the `{}`s contained. Additional parameters passed to `format!` replace the `{}`s within the formatting string in the order given unless named or positional parameters are used.

See the formatting syntax documentation in `std::fmt` for details.

A common use for `format!` is concatenation and interpolation of strings. The same convention is used with `print!` and `write!` macros, depending on the intended destination of the string; all these macros internally use `format_args!`.

To convert a single value to a string, use the `to_string` method. This will use the `Display` formatting trait.

To concatenate literals into a `&'static str`, use the `concat!` macro.

## §Panics

`format!` panics if a formatting trait implementation returns an error. This indicates an incorrect implementation since `fmt::Write for String` never returns an error itself.

## §Examples

    format!("test");                             // => "test"
    format!("hello {}", "world!");               // => "hello world!"
    format!("x = {}, y = {val}", 10, val = 30);  // => "x = 10, y = 30"
    let (x, y) = (1, 2);
    format!("{x} + {y} = 3");                    // => "1 + 2 = 3"