macros.rs - source
Description: Source of the Rust file `library/alloc/src/macros.rs`.

#
alloc/
macros.rs

    1/// Creates a [`Vec`] containing the arguments.
    2///
    3/// `vec!` allows `Vec`s to be defined with the same syntax as array expressions.
    4/// There are two forms of this macro:
    5///
    6/// - Create a [`Vec`] containing a given list of elements:
    7///
    8/// ```
    9/// let v = vec![1, 2, 3];
    10/// assert_eq!(v[0], 1);
    11/// assert_eq!(v[1], 2);
    12/// assert_eq!(v[2], 3);
    13/// ```
    14///
    15/// - Create a [`Vec`] from a given element and size:
    16///
    17/// ```
    18/// let v = vec![1; 3];
    19/// assert_eq!(v, [1, 1, 1]);
    20/// ```
    21///
    22/// Note that unlike array expressions this syntax supports all elements
    23/// which implement [`Clone`] and the number of elements doesn't have to be
    24/// a constant.
    25///
    26/// This will use `clone` to duplicate an expression, so one should be careful
    27/// using this with types having a nonstandard `Clone` implementation. For
    28/// example, `vec![Rc::new(1); 5]` will create a vector of five references
    29/// to the same boxed integer value, not five references pointing to independently
    30/// boxed integers.
    31///
    32/// Also, note that `vec![expr; 0]` is allowed, and produces an empty vector.
    33/// This will still evaluate `expr`, however, and immediately drop the resulting value, so
    34/// be mindful of side effects.
    35///
    36/// [`Vec`]: crate::vec::Vec
    37#[cfg(not(no_global_oom_handling))]
    38#[macro_export]
    39#[stable(feature = "rust1", since = "1.0.0")]
    40#[rustc_diagnostic_item = "vec_macro"]
    41#[allow_internal_unstable(rustc_attrs, liballoc_internals)]
    42macro_rules! vec {
    43    () => (
    44        $crate::vec::Vec::new()
    45    );
    46    ($elem:expr; $n:expr) => (
    47        $crate::vec::from_elem($elem, $n)
    48    );
    49    ($($x:expr),+ $(,)?) => (
    50        <[_]>::into_vec(
    51            // Using the intrinsic produces a dramatic improvement in stack usage for
    52            // unoptimized programs using this code path to construct large Vecs.
    53            $crate::boxed::box_new([$($x),+])
    54        )
    55    );
    56}
    57
    58/// Creates a `String` using interpolation of runtime expressions.
    59///
    60/// The first argument `format!` receives is a format string. This must be a string
    61/// literal. The power of the formatting string is in the `{}`s contained.
    62/// Additional parameters passed to `format!` replace the `{}`s within the
    63/// formatting string in the order given unless named or positional parameters
    64/// are used.
    65///
    66/// See [the formatting syntax documentation in `std::fmt`](../std/fmt/index.html)
    67/// for details.
    68///
    69/// A common use for `format!` is concatenation and interpolation of strings.
    70/// The same convention is used with [`print!`] and [`write!`] macros,
    71/// depending on the intended destination of the string; all these macros internally use [`format_args!`].
    72///
    73/// To convert a single value to a string, use the [`to_string`] method. This
    74/// will use the [`Display`] formatting trait.
    75///
    76/// To concatenate literals into a `&'static str`, use the [`concat!`] macro.
    77///
    78/// [`print!`]: ../std/macro.print.html
    79/// [`write!`]: core::write
    80/// [`format_args!`]: core::format_args
    81/// [`to_string`]: crate::string::ToString
    82/// [`Display`]: core::fmt::Display
    83/// [`concat!`]: core::concat
    84///
    85/// # Panics
    86///
    87/// `format!` panics if a formatting trait implementation returns an error.
    88/// This indicates an incorrect implementation
    89/// since `fmt::Write for String` never returns an error itself.
    90///
    91/// # Examples
    92///
    93/// ```
    94/// # #![allow(unused_must_use)]
    95/// format!("test");                             // => "test"
    96/// format!("hello {}", "world!");               // => "hello world!"
    97/// format!("x = {}, y = {val}", 10, val = 30);  // => "x = 10, y = 30"
    98/// let (x, y) = (1, 2);
    99/// format!("{x} + {y} = 3");                    // => "1 + 2 = 3"
    100/// ```
    101#[macro_export]
    102#[stable(feature = "rust1", since = "1.0.0")]
    103#[allow_internal_unstable(hint_must_use, liballoc_internals)]
    104#[rustc_diagnostic_item = "format_macro"]
    105macro_rules! format {
    106    ($($arg:tt)*) => {
    107        $crate::__export::must_use({
    108            $crate::fmt::format($crate::__export::format_args!($($arg)*))
    109        })
    110    }
    111}