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document lifetime extension for format_args!'s arguments

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dianne 2025-08-27 12:06:52 -07:00
parent 3c91be712d
commit ee9986b4a2
1 changed files with 12 additions and 7 deletions
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19
library/core/src/macros/mod.rs
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@ -951,8 +951,9 @@ pub(crate) mod builtin {
/// format string in `format_args!`.
///
/// ```rust
/// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
/// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
/// 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);
/// ```
@ -976,13 +977,17 @@ pub(crate) mod builtin {
/// assert_eq!(s, format!("hello {}", "world"));
/// ```
///
/// # Lifetime limitation
/// # Argument lifetimes
///
/// Except when no formatting arguments are used,
/// the produced `fmt::Arguments` value borrows temporary values,
/// which means it can only be used within the same expression
/// and cannot be stored for later use.
/// This is a known limitation, see [#92698](https://github.com/rust-lang/rust/issues/92698).
/// 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].
///
/// [extended]: ../reference/destructors.html#temporary-lifetime-extension
/// [Reference]: ../reference/destructors.html#extending-based-on-expressions
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "format_args_macro"]
#[allow_internal_unsafe]