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Rollup merge of #106823 - m-ou-se:format-args-as-str-guarantees, r=dtolnay

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Allow fmt::Arguments::as_str() to return more Some(_).

This adjusts the documentation to allow optimization of format_args!() to be visible through fmt::Arguments::as_str().

This allows for future changes like https://github.com/rust-lang/rust/pull/106824.
This commit is contained in:
Dylan DPC 2023-01-25 17:01:42 +05:30 committed by GitHub
commit 0c53b215de
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1 changed files with 20 additions and 3 deletions
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23
library/core/src/fmt/mod.rs
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@ -489,9 +489,26 @@ pub struct Arguments<'a> {
}
impl<'a> Arguments<'a> {
/// Get the formatted string, if it has no arguments to be formatted.
/// Get the formatted string, if it has no arguments to be formatted at runtime.
///
/// This can be used to avoid allocations in the most trivial case.
/// This can be used to avoid allocations in some cases.
///
/// # Guarantees
///
/// For `format_args!("just a literal")`, this function is guaranteed to
/// return `Some("just a literal")`.
///
/// For most cases with placeholders, this function will return `None`.
///
/// However, the compiler may perform optimizations that can cause this
/// function to return `Some(_)` even if the format string contains
/// placeholders. For example, `format_args!("Hello, {}!", "world")` may be
/// optimized to `format_args!("Hello, world!")`, such that `as_str()`
/// returns `Some("Hello, world!")`.
///
/// The behavior for anything but the trivial case (without placeholders)
/// is not guaranteed, and should not be relied upon for anything other
/// than optimization.
///
/// # Examples
///
@ -512,7 +529,7 @@ impl<'a> Arguments<'a> {
/// ```rust
/// assert_eq!(format_args!("hello").as_str(), Some("hello"));
/// assert_eq!(format_args!("").as_str(), Some(""));
/// assert_eq!(format_args!("{}", 1).as_str(), None);
/// assert_eq!(format_args!("{:?}", std::env::current_dir()).as_str(), None);
/// ```
#[stable(feature = "fmt_as_str", since = "1.52.0")]
#[rustc_const_unstable(feature = "const_arguments_as_str", issue = "103900")]