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1 change: 1 addition & 0 deletions library/alloc/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -85,6 +85,7 @@
//
// Library features:
// tidy-alphabetical-start
#![cfg_attr(not(no_global_oom_handling), feature(string_replace_in_place))]
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@tgross35 tgross35 Oct 21, 2025

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Fwiw I think you may as well drop the cfg_attr and enable the feature unconditionally, usually we don't gate these too strictly. And we'll wind up using it elsewhere anyway.

I'll merge after PR CI finishes, with or without this (just ping me once it does if I miss it)

#![feature(alloc_layout_extra)]
#![feature(allocator_api)]
#![feature(array_into_iter_constructors)]
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61 changes: 61 additions & 0 deletions library/alloc/src/string.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2090,6 +2090,67 @@ impl String {
unsafe { self.as_mut_vec() }.splice((start, end), replace_with.bytes());
}

/// Replaces the leftmost occurrence of a pattern with another string, in-place.
///
/// This method can be preferred over [`string = string.replacen(..., 1);`][replacen],
/// as it can use the `String`'s existing capacity to prevent a reallocation if
/// sufficient space is available.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(string_replace_in_place)]
///
/// let mut s = String::from("Test Results: ❌❌❌");
///
/// // Replace the leftmost ❌ with a ✅
/// s.replace_first('❌', "✅");
/// assert_eq!(s, "Test Results: ✅❌❌");
/// ```
///
/// [replacen]: ../../std/primitive.str.html#method.replacen
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "string_replace_in_place", issue = "147949")]
pub fn replace_first<P: Pattern>(&mut self, from: P, to: &str) {
let range = match self.match_indices(from).next() {
Some((start, match_str)) => start..start + match_str.len(),
None => return,
};

self.replace_range(range, to);
}

/// Replaces the rightmost occurrence of a pattern with another string, in-place.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(string_replace_in_place)]
///
/// let mut s = String::from("Test Results: ❌❌❌");
///
/// // Replace the rightmost ❌ with a ✅
/// s.replace_last('❌', "✅");
/// assert_eq!(s, "Test Results: ❌❌✅");
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "string_replace_in_place", issue = "147949")]
pub fn replace_last<P: Pattern>(&mut self, from: P, to: &str)
where
for<'a> P::Searcher<'a>: core::str::pattern::ReverseSearcher<'a>,
{
let range = match self.rmatch_indices(from).next() {
Some((start, match_str)) => start..start + match_str.len(),
None => return,
};

self.replace_range(range, to);
}

/// Converts this `String` into a <code>[Box]<[str]></code>.
///
/// Before doing the conversion, this method discards excess capacity like [`shrink_to_fit`].
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1 change: 1 addition & 0 deletions library/alloctests/tests/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,7 @@
#![feature(local_waker)]
#![feature(str_as_str)]
#![feature(strict_provenance_lints)]
#![feature(string_replace_in_place)]
#![feature(vec_deque_pop_if)]
#![feature(vec_deque_truncate_front)]
#![feature(unique_rc_arc)]
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34 changes: 34 additions & 0 deletions library/alloctests/tests/string.rs
Original file line number Diff line number Diff line change
Expand Up @@ -719,6 +719,40 @@ fn test_replace_range_evil_end_bound() {
assert_eq!(Ok(""), str::from_utf8(s.as_bytes()));
}

#[test]
fn test_replace_first() {
let mut s = String::from("~ First ❌ Middle ❌ Last ❌ ~");
s.replace_first("❌", "✅✅");
assert_eq!(s, "~ First ✅✅ Middle ❌ Last ❌ ~");
s.replace_first("🦀", "😳");
assert_eq!(s, "~ First ✅✅ Middle ❌ Last ❌ ~");

let mut s = String::from("❌");
s.replace_first('❌', "✅✅");
assert_eq!(s, "✅✅");

let mut s = String::from("");
s.replace_first('🌌', "❌");
assert_eq!(s, "");
}

#[test]
fn test_replace_last() {
let mut s = String::from("~ First ❌ Middle ❌ Last ❌ ~");
s.replace_last("❌", "✅✅");
assert_eq!(s, "~ First ❌ Middle ❌ Last ✅✅ ~");
s.replace_last("🦀", "😳");
assert_eq!(s, "~ First ❌ Middle ❌ Last ✅✅ ~");

let mut s = String::from("❌");
s.replace_last::<char>('❌', "✅✅");
assert_eq!(s, "✅✅");

let mut s = String::from("");
s.replace_last::<char>('🌌', "❌");
assert_eq!(s, "");
}

#[test]
fn test_extend_ref() {
let mut a = "foo".to_string();
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