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str: Mark unstable round_char_boundary feature functions as const #144472

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Merged
merged 1 commit into from
Jul 28, 2025
+26 −14
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str: Mark unstable round_char_boundary feature functions as const #144472

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40 changes: 26 additions & 14 deletions library/core/src/str/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -407,17 +407,22 @@ impl str {
/// ```
#[unstable(feature = "round_char_boundary", issue = "93743")]
#[inline]
pub fn floor_char_boundary(&self, index: usize) -> usize {
pub const fn floor_char_boundary(&self, index: usize) -> usize {
if index >= self.len() {
self.len()
} else {
let lower_bound = index.saturating_sub(3);
let new_index = self.as_bytes()[lower_bound..=index]
.iter()
.rposition(|b| b.is_utf8_char_boundary());

// SAFETY: we know that the character boundary will be within four bytes
unsafe { lower_bound + new_index.unwrap_unchecked() }
let mut i = index;
while i > 0 {
if self.as_bytes()[i].is_utf8_char_boundary() {
break;
}
i -= 1;
}

// The character boundary will be within four bytes of the index
debug_assert!(i >= index.saturating_sub(3));

i
}
}

Expand Down Expand Up @@ -445,15 +450,22 @@ impl str {
/// ```
#[unstable(feature = "round_char_boundary", issue = "93743")]
#[inline]
pub fn ceil_char_boundary(&self, index: usize) -> usize {
pub const fn ceil_char_boundary(&self, index: usize) -> usize {
if index >= self.len() {
self.len()
} else {
let upper_bound = Ord::min(index + 4, self.len());
self.as_bytes()[index..upper_bound]
.iter()
.position(|b| b.is_utf8_char_boundary())
.map_or(upper_bound, |pos| pos + index)
let mut i = index;
while i < self.len() {
if self.as_bytes()[i].is_utf8_char_boundary() {
break;
}
i += 1;
}

// The character boundary will be within four bytes of the index
debug_assert!(i <= index + 3);
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Personally, I think that simply adding this as a condition to the loop itself might be helpful (and similarly above) to ensure that the optimiser knows this precondition as well. Not sure how it'd affect codegen, but it feels like the safest approach compared to, for example, assert_unchecked.


i
}
}

Expand Down
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