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Auto merge of #131929 - LaihoE:replace_default_capacity, r=joboet

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better default capacity for str::replace

Adds smarter capacity for str::replace in cases where we know that the output will be at least as long as the original string.
This commit is contained in:
bors 2024-10-23 01:03:48 +00:00
commit b13176595d
1 changed files with 8 additions and 4 deletions
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12
library/alloc/src/str.rs
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@ -269,8 +269,7 @@ pub fn into_boxed_bytes(self: Box<str>) -> Box<[u8]> {
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
pub fn replace<P: Pattern>(&self, from: P, to: &str) -> String {
// Fast path for ASCII to ASCII case.
// Fast path for replacing a single ASCII character with another.
if let Some(from_byte) = match from.as_utf8_pattern() {
Some(Utf8Pattern::StringPattern([from_byte])) => Some(*from_byte),
Some(Utf8Pattern::CharPattern(c)) => c.as_ascii().map(|ascii_char| ascii_char.to_u8()),
@ -280,8 +279,13 @@ pub fn replace<P: Pattern>(&self, from: P, to: &str) -> String {
return unsafe { replace_ascii(self.as_bytes(), from_byte, *to_byte) };
}
}
let mut result = String::new();
// Set result capacity to self.len() when from.len() <= to.len()
let default_capacity = match from.as_utf8_pattern() {
Some(Utf8Pattern::StringPattern(s)) if s.len() <= to.len() => self.len(),
Some(Utf8Pattern::CharPattern(c)) if c.len_utf8() <= to.len() => self.len(),
_ => 0,
};
let mut result = String::with_capacity(default_capacity);
let mut last_end = 0;
for (start, part) in self.match_indices(from) {
result.push_str(unsafe { self.get_unchecked(last_end..start) });