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Prevent unnecessary allocation in PathBuf::set_extension. #65731

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Oct 25, 2019
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Prevent unnecessary allocation in PathBuf::set_extension. #65731

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26 changes: 15 additions & 11 deletions src/libstd/path.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1363,20 +1363,24 @@ impl PathBuf {
}

fn _set_extension(&mut self, extension: &OsStr) -> bool {
if self.file_name().is_none() {
return false;
}

let mut stem = match self.file_stem() {
Some(stem) => stem.to_os_string(),
None => OsString::new(),
let file_stem = match self.file_stem() {
None => return false,
Some(f) => os_str_as_u8_slice(f),
};

if !os_str_as_u8_slice(extension).is_empty() {
stem.push(".");
stem.push(extension);
// truncate until right after the file stem
let end_file_stem = file_stem[file_stem.len()..].as_ptr() as usize;
let start = os_str_as_u8_slice(&self.inner).as_ptr() as usize;
let v = self.as_mut_vec();
v.truncate(end_file_stem.wrapping_sub(start));

// add the new extension, if any
let new = os_str_as_u8_slice(extension);
if !new.is_empty() {
v.reserve_exact(new.len() + 1);
v.push(b'.');
v.extend_from_slice(new);
}
self.set_file_name(&stem);

true
}
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