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Guarantee vec.clear/truncate is O(1) for trivial types

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This commit is contained in:
Kornel 2019-09-11 11:46:53 +01:00
parent 74d5c70b17
commit 223600ac2c
1 changed files with 17 additions and 13 deletions
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30
src/liballoc/vec.rs
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@ -685,21 +685,25 @@ impl<T> Vec<T> {
/// [`drain`]: #method.drain
#[stable(feature = "rust1", since = "1.0.0")]
pub fn truncate(&mut self, len: usize) {
let current_len = self.len;
unsafe {
let mut ptr = self.as_mut_ptr().add(self.len);
// Set the final length at the end, keeping in mind that
// dropping an element might panic. Works around a missed
// optimization, as seen in the following issue:
// https://github.com/rust-lang/rust/issues/51802
let mut local_len = SetLenOnDrop::new(&mut self.len);
if mem::needs_drop::<T>() {
let current_len = self.len;
unsafe {
let mut ptr = self.as_mut_ptr().add(self.len);
// Set the final length at the end, keeping in mind that
// dropping an element might panic. Works around a missed
// optimization, as seen in the following issue:
// https://github.com/rust-lang/rust/issues/51802
let mut local_len = SetLenOnDrop::new(&mut self.len);
// drop any extra elements
for _ in len..current_len {
local_len.decrement_len(1);
ptr = ptr.offset(-1);
ptr::drop_in_place(ptr);
// drop any extra elements
for _ in len..current_len {
local_len.decrement_len(1);
ptr = ptr.offset(-1);
ptr::drop_in_place(ptr);
}
}
} else if len <= self.len {
self.len = len;
}
}