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Optimize Vec::insert for the case where index == len. #98755

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Jul 3, 2022
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Optimize Vec::insert for the case where index == len. #98755

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15 changes: 9 additions & 6 deletions library/alloc/src/vec/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1379,9 +1379,6 @@ impl<T, A: Allocator> Vec<T, A> {
}

let len = self.len();
if index > len {
assert_failed(index, len);
}

// space for the new element
if len == self.buf.capacity() {
Expand All @@ -1393,9 +1390,15 @@ impl<T, A: Allocator> Vec<T, A> {
// The spot to put the new value
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This "infallible" comment is now a lie and needs extra justification.

{
let p = self.as_mut_ptr().add(index);
// Shift everything over to make space. (Duplicating the
// `index`th element into two consecutive places.)
ptr::copy(p, p.offset(1), len - index);
if index < len {
// Shift everything over to make space. (Duplicating the
// `index`th element into two consecutive places.)
ptr::copy(p, p.offset(1), len - index);
} else if index == len {
// No elements need shifting.
} else {
assert_failed(index, len);
}
// Write it in, overwriting the first copy of the `index`th
// element.
ptr::write(p, element);
Expand Down