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Bail out early from truncate if the new length is equal to the old #74172

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Bail out early from truncate if the new length is equal to the old #74172

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8d0eded
Don't truncate if new length is equal to old
ecstatic-morse Jul 9, 2020
d88f09b
Implement `Vec::clear` by hand
ecstatic-morse Jul 9, 2020
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6 changes: 3 additions & 3 deletions src/liballoc/collections/vec_deque.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -913,12 +913,12 @@ impl<T> VecDeque<T> {
// Safe because:
//
// * Any slice passed to `drop_in_place` is valid; the second case has
// `len <= front.len()` and returning on `len > self.len()` ensures
// `begin <= back.len()` in the first case
// `len <= front.len()` and returning on `len >= self.len()` ensures
// `begin < back.len()` in the first case
// * The head of the VecDeque is moved before calling `drop_in_place`,
// so no value is dropped twice if `drop_in_place` panics
unsafe {
if len > self.len() {
if len >= self.len() {
return;
}
let num_dropped = self.len() - len;
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8 changes: 5 additions & 3 deletions src/liballoc/string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1149,10 +1149,12 @@ impl String {
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn truncate(&mut self, new_len: usize) {
if new_len <= self.len() {
assert!(self.is_char_boundary(new_len));
self.vec.truncate(new_len)
if new_len >= self.len() {
return;
}

assert!(self.is_char_boundary(new_len));
self.vec.truncate(new_len)
}

/// Removes the last character from the string buffer and returns it.
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18 changes: 15 additions & 3 deletions src/liballoc/vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -736,13 +736,13 @@ impl<T> Vec<T> {
pub fn truncate(&mut self, len: usize) {
// This is safe because:
//
// * the slice passed to `drop_in_place` is valid; the `len > self.len`
// * the slice passed to `drop_in_place` is valid; the `len >= self.len`
// case avoids creating an invalid slice, and
// * the `len` of the vector is shrunk before calling `drop_in_place`,
// such that no value will be dropped twice in case `drop_in_place`
// were to panic once (if it panics twice, the program aborts).
unsafe {
if len > self.len {
if len >= self.len {
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@ecstatic-morse Can you please check on this? I think this also impacts resize where it runs truncate even though the length is the same. CC @lzutao

pub fn resize(&mut self, new_len: usize, value: T) {
    let len = self.len();

    if new_len > len {
        self.extend_with(new_len - len, ExtendElement(value))
    } else {
        self.truncate(new_len); // truncate run the rest of the code even though the length is same
    }
}

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@ecstatic-morse ecstatic-morse Aug 29, 2020 •
edited
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@pickfire What would you like me to do exactly? This PR was closed because the reviewer wouldn't sign off on it. Talk to them.

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@ecstatic-morse The reviewer is leaving the team already. See the message on zulip.

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@ecstatic-morse ecstatic-morse Aug 30, 2020 •
edited
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That's too bad. Still, I don't see what you want from me. While I think this PR was fine as-is and was held to an unusually high standard during the review process, stepping away from the project does not invalidate one's opinion. If you can find someone with r+ rights who feels strongly that some version of these changes should be merged, then I guess we can reopen this, but I'm not going to try to cherry pick a reviewer.

return;
}
let remaining_len = self.len - len;
Expand Down Expand Up @@ -1375,7 +1375,19 @@ impl<T> Vec<T> {
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn clear(&mut self) {
self.truncate(0)
let elems: *mut [T] = self.as_mut_slice();

// SAFETY:
//
// * `elems` comes directly from `as_mut_slice` and is therefore valid.
//
// * Setting `self.len = 0` before calling `drop_in_place` means that, if an element's
// `Drop` impl panics, the vector's `Drop` impl will do nothing (leaking the rest of
// the elements) instead of dropping some twice.
unsafe {
self.len = 0;
ptr::drop_in_place(elems);
}
}

/// Returns the number of elements in the vector, also referred to
Expand Down