Rollup merge of rust-lang#64375 - kornelski:vecdrop, r=rkruppe

Fast path for vec.clear/truncate

For trivial types like `u8`, `vec.truncate()`/`vec.clear()` relies on the optimizer to remove the loop. This means more work in debug builds, and more work for the optimizer.

Avoiding this busywork is exactly what `mem::needs_drop::<T>()` is for.

src/liballoc/vec.rs

@@ -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;
  }
  }