Vec::from_raw_parts docs do not correctly handle empty buffers

[oskgo]

Location

https://doc.rust-lang.org/std/vec/struct.Vec.html#method.from_raw_parts

Summary

The docs state the precondition "ptr must have been allocated using the global allocator, such as via the alloc::alloc function", which means that the following code is unsound since Vec::new does not allocate to produce the pointer:

fn reassemble<T>(mut v: Vec<T>) -> Vec<T> {
  let capacity = v.capacity();
  let ptr = v.as_mut_ptr();
  let length = v.len();
  std::mem::forget(v);
  unsafe {Vec::from_raw_parts(ptr, length, capacity)}
}

I believe this to be highly surprising, and probably unintended.

Vec::from_raw_parts should allow ptr not to be obtained from an allocation if capacity times the size of T is zero.

If the documentation is correct InPlaceDstBufDrop is unsound: https://github.com/rust-lang/rust/blob/master/library/alloc/src/vec/in_place_drop.rs#L37

String::from_raw_parts and Vec::from_raw_parts_in have the same issue.

[the8472]

This is a documentation error. If the vec has zero bytes capacity then it must be a dangling pointer. See #113113 for a recent PR adjusting its behavior towards that.

For an unallocated Vec Drop does nothing:

rust/library/alloc/src/raw_vec.rs

Lines 529 to 533 in e4c626d

	fn drop(&mut self) {
	if let Some((ptr, layout)) = self.current_memory() {
	unsafe { self.alloc.deallocate(ptr, layout) }
	}
	}

[lolbinarycat]

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