std::alloc::Global.shrink returns an error if realloc returns null

[mahkoh]

Some allocators do not (wish to) permit reducing the size of a large allocation below a certain threshold. This leaves the GlobalAlloc::realloc implementation with two choices to handle such shrink operations:

• Create a new, smaller allocation, copy the contents from the large to the small allocation, free the large allocation
• Return a null pointer to indicate that the allocation has not changed

The second choice corresponds to the allocator behavior. Unfortunately std::alloc::Global.shrink translates this into an error instead of returning the old pointer:

rust/library/alloc/src/alloc.rs

Lines 289 to 290 in b2dd829

	let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
	let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;

Downstream (e.g. RawVec) aborts the process if this happens even though there is still plenty of memory available.

Until GlobalAlloc has been completely replaced by AllocRef, I believe that it would be better if std::alloc::Global.shrink handled null pointer return values of the global allocator by returning the original allocation.

[mahkoh]

Unfortunately I believe the current implementation is correct and necessary because the Layout "fitting" requirements of AllocRef are a bit too permissive. I'll open an issue in wg-allocators shortly.

[mahkoh]

rust-lang/wg-allocators#78

[Amanieu]

I think this should probably be fixed in RawVec itself: a failed shrink should be non-fatal and just keep the old length.