[tialaramex]

The global allocator GlobalAlloc::alloc method does indeed return a naked pointer

But the (not yet stable) Allocator::allocate returns Result<NonNull<[u8]>, AllocError> --- that is, either a slice of bytes OR a failure.

Vec actually relies on Allocator not GlobalAlloc (it's part of the standard library so it's allowed to use unstable features)

So that interface is allowed to say you asked for 128 bytes but here's 256. Or, more likely, you asked for 940 bytes, but here's 1024. So if you were trying to make a Vec<TwentyByteThing> and Vec::with_capacity(47) it would be practical to adjust this so that when the allocator has 1024 bytes available but not 940 we get back a Vec with capacity 51 not 47.