Check for overflow in DroplessArena and align returned memory
* Check for overflow when calculating the slice start & end position.
* Align the pointer obtained from the allocator, ensuring that it
satisfies user requested alignment (the allocator is only asked for
layout compatible with u8 slice).
* Remove an incorrect assertion from DroplessArena::align.
* Avoid forming references to an uninitialized memory in DroplessArena.
Helps with #73007, #72624.
Use min_specialization in the remaining rustc crates
This adds a lot of `transmute` calls to replace the unsound uses of specialization.
It's ugly, but at least it's honest about what's going on.
cc #71420, @RalfJung
Return a pointer from `alloc_raw` instead of a slice. There is no
practical use for slice as a return type and changing it to a pointer
avoids forming references to an uninitialized memory.
* Check for overflow when calculating the slice start & end position.
* Align the pointer obtained from the allocator, ensuring that it
satisfies user requested alignment (the allocator is only asked for
layout compatible with u8 slice).
* Remove an incorrect assertion from DroplessArena::align.
- Use `len` more consistently for the number of elements in a vector,
because that's the usual name.
- Use `additional` more consistently for the number of elements we want
to add, because that's what `Vec::reserve()` uses.
- Use `cap` consistently rather than `capacity`.
- Plus a few other tweaks.
This increases consistency and conciseness.
They are pointless. No reasonable allocator will be able to satisfy a
`reserve_in_place` request that *doubles* the size of an allocation when
dealing with allocations that are 4 KiB and larger.
Just to be sure, I confirmed on Linux that the `reserve_in_place` calls
never succeed.
(Note however that the `reserve_in_place` call for `DroplessArena::grow`
did occasionally succeed prior to the off-by-one fix in the previous
commit, because we would sometimes do a `reserve_in_place` request for
the chunk's current size, which would trivially succeed!)
