Re: [RFC PATCH] rust: alloc: pass `old_layout` to `Allocator`

[Gary Guo]

On Mon, 23 Sep 2024 15:56:28 +0200
Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:

> On Sat, Sep 21, 2024 at 5:33 PM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
> > @@ -84,11 +92,18 @@ unsafe fn call(
> >          &self,
> >          ptr: Option<NonNull<u8>>,
> >          layout: Layout,
> > +        old_layout: Layout,
> >          flags: Flags,
> >      ) -> Result<NonNull<[u8]>, AllocError> {
> >          let size = aligned_size(layout);
> >          let ptr = match ptr {
> > -            Some(ptr) => ptr.as_ptr(),
> > +            Some(ptr) => {
> > +                if old_layout.size() == 0 {
> > +                    ptr::null()
> > +                } else {
> > +                    ptr.as_ptr()
> > +                }
> > +            }  
> 
> This is making Allocator work with zero-sized types, which deviates
> from std. We should not do that without a reason. What is the reason?
> 
> Alice

As Benno said, this makes the API closer to Rust `allocator_api`
Allocator trait as opposed to deviation.

There's one benefit of doing this (discussed with Danilo off-list),
which is it removes ZST special casing from caller. This RFC patch
simplifies `Box` handling, and if we add this line to the safety doc

	`ptr` does not need to be a pointer returned by this
	allocator if the layout is zero-sized.

then the `Vec` can also be simplified, removing all logics handling ZST
specially, except for `Vec::new()` which it forges a well-aligned
dangling pointer from nowhere.

Best,
Gary