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

[Danilo Krummrich <dakr@kernel.org>]

On Mon, Sep 23, 2024 at 05:13:15PM +0100, Gary Guo wrote:
> On Mon, 23 Sep 2024 15:56:28 +0200
> Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > On Sat, Sep 21, 2024 at 5:33 PM Danilo Krummrich <dakr@kernel.org> 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.

Partially, we still need the additional `Layout` for `Allocator::free`, which
in `Vec::drop` and `IntoIter::drop` looks like this:

`let layout = Layout::array::<T>(self.cap).unwrap();`

I really dislike that this can potentially transform into `BUG()`, but that's
probably unrelated to this patch series.

> 
> Best,
> Gary
>