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

[Danilo Krummrich]

On Tue, Sep 24, 2024 at 03:31:55PM +0200, Danilo Krummrich wrote:
> 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@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.
> 
> 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();`

Adding in the diff:

-        // If `cap == 0` we never allocated any memory in the first place.
-        if self.cap != 0 {
-            // SAFETY: `self.ptr` was previously allocated with `A`.
-            unsafe { A::free(self.ptr.cast()) };
-        }
+        // This can never fail; since this `Layout` is equivalent to the one of the original
+        // allocation.
+        let layout = Layout::array::<T>(self.cap).unwrap();
+
+        // SAFETY: `self.ptr` was previously allocated with `A`.
+        unsafe { A::free(self.ptr.cast(), layout) };

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