The GlobalAlloc trait is stable with this method:

unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8

(And similarly the std::alloc::realloc function.)

Note new_size: usize instead of new_layout: Layout. This is because this method does not support changing the alignment.

When (re)allocation fails (when the method returns null), the caller typically wants to call handle_alloc_error which takes a Layout. This value needs to be created with code like Layout::from_size_align(new_size, layout.align()).unwrap(). It would be better if the value passed to the error handler was already available. This would be the case if the parameters to realloc were old_size: usize, new_layout: Layout instead.

For the Alloc trait, we could:

• Keep consistency with GlobalAlloc, or
• Change to old_size: usize, new_layout: Layout, or
• If we also decide to lift the same-alignment restriction (which should be discussed in Support reallocating to a different alignment? #5), then the signature becomes old_layout: Layout, new_layout: Layout and this issue is moot.