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

[Benno Lossin <benno.lossin@proton.me>]

On 23.09.24 15:56, Alice Ryhl 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?

The global allocator doesn't support it, but the `Allocator` trait from
std handles zero-sized allocations. For example, this code runs as
expected:

    #![feature(allocator_api)]
    
    use std::alloc::{self, Allocator, Layout};
    
    fn main() {
        let alloc: &dyn Allocator = &alloc::Global;
        let ptr = alloc.allocate(Layout::new::<()>()).unwrap().cast::<u8>();
        unsafe { alloc.deallocate(ptr, Layout::new::<()>()) };
    }

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0a2d12ee6dabf16f2ebd67cc6faa864e

---
Cheers,
Benno