Re: [RFC 06/13] rust: add bindings for memattrs

[Paolo Bonzini]

On 1/23/25 16:10, Zhao Liu wrote:
> > Another solution would be to implement Zeroable for __BindgenBitfieldUnit in
> > bindings.rs, but this is much nicer!  It works even with old Rust versions
> > and, even though it needs manual implementation of the trait each type, it
> > doesn't require enumerating the fields one by one.  So it's better than the
> > current version of Zeroable and, if you wish, you can also replace existing
> > implementations of Zeroable with const_zero.
>
> I'm working on this, and it's just a simple patch.
>
> But I'm not sure why Zeroable needs a Default constraint. I think Sized
> seems to be enough, doesn't it?

It's not needed but I think it makes sense semantically:

1) the idea was that Default::default() would always return the same 
value as Zeroable::ZERO, apart possibly from padding.  That is, for 
something that's Zeroable it should always make sense to use an all-zero 
value for Default::default().  And since there are standard library 
methods that use Default, the bound makes it possible to use them and 
know that they fill-with-zeroes.

2) I wasn't sure in which cases bindgen doesn't generate Default, if 
any; and with Default as a supertrait we'd have to double-check the 
first occurrence of a non-Default but zeroable struct.

Paolo