Re: [PATCH v10 25/27] x86: enable initial Rust support

[H. Peter Anvin]

On October 13, 2023 11:54:46 AM PDT, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Fri, 13 Oct 2023 at 05:18, Ramon de C Valle <rcvalle@xxxxxxxxxx> wrote:
>>
>> Both C and repr(C) Rust structs have this encoding, but I understand
>> the problems with doing this in C since it doesn't have
>> repr(transparent) structs so there would be a lot of casting back and
>> forth. Maybe there is an alternative or this could be done for less
>> used function pairs?
>
>We actually have some C variations of what I think people want to use
>"repr(transparent) struct" for in Rust.
>
>Of course, that is depending on what kind of context you want to use
>it for, and I might have lost some background. But I'm assuming you're
>talking about the situation where you want to treat two or more types
>as being "compatible" within certain contexts.
>
>There's the actual standard C "_Generic()" alternative, which allows
>you to make macros etc that use different types transparently.
>
>It's not very widely used in the kernel, because we only fairly
>recently moved to require recent enough compiler versions, but we do
>use it now in a couple of places.
>
>And there's the much more traditional gcc extension in the form of the
>__attribute__((__transparent_union__)) thing. In the kernel, that one
>is even less used, and that one use is likely going away since the
>need for it is going away.
>
>But while it's not standard C, it's actually been supported by
>relevant compilers for much longer than "_Generic" has, and is
>designed exactly for the "I have a function that can take arguments of
>different types", either because the types are bitwise identical (even
>if _conceptually_ not the same), or simply because you have a
>different argument that describes the type (the traditional C union
>model).
>
>I suspect, for example, that we *should* have used those transparent
>unions for the "this function can take either a folio or a page" case,
>instead of duplicating functions for the two uses.
>
>But probably because few people aren familiar with the syntax, that's
>not what happened.
>
>             Linus

Transparent unions have been standard C since C99.