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    SubjectRe: [PATCH 04/13] Kbuild: Rust support
    On Fri, Apr 16, 2021 at 11:39:00PM +0200, Miguel Ojeda wrote:
    > On Fri, Apr 16, 2021 at 10:58 PM Willy Tarreau <w@1wt.eu> wrote:
    > >
    > > No, two:
    > > - ok in %rax (seems like it's "!ok" technically speaking since it
    > > returns 1 on !ok and 0 on ok)
    > > - foo_or_err in %rdx
    >
    > Yes, but that is the implementation -- conceptually you only have one
    > or the other, and Rust won't allow you to use the wrong one.

    OK so for unions you always pass two values along the whole chain, a
    selector and the value itself.

    But my point remains that the point of extreme care is at the interface
    with the rest of the kernel because there is a change of semantics
    there.

    > > However then I'm bothered because Miguel's example showed that regardless
    > > of OK, EINVAL was always returned in foo_or_err, so maybe it's just
    > > because his example was not well chosen but it wasn't very visible from
    > > the source:
    >
    > That is the optimizer being fancy since the error can be put
    > unconditionally in `rdx`.

    Yes that's what I understood as well. I just didn't know that it had
    to be seen as a union.

    On Fri, Apr 16, 2021 at 11:19:18PM +0200, Miguel Ojeda wrote:
    > On Fri, Apr 16, 2021 at 10:22 PM Willy Tarreau <w@1wt.eu> wrote:
    > >
    > > So it simply does the equivalent of:
    > >
    > > struct result {
    > > int status;
    > > int error;
    > > };
    >
    > Not exactly, it is more like a tagged union, as Connor mentioned.
    >
    > However, and this is the critical bit: it is a compile-time error to
    > access the inactive variants (in safe code). In C, it is on you to
    > keep track which one is the current one.

    Sure but as I said most often (due to API or ABI inheritance), both
    are already exclusive and stored as ranges. Returning 1..4095 for
    errno or a pointer including NULL for a success doesn't shock me at
    all.

    Along thes lines I hardly see how you'd tag pointers by manipulating
    their lower unused bits. That's something important both for memory
    usage and performance (supports atomic opts).

    > > kill_foo(); // only for rust, C doesn't need it
    >
    > Please note that `kill_foo()` is not needed in Rust -- it was an
    > example of possible cleanup (since Al mentioned resources/cleanup)
    > using RAII.

    Yep but I kept it just to have comparable output code since in C
    you'd simply use "goto leave" and not have this function call to
    do the cleanup.

    Willy

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