Re: [PATCH 32/33] rust: kbuild: support global per-version flags

[Miguel Ojeda]

On Thu, Apr 2, 2026 at 12:28 AM Gary Guo <gary@xxxxxxxxxxx> wrote:
>
> I think I would prefer moving these down.
>
> The current approach append the flags to all variables, which will cause the
> following equivalence to stop holding after the flag update.
>
> KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
>                         -Zallow-features= $(HOSTRUSTFLAGS)
>
> (Per version flags doesn't go before -O anymore, it comes after HOSTRUSTFLAGS).

[ For context for others, Sashiko reported the same and we also talked
about it in a Rust for Linux call. ]

I have been thinking about this, and about potential other ways to
achieve the same thing. I think the best at the moment is to move just
the `$(HOSTRUSTFLAGS)` below, but not the rest.

The reason is that it is closer to what we do with other user (kernel)
flags (e.g. arch flags come after the general ones). But I am
wondering if we should/could set all the user variables later in the
`Makefile` in general `HOST*FLAGS` later in the `Makefile`.

In fact, there is already a limitation with the host flags: `-Werror`,
i.e. that one gets appended later, and so users cannot override it.

This may be considered a bug, because commit 7ded7d37e5f5
("scripts/Makefile.extrawarn: Respect CONFIG_WERROR / W=e for
hostprogs") says:

    While it is
    possible to avoid this behavior by passing HOSTCFLAGS=-w or
    HOSTCFLAGS=-Wno-error, it may not be the most intuitive for regular
    users not intimately familiar with Kbuild.

But passing `HOSTCFLAGS=-Wno-error` doesn't work, since it comes
earlier (`-w` does work, but because it turns off everything).

I am also a bit confused with:

    While this means there is a small portion of
    the build that does not have -Werror enabled (namely scripts/kconfig/*
    and scripts/basic/fixdep)

because it gets enabled in the build (I think it is referring to other
targets like the config ones).

Anyway, for now I moved the expansion of `HOSTRUSTFLAGS` in v2. If
Kbuild (Nathan, Nicolas) think it is a good idea to do one of the
bigger changes (e.g. for more `HOST*` flags, appending it even later),
then we can do it afterwards.

Cheers,
Miguel