Re: [PATCH v3] Kbuild: fix issues with rustc-option

[Miguel Ojeda]

On Wed, Oct 9, 2024 at 1:42 PM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> Fix a few different compiler errors that cause rustc-option to give
> wrong results.
>
> If KBUILD_RUSTFLAGS or the flags being tested contain any -Z flags, then
> the error below is generated. The RUSTC_BOOTSTRAP environment variable
> is added to fix this error.
>
>         error: the option `Z` is only accepted on the nightly compiler
>         help: consider switching to a nightly toolchain: `rustup default nightly`
>         note: selecting a toolchain with `+toolchain` arguments require a rustup proxy;
>               see <https://rust-lang.github.io/rustup/concepts/index.html>
>         note: for more information about Rust's stability policy, see
>               <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>
>         error: 1 nightly option were parsed
>
> Note that RUSTC_BOOTSTRAP is also defined in the top-level Makefile, but
> its value is unfortunately *not* inherited from the environment. That
> said, this is changing as of commit 98da874c4303 ("[SV 10593] Export
> variables to $(shell ...) commands"), which is part of Make 4.4.
>
> The probe may also fail with the error message below. To fix it, the
> /dev/null argument is replaced with a file containing the crate
> attribute #![no_core]. The #![no_core] attribute ensures that rustc does
> not look for the standard library. It's not possible to instead supply a
> standard library to rustc, as we need `rustc-option` before the Rust
> standard library is compiled.
>
>         error[E0463]: can't find crate for `std`
>           |
>           = note: the `aarch64-unknown-none` target may not be installed
>           = help: consider downloading the target with `rustup target add aarch64-unknown-none`
>           = help: consider building the standard library from source with `cargo build -Zbuild-std`
>
> The -o and --out-dir parameters are altered to fix this warning:
>
>         warning: ignoring --out-dir flag due to -o flag
>
> The --sysroot flag is provided as we would otherwise require it to be
> present in KBUILD_RUSTFLAGS. The --emit=obj flag is used to write the
> resulting rlib to /dev/null instead of writing it to a file in
> $(TMPOUT).
>
> I verified that the Kconfig version of rustc-option doesn't have the
> same issues.
>
> Fixes: c42297438aee ("kbuild: rust: Define probing macros for rustc")
> Co-developed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

Applied to `rust-fixes` -- thanks everyone!

    [ Reworded as discussed in the list. - Miguel ]

Cheers,
Miguel