Re: [PATCH] Kbuild: add RUSTC_BOOTSTRAP to rustc-option

[Alice Ryhl <alice@ryhl.io>]

On 10/8/24 4:24 PM, Alice Ryhl wrote:
> On Tue, Oct 8, 2024 at 3:49 PM Alice Ryhl <aliceryhl@google.com> wrote:
>>
>> When using unstable features with the Rust compiler, you must either use
>> a nightly compiler release or set the RUSTC_BOOTSTRAP environment
>> variable. Otherwise, the compiler will emit a compiler error. This
>> environment variable is missing when rustc-option is executed, so add
>> the environment variable.
>>
>> This change is necessary to avoid two kinds of problems:
>>
>> 1. When using rustc-option to test whether a -Z flag is available, the
>>     check will always fail, even if the flag is available.
>> 2. If KBUILD_RUSTFLAGS happens to contain any -Z flags from the
>>     environment, even if unrelated to the flag being tested, then all
>>     invocations of rustc-option everywhere will fail.
>>
>> I was not actually able to trigger the second kind of problem with the
>> makefiles that exist today, but it seems like it could easily start
>> being a problem due to complicated interactions between changes. It is
>> better to fix this now so it doesn't surprise us later.
>>
>> I added the flag under try-run as this seemed like the easiest way to
>> make sure that the fix applies to all variations of rustc-option,
>> including new ones added in the future.
>>
>> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> 
> It should be "export". Also, this doesn't seem to be sufficient at
> all. Now I'm running into this error:
> 
> make[1]: Entering directory '/home/aliceryhl/lout'
> warning: ignoring --out-dir flag due to -o flag
> 
> 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`
> 
> error: aborting due to 1 previous error; 1 warning emitted
> 
> For more information about this error, try `rustc --explain E0463`.
> 
> I think this patch is going to need more work. Sorry for sending it prematurely.
v2 is here:
https://lore.kernel.org/all/20241008-rustc-option-bootstrap-v2-1-e6e155b8f9f3@google.com/