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References: <20240217002638.57373-1-ojeda@kernel.org>
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From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date: Thu, 29 Feb 2024 22:33:18 +0100
Message-ID: <CANiq72nPiVDV2WY_jCuONz_arWRVyMMiRT5iVb=2xqoXnZYW+g@mail.gmail.com>
Subject: Re: [PATCH 1/2] kbuild: mark `rustc` (and others) invocations as recursive
To: Miguel Ojeda <ojeda@kernel.org>
Cc: Wedson Almeida Filho <wedsonaf@gmail.com>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Alice Ryhl <aliceryhl@google.com>, Masahiro Yamada <masahiroy@kernel.org>, 
	linux-kbuild@vger.kernel.org, rust-for-linux@vger.kernel.org, 
	linux-kernel@vger.kernel.org, patches@lists.linux.dev
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On Sat, Feb 17, 2024 at 1:26=E2=80=AFAM Miguel Ojeda <ojeda@kernel.org> wro=
te:
>
> `rustc` (like Cargo) may take advantage of the jobserver at any time
> (e.g. for backend parallelism, or eventually frontend too). In the kernel=
,
> we call `rustc` with `-Ccodegen-units=3D1` (and `-Zthreads` is 1 so far),
> so we do not expect parallelism. However, in the upcoming Rust 1.76.0, a
> warning is emitted by `rustc` when it cannot connect to the jobserver it
> was passed (in many cases, but not all: compiling and `--print sysroot`
> do, but `--version` does not). And given GNU Make always passes
> the jobserver in the environment variable (even when a line is deemed
> non-recursive), `rustc` will end up complaining about it (in particular
> in Make 4.3 where there is only the simple pipe jobserver style).
>
> One solution is to remove the jobserver from `MAKEFLAGS`. However, we
> can mark the lines with calls to `rustc` (and Cargo) as recursive, which
> looks simpler. This is what is now documented and recommended by `rustc`
> and allows us to be ready for the time we may use parallelism inside
> `rustc` (potentially now, if a user passes `-Zthreads`). Thus do so.
>
> Similarly, do the same for `rustdoc` and `cargo` calls.
>
> Finally, there is one case that the solution does not cover, which is the
> `$(shell ...)` call we have. Thus, for that one, set an empty `MAKEFLAGS`
> environment variable.
>
> Link: https://github.com/rust-lang/rust/issues/120515
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Applied (i.e. including the upgrade to 1.76.0) to `rust-next` --
thanks everyone!

[ Reworded to add link to PR documenting the recommendation. ]

Cheers,
Miguel