linux-kernel - [PATCH v2 4/6] kbuild: rust: replace proc macros dependency on `core.o` with the version text

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Message-ID: <20240902165535.1101978-5-ojeda@kernel.org>
Date: Mon,  2 Sep 2024 18:55:31 +0200
From: Miguel Ojeda <ojeda@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Masahiro Yamada <masahiroy@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	rust-for-linux@...r.kernel.org,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev
Subject: [PATCH v2 4/6] kbuild: rust: replace proc macros dependency on `core.o` with the version text

With the `RUSTC_VERSION_TEXT` rebuild support in place, now proc macros
can depend on that instead of `core.o`.

This means that both the `core` and `macros` crates can be built in
parallel, and that touching `core.o` does not trigger a rebuild of the
proc macros.

This could be accomplished using the same approach as for `core`
(i.e. depending directly on `include/config/RUSTC_VERSION_TEXT`). However,
that is considered an implementation detail [1], and thus it is best
to avoid it. Instead, let fixdep find a string that we explicitly
write down in the source code for this purpose (like it is done for
`include/linux/compiler-version.h`), which we can easily do (unlike for
`core`) since this is our own source code.

Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
Link: https://lore.kernel.org/rust-for-linux/CAK7LNAQBG0nDupXSgAAk-6nOqeqGVkr3H1RjYaqRJ1OxmLm6xA@mail.gmail.com/ [1]
Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
 rust/Makefile      | 4 +---
 rust/macros/lib.rs | 4 ++++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile
index bb57a7c30f1a..4eae318f36ff 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -342,9 +342,7 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
 		--crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
 
 # Procedural macros can only be used with the `rustc` that compiled it.
-# Therefore, to get `libmacros.so` automatically recompiled when the compiler
-# version changes, we add `core.o` as a dependency (even if it is not needed).
-$(obj)/libmacros.so: $(src)/macros/lib.rs $(obj)/core.o FORCE
+$(obj)/libmacros.so: $(src)/macros/lib.rs FORCE
 	+$(call if_changed_dep,rustc_procmacro)
 
 quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index 5be0cb9db3ee..a626b1145e5c 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -2,6 +2,10 @@
 
 //! Crate for all kernel procedural macros.
 
+// When fixdep scans this, it will find this string `CONFIG_RUSTC_VERSION_TEXT`
+// and thus add a dependency on `include/config/RUSTC_VERSION_TEXT`, which is
+// touched by Kconfig when the version string from the compiler changes.
+
 #[macro_use]
 mod quote;
 mod concat_idents;
-- 
2.46.0