[PATCH v2 1/6] kbuild: rust: add `CONFIG_RUSTC_VERSION`

[Miguel Ojeda <ojeda@kernel.org>]

Now that we support several Rust versions, introduce
`CONFIG_RUSTC_VERSION` so that it can be used in Kconfig to enable and
disable configuration options based on the `rustc` version.

The approach taken resembles `pahole`'s -- see commit 613fe1692377
("kbuild: Add CONFIG_PAHOLE_VERSION"), i.e. a simple version parsing
without trying to identify several kinds of compilers, since so far
there is only one (`rustc`).

However, unlike `pahole`'s, we also print a zero if executing failed for
any reason, rather than checking if the command is found and executable
(which still leaves things like a file that exists and is executable,
but e.g. is built for another platform [1]). An equivalent approach to
the one here was also submitted for `pahole` [2].

Link: https://lore.kernel.org/rust-for-linux/CANiq72=4vX_tJMJLE6e+bg7ZECHkS-AQpm8GBzuK75G1EB7+Nw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/linux-kbuild/20240728125527.690726-1-ojeda@kernel.org/ [2]
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 init/Kconfig             |  7 +++++++
 scripts/rustc-version.sh | 26 ++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100755 scripts/rustc-version.sh

diff --git a/init/Kconfig b/init/Kconfig
index 839c83034006..38c1cfcce821 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -60,6 +60,13 @@ config LLD_VERSION
 	default $(ld-version) if LD_IS_LLD
 	default 0
 
+config RUSTC_VERSION
+	int
+	default $(shell,$(srctree)/scripts/rustc-version.sh $(RUSTC))
+	help
+	  It does not depend on `RUST` since that one may need to use the version
+	  in a `depends on`.
+
 config RUST_IS_AVAILABLE
 	def_bool $(success,$(srctree)/scripts/rust_is_available.sh)
 	help
diff --git a/scripts/rustc-version.sh b/scripts/rustc-version.sh
new file mode 100755
index 000000000000..62ea510be71b
--- /dev/null
+++ b/scripts/rustc-version.sh
@@ -0,0 +1,26 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
+# Usage: $ ./rustc-version.sh rustc
+#
+# Print the Rust compiler name and its version in a 6 or 7-digit form.
+
+# Convert the version string x.y.z to a canonical up-to-7-digits form.
+#
+# Note that this function uses one more digit (compared to other
+# instances in other version scripts) to give a bit more space to
+# `rustc` since it will reach 1.100.0 in late 2026.
+get_canonical_version()
+{
+	IFS=.
+	set -- $1
+	echo $((100000 * $1 + 100 * $2 + $3))
+}
+
+if output=$("$@" --version 2>/dev/null); then
+	set -- $output
+	get_canonical_version $2
+else
+	echo 0
+	exit 1
+fi
-- 
2.46.0