kbuild: rust: add rustc-version support

Now that we are starting to support several Rust versions, introduce
`rustc-version` support, mimicking the C side:

  - `scripts/rustc-version.sh`, that mimics the other version scripts
     (with one more digit, e.g. Rust 1.79.0 is 107900).

  - `rustc-{info,name,version}` Kbuild macros.

  - `CONFIG_RUSTC_VERSION` Kconfig symbol that calls `rustc-version`.

  - `rustc-min-version` Kbuild macro that uses `CONFIG_RUSTC_VERSION`.

With these, we can easily support flags conditionally depending on
`rustc`'s version -- a user comes in the next patch.

Another user will be the `-Ctarget-feature=+reserve-x18`/`-Zfixed-x18`
arm64 flags [1].

Link: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Signed-off-by: Miguel Ojeda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ojeda

init/Kconfig

@@ -1920,6 +1920,12 @@ config RUST
 
 	  If unsure, say N.
 
+config RUSTC_VERSION
+	int
+	depends on RUST
+	default $(rustc-version)
+	default 0
+
 config RUSTC_VERSION_TEXT
 	string
 	depends on RUST

scripts/Kconfig.include

@@ -45,6 +45,12 @@ $(error-if,$(success,test -z "$(cc-info)"),Sorry$(comma) this C compiler is not
 cc-name := $(shell,set -- $(cc-info) && echo $1)
 cc-version := $(shell,set -- $(cc-info) && echo $2)
 
+# Get the Rust compiler name, version, and error out if it is not supported.
+rustc-info := $(shell,$(srctree)/scripts/rustc-version.sh $(RUSTC))
+$(error-if,$(success,test -z "$(rustc-info)"),Sorry$(comma) this Rust compiler is not supported.)
+rustc-name := $(shell,set -- $(rustc-info) && echo $1)
+rustc-version := $(shell,set -- $(rustc-info) && echo $2)
+
 # Get the assembler name, version, and error out if it is not supported.
 as-info := $(shell,$(srctree)/scripts/as-version.sh $(CC) $(CLANG_FLAGS))
 $(error-if,$(success,test -z "$(as-info)"),Sorry$(comma) this assembler is not supported.)

scripts/Makefile.compiler

@@ -69,6 +69,10 @@ gcc-min-version = $(call test-ge, $(CONFIG_GCC_VERSION), $1)
 # Usage: cflags-$(call clang-min-version, 110000) += -foo
 clang-min-version = $(call test-ge, $(CONFIG_CLANG_VERSION), $1)
 
+# rustc-min-version
+# Usage: rustflags-$(call rustc-min-version, 107900) += -foo
+rustc-min-version = $(call test-ge, $(CONFIG_RUSTC_VERSION), $1)
+
 # ld-option
 # Usage: KBUILD_LDFLAGS += $(call ld-option, -X, -Y)
 ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))

scripts/rustc-version.sh

@@ -0,0 +1,52 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
+# Print the Rust compiler name and its version in a 5 or 6-digit form.
+# Also, perform the minimum version check.
+
+set -e
+
+# 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 + $3))
+}
+
+orig_args="$@"
+
+set -- $("$@" --version)
+
+name=$1
+
+min_tool_version=$(dirname $0)/min-tool-version.sh
+
+case "$name" in
+rustc)
+	version=$2
+	min_version=$($min_tool_version rustc)
+	;;
+*)
+	echo "$orig_args: unknown Rust compiler" >&2
+	exit 1
+	;;
+esac
+
+rustcversion=$(get_canonical_version $version)
+min_rustcversion=$(get_canonical_version $min_version)
+
+if [ "$rustcversion" -lt "$min_rustcversion" ]; then
+	echo >&2 "***"
+	echo >&2 "*** Rust compiler is too old."
+	echo >&2 "***   Your $name version:    $version"
+	echo >&2 "***   Minimum $name version: $min_version"
+	echo >&2 "***"
+	exit 1
+fi
+
+echo $name $rustcversion