[PATCH v1] tools build: Fix feature test for rust compiler

[Dmitrii Dolgov <9erthalion6@gmail.com>]

Currently a dummy rust code is compiled to detect if the rust feature
could be enabled. It turns out that in this case rust emits a dependency
file without any external references:

    /perf/feature/test-rust.d: test-rust.rs

    /perf/feature/test-rust.bin: test-rust.rs

    test-rust.rs:

This can lead to a situation, when rustc was removed after a successful build,
but the build process still thinks it's there and the feature is enabled on
subsequent runs.

Instead simply check the compiler presence to detect the feature, as
suggested by Arnaldo Carvalho de Melo. This way no actual test-rust.bin
will be created, meaning the feature check will not be cached and always
performed. That's exactly what we want, and the overhead of doing this
every time is minimal.

Tested with multiple rounds of install/remove of the rust package.

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 tools/build/feature/Makefile     | 10 ++++++----
 tools/build/feature/test-rust.rs |  4 ----
 2 files changed, 6 insertions(+), 8 deletions(-)
 delete mode 100644 tools/build/feature/test-rust.rs

diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index e959caa7f1c..698e89da015 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -113,9 +113,6 @@ __BUILD = $(CC) $(CFLAGS) -MD -Wall -Werror -o $@ $(patsubst %.bin,%.c,$(@F)) $(
 __BUILDXX = $(CXX) $(CXXFLAGS) -MD -Wall -Werror -o $@ $(patsubst %.bin,%.cpp,$(@F)) $(LDFLAGS)
   BUILDXX = $(__BUILDXX) > $(@:.bin=.make.output) 2>&1
 
-__BUILDRS = $(RUSTC) $(RUSTC_FLAGS) --emit=dep-info=$(patsubst %.bin,%.d,$(@F)),link -o $@ $(patsubst %.bin,%.rs,$(@F))
-  BUILDRS = $(__BUILDRS) > $(@:.bin=.make.output) 2>&1
-
 ###############################
 
 $(OUTPUT)test-all.bin:
@@ -393,8 +390,13 @@ $(OUTPUT)test-bpftool-skeletons.bin:
 	$(SYSTEM_BPFTOOL) version | grep '^features:.*skeletons' \
 		> $(@:.bin=.make.output) 2>&1
 
+# Testing Rust is special: we don't compile anything, it's enough to check the
+# compiler presence. Compiling a test code for this purposes is problematic,
+# because Rust will emit a depedency file without any external references,
+# meaning that if rustc will be removed the build process will still think it's
+# there.
 $(OUTPUT)test-rust.bin:
-	$(BUILDRS) > $(@:.bin=.make.output) 2>&1
+	$(RUSTC) --version > /dev/null 2>&1
 
 ###############################
 
diff --git a/tools/build/feature/test-rust.rs b/tools/build/feature/test-rust.rs
deleted file mode 100644
index f2fc91cc4f6..00000000000
--- a/tools/build/feature/test-rust.rs
+++ /dev/null
@@ -1,4 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-fn main() {
-    println!("hi")
-}

base-commit: 5d1ab659fb93eed85d6d8b2937013360157032f4
-- 
2.52.0