[PATCH] Makefile: rust-analyzer target: better error handling and comments

[John Hubbard <jhubbard@nvidia.com>]

1) Provide a more self-explanatory error message for the "Rust not
available" case. Without this patch, if Rust is not set up properly
(which happens a lot, seeing as how one must routinely run "rustup
override ..." with each new kernel release), the "make rust-analyzer"
invocation generates a somewhat confusing message:

    "No rule to make target 'rust-analyzer"

This is confusing at first, because there is, in fact, a rust-analyzer
build target. It's just not set up to handle errors gracefully.

Instead of inflicting that on the developer, just print that Rust is
not available, with a blank line above and below, so it doesn't get lost
in the noise. Now the error case looks like this:

    $ make rust-analyzer

    Rust is not available

    make[1]: *** [/kernel_work/linux-github/Makefile:1975: rust-analyzer] Error 1
    make: *** [Makefile:240: __sub-make] Error 2

2) As long as I'm there, also add some documentation about what
rust-analyzer provides.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f975b6396328..aca2c96820aa 100644
--- a/Makefile
+++ b/Makefile
@@ -1967,9 +1967,13 @@ quiet_cmd_tags = GEN     $@
 tags TAGS cscope gtags: FORCE
 	$(call cmd,tags)
 
-# IDE support targets
+# Generate rust-project.json, which does for Rust what clangd's
+# compile_commands.json does for C/C++: provides a browsing database for code
+# editors and IDEs.
 PHONY += rust-analyzer
 rust-analyzer:
+	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh 2>/dev/null || \
+	    { echo; echo "Rust is not available"; echo; false; }
 	$(Q)$(MAKE) $(build)=rust $@
 
 # Script to generate missing namespace dependencies

base-commit: b050496579632f86ee1ef7e7501906db579f3457
-- 
2.45.1