rust: kbuild: simplify `--cfg` handling

We need to handle `cfg`s in both `rustc` and `rust-analyzer`, and in
future commits some of those contain double quotes, which complicates
things further.

Thus, instead of removing the `--cfg ` part in the rust-analyzer
generation script, have the `*-cfgs` variables contain just the actual
`cfg`, and use that to generate the actual flags in `*-flags`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-3-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -60,8 +60,10 @@ rustdoc_test_quiet=--test-args -q
 rustdoc_test_kernel_quiet=>/dev/null
 endif
 
+cfgs-to-flags = $(patsubst %,--cfg='%',$1)
+
 core-cfgs := \
-    --cfg no_fp_fmt_parse
+    no_fp_fmt_parse
 
 core-edition := $(if $(call rustc-min-version,108700),2024,2021)
 
@@ -72,7 +74,7 @@ core-skip_flags := \
 
 core-flags := \
     --edition=$(core-edition) \
-    $(core-cfgs)
+    $(call cfgs-to-flags,$(core-cfgs))
 
 # `rustdoc` did not save the target modifiers, thus workaround for
 # the time being (https://github.com/rust-lang/rust/issues/144521).

scripts/generate_rust_analyzer.py

@@ -15,7 +15,7 @@ def args_crates_cfgs(cfgs):
     crates_cfgs = {}
     for cfg in cfgs:
         crate, vals = cfg.split("=", 1)
-        crates_cfgs[crate] = vals.replace("--cfg", "").split()
+        crates_cfgs[crate] = vals.split()
 
     return crates_cfgs