rust: raise a warning if clippy is used instead of rustc

clippy-driver is not meant to be a general-purpose compiler front-end.
Since Meson can now provide natively the ability to invoke clippy,
raise a warning if someone uses it that way.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/markdown/snippets/clippy.md

@@ -3,3 +3,6 @@
 Meson now defines a `clippy` target if the project uses the Rust programming
 language.  The target runs clippy on all Rust sources, using the `clippy-driver`
 program from the same Rust toolchain as the `rustc` compiler.
+
+Using `clippy-driver` as the Rust compiler will now emit a warning, as it
+is not meant to be a general-purpose compiler front-end.

mesonbuild/compilers/detect.py

@@ -1042,6 +1042,10 @@ def detect_rust_compiler(env: 'Environment', for_machine: MachineChoice) -> Rust
             version = search_version(out)
 
             cls = rust.ClippyRustCompiler
+            mlog.warning(
+                'clippy-driver is not intended as a general purpose compiler. '
+                'You can use "ninja clippy" in order to run clippy on a '
+                'meson project.')
 
         if 'rustc' in out:
             # On Linux and mac rustc will invoke gcc (clang for mac