cargo: allow overriding Meson's Cargo interpreter

Some projects may want to override Meson's AST generation for Cargo
projects.  This was not really doable before without hard coding the
results of feature resolution; however, now it will be possible by
accessing the results of the global feature resolution from the Rust
module's workspace object.

At the same time, the subproject must keep using the Cargo method,
which is forced by the workspace object's subproject() method,
because otherwise the interpreter is not propagated.  So just
skip the interpretation phase if a Meson.build is present.

Signed-off-by: Paolo Bonzini <[email protected]>

Author: bonzini

mesonbuild/interpreter/interpreter.py

@@ -1060,7 +1060,11 @@ def _do_subproject_cargo(self, subp_name: str, subdir: str,
             except cargo.TomlImplementationMissing as e:
                 raise MesonException(f'Failed to load Cargo.lock: {e!s}')
 
-            ast = cargo_int.interpret(subdir)
+            if os.path.exists(os.path.join(self.environment.get_source_dir(), subdir, environment.build_filename)):
+                ast = None
+            else:
+                ast = cargo_int.interpret(subdir)
+
             return self._do_subproject_meson(
                 subp_name, subdir, default_options, kwargs, ast,
                 relaxations={InterpreterRuleRelaxation.CARGO_SUBDIR},