modules/rust: Specify the compiler version to bindgen when possible

bindgen by default may output code that an older rustc cannot
successfully consume. To avoid this, we check if bindgen supports the
rustc version we're using, if so, and if the user didn't set the
`--rust-target` option, we will supply it to ensure that bindgen writes
out code our rustc can use. If it does not support that version
explicitly, we leave it at the default, assuming that our compiler
version is newer than bindgen.

mesonbuild/modules/rust.py

@@ -62,6 +62,10 @@ class RustModule(ExtensionModule):
     def __init__(self, interpreter: Interpreter) -> None:
         super().__init__(interpreter)
         self._bindgen_bin: T.Optional[T.Union[ExternalProgram, Executable, OverrideProgram]] = None
+        if 'rust' in interpreter.compilers.host:
+            self._bindgen_rust_target: T.Optional[str] = interpreter.compilers.host['rust'].version
+        else:
+            self._bindgen_rust_target = None
         self.methods.update({
             'test': self.test,
             'bindgen': self.bindgen,
@@ -250,6 +254,15 @@ def bindgen(self, state: ModuleState, args: T.List, kwargs: FuncBindgen) -> Modu
 
         if self._bindgen_bin is None:
             self._bindgen_bin = state.find_program('bindgen', wanted=kwargs['bindgen_version'])
+            if self._bindgen_rust_target is not None:
+                # ExternalCommand.command's type is bonkers
+                _, _, err = mesonlib.Popen_safe(
+                    T.cast('T.List[str]', self._bindgen_bin.get_command()) +
+                    ['--rust-target', self._bindgen_rust_target])
+                # Sometimes this is "invalid Rust target" and sometimes "invalid
+                # rust target"
+                if 'Got an invalid' in err:
+                    self._bindgen_rust_target = None
 
         name: str
         if isinstance(header, File):
@@ -317,9 +330,13 @@ def bindgen(self, state: ModuleState, args: T.List, kwargs: FuncBindgen) -> Modu
                 '@INPUT@', '--output',
                 os.path.join(state.environment.build_dir, '@OUTPUT0@')
             ] + \
-            kwargs['args'] + inline_wrapper_args + ['--'] + \
-            kwargs['c_args'] + clang_args + \
-            ['-MD', '-MQ', '@INPUT@', '-MF', '@DEPFILE@']
+            kwargs['args'] + inline_wrapper_args
+        if self._bindgen_rust_target and '--rust-target' not in cmd:
+            cmd.extend(['--rust-target', self._bindgen_rust_target])
+        cmd.append('--')
+        cmd.extend(kwargs['c_args'])
+        cmd.extend(clang_args)
+        cmd.extend(['-MD', '-MQ', '@INPUT@', '-MF', '@DEPFILE@'])
 
         target = CustomTarget(
             f'rustmod-bindgen-{name}'.replace('/', '_'),