Rust: Fix proc-macro usage when cross compiling

Force BUILD machine and allow to link with HOST machine.

mesonbuild/build.py

@@ -1474,7 +1474,7 @@ class BuildTarget(Target):
         links_with_rust_abi = isinstance(t, BuildTarget) and t.uses_rust_abi()
         if not self.uses_rust() and links_with_rust_abi:
             raise InvalidArguments(f'Try to link Rust ABI library {t.name!r} with a non-Rust target {self.name!r}')
-        if self.for_machine is not t.for_machine:
+        if self.for_machine is not t.for_machine and (not links_with_rust_abi or t.rust_crate_type != 'proc-macro'):
             msg = f'Tried to mix libraries for machines {self.for_machine} and {t.for_machine} in target {self.name!r}'
             if self.environment.is_cross_build():
                 raise InvalidArguments(msg + ' This is not possible in a cross build.')

mesonbuild/interpreter/interpreter.py

@@ -3247,6 +3247,10 @@ class Interpreter(InterpreterBase, HoldableObject):
 
         name, sources = args
         for_machine = self.machine_from_native_kwarg(kwargs)
+        if kwargs.get('rust_crate_type') == 'proc-macro':
+            # Silently force to native because that's the only sensible value
+            # and rust_crate_type is deprecated any way.
+            for_machine = MachineChoice.BUILD
         if 'sources' in kwargs:
             sources += listify(kwargs['sources'])
         if any(isinstance(s, build.BuildTarget) for s in sources):