rpc_proto: serialize keywords correctly.

Fixes #1109.

artiq/firmware/libproto_artiq/rpc_proto.rs

@@ -151,11 +151,11 @@ unsafe fn send_value<W>(writer: &mut W, tag: Tag, data: &mut *const ())
             Ok(())
         }
         Tag::Keyword(it) => {
-            struct Keyword<'a> { name: CSlice<'a, u8>, contents: () };
+            struct Keyword<'a> { name: CSlice<'a, u8> };
             consume_value!(Keyword, |ptr| {
                 writer.write_string(str::from_utf8((*ptr).name.as_ref()).unwrap())?;
                 let tag = it.clone().next().expect("truncated tag");
-                let mut data = &(*ptr).contents as *const ();
+                let mut data = ptr.offset(1) as *const ();
                 send_value(writer, tag, &mut data)
             })
             // Tag::Keyword never appears in composite types, so we don't have

artiq/test/coredevice/test_portability.py

@@ -202,6 +202,20 @@ class _RPCExceptions(EnvExperiment):
             self.success = True
 
 
+class _Keywords(EnvExperiment):
+    def build(self, value, output):
+        self.setattr_device("core")
+        self.value  = value
+        self.output = output
+
+    def rpc(self, kw):
+        self.output.append(kw)
+
+    @kernel
+    def run(self):
+        self.rpc(kw=self.value)
+
+
 class HostVsDeviceCase(ExperimentCase):
     def test_primes(self):
         l_device, l_host = [], []
@@ -245,3 +259,18 @@ class HostVsDeviceCase(ExperimentCase):
                 f(_RPCExceptions, catch=False)
             uut = self.execute(_RPCExceptions, catch=True)
             self.assertTrue(uut.success)
+
+    def test_keywords(self):
+        for f in self.execute, _run_on_host:
+            output = []
+            f(_Keywords, value=0, output=output)
+            self.assertEqual(output, [0])
+            output = []
+            f(_Keywords, value=1, output=output)
+            self.assertEqual(output, [1])
+            output = []
+            f(_Keywords, value=False, output=output)
+            self.assertEqual(output, [False])
+            output = []
+            f(_Keywords, value=True, output=output)
+            self.assertEqual(output, [True])