Rust: Add validation support to code generator

mojom/generators/rust/templates/union.tmpl.go

Index: mojom/generators/rust/templates/union.tmpl.go
diff --git a/mojom/generators/rust/templates/union.tmpl.go b/mojom/generators/rust/templates/union.tmpl.go
index d395346daa24cd81903e9b1376dcc3f07059862f..0e957be54b64cb9bd26813e7f62d5a70a71c8919 100644
--- a/mojom/generators/rust/templates/union.tmpl.go
+++ b/mojom/generators/rust/templates/union.tmpl.go
@@ -31,18 +31,24 @@ impl MojomUnion for {{$union.Name}} {
 {{end}}            {{$union.Name}}::_Unknown(val) => MojomEncodable::encode(val, encoder, context.clone()),
         }
     }
-    fn decode_value(decoder: &mut Decoder, context: Context) -> Self {
-        // TODO(mknyszek): Validate bytes and tag
-        let (_bytes, tag) = {
+    fn decode_value(decoder: &mut Decoder, context: Context) -> Result<Self, ValidationError> {
+        let tag = {
             let mut state = decoder.get_mut(&context);
             let bytes = state.decode::<u32>();
-	    let tag = state.decode::<u32>();
-	    (bytes, tag)
+            if (bytes as usize) != UNION_SIZE {
+                return Err(ValidationError::UnexpectedNullUnion);
+            }
+	    state.decode::<u32>()
         };
-        match tag {
-{{range $field := $union.Fields}}            {{$union.TagsEnum.Name}}_{{$field.Name}} => {{$union.Name}}::{{$field.Name}}(<{{$field.Type}}>::decode(decoder, context.clone())),
-{{end}}        _ => {{$union.Name}}::_Unknown(u64::decode(decoder, context.clone())),
-        }
+        Ok(match tag {
+{{range $field := $union.Fields}}            {{$union.TagsEnum.Name}}_{{$field.Name}} => {{$union.Name}}::{{$field.Name}}({
+                match <{{$field.Type}}>::decode(decoder, context.clone()) {
+                    Ok(value) => value,
+                    Err(err) => return Err(err),
+                }
+            }),
+{{end}}        _ => {{$union.Name}}::_Unknown(u64::decode(decoder, context.clone()).unwrap()),
+        })
     }
 }