Rust: Add validation support to code generator

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package templates
 
 const GenerateUnion = `
 {{/* . (dot) refers to a the Go type |rustgen.UnionTemplate|  */}}
 {{- define "GenerateUnion" -}}
 {{- $union := . -}}
@@ skipping 13 matching lines @@
 {{range $field := $union.Fields}}            {{$union.Name}}::{{$field.Name}}(_) => {{$union.TagsEnum.Name}}_{{$field.Name}},
 {{end}}            {{$union.Name}}::_Unknown(_) => {{$union.TagsEnum.Name}}__UNKNOWN,
         }
     }
     fn encode_value(self, encoder: &mut Encoder, context: Context) {
         match self {
 {{range $field := $union.Fields}}            {{$union.Name}}::{{$field.Name}}(val) => MojomEncodable::encode(val, encoder, context.clone()),
 {{end}}            {{$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()),
+        })
     }
 }
 
 impl MojomEncodable for {{$union.Name}} {
     impl_encodable_for_union!();
     fn compute_size(&self, context: Context) -> usize {
         UNION_SIZE +
         match *self {
 {{range $field := $union.Fields}}            {{$union.Name}}::{{$field.Name}}(ref val) => val.compute_size(context.clone()),
 {{end}}            {{$union.Name}}::_Unknown(ref val) => 0,
         }
     }
 }
 {{end}}
 `