[New go generator] Implement declaring unions.

mojom/generators/go/templates/declarations.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 structDeclTmplText = `
 {{- define "StructDecl" -}}
 {{$struct := . -}}
 type {{$struct.Name}} struct {
 {{- range $field := $struct.Fields}}
         {{$field.Name}} {{$field.Type}}
 {{- end}}
 }
 {{- end -}}
 `
+
+const unionDeclTmplText = `
+{{- define "UnionDecl" -}}
+{{- $struct := . }}
+{{ template "UnionInterfaceDecl" $struct }}
+
+{{ template "UnknownUnionFieldDecl" $struct }}
+
+{{- range $field := $struct.Field}}
+{{ template "UnionFieldDecl" $field }}
+{{- end}}
+{{- end -}}
+`
+
+const unionInterfaceDeclTmplText = `
+{{- define "UnionInterfaceDecl" -}}
+{{$union := . -}}
+type {{$union.Name}} interface {
+        Tag() uint32
+        Interface() interface{}
+        __Reflect(__SomeUnionReflect)
+        Encode(encoder *bindings.Encoder) error
+}
+
+type __{{$union.Name}}Reflect struct {
+{{- range $field := $union.Fields}}
+        {{$field.Name}} {{$field.Type}}
+{{- end}}
+}
+{{- end -}}
+`
+
+const unionFieldDeclTmplText = `
+{{- define "UnionFieldDecl" -}}
+{{$field := . -}}
+{{$unionName := $field.Union.Name -}}
+type {{$unionName}}{{$field.Name}} struct{ Value {{$field.Type}} }
+func (u *{{$unionName}}{{$field.Name}}) Tag() uint32 { return {{$field.Tag}} }
+func (u *{{$unionName}}{{$field.Name}}) Interface() interface{} { return u.Value }
+func (u *{{$unionName}}{{$field.Name}}) __Reflect(__{{$unionName}}Reflect) {}
+{{- end -}}
+`
+
+const unknownUnionFieldDeclTmplText = `
+{{- define "UnknownUnionFieldDecl" -}}
+{{$union := . -}}
+type {{$union.Name}}Unknown struct { tag uint32 }
+func (u *{{$union.Name}}Unknown) Tag() uint32 { return u.tag }
+func (u *{{$union.Name}}Unknown) Interface() interface{} { return nil }
+func (u *{{$union.Name}}Unknown) __Reflect(__{{$union.Name}}Reflect) {}
+{{- end -}}
+`