[metal-writer] Add identifier expression.

This CL adds the start of identifier expressions to the Metal backend.
Identifiers with paths are currently not supported.

Bug: tint:8
Change-Id: I4df8b6a3c32251d454d3dae5fa8933dad36094f8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23703
Reviewed-by: David Neto <dneto@google.com>

BUILD.gn

@@ -885,6 +885,7 @@ source_set("tint_unittests_wgsl_writer_src") {
 
 source_set("tint_unittests_msl_writer_src") {
   sources = [
+    "src/writer/msl/generator_impl_identifier_test.cc",
     "src/writer/msl/generator_impl_return_test.cc",
     "src/writer/msl/generator_impl_test.cc",
     "src/writer/msl/generator_impl_type_test.cc",

src/CMakeLists.txt

@@ -493,6 +493,7 @@ endif()
 
 if(${TINT_BUILD_MSL_WRITER})
   list(APPEND TINT_TEST_SRCS
+    writer/msl/generator_impl_identifier_test.cc
     writer/msl/generator_impl_return_test.cc
     writer/msl/generator_impl_test.cc
     writer/msl/generator_impl_type_test.cc

src/writer/msl/generator_impl.cc

@@ -14,6 +14,7 @@
 
 #include "src/writer/msl/generator_impl.h"
 
+#include "src/ast/identifier_expression.h"
 #include "src/ast/return_statement.h"
 #include "src/ast/type/alias_type.h"
 #include "src/ast/type/array_type.h"
@@ -39,11 +40,26 @@ bool GeneratorImpl::Generate(const ast::Module&) {
   return true;
 }
 
-bool GeneratorImpl::EmitExpression(ast::Expression*) {
+bool GeneratorImpl::EmitExpression(ast::Expression* expr) {
+  if (expr->IsIdentifier()) {
+    return EmitIdentifier(expr->AsIdentifier());
+  }
+
   error_ = "unknown expression type";
   return false;
 }
 
+bool GeneratorImpl::EmitIdentifier(ast::IdentifierExpression* expr) {
+  auto* ident = expr->AsIdentifier();
+  if (ident->has_path()) {
+    // TODO(dsinclair): Handle identifier with path
+    error_ = "Identifier paths not handled yet.";
+    return false;
+  }
+  out_ << ident->name();
+  return true;
+}
+
 bool GeneratorImpl::EmitReturn(ast::ReturnStatement* stmt) {
   make_indent();
 

src/writer/msl/generator_impl.h

@@ -41,6 +41,10 @@ class GeneratorImpl : public TextGenerator {
   /// @param expr the expression
   /// @returns true if the expression was emitted
   bool EmitExpression(ast::Expression* expr);
+  /// Handles generating an identifier expression
+  /// @param expr the identifier expression
+  /// @returns true if the identifeir was emitted
+  bool EmitIdentifier(ast::IdentifierExpression* expr);
   /// Handles return statements
   /// @param stmt the statement to emit
   /// @returns true if the statement was successfully emitted

src/writer/msl/generator_impl_identifier_test.cc

@@ -0,0 +1,54 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "gtest/gtest.h"
+#include "src/ast/identifier_expression.h"
+#include "src/writer/msl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace msl {
+namespace {
+
+using MslGeneratorImplTest = testing::Test;
+
+TEST_F(MslGeneratorImplTest, DISABLED_EmitExpression_Identifier) {
+  ast::IdentifierExpression i(std::vector<std::string>{"std", "glsl"});
+
+  GeneratorImpl g;
+  ASSERT_TRUE(g.EmitExpression(&i)) << g.error();
+  EXPECT_EQ(g.result(), "std::glsl");
+}
+
+TEST_F(MslGeneratorImplTest, EmitIdentifierExpression_Single) {
+  ast::IdentifierExpression i("foo");
+
+  GeneratorImpl g;
+  ASSERT_TRUE(g.EmitExpression(&i)) << g.error();
+  EXPECT_EQ(g.result(), "foo");
+}
+
+// TODO(dsinclair): Handle import names
+TEST_F(MslGeneratorImplTest, DISABLED_EmitIdentifierExpression_MultipleNames) {
+  ast::IdentifierExpression i({"std", "glsl", "init"});
+
+  GeneratorImpl g;
+  ASSERT_TRUE(g.EmitExpression(&i)) << g.error();
+  EXPECT_EQ(g.result(), "std::glsl::init");
+}
+
+}  // namespace
+}  // namespace msl
+}  // namespace writer
+}  // namespace tint

src/writer/msl/generator_impl_return_test.cc

@@ -37,7 +37,7 @@ TEST_F(MslGeneratorImplTest, Emit_Return) {
   EXPECT_EQ(g.result(), "  return;\n");
 }
 
-TEST_F(MslGeneratorImplTest, DISABLED_Emit_ReturnWithValue) {
+TEST_F(MslGeneratorImplTest, Emit_ReturnWithValue) {
   auto expr = std::make_unique<ast::IdentifierExpression>("expr");
   ast::ReturnStatement r(std::move(expr));