[metal-writer] Emit assignment statements.

This CL adds assignment statements to the Metal backend.

Bug: tint:8
Change-Id: Iaf4faa62124948fd0e785d5bebd20aae778ba050
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23705
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_assign_test.cc",
     "src/writer/msl/generator_impl_function_test.cc",
     "src/writer/msl/generator_impl_identifier_test.cc",
     "src/writer/msl/generator_impl_return_test.cc",

src/CMakeLists.txt

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

src/writer/msl/generator_impl.cc

@@ -14,6 +14,7 @@
 
 #include "src/writer/msl/generator_impl.h"
 
+#include "src/ast/assignment_statement.h"
 #include "src/ast/function.h"
 #include "src/ast/identifier_expression.h"
 #include "src/ast/return_statement.h"
@@ -47,6 +48,24 @@ bool GeneratorImpl::Generate(const ast::Module& module) {
   return true;
 }
 
+bool GeneratorImpl::EmitAssign(ast::AssignmentStatement* stmt) {
+  make_indent();
+
+  if (!EmitExpression(stmt->lhs())) {
+    return false;
+  }
+
+  out_ << " = ";
+
+  if (!EmitExpression(stmt->rhs())) {
+    return false;
+  }
+
+  out_ << ";" << std::endl;
+
+  return true;
+}
+
 bool GeneratorImpl::EmitExpression(ast::Expression* expr) {
   if (expr->IsIdentifier()) {
     return EmitIdentifier(expr->AsIdentifier());
@@ -138,6 +157,9 @@ bool GeneratorImpl::EmitStatementBlockAndNewline(
 }
 
 bool GeneratorImpl::EmitStatement(ast::Statement* stmt) {
+  if (stmt->IsAssign()) {
+    return EmitAssign(stmt->AsAssign());
+  }
   if (stmt->IsReturn()) {
     return EmitReturn(stmt->AsReturn());
   }

src/writer/msl/generator_impl.h

@@ -37,6 +37,10 @@ class GeneratorImpl : public TextGenerator {
   /// @returns true on successful generation; false otherwise
   bool Generate(const ast::Module& module);
 
+  /// Handles an assignment statement
+  /// @param stmt the statement to emit
+  /// @returns true if the statement was emitted successfully
+  bool EmitAssign(ast::AssignmentStatement* stmt);
   /// Handles generate an Expression
   /// @param expr the expression
   /// @returns true if the expression was emitted

src/writer/msl/generator_impl_assign_test.cc

@@ -0,0 +1,45 @@
+// 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 <memory>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "src/ast/assignment_statement.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, Emit_Assign) {
+  auto lhs = std::make_unique<ast::IdentifierExpression>("lhs");
+  auto rhs = std::make_unique<ast::IdentifierExpression>("rhs");
+  ast::AssignmentStatement assign(std::move(lhs), std::move(rhs));
+
+  GeneratorImpl g;
+  g.increment_indent();
+
+  ASSERT_TRUE(g.EmitStatement(&assign)) << g.error();
+  EXPECT_EQ(g.result(), "  lhs = rhs;\n");
+}
+
+}  // namespace
+}  // namespace msl
+}  // namespace writer
+}  // namespace tint