[msl-writer] Add generation of as casts.

This CL adds the MSL conversion of as casts to `as_cast`.

Bug: tint:8
Change-Id: Iaa8ee1fa3077e4471bbead9d24fcf1e2d68998b0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23821
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_as_test.cc",
     "src/writer/msl/generator_impl_assign_test.cc",
     "src/writer/msl/generator_impl_binary_test.cc",
     "src/writer/msl/generator_impl_constructor_test.cc",

src/CMakeLists.txt

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

src/writer/msl/generator_impl.cc

@@ -14,6 +14,7 @@
 
 #include "src/writer/msl/generator_impl.h"
 
+#include "src/ast/as_expression.h"
 #include "src/ast/assignment_statement.h"
 #include "src/ast/binary_expression.h"
 #include "src/ast/bool_literal.h"
@@ -57,6 +58,21 @@ bool GeneratorImpl::Generate(const ast::Module& module) {
   return true;
 }
 
+bool GeneratorImpl::EmitAs(ast::AsExpression* expr) {
+  out_ << "as_type<";
+  if (!EmitType(expr->type(), "")) {
+    return false;
+  }
+
+  out_ << ">(";
+  if (!EmitExpression(expr->expr())) {
+    return false;
+  }
+
+  out_ << ")";
+  return true;
+}
+
 bool GeneratorImpl::EmitAssign(ast::AssignmentStatement* stmt) {
   make_indent();
 
@@ -217,6 +233,9 @@ bool GeneratorImpl::EmitLiteral(ast::Literal* lit) {
 }
 
 bool GeneratorImpl::EmitExpression(ast::Expression* expr) {
+  if (expr->IsAs()) {
+    return EmitAs(expr->AsAs());
+  }
   if (expr->IsBinary()) {
     return EmitBinary(expr->AsBinary());
   }

src/writer/msl/generator_impl.h

@@ -40,6 +40,10 @@ class GeneratorImpl : public TextGenerator {
   /// @returns true on successful generation; false otherwise
   bool Generate(const ast::Module& module);
 
+  /// Handles generating an as expression
+  /// @param expr the as expression
+  /// @returns true if the as was emitted
+  bool EmitAs(ast::AsExpression* expr);
   /// Handles an assignment statement
   /// @param stmt the statement to emit
   /// @returns true if the statement was emitted successfully

src/writer/msl/generator_impl_as_test.cc

@@ -0,0 +1,43 @@
+// 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 "gtest/gtest.h"
+#include "src/ast/as_expression.h"
+#include "src/ast/identifier_expression.h"
+#include "src/ast/type/f32_type.h"
+#include "src/writer/msl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace msl {
+namespace {
+
+using MslGeneratorImplTest = testing::Test;
+
+TEST_F(MslGeneratorImplTest, EmitExpression_As) {
+  ast::type::F32Type f32;
+  auto id = std::make_unique<ast::IdentifierExpression>("id");
+  ast::AsExpression as(&f32, std::move(id));
+
+  GeneratorImpl g;
+  ASSERT_TRUE(g.EmitExpression(&as)) << g.error();
+  EXPECT_EQ(g.result(), "as_type<float>(id)");
+}
+
+}  // namespace
+}  // namespace msl
+}  // namespace writer
+}  // namespace tint