Add generation of the 'as' expression.

This CL adds 'as' generation into the WGSL generator.

Bug: tint:4
Change-Id: Ica9e615e30aae1882681d8da74b26a51cff6dc30
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16880
Reviewed-by: David Neto <dneto@google.com>

src/CMakeLists.txt

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index d51da20..8731960 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -406,6 +406,7 @@
     writer/wgsl/generator_impl_test.cc
     writer/wgsl/generator_impl_alias_type_test.cc
     writer/wgsl/generator_impl_array_accessor_test.cc
+    writer/wgsl/generator_impl_as_test.cc
     writer/wgsl/generator_impl_entry_point_test.cc
     writer/wgsl/generator_impl_identifier_test.cc
     writer/wgsl/generator_impl_import_test.cc

src/writer/wgsl/generator_impl.cc

diff --git a/src/writer/wgsl/generator_impl.cc b/src/writer/wgsl/generator_impl.cc
index 833b821..e0257f3 100644
--- a/src/writer/wgsl/generator_impl.cc
+++ b/src/writer/wgsl/generator_impl.cc
@@ -18,6 +18,7 @@
 #include <limits>
 
 #include "src/ast/array_accessor_expression.h"
+#include "src/ast/as_expression.h"
 #include "src/ast/binding_decoration.h"
 #include "src/ast/bool_literal.h"
 #include "src/ast/builtin_decoration.h"
@@ -118,6 +119,9 @@
   if (expr->IsArrayAccessor()) {
     return EmitArrayAccessor(expr->AsArrayAccessor());
   }
+  if (expr->IsAs()) {
+    return EmitAs(expr->AsAs());
+  }
   if (expr->IsIdentifier()) {
     return EmitIdentifier(expr->AsIdentifier());
   }
@@ -143,6 +147,21 @@
   return true;
 }
 
+bool GeneratorImpl::EmitAs(ast::AsExpression* expr) {
+  out_ << "as<";
+  if (!EmitType(expr->type())) {
+    return false;
+  }
+
+  out_ << ">(";
+  if (!EmitExpression(expr->expr())) {
+    return false;
+  }
+
+  out_ << ")";
+  return true;
+}
+
 bool GeneratorImpl::EmitInitializer(ast::InitializerExpression* expr) {
   if (expr->IsConstInitializer()) {
     return EmitConstInitializer(expr->AsConstInitializer());

src/writer/wgsl/generator_impl.h

diff --git a/src/writer/wgsl/generator_impl.h b/src/writer/wgsl/generator_impl.h
index 3ecc17c..25203d9 100644
--- a/src/writer/wgsl/generator_impl.h
+++ b/src/writer/wgsl/generator_impl.h
@@ -74,6 +74,10 @@
   /// @param expr the expression to emit
   /// @returns true if the array accessor was emitted
   bool EmitArrayAccessor(ast::ArrayAccessorExpression* expr);
+  /// Handles generating an as expression
+  /// @param expr the as expression
+  /// @returns true if the as was emitted
+  bool EmitAs(ast::AsExpression* expr);
   /// Handles generating a const initializer
   /// @param expr the const initializer expression
   /// @returns true if the initializer is emitted

src/writer/wgsl/generator_impl_as_test.cc

diff --git a/src/writer/wgsl/generator_impl_as_test.cc b/src/writer/wgsl/generator_impl_as_test.cc
new file mode 100644
index 0000000..740dde0
--- /dev/null
+++ b/src/writer/wgsl/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/wgsl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace wgsl {
+namespace {
+
+using GeneratorImplTest = testing::Test;
+
+TEST_F(GeneratorImplTest, 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<f32>(id)");
+}
+
+}  // namespace
+}  // namespace wgsl
+}  // namespace writer
+}  // namespace tint