[wgsl-writer] Add DiscardStatement support

This CL adds the DiscardStatement to the WGSL writer.

Bug: tint:165
Change-Id: I012bc030482e097fd9a957ed96f15d8360f621a5
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25603
Reviewed-by: David Neto <dneto@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index c329fa0..a2a339b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -944,6 +944,7 @@
     "src/writer/wgsl/generator_impl_cast_test.cc",
     "src/writer/wgsl/generator_impl_constructor_test.cc",
     "src/writer/wgsl/generator_impl_continue_test.cc",
+    "src/writer/wgsl/generator_impl_discard_test.cc",
     "src/writer/wgsl/generator_impl_entry_point_test.cc",
     "src/writer/wgsl/generator_impl_fallthrough_test.cc",
     "src/writer/wgsl/generator_impl_function_test.cc",

src/CMakeLists.txt

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index aa92225..062b0b5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -494,6 +494,7 @@
     writer/wgsl/generator_impl_cast_test.cc
     writer/wgsl/generator_impl_constructor_test.cc
     writer/wgsl/generator_impl_continue_test.cc
+    writer/wgsl/generator_impl_discard_test.cc
     writer/wgsl/generator_impl_entry_point_test.cc
     writer/wgsl/generator_impl_fallthrough_test.cc
     writer/wgsl/generator_impl_function_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 4eb9583..7d6c0f4 100644
--- a/src/writer/wgsl/generator_impl.cc
+++ b/src/writer/wgsl/generator_impl.cc
@@ -643,6 +643,9 @@
   if (stmt->IsContinue()) {
     return EmitContinue(stmt->AsContinue());
   }
+  if (stmt->IsDiscard()) {
+    return EmitDiscard(stmt->AsDiscard());
+  }
   if (stmt->IsFallthrough()) {
     return EmitFallthrough(stmt->AsFallthrough());
   }
@@ -768,6 +771,12 @@
   return true;
 }
 
+bool GeneratorImpl::EmitDiscard(ast::DiscardStatement*) {
+  make_indent();
+  out_ << "discard;" << std::endl;
+  return true;
+}
+
 bool GeneratorImpl::EmitKill(ast::KillStatement*) {
   make_indent();
   out_ << "kill;" << std::endl;

src/writer/wgsl/generator_impl.h

diff --git a/src/writer/wgsl/generator_impl.h b/src/writer/wgsl/generator_impl.h
index 9d394ad..69243c4 100644
--- a/src/writer/wgsl/generator_impl.h
+++ b/src/writer/wgsl/generator_impl.h
@@ -127,6 +127,10 @@
   /// @param expr the constructor expression
   /// @returns true if the expression was emitted
   bool EmitConstructor(ast::ConstructorExpression* expr);
+  /// Handles generating a discard statement
+  /// @param stmt the discard statement
+  /// @returns true if the statement was successfully emitted
+  bool EmitDiscard(ast::DiscardStatement* stmt);
   /// Handles generating a kill statement
   /// @param stmt the kill statement
   /// @returns true if the statement was successfully emitted

src/writer/wgsl/generator_impl_discard_test.cc

diff --git a/src/writer/wgsl/generator_impl_discard_test.cc b/src/writer/wgsl/generator_impl_discard_test.cc
new file mode 100644
index 0000000..9e87437
--- /dev/null
+++ b/src/writer/wgsl/generator_impl_discard_test.cc
@@ -0,0 +1,39 @@
+// 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/discard_statement.h"
+#include "src/writer/wgsl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace wgsl {
+namespace {
+
+using WgslGeneratorImplTest = testing::Test;
+
+TEST_F(WgslGeneratorImplTest, Emit_Discard) {
+  ast::DiscardStatement k;
+
+  GeneratorImpl g;
+  g.increment_indent();
+
+  ASSERT_TRUE(g.EmitStatement(&k)) << g.error();
+  EXPECT_EQ(g.result(), "  discard;\n");
+}
+
+}  // namespace
+}  // namespace wgsl
+}  // namespace writer
+}  // namespace tint