[ast] Add DiscardStatement AST node.

This CL adds DiscardStatement to the AST.

Bug: tint:162
Change-Id: I1905023eb8297d9a983884e77c6a2267fd43a076
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25601
Reviewed-by: David Neto <dneto@google.com>

src/CMakeLists.txt

@@ -70,6 +70,8 @@ set(TINT_LIB_SRCS
   ast/continue_statement.h
   ast/decorated_variable.cc
   ast/decorated_variable.h
+  ast/discard_statement.cc
+  ast/discard_statement.h
   ast/else_statement.cc
   ast/else_statement.h
   ast/entry_point.cc
@@ -286,6 +288,7 @@ set(TINT_TEST_SRCS
   ast/case_statement_test.cc
   ast/cast_expression_test.cc
   ast/continue_statement_test.cc
+  ast/discard_statement_test.cc
   ast/decorated_variable_test.cc
   ast/else_statement_test.cc
   ast/entry_point_test.cc

src/ast/discard_statement.cc

@@ -0,0 +1,40 @@
+// 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 "src/ast/discard_statement.h"
+
+namespace tint {
+namespace ast {
+
+DiscardStatement::DiscardStatement() : Statement() {}
+
+DiscardStatement::DiscardStatement(const Source& source) : Statement(source) {}
+
+DiscardStatement::~DiscardStatement() = default;
+
+bool DiscardStatement::IsDiscard() const {
+  return true;
+}
+
+bool DiscardStatement::IsValid() const {
+  return true;
+}
+
+void DiscardStatement::to_str(std::ostream& out, size_t indent) const {
+  make_indent(out, indent);
+  out << "Discard{}" << std::endl;
+}
+
+}  // namespace ast
+}  // namespace tint

src/ast/discard_statement.h

@@ -0,0 +1,53 @@
+// 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.
+
+#ifndef SRC_AST_DISCARD_STATEMENT_H_
+#define SRC_AST_DISCARD_STATEMENT_H_
+
+#include "src/ast/statement.h"
+
+namespace tint {
+namespace ast {
+
+/// A discard statement
+class DiscardStatement : public Statement {
+ public:
+  /// Constructor
+  DiscardStatement();
+  /// Constructor
+  /// @param source the discard statement source
+  explicit DiscardStatement(const Source& source);
+  /// Move constructor
+  DiscardStatement(DiscardStatement&&) = default;
+  ~DiscardStatement() override;
+
+  /// @returns true if this is a discard statement
+  bool IsDiscard() const override;
+
+  /// @returns true if the node is valid
+  bool IsValid() const override;
+
+  /// Writes a representation of the node to the output stream
+  /// @param out the stream to write to
+  /// @param indent number of spaces to indent the node when writing
+  void to_str(std::ostream& out, size_t indent) const override;
+
+ private:
+  DiscardStatement(const DiscardStatement&) = delete;
+};
+
+}  // namespace ast
+}  // namespace tint
+
+#endif  // SRC_AST_DISCARD_STATEMENT_H_

src/ast/discard_statement_test.cc

@@ -0,0 +1,59 @@
+// 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 "src/ast/discard_statement.h"
+
+#include <sstream>
+
+#include "gtest/gtest.h"
+
+namespace tint {
+namespace ast {
+namespace {
+
+using DiscardStatementTest = testing::Test;
+
+TEST_F(DiscardStatementTest, Creation) {
+  DiscardStatement stmt;
+  EXPECT_EQ(stmt.line(), 0u);
+  EXPECT_EQ(stmt.column(), 0u);
+}
+
+TEST_F(DiscardStatementTest, Creation_WithSource) {
+  DiscardStatement stmt(Source{20, 2});
+  EXPECT_EQ(stmt.line(), 20u);
+  EXPECT_EQ(stmt.column(), 2u);
+}
+
+TEST_F(DiscardStatementTest, IsDiscard) {
+  DiscardStatement stmt;
+  EXPECT_TRUE(stmt.IsDiscard());
+}
+
+TEST_F(DiscardStatementTest, IsValid) {
+  DiscardStatement stmt;
+  EXPECT_TRUE(stmt.IsValid());
+}
+
+TEST_F(DiscardStatementTest, ToStr) {
+  DiscardStatement stmt;
+  std::ostringstream out;
+  stmt.to_str(out, 2);
+  EXPECT_EQ(out.str(), R"(  Discard{}
+)");
+}
+
+}  // namespace
+}  // namespace ast
+}  // namespace tint

src/ast/statement.cc

@@ -21,6 +21,7 @@
 #include "src/ast/call_statement.h"
 #include "src/ast/case_statement.h"
 #include "src/ast/continue_statement.h"
+#include "src/ast/discard_statement.h"
 #include "src/ast/else_statement.h"
 #include "src/ast/fallthrough_statement.h"
 #include "src/ast/if_statement.h"
@@ -61,6 +62,10 @@ bool Statement::IsContinue() const {
   return false;
 }
 
+bool Statement::IsDiscard() const {
+  return false;
+}
+
 bool Statement::IsElse() const {
   return false;
 }
@@ -118,6 +123,11 @@ const ContinueStatement* Statement::AsContinue() const {
   return static_cast<const ContinueStatement*>(this);
 }
 
+const DiscardStatement* Statement::AsDiscard() const {
+  assert(IsDiscard());
+  return static_cast<const DiscardStatement*>(this);
+}
+
 const ElseStatement* Statement::AsElse() const {
   assert(IsElse());
   return static_cast<const ElseStatement*>(this);
@@ -183,6 +193,11 @@ ContinueStatement* Statement::AsContinue() {
   return static_cast<ContinueStatement*>(this);
 }
 
+DiscardStatement* Statement::AsDiscard() {
+  assert(IsDiscard());
+  return static_cast<DiscardStatement*>(this);
+}
+
 ElseStatement* Statement::AsElse() {
   assert(IsElse());
   return static_cast<ElseStatement*>(this);

src/ast/statement.h

@@ -28,6 +28,7 @@ class BreakStatement;
 class CallStatement;
 class CaseStatement;
 class ContinueStatement;
+class DiscardStatement;
 class ElseStatement;
 class FallthroughStatement;
 class IfStatement;
@@ -52,6 +53,8 @@ class Statement : public Node {
   virtual bool IsCase() const;
   /// @returns true if this is a continue statement
   virtual bool IsContinue() const;
+  /// @returns true if this is a discard statement
+  virtual bool IsDiscard() const;
   /// @returns true if this is an else statement
   virtual bool IsElse() const;
   /// @returns true if this is a fallthrough statement
@@ -79,6 +82,8 @@ class Statement : public Node {
   const CaseStatement* AsCase() const;
   /// @returns the statement as a const continue statement
   const ContinueStatement* AsContinue() const;
+  /// @returns the statement as a const discard statement
+  const DiscardStatement* AsDiscard() const;
   /// @returns the statement as a const else statement
   const ElseStatement* AsElse() const;
   /// @returns the statement as a const fallthrough statement
@@ -106,6 +111,8 @@ class Statement : public Node {
   CaseStatement* AsCase();
   /// @returns the statement as a continue statement
   ContinueStatement* AsContinue();
+  /// @returns the statement as a discard statement
+  DiscardStatement* AsDiscard();
   /// @returns the statement as a else statement
   ElseStatement* AsElse();
   /// @returns the statement as a fallthrough statement