Add transform to substitute overrides with const expressions.

This CL adds a SubstituteOverride transform which will convert
an `override` into a `const`. The transform is provided a map of
(string, double) which matches what the WebGPU API accepts as
data for overrides.

Bug: tint:1582
Change-Id: I6e6bf51b98ce4d4746f8de55128666c36735e585
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96760
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/ast/override.cc

@@ -46,4 +46,11 @@ const Override* Override::Clone(CloneContext* ctx) const {
     return ctx->dst->create<Override>(src, sym, ty, ctor, attrs);
 }
 
+std::string Override::Identifier(const SymbolTable& symbols) const {
+    if (auto* id = ast::GetAttribute<ast::IdAttribute>(attributes)) {
+        return std::to_string(id->value);
+    }
+    return symbols.NameFor(symbol);
+}
+
 }  // namespace tint::ast

src/tint/ast/override.h

@@ -15,6 +15,8 @@
 #ifndef SRC_TINT_AST_OVERRIDE_H_
 #define SRC_TINT_AST_OVERRIDE_H_
 
+#include <string>
+
 #include "src/tint/ast/variable.h"
 
 namespace tint::ast {
@@ -60,6 +62,12 @@ class Override final : public Castable<Override, Variable> {
     /// @param ctx the clone context
     /// @return the newly cloned node
     const Override* Clone(CloneContext* ctx) const override;
+
+    /// @param symbols the symbol table to retrieve the name from
+    /// @returns the identifier string for the override. If the override has
+    /// an ID attribute, the string is the id-stringified. Otherwise, the ID
+    /// is the symbol.
+    std::string Identifier(const SymbolTable& symbols) const;
 };
 
 }  // namespace tint::ast

src/tint/ast/override_test.cc

@@ -0,0 +1,35 @@
+// Copyright 2022 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/tint/ast/override.h"
+
+#include "src/tint/ast/test_helper.h"
+
+namespace tint::ast {
+namespace {
+
+using OverrideTest = TestHelper;
+
+TEST_F(OverrideTest, Identifier_NoId) {
+    auto* o = Override("o", nullptr, Expr(f32(1.0)));
+    EXPECT_EQ(std::string("o"), o->Identifier(Symbols()));
+}
+
+TEST_F(OverrideTest, Identifier_WithId) {
+    auto* o = Override("o", nullptr, Expr(f32(1.0)), {Id(4u)});
+    EXPECT_EQ(std::string("4"), o->Identifier(Symbols()));
+}
+
+}  // namespace
+}  // namespace tint::ast