tint/writer/wgsl: Emit 'const' variables

Bug: tint:1580
Change-Id: Iea2a38814757f79ba645c94cfa89fe84b5d200d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94681
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/writer/wgsl/generator_impl.cc

@@ -645,14 +645,6 @@ bool GeneratorImpl::EmitVariable(std::ostream& out, const ast::Variable* v) {
 
     bool ok = Switch(
         v,  //
-        [&](const ast::Let* ) {
-            out << "let";
-            return true;
-        },
-        [&](const ast::Override* ) {
-            out << "override";
-            return true;
-        },
         [&](const ast::Var* var) {
             out << "var";
             auto sc = var->declared_storage_class;
@@ -669,6 +661,18 @@ bool GeneratorImpl::EmitVariable(std::ostream& out, const ast::Variable* v) {
             }
             return true;
         },
+        [&](const ast::Let*) {
+            out << "let";
+            return true;
+        },
+        [&](const ast::Override*) {
+            out << "override";
+            return true;
+        },
+        [&](const ast::Const*) {
+            out << "const";
+            return true;
+        },
         [&](Default) {
             TINT_ICE(Writer, diagnostics_) << "unhandled variable type " << v->TypeInfo().name;
             return false;

src/tint/writer/wgsl/generator_impl_global_decl_test.cc

@@ -131,6 +131,36 @@ TEST_F(WgslGeneratorImplTest, Emit_Global_Texture) {
     EXPECT_EQ(gen.result(), "  @group(0) @binding(0) var t : texture_1d<f32>;\n");
 }
 
+TEST_F(WgslGeneratorImplTest, Emit_GlobalLet) {
+    GlobalLet("explicit", ty.f32(), Expr(1_f));
+    GlobalLet("inferred", nullptr, Expr(1_f));
+
+    GeneratorImpl& gen = Build();
+
+    gen.increment_indent();
+
+    ASSERT_TRUE(gen.Generate()) << gen.error();
+    EXPECT_EQ(gen.result(), R"(  let explicit : f32 = 1.0f;
+
+  let inferred = 1.0f;
+)");
+}
+
+TEST_F(WgslGeneratorImplTest, Emit_GlobalConst) {
+    GlobalConst("explicit", ty.f32(), Expr(1_f));
+    GlobalConst("inferred", nullptr, Expr(1_f));
+
+    GeneratorImpl& gen = Build();
+
+    gen.increment_indent();
+
+    ASSERT_TRUE(gen.Generate()) << gen.error();
+    EXPECT_EQ(gen.result(), R"(  const explicit : f32 = 1.0f;
+
+  const inferred = 1.0f;
+)");
+}
+
 TEST_F(WgslGeneratorImplTest, Emit_OverridableConstants) {
     Override("a", ty.f32(), nullptr);
     Override("b", ty.f32(), nullptr, {Id(7u)});

src/tint/writer/wgsl/generator_impl_variable_test.cc

@@ -111,9 +111,9 @@ TEST_F(WgslGeneratorImplTest, EmitVariable_Constructor) {
     EXPECT_EQ(out.str(), R"(var<private> a : f32 = 1.0f;)");
 }
 
-TEST_F(WgslGeneratorImplTest, EmitVariable_Const) {
+TEST_F(WgslGeneratorImplTest, EmitVariable_Let_Explicit) {
     auto* v = Let("a", ty.f32(), Expr(1_f));
-    WrapInFunction(Decl(v));
+    WrapInFunction(v);
 
     GeneratorImpl& gen = Build();
 
@@ -122,5 +122,38 @@ TEST_F(WgslGeneratorImplTest, EmitVariable_Const) {
     EXPECT_EQ(out.str(), R"(let a : f32 = 1.0f;)");
 }
 
+TEST_F(WgslGeneratorImplTest, EmitVariable_Let_Inferred) {
+    auto* v = Let("a", nullptr, Expr(1_f));
+    WrapInFunction(v);
+
+    GeneratorImpl& gen = Build();
+
+    std::stringstream out;
+    ASSERT_TRUE(gen.EmitVariable(out, v)) << gen.error();
+    EXPECT_EQ(out.str(), R"(let a = 1.0f;)");
+}
+
+TEST_F(WgslGeneratorImplTest, EmitVariable_Const_Explicit) {
+    auto* v = Const("a", ty.f32(), Expr(1_f));
+    WrapInFunction(v);
+
+    GeneratorImpl& gen = Build();
+
+    std::stringstream out;
+    ASSERT_TRUE(gen.EmitVariable(out, v)) << gen.error();
+    EXPECT_EQ(out.str(), R"(const a : f32 = 1.0f;)");
+}
+
+TEST_F(WgslGeneratorImplTest, EmitVariable_Const_Inferred) {
+    auto* v = Const("a", nullptr, Expr(1_f));
+    WrapInFunction(v);
+
+    GeneratorImpl& gen = Build();
+
+    std::stringstream out;
+    ASSERT_TRUE(gen.EmitVariable(out, v)) << gen.error();
+    EXPECT_EQ(out.str(), R"(const a = 1.0f;)");
+}
+
 }  // namespace
 }  // namespace tint::writer::wgsl