tint: Deprecated module-scope 'let' for 'const'

Enable the parsing of 'const'.
Warn on use of module-scope 'let', and automatically replace with 'const'.

Fixed: tint:1580
Change-Id: I214aabca80686dc6b60ae21a7a57fbfb4898ea83
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93786
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/const/inferred/function.wgsl

@@ -0,0 +1,21 @@
+type MyArray = array<f32, 10>;
+
+// Function-scope consts
+fn const_decls() {
+    const v1 = 1;
+    const v2 = 1u;
+    const v3 = 1.0;
+
+    const v4 = vec3<i32>(1, 1, 1);
+    const v5 = vec3<u32>(1u, 1u, 1u);
+    const v6 = vec3<f32>(1.0, 1.0, 1.0);
+
+    const v7 = mat3x3<f32>(v6, v6, v6);
+
+    const v8 = MyArray();
+}
+
+@fragment
+fn main() -> @location(0) vec4<f32> {
+    return vec4<f32>(0.0,0.0,0.0,0.0);
+}

test/tint/const/inferred/function.wgsl.expected.glsl

@@ -0,0 +1,13 @@
+#version 310 es
+precision mediump float;
+
+layout(location = 0) out vec4 value;
+vec4 tint_symbol() {
+  return vec4(0.0f);
+}
+
+void main() {
+  vec4 inner_result = tint_symbol();
+  value = inner_result;
+  return;
+}

test/tint/const/inferred/function.wgsl.expected.hlsl

@@ -0,0 +1,17 @@
+void const_decls() {
+}
+
+struct tint_symbol {
+  float4 value : SV_Target0;
+};
+
+float4 main_inner() {
+  return (0.0f).xxxx;
+}
+
+tint_symbol main() {
+  const float4 inner_result = main_inner();
+  tint_symbol wrapper_result = (tint_symbol)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}

test/tint/const/inferred/function.wgsl.expected.msl

@@ -0,0 +1,21 @@
+#include <metal_stdlib>
+
+using namespace metal;
+void const_decls() {
+}
+
+struct tint_symbol_1 {
+  float4 value [[color(0)]];
+};
+
+float4 tint_symbol_inner() {
+  return float4(0.0f);
+}
+
+fragment tint_symbol_1 tint_symbol() {
+  float4 const inner_result = tint_symbol_inner();
+  tint_symbol_1 wrapper_result = {};
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}
+

test/tint/const/inferred/function.wgsl.expected.spvasm

@@ -0,0 +1,36 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 16
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %value
+               OpExecutionMode %main OriginUpperLeft
+               OpName %value "value"
+               OpName %const_decls "const_decls"
+               OpName %main_inner "main_inner"
+               OpName %main "main"
+               OpDecorate %value Location 0
+      %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+          %5 = OpConstantNull %v4float
+      %value = OpVariable %_ptr_Output_v4float Output %5
+       %void = OpTypeVoid
+          %6 = OpTypeFunction %void
+         %10 = OpTypeFunction %v4float
+%const_decls = OpFunction %void None %6
+          %9 = OpLabel
+               OpReturn
+               OpFunctionEnd
+ %main_inner = OpFunction %v4float None %10
+         %12 = OpLabel
+               OpReturnValue %5
+               OpFunctionEnd
+       %main = OpFunction %void None %6
+         %14 = OpLabel
+         %15 = OpFunctionCall %v4float %main_inner
+               OpStore %value %15
+               OpReturn
+               OpFunctionEnd

test/tint/const/inferred/function.wgsl.expected.wgsl

@@ -0,0 +1,17 @@
+type MyArray = array<f32, 10>;
+
+fn const_decls() {
+  const v1 = 1;
+  const v2 = 1u;
+  const v3 = 1.0;
+  const v4 = vec3<i32>(1, 1, 1);
+  const v5 = vec3<u32>(1u, 1u, 1u);
+  const v6 = vec3<f32>(1.0, 1.0, 1.0);
+  const v7 = mat3x3<f32>(v6, v6, v6);
+  const v8 = MyArray();
+}
+
+@fragment
+fn main() -> @location(0) vec4<f32> {
+  return vec4<f32>(0.0, 0.0, 0.0, 0.0);
+}