tint/writer/msl: Generate an array<T,N> helper

And remove the WrapArraysInStructs transform.

Wrapping arrays in structures becomes troublesome for `const` arrays, as
currently WGSL does not allow `const` structures.

MSL 2.0+ has a builtin array<> helper, but we're targetting MSL 1.2, so
we have to emit our own. Fortunately, it can be done with a few lines of
templated code.

This produces significantly cleaner output.

Change-Id: Ifc92ef21e09befa252a07c856c4b5afdc51cc2e4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94540
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>

test/tint/expressions/zero_init/array/bool.wgsl.expected.msl

@@ -1,11 +1,20 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  bool arr[4];
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
 };
 
 void f() {
-  tint_array_wrapper v = {.arr={}};
+  tint_array<bool, 4> v = tint_array<bool, 4>{};
 }
 

test/tint/expressions/zero_init/array/f32.wgsl.expected.msl

@@ -1,11 +1,20 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  float arr[4];
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
 };
 
 void f() {
-  tint_array_wrapper v = {.arr={}};
+  tint_array<float, 4> v = tint_array<float, 4>{};
 }
 

test/tint/expressions/zero_init/array/i32.wgsl.expected.msl

@@ -1,11 +1,20 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  int arr[4];
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
 };
 
 void f() {
-  tint_array_wrapper v = {.arr={}};
+  tint_array<int, 4> v = tint_array<int, 4>{};
 }
 

test/tint/expressions/zero_init/array/struct.wgsl.expected.msl

@@ -1,6 +1,19 @@
 #include <metal_stdlib>
 
 using namespace metal;
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
+};
+
 struct S {
   int i;
   uint u;
@@ -8,11 +21,7 @@ struct S {
   bool b;
 };
 
-struct tint_array_wrapper {
-  S arr[4];
-};
-
 void f() {
-  tint_array_wrapper v = {.arr={}};
+  tint_array<S, 4> v = tint_array<S, 4>{};
 }
 

test/tint/expressions/zero_init/array/u32.wgsl.expected.msl

@@ -1,11 +1,20 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  uint arr[4];
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
 };
 
 void f() {
-  tint_array_wrapper v = {.arr={}};
+  tint_array<uint, 4> v = tint_array<uint, 4>{};
 }
 

test/tint/expressions/zero_init/struct/array.wgsl.expected.msl

@@ -1,12 +1,21 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  float arr[4];
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
 };
 
 struct S {
-  tint_array_wrapper a;
+  tint_array<float, 4> a;
 };
 
 void f() {