Forbid unsized structs as final members of structs. (#1443)

WGSL says:

> - The last member of the structure type defining the store type for a variable
>   ... may be a runtime-sized array.
>
> - A runtime-sized array must not be used as the store type or contained within
>   a store type in any other cases.

Thus, a struct whose final member is a struct whose final member is a
runtime-sized array is verboten.

src/valid/type.rs

@@ -473,9 +473,13 @@ impl super::Validator {
                         handle,
                     );
 
-                    // only the last field can be unsized
+                    // The last field may be an unsized array.
                     if !base_info.flags.contains(TypeFlags::SIZED) {
-                        if i + 1 != members.len() {
+                        let is_array = match types[member.ty].inner {
+                            crate::TypeInner::Array { .. } => true,
+                            _ => false,
+                        };
+                        if !is_array || i + 1 != members.len() {
                             let name = member.name.clone().unwrap_or_default();
                             return Err(TypeError::InvalidDynamicArray(name, member.ty));
                         }

tests/wgsl-errors.rs

@@ -913,3 +913,35 @@ fn dead_code() {
         })
     }
 }
+
+#[test]
+fn invalid_runtime_sized_arrays() {
+    // You can't have structs whose last member is an unsized struct. An unsized
+    // array may only appear as the last member of a struct used directly as a
+    // variable's store type.
+    check_validation_error! {
+        "
+        struct Unsized {
+            arr: array<f32>;
+        };
+
+        [[block]]
+        struct Outer {
+            legit: i32;
+            unsized: Unsized;
+        };
+
+        [[group(0), binding(0)]] var<storage> outer: Outer;
+
+        fn fetch(i: i32) -> f32 {
+           return outer.unsized.arr[i];
+        }
+        ":
+        Err(naga::valid::ValidationError::Type {
+            name: struct_name,
+            error: naga::valid::TypeError::InvalidDynamicArray(member_name, _),
+            ..
+        })
+        if struct_name == "Outer" && member_name == "unsized"
+    }
+}