diff --git a/src/InternPool.zig b/src/InternPool.zig
index 9a0aa78e814291caacd7d2e852697f7b37e87586..be6e45441e55ec373e533643786528d0abeb310c 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -2147,9 +2147,11 @@ pub const Key = union(enum) {
         };
     };
 
-    pub const SpirvType = struct {
+    pub const SpirvType = extern struct {
         /// A `spirv_reify` instruction.
         zir_index: TrackedInst.Index,
+        /// Always 0.
+        padding: u32 = 0,
         /// A hash of this type's attributes generated by Sema.
         type_hash: u64,
     };
@@ -2591,7 +2593,6 @@ pub const Key = union(enum) {
         const KeyTag = @typeInfo(Key).@"union".tag_type.?;
         const seed = @intFromEnum(@as(KeyTag, key));
         return switch (key) {
-            // TODO: assert no padding in these types
             inline .ptr_type,
             .array_type,
             .vector_type,
@@ -2608,7 +2609,11 @@ pub const Key = union(enum) {
             .enum_tag,
             .inferred_error_set_type,
             .un,
-            => |x| Hash.hash(seed, asBytes(&x)),
+            => |x| {
+                _ = extern struct { is_extern: @TypeOf(x) };
+                comptime assert(std.meta.hasUniqueRepresentation(@TypeOf(x)));
+                return Hash.hash(seed, asBytes(&x));
+            },
 
             .int_type => |x| Hash.hash(seed + @intFromEnum(x.signedness), asBytes(&x.bits)),