Fixup single element swizzle type determination.

For a swizzle with one element (eg vec.x) the result type is just the
type of the vector, instead of a new vector.

Change-Id: I04ddb22da61db1c3553d465e4e5f9d6b32beae83
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20062
Reviewed-by: David Neto <dneto@google.com>

src/type_determiner.cc

@@ -400,12 +400,17 @@ bool TypeDeterminer::DetermineMemberAccessor(
   if (data_type->IsVector()) {
     auto* vec = data_type->AsVector();
 
+    auto size = expr->member()->name().size();
+    if (size == 1) {
+      // A single element swizzle is just the type of the vector.
+      expr->set_result_type(vec->type());
+    } else {
       // The vector will have a number of components equal to the length of the
       // swizzle. This assumes the validator will check that the swizzle
       // is correct.
-    expr->set_result_type(
-        ctx_.type_mgr().Get(std::make_unique<ast::type::VectorType>(
-            vec->type(), expr->member()->name().size())));
+      expr->set_result_type(ctx_.type_mgr().Get(
+          std::make_unique<ast::type::VectorType>(vec->type(), size)));
+    }
     return true;
   }
 

src/type_determiner_test.cc

@@ -745,6 +745,26 @@ TEST_F(TypeDeterminerTest, Expr_MemberAccessor_VectorSwizzle) {
   EXPECT_EQ(mem.result_type()->AsVector()->size(), 2u);
 }
 
+TEST_F(TypeDeterminerTest, Expr_MemberAccessor_VectorSwizzle_SingleElement) {
+  ast::type::F32Type f32;
+  ast::type::VectorType vec3(&f32, 3);
+
+  auto var = std::make_unique<ast::Variable>("my_vec", ast::StorageClass::kNone,
+                                             &vec3);
+  mod()->AddGlobalVariable(std::move(var));
+
+  // Register the global
+  EXPECT_TRUE(td()->Determine());
+
+  auto ident = std::make_unique<ast::IdentifierExpression>("my_vec");
+  auto swizzle = std::make_unique<ast::IdentifierExpression>("x");
+
+  ast::MemberAccessorExpression mem(std::move(ident), std::move(swizzle));
+  EXPECT_TRUE(td()->DetermineResultType(&mem)) << td()->error();
+  ASSERT_NE(mem.result_type(), nullptr);
+  ASSERT_TRUE(mem.result_type()->IsF32());
+}
+
 TEST_F(TypeDeterminerTest, Expr_MultiLevel) {
   // struct b {
   //   vec4<f32> foo