Add `intrinsics.type_field_type`

core/intrinsics/intrinsics.odin

@@ -153,6 +153,7 @@ type_is_specialization_of :: proc($T, $S: typeid) -> bool ---
 type_is_variant_of :: proc($U, $V: typeid) -> bool where type_is_union(U) ---
 
 type_has_field :: proc($T: typeid, $name: string) -> bool ---
+type_field_type :: proc($T: typeid, $name: string) -> typeid ---
 
 type_proc_parameter_count :: proc($T: typeid) -> int where type_is_proc(T) ---
 type_proc_return_count    :: proc($T: typeid) -> int where type_is_proc(T) ---

src/check_builtin.cpp

@@ -3926,6 +3926,37 @@ bool check_builtin_procedure(CheckerContext *c, Operand *operand, Ast *call, i32
 			break;
 		}
 		break;
+	case BuiltinProc_type_field_type:
+		{
+			Operand op = {};
+			Type *bt = check_type(c, ce->args[0]);
+			Type *type = base_type(bt);
+			if (type == nullptr || type == t_invalid) {
+				error(ce->args[0], "Expected a type for '%.*s'", LIT(builtin_name));
+				return false;
+			}
+			Operand x = {};
+			check_expr(c, &x, ce->args[1]);
+
+			if (!is_type_string(x.type) || x.mode != Addressing_Constant || x.value.kind != ExactValue_String) {
+				error(ce->args[1], "Expected a const string for field argument");
+				return false;
+			}
+
+			String field_name = x.value.value_string;
+
+			Selection sel = lookup_field(type, field_name, false);
+			if (sel.index.count == 0) {
+				gbString t = type_to_string(type);
+				error(ce->args[1], "'%.*s' is not a field of type %s", LIT(field_name), t);
+				gb_string_free(t);
+				return false;
+			}
+			operand->mode = Addressing_Type;
+			operand->type = sel.entity->type;
+			break;
+		}
+		break;
 
 	case BuiltinProc_type_is_specialization_of:
 		{

src/checker_builtin_procs.hpp

@@ -179,6 +179,7 @@ BuiltinProc__type_simple_boolean_begin,
 BuiltinProc__type_simple_boolean_end,
 
 	BuiltinProc_type_has_field,
+	BuiltinProc_type_field_type,
 
 	BuiltinProc_type_is_specialization_of,
 
@@ -395,6 +396,7 @@ gb_global BuiltinProc builtin_procs[BuiltinProc_COUNT] = {
 	{STR_LIT(""), 0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
 
 	{STR_LIT("type_has_field"),            2, false, Expr_Expr, BuiltinProcPkg_intrinsics},
+	{STR_LIT("type_field_type"),           2, false, Expr_Expr, BuiltinProcPkg_intrinsics},
 
 	{STR_LIT("type_is_specialization_of"), 2, false, Expr_Expr, BuiltinProcPkg_intrinsics},