add doc comment and tests for ast.ContainsExpression function

ast.go

@@ -42,6 +42,8 @@ type String struct {
 	Pos *Pos
 }
 
+// ContainsExpression checks if the given string contains a ${{ }} placeholder or not. This function
+// is identical to String.ContainsExpression method except for taking a standard string value.
 func ContainsExpression(s string) bool {
 	i := strings.Index(s, "${{")
 	return i >= 0 && i < strings.Index(s, "}}")

ast_test.go

@@ -48,9 +48,11 @@ func TestStringContainsExpression(t *testing.T) {
 	for _, tc := range tests {
 		t.Run(tc.input, func(t *testing.T) {
 			s := &String{Value: tc.input}
-			have := s.ContainsExpression()
-			if tc.want != have {
-				t.Fatalf("wanted %v but got %v for input %q", tc.want, have, tc.input)
+			if have := s.ContainsExpression(); tc.want != have {
+				t.Fatalf("wanted %v but the method returned %v for input %q", tc.want, have, tc.input)
+			}
+			if have := ContainsExpression(tc.input); tc.want != have {
+				t.Fatalf("wanted %v but the function returned %v for input %q", tc.want, have, tc.input)
 			}
 		})
 	}