Change question to FIXME

Author: phansch

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -25,8 +25,8 @@ fn two() -> i32 {
     abc
 }
 
-// TODO: Why can this be const? because it's a zero sized type?
-// There is the `const_string_new` feature, but it seems that this already works in const fns?
+// FIXME: This is a false positive in the `is_min_const_fn` function.
+// At least until the `const_string_new` feature is stabilzed.
 fn string() -> String {
     String::new()
 }