Missing Hint for things within "Function Type"

[vi]

Source code

 1 | fn lol(a: Box<for<'a> fn(x:&'a u32)->()>) {
   |                ↑
 2 | }

Location: line 1, column 15

What I got

"Function Type. The type of a free standing function." spanning the whole generic argument of the Box.

What I expected

"The type of a free standing function" can contain many tricky things inside it. I expect some individual tokens within that type to be selectable.