{"ast_node_type":"impl","tokens":[{"end":39,"kind":"keyword","start":0,"text":"///"}]}
    /// in an `impl FnOnce()` position.
    /// bounds, suggest borrowing. This covers, e.g., borrowing an `impl Fn()` argument being passed
    /// Returns `Some(mutability)` when suggesting to borrow with mutability `mutability`, or `None`