New users' code or refactored code might end up containing e.g. functions taking a &Fn() as an argument, when they could take a generic &T that has a T: Fn() trait bound instead.

Linting this in a "perfect" way is obviously (too?) hard. But simple cases could be caught for now and the lint could be replaced by a full blown proof $whenever_someone_is_highly_motivated

Related RFC: rust-lang/rfcs#1603 (comment)