rustc comiler internals helpfully tell us how to fix the issue:
to get the signature of a closure, use `substs.as_closure().sig()` not `fn_sig()`
Fixes ICE in #9041
* Track the argument when used to initialize simple `let` bindings
* Check if the argument is passed to a function requiring the original type
* Use `multipart_suggestion` rather than multiple suggestions
* Check if the name given in the source code matches the name of the actual type
