Function recognize_fn_trait_impl_proof 

pub fn recognize_fn_trait_impl_proof<'a, 'tcx>(
    trait_proof: &'a TraitProof<'tcx>,
) -> Option<(Binder<&'a Ty<'tcx>>, ClosureKind)>

If this trait proof is a built-in impl of a Fn* trait, return the Self type it is implemented for and the kind of the implemented trait.