Function callable_sig_from_fn_trait 

pub fn callable_sig_from_fn_trait<'db>(
    self_ty: Ty<'db>,
    trait_env: ParamEnvAndCrate<'db>,
    db: &'db dyn HirDatabase,
) -> Option<(FnTrait, PolyFnSig<'db>)>

To be used from hir only.