Function callable_sig_from_fn_trait 

pub fn callable_sig_from_fn_trait<'db>(
    self_ty: Ty<'db>,
    param_env: ParamEnvAndCrate<'db>,
    db: &'db (dyn HirDatabase + 'static),
) -> Option<(FnTrait, Binder<DbInterner<'db>, FnSig<DbInterner<'db>>>)>

To be used from hir only.