Function parse_fn_sig

pub(crate) fn parse_fn_sig(cx: &mut ParseCtxt<'_>) -> ParseResult<FnSig>

⟨fn_sig⟩ := ⟨asyncness⟩ fn ⟨ident⟩?
            ⟨ [ ⟨refine_param⟩,* ] ⟩?
            ( ⟨fn_inputs⟩,* )
            ⟨-> ⟨ty⟩⟩?
            ⟨requires⟩ ⟨ensures⟩ ⟨where⟩