Function parse_fn_sig 

pub(crate) fn parse_fn_sig<T: PeekExpected>(
    cx: &mut ParseCtxt<'_>,
    end: T,
) -> ParseResult<FnSig>

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