Function parse_one_tt 

pub(super) fn parse_one_tt(
    input: TokenTree,
    part: RulePart,
    sess: &Session,
    node_id: NodeId,
    features: &Features,
    edition: Edition,
) -> TokenTree

Takes a tokenstream::TokenTree and returns a self::TokenTree. Like parse, but for a single token tree. Emits errors to sess if needed.