Cursor::next_with_spacing_ref
This lets us clone just the parts within a `TokenTree` that need cloning, rather than the entire thing. This is a surprisingly large performance win, up to 4% on `async-std-1.10.0`.
TokenCursor::{next,next_desugared}
Nonterminal
to_string
format!
Parser::nonterminal_may_begin_with
const_extern_fn