Split `$crate` into two tokens.

[ehuss]

The grammar rules should be clear that $ crate is two tokens. I think putting a space between them would be sufficient (such as in paths.md)

[petrochenkov]

This is tricky.
$ crate is two tokens in macro_rules input, but it turns into a single identifier token in the macro output after the macro is expanded, see rust-lang/rust#55640 for more details.