Compiler plugins example doesn't compile

[frewsxcv]

https://doc.rust-lang.org/nightly/book/compiler-plugins.html

The first example

   Compiling url_plugin v0.1.0 (file:///Users/coreyf/Downloads/testing)
/Users/coreyf/Development/rust/rust-url/plugin/src/lib.rs:86:9: 86:50 error: slice pattern syntax is experimental (see issue #23121)
/Users/coreyf/Development/rust/rust-url/plugin/src/lib.rs:86         [TokenTree::Token(_, token::Ident(s, _))] => s.to_string(),
                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/coreyf/Development/rust/rust-url/plugin/src/lib.rs:86:9: 86:50 help: add #![feature(slice_patterns)] to the crate attributes to enable
error: aborting due to previous error
Could not compile `url_plugin`.

To learn more, run the command again with --verbose.

[steveklabnik]

Thanks, these are hard to keep in sync, as they don't get automated testing :/