Using $crate in Rust’s procedural macros?

Since Rust 1.34, you can use extern crate self as my_crate, and use my_crate::Foo instead of $crate::Foo.

https://github.com/rust-lang/rust/issues/54647

https://github.com/rust-lang/rust/pull/57407

(Credit: Neptunepink ##rust irc.freenode.net)