Well, you can still have your syntax extension live in the same repo as the rest of your code. For example, the Servo repo has a "components/plugins" directory

Well, you can still have your syntax extension live in the same repo as the rest of your code. For example, the Servo repo has a "components/plugins" directory that contains our custom lints and other extensions. And other code in the repo just links to it like this: Cargo.toml: [dependencies.plugins] path = "../plugins" lib.rs: #![plugin(plugins)] This doesn't seem too different from the situation in C where you put your cpp macros in a header file so you can #include them in multiple source files.