This is a bit hacky. Injections cannot stack on each other like highlights because layers can have their own injections. So this new language `rust-format-args-macro` emulates that. It unconditionally injects `rust-format-args` into all strings. Rust injects this new language into known format-args macros like `println!`. The downside is that this can cause false-positive highlights within these macros for strings which happen to contain format-args syntax println!("Hello, {}!", "{}"); // ^ format args syntax // ^ not format args syntax, but highlighted // as if it were :( This false-positive case is expected to be rare. Injecting this fake language fixes regular non-string highlights in macro invocations: macro invocations need to inject the entire token tree and use `injection.include-children` for proper highlighting.