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.
Michael Davis