I've said for a while that I think that the "real" issue with compile times is that there's a ton of dead code getting compiled across the dependency tree. I personally blame Cargo features not being ergonomic enough, because on paper they're perfectly suited to solve this problem, but in practice the amount of boilerplate needed to use them for this is infeasible. I wrote a manifesto about this a while back: https://saghm.com/cargo-features-rust-compile-times/

A cross-crate dead code analysis would mean that compilation of a crate now depends on information about its dependents. This would break reuse of compiled crates and cause recompiles when the analysis changes.

Something does seem a little off about this, though. Ideally for this `Debug` case there would be an annotation that says "compile this lazily, don't inline". Maybe there doesn't even need to be a new annotation, just `#[inline] #[cold]`. Which looks pretty weird, but might work already.