How much code is never used and compilation could be skipped entirely? Maybe applying a reachability pass to skip compiling unused code would be helpful.

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.