I feel like Debug should be lazily emitted altogether when it’s first used - just a special marker that’s never expanded since 99% of the Debug implementations aren’t used and having the rest marked #[cold] as inline is obviously wrong. Of course implementing it in practice sounds exceptionally difficult.

That being said, even the justifying performance improvement PR was itself a mix of improvements and regressions

Lazily emitted from what? You would need information about the type at runtime to derive the implementation, and normally that information is not available at runtime. The debug implementation itself is probably within spitting distance of any other representation that would be sufficient to lazily emit the debug implementation.