I’m fairly convinced that Debug should never be inlined. Display probably neither, the fmt machinery is heavy enough that not inlining is probably not a bottleneck even in serialization-heavy workloads. I’ve had to #[inline(never)] some of my own Debug/Display impls, shrinking the binary by tens of kilobytes (out of a few hundred, so relatively a significant reduction).
For what it's worth, according to the PR that added the annotation [0] doing so generally resulted in decreases in compile times and binary sizes on benchmarks. Furthermore, an additional experiment that avoided emitting the inline attribute on structs with >5 fields resulted in benchmark regressions compared to always emitting the attribute [1]. I'd guess this is one of those things which may help in aggregate but hurts for specific cases.
That being said, one of the Rust devs indicated in the corresponding lobste.rs discussion [2] that they're open to revisiting/rebalancing things if they get enough bug reports indicating something is up, so it might not hurt to tag onto the bug report the author will (hopefully) eventually submit.
Or just try to avoid all of these optimization guesses by using Profile-Guided Optimization (PGO), that inserts/deletes all inlines based on actual application runtime profile.
That being said, one of the Rust devs indicated in the corresponding lobste.rs discussion [2] that they're open to revisiting/rebalancing things if they get enough bug reports indicating something is up, so it might not hurt to tag onto the bug report the author will (hopefully) eventually submit.
[0]: https://github.com/rust-lang/rust/pull/117727
[1]: https://github.com/rust-lang/rust/pull/118031
[2]: https://lobste.rs/s/dldhpw/rust_s_derive_often_implies_inlin...