The vast majority of Debug traits never get used. All the standard library containers (collections, smart pointers, refcell, etc) have Debug traits auto-emitted if the underlying type has Debug. Even if the type never gets formatted.

I really doubt lazily code genning the Debug trait makes things worse or is super expensive to track. The main tension is more how this interplays with LTO (which I'm guessing would be unable to do this) and designing a generic language feature here for macros.

A rough sketch of the idea would be that you can annotate functions with #[lazy] and the Rust compiler knows to save off the function body for later and only keep the function definition around (no codegen). Then if it does need to codegen because it gets invoked from a non-#[lazy] function, the function is treated as inlineable and #[cold] so that if it's not inlined it's put in a cold region of the binary. It's non-trivial because you somehow need to put the codegen back into the original crate's object file which is probably tricky.

I don't understand the concern regarding the orphan rule or "downstream usage by dependents" - that's not relevant here and no language semantics change.