Pretty sure what is meant by "lazily" is that it should just be added by default and then those that are not used should be optimized away by the linker.
Rust already does dead code elimination at a couple different levels by default, and generic link-time optimization is always an option on top of that, so I assumed GP was talking about something that was not already the status quo.
What they're saying should change is that the Debug trait should be added to structs by default, instead of having to be explicitly specified.