Building core with defmt support

[ia0]

I'm not sure if it's feasible or if it even makes sense, but I was wondering if it is (or will be) possible to build core (and alloc) with defmt support. I mean something like -Z build-std=core,alloc -Z build-std-features=panic_defmt_abort.

The motivation would be to reduce the footprint of core (and alloc) panic messages, by removing the string literals as well as formatting code from the final binary.

I can see how this could not be a priority because using panic_immediate_abort and defmt::panic! in the user crates already covers quite a lot of the usecases. So the benefit might be small for a possibly impossible compiler change. But I felt like asking just in case I'm wrong (and it's either easy to do / already exists, or it's actually more useful than I think it is).