Allow disabling the panic_impl defined by std

[puppymati]

Proposal

Problem statement

It's not currently possible to mark a function with #[panic_handler] in std programs. The reason is quite simple: Only one panic_impl can be defined per program and std already defines one. The obvious solution to this problem is to allow users to disable the panic_impl defined by std

Motivation, use-cases

It is already possible to define custom panic_impl in no_std environments. There are certain situations where it would be useful in normal programs:

• Bypass the default implementation as it might be desirable to avoid all the other stuff that std sets up
• Allow the use of rust programs in more constrained environments without having to resort to no_std
• Allow to assert at compile time that a program cannot panic with tricks like this, which currently only work in no_std environments.

Furthermore I believe that making std more configurable in general is always a good thing.

Solution sketches

The best solution, that doesn't require a rework of how std works, is making the panic_impl togglable via a cargo feature. Any user that wishes to disable it would need to build std themself + enabling the corresponding feature. Thus making it impossible to accidentally enable.

I opened a pull request over at rust-lang/rust#100156 that does what described above by adding a feature called no_panic_impl.

Right now this would require nightly rust to actually use since it relies on build-std and build-std-features.

Example usage:

cargo +nightly build -Zbuild-std=no_panic_impl

Links and related work

I was asked here to open an ACP since this should qualify as unstable api change.

I also asked some questions over on zulip so the conversation might be continued there:
https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Allow.20custom.20panic_handler.20when.20building.20std

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[joshtriplett]

We haven't committed to supporting feature flags in build-std, but this is a nightly feature, and it seems fine to have it in nightly for now. We can consider it further when we have stabilized build-std and then start considering how or if to support configurability of std. (Note for such future consideration: we're unlikely to want no_* negative features.)

That said, could you please confirm that you've usefully built programs with this that call into functionality from std and don't actually need a panic handler?