Unstably add -Cpanic=immediate-abort

[saethlin]

Draft implementation: rust-lang/rust#146317

Proposal

We currently have a standard library feature called panic_immediate_abort. The goal of this feature is to minimize code size, and it does so by replacing the entire panic runtime with intrinsics::abort(). For example, on x86 this aborts with just the ud2 instruction, not a call to libc::abort(). In addition, panic hooks are not called which means that panic messages and backtraces are not printed. Finally, the feature adjusts some inlining attributes on panic wrappers in the standard library so that they completely optimize away.

-Zbuild-std-features=panic_immediate_abort is really a new panic strategy, kludged in as a standard library feature. I'm proposing that we elevate it to an actual panic option, so that it integrates better with the toolchain. This will let users set the option in the normal way that panics are customized, and let us customize diagnostics about it.

It also opens up opportunities to teach codegen about the panic mode, instead of managing an ever-growing amount of cfg_attr( in the standard library source.

And if eventually libraries want to customize their behavior to facilitate the code size optimizations that the option opens up, they could use cfg(panic = "immediate_abort") like the standard library.

Mentors or Reviewers

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[rustbot]

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cc @rust-lang/compiler

[jieyouxu]

I'm very much in favor of making this a proper toolchain-level knob to make this more maintainable in the long run, instead of adding a bunch of adhoc std features.

@rustbot second

[rustbot]

The final comment period is now complete, this major change is now accepted.

As the automated representative, I would like to thank the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.