Tracking Issue for feature(ub_checks) and -Zub-checks

[saethlin]

The feature gate for the issue is #![feature(ub_checks)].

It allows write code that detects if the compiler will emit UB checks, simlar to how `cfg(debug_assertions) detects if debug assertions are enabled.

We also track the -Zub-checks flag here; that should eventually become -Cub-checks similar to -Coverflow-checks.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement feature.
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

#123411