`or unreachable` in a release build

Decided

D-0020, settled by the language’s author: part of spec §18. Older text cites it as 18.20.

The compiler assumes the success path — __builtin_unreachable, not merely a skipped check. Debug still panics with the location. Chosen for speed: or unreachable is already the loud spelling of “this cannot fail”, and §1.1 does not promise safety (§9.4)

Revised by D-0094 point 4 (r108): or unreachable panics with the location in every build. The assumption is kept only inside unsafe, a block or an unsafe fn, where an optimised build still lowers it to __builtin_unreachable: a release build that assumed a failed search succeeded ran the loop past the end of its list, undefined behaviour reached from safe code (T-0128).