Stabilize std::hint::unreachable_unchecked.

[kennytm]

Closes#43751.

[rust-highfive]

r? @Mark-Simulacrum

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

r? @sfackler

[lukaslueg]

Maybe the docs should point out that undefined behaviour can occur if runtime could reach the point that was marked unreachable_unchecked, not just if. That is, the program might behave weirdly even if it does not reach the unreachable part, because the compiler made assumptions somewhere else.

It's a nuance in wording, yet people have been banging their heads with "catching" code reaching unreachable parts while debugging random segfaults: It's spooky action at a distance.

[hanna-kruppe]

It's certainly worth reminding people of that since it's often not untuitive, but I want to be clear that this is not specific to unreachable: all Undefined Behavior is "time travelling" in the sense that a program that eventually executes UB has never had any meaning to begin with and thus implementations don't have to try and faithfully implement the part of the program trace before the UB operation. This is worth stressing, but if it's stressed in the documentation of one particular function, the docs should be careful to not imply that this is specific to said function.

[emilyalbini]

Ping from triage @sfackler! This PR needs your review!

[sfackler]

@bors r+

[bors]

📌 Commit 5fe8c59 has been approved by sfackler

[aturon]

@bors: rollup