Prevent miscompilation in trivial loop {}

[Mark-Simulacrum]

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.

Performance results indicate this is not a major regression, if at all, so it should be safe to land.

cc #28728

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit e2efec8 with merge b6bc5e04aa420f81b5241f406c9ef4e2e07dd8d1...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: b6bc5e04aa420f81b5241f406c9ef4e2e07dd8d1 (b6bc5e04aa420f81b5241f406c9ef4e2e07dd8d1)

[rust-timer]

Queued b6bc5e04aa420f81b5241f406c9ef4e2e07dd8d1 with parent 7f58716, future comparison URL.

[rust-timer]

Finished benchmarking try commit (b6bc5e04aa420f81b5241f406c9ef4e2e07dd8d1): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[Mark-Simulacrum]

Okay, this seems to be performance neutral or non-regressing, at least. I am inclined to land it; let's try r? @nagisa perhaps?

This is definitely not a fix for #28728, of course, because empty loops are by far not the only ones that miscompile, but it seems like a clear win which we should take. I believe this likely means that in the commonly seen "intentional" cases at least people are likely to get the behavior they want, even if in general their code may break.

[nagisa]

LGTM. Sounds like high impact, low-effort change to make. Even if it does not close out all of the non-progress related issues, it should help with the otherwise very common scenario.

@bors r+

[bors]

📌 Commit e2efec8 has been approved by nagisa

[bors]

⌛ Testing commit e2efec8 with merge a78a62f...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: nagisa
Pushing a78a62f to master...