Transform post order walk to an iterative approach

[HeroicKatora]

The previous recursive approach might overflow the stack when walking a
particularly deep, list-like, graph. In particular, dominator
calculation for borrow checking does such a traversal and very long
functions might lead to a region dependency graph with in this
problematic structure.

This addresses what appears to be the cause of #78567 (@SunHao-0 thanks for the stack trace).

[rust-highfive]

r? @davidtwco

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[davidtwco]

Implementation LGTM, have you confirmed that this fixes the issue reported? Going to trigger a perf run to see the impact also (though if this fixes the issue, then I think we'd likely land it anyway).

[davidtwco]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit af72a70 with merge b0d26b01718f8a86b4ee8dfdb0508d3a5d49e19b...

[HeroicKatora]

Implementation LGTM, have you confirmed that this fixes the issue reported?

Unfortunately there is no simplified reproduction. It's not entirely clear to me how build it and the file does appear to have dependencies. In any case, if there's any perf trouble I would like to try out a small-vec solution before admitting a regression in the fix.

[bors]

☀️ Try build successful - checks-actions
Build commit: b0d26b01718f8a86b4ee8dfdb0508d3a5d49e19b (b0d26b01718f8a86b4ee8dfdb0508d3a5d49e19b)

[rust-timer]

Queued b0d26b01718f8a86b4ee8dfdb0508d3a5d49e19b with parent 234099d, future comparison URL.

[rust-timer]

Finished benchmarking try commit (b0d26b01718f8a86b4ee8dfdb0508d3a5d49e19b): 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
@rustbot modify labels: +S-waiting-on-review -S-waiting-on-perf

[davidtwco]

Looks good to me.

@bors r+

[bors]

📌 Commit af72a70 has been approved by davidtwco

[bors]

⌛ Testing commit af72a70 with merge 338f939...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 338f939 to master...

[spastorino]

Adding T-compiler label so this can be picked up by our automated prioritization tool and added to the following T-compiler weekly meeting agenda.

[spastorino]

discussed in T-compiler meeting.

@rustbot modify labels: stable-accepted