Revert implementing Iterator::nth[_back] in terms of advance_by[_back]

[timvermeulen]

See #76909 (comment).

cc @ecstatic-morse @scottmcm

r? @ghost

[ecstatic-morse]

@bors try
@rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit ee7dfce with merge cb0908be81ddf2f8624d49cf8ed5df8b49cc2e8d...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: cb0908be81ddf2f8624d49cf8ed5df8b49cc2e8d (cb0908be81ddf2f8624d49cf8ed5df8b49cc2e8d)

[rust-timer]

Queued cb0908be81ddf2f8624d49cf8ed5df8b49cc2e8d with parent a14bf48, future comparison URL.

[rust-timer]

Finished benchmarking try commit (cb0908be81ddf2f8624d49cf8ed5df8b49cc2e8d): 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]

r? @scottmcm

It looks like this does not fix the regression noted in #76909 (comment). I am unsure if we want to revert that PR entirely or just eat this 15% loss. It would seem unfortunate to do so, but I suspect there may be nothing we can do.

[Dylan-DPC-zz]

r? @KodrAus

[Dylan-DPC-zz]

r? @m-ou-se

[jyn514]

I think that one will regress massively any time an object-safe method is added to Iterator, since it uses Box so all those methods need to be codegened for the vtable. So I don't think reverting the nth changes will fix that part; we might just have to accept it -- unless we decide we just can't add another object-safe method ever again. (Also, that test is massively unrealistic because it's an empty iterator of ()s, and even changing it to an empty iterator of i32 already makes it take 30+ minutes, let alone making it actually have elements.)

Given that this doesn't fix the perf regression, I don't think there's any point in merging or keeping it open.