Rework beautify_doc_string so that it returns a Symbol instead of a String

[GuillaumeGomez]

This commit comes from #80261, the goal here is to inspect the impact on performance of this change on its own.

The idea of rewriting beautify_doc_string is to not go through String if we don't need to update the doc comment to be able to keep the original Symbol and also to have better performance.

r? @jyn514

[jyn514]

r? @petrochenkov

[jyn514]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

[bors]

⌛ Trying commit 64afded with merge bfedb49624a98ca1cdf9865df9704c425f540444...

[bors]

☀️ Try build successful - checks-actions
Build commit: bfedb49624a98ca1cdf9865df9704c425f540444 (bfedb49624a98ca1cdf9865df9704c425f540444)

[rust-timer]

Queued bfedb49624a98ca1cdf9865df9704c425f540444 with parent 75e1acb, future comparison URL.

@rustbot label: +S-waiting-on-perf

[rust-timer]

Finished benchmarking try commit (bfedb49624a98ca1cdf9865df9704c425f540444): 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 label: +S-waiting-on-review -S-waiting-on-perf

[jyn514]

Perf look like noise, probably any improvements here are being lost because we immediately call to_string(). I think this would be worthwhile as part of #80261 though.

[GuillaumeGomez]

Just like I said. As is, it's pretty useless unfortunately. So I suggest to merge this so I can integrate it into #80261.

[GuillaumeGomez]

We need someone from the compiler team. Ping @oli-obk :)

[GuillaumeGomez]

Oh and ping @petrochenkov too. :)

[petrochenkov]

So, the goal is to not just return a Symbol instead of String, the goal is to return the original Symbol without going through String at all if nothing changed?
This is a good idea since this optimization should cover the common case.
I only wish this was written in the PR description, because I initially assumed that the goal is keeping a Symbol in DocFragment, but that doesn't require changing beautify_doc_string - the interning can be done on rustdoc side.

[GuillaumeGomez]

Sorry, I opened it to make the review of #80261 simpler for @jyn514. I'll add a more extensive description.

[GuillaumeGomez]

Done! Anything that needs to be updated here @petrochenkov (is the new description enough too?)?

[GuillaumeGomez]

Ok, let's move on then!

@bors: r=petrochenkov

[bors]

📌 Commit 64afded has been approved by petrochenkov

[bors]

⌛ Testing commit 64afded with merge 2acf6ee...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 2acf6ee to master...