Cow'ify some pprust methods

[klensy]

Reduce number of potential needless de/allocations by using Cow<'static, str> instead of explicit String type.

[rust-highfive]

r? @nagisa

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[nagisa]

Could you please adjust the PR description to include some motivation for this change (i.e. the why?)

[ghost]

Could you please adjust the PR description to include some motivation for this change (i.e. the why?)

I'm confident that the goal of this PR is performance. It might be worth running a benchmark on this.

[klensy]

@r00ster91 I've edited PR description, previously it was empty.

[nagisa]

Right, the goal of my request was to make sure there's less guessing involved as to what the motivation really is.

---

I don't think pretty printer is at all performance critical and therefore value of a benchmark here seems fairly low (and also pretty high-effort). The changes also don't seem all that intrusive either, so I'm happy to approve this based on the intuition that things are better this way.

Can you please also squash the commits together? r=me afterwards.

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 5ca71fed1af4c962d0fb4dfccdb084146894066e with merge f91948fcb5e5a8daf32eabb6124dff78604d248e...

[bors]

☀️ Try build successful - checks-actions
Build commit: f91948fcb5e5a8daf32eabb6124dff78604d248e (f91948fcb5e5a8daf32eabb6124dff78604d248e)

[rust-timer]

Queued f91948fcb5e5a8daf32eabb6124dff78604d248e with parent 47ab5f7, future comparison URL.

[rust-timer]

Finished benchmarking try commit (f91948fcb5e5a8daf32eabb6124dff78604d248e): comparison url.

Summary: This change led to moderate relevant improvements 🎉 in compiler performance.

• Moderate improvement in instruction counts (up to -0.9% on incr-full builds of coercions)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[klensy]

Squashed.

[nagisa]

@bors r+

[bors]

📌 Commit c565339 has been approved by nagisa

[bors]

⌛ Testing commit c565339 with merge ae0b03b...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing ae0b03b to master...