Optimize escape_string_symbol()

[nikic]

Going through escape_default() + to_string(), which involves going through formatting machinery and a flat_map of a complex iterator, ends up being quite slow. Directly pushing to a string, even with a naive implementation, is significantly faster.

This shows up in the include-blob benchmark, which mostly exercises this one function.

[nikic]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ab58777 (ab587773e58bf51cb3d98b08ae776394d0df33db)
Base parent: 9e71b3b (9e71b3bc704eea68d39bd0f6a46703c7d22f5d3b)

[rust-timer]

Finished benchmarking commit (ab58777): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-21.2%	[-33.6%, -6.0%]	13
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 4.9%, secondary -2.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.9%	[4.9%, 4.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-4.8%, -0.8%]	4
All ❌✅ (primary)	4.9%	[4.9%, 4.9%]	1

Cycles

Results (secondary -5.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.6%	[5.6%, 5.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-11.7%	[-12.2%, -11.3%]	2
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 486.168s -> 486.987s (0.17%)
Artifact size: 392.47 MiB -> 391.76 MiB (-0.18%)

[rustbot]

r? @folkertdev

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, parser
• compiler, parser expanded to 74 candidates
• Random selection from 17 candidates

[Kobzol]

Nice find! I wonder if .default_escape().collect() would be similar? 🤔

Edit: it doesn't, I tried.

[folkertdev]

Neat!

@bors r+

[rust-bors]

📌 Commit bfe8ccb has been approved by folkertdev

It is now in the queue for this repository.

[ZuseZ4]

Would it be reasonable to bump the priority a bit, given that this seems to be the last PR blocking LLVM 23? 23 gives a nice perf improvement and it would unblock some offload work? Plus, there are some autodiff CI PRs where I have a slight preference to enable them after the bump. @Kobzol

[Kobzol]

Sure, makes sense.

@bors p=1 Blocks LLVM 23 update.

[rust-bors]

⌛ Testing commit bfe8ccb with merge b803f36...

Workflow: https://github.com/rust-lang/rust/actions/runs/29896556061