Remove allocation from mbe::quoted::parse

[Kobzol]

Something I noticed while working on #159378. Probably won't help much, but let's try.

[rustbot]

r? @TaKO8Ki

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[Kobzol]

@bors try @rust-timer queue

[rust-log-analyzer]

The job test-tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 7269 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [alphabetical (compiler)]: /checkout/compiler/rustc_expand/src/lib.rs:6: line not in alphabetical order (tip: use --bless to sort this list)
tidy [alphabetical (compiler)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.12' and 'venv'
Requirement already satisfied: pip in ./build/venv/lib/python3.12/site-packages (24.0)
Collecting pip
  Downloading pip-26.2.1-py3-none-any.whl.metadata (4.6 kB)
---
typechecking javascript files
tidy: The following check failed: alphabetical (compiler)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Currently active steps:
test::Tidy {  } at src/bootstrap/src/core/build_steps/test.rs:1665
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1627:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1747:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:42
  local time: Mon Sep 14 18:33:54 UTC 2026
  network time: Mon, 14 Sep 2026 18:33:54 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[rust-bors]

☀️ Try build successful (CI)
Build commit: a34f031 (a34f03154bd5ade55e0655a23f7bfe3972a209c8)
Base parent: a8a1e6f (a8a1e6fd9df2e094d6f09c0d57991508680acc1c)

[rust-timer]

Finished benchmarking commit (a34f031): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.2%	[0.1%, 0.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.2%, secondary -1.3%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	3.1%	[1.4%, 7.8%]	6
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	2
Improvements ✅ (secondary)	-3.9%	[-7.2%, -2.1%]	10
All ❌✅ (primary)	-1.2%	[-2.2%, 1.0%]	3

Cycles

Results (secondary 3.0%)

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)	6.2%	[3.1%, 10.5%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.2%	[-8.2%, -8.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 495.556s -> 493.972s (-0.32%)
Artifact size: 406.86 MiB -> 406.92 MiB (0.01%)

[Kobzol]

Nevermind.