Perf: Reduce allocations while canonicaliziong external_constraints

[ShoyuVanilla]

No description provided.

[ShoyuVanilla]

@bors try @rust-timer queue

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

[rust-bors]

☀️ Try build successful (CI)
Build commit: ff00ddf (ff00ddf69bc64dc5ea1c333678e4f53343f0afe1)
Base parent: 2e071b2 (2e071b28ef7e8a066b49a179e1da753c53500c62)

[rust-timer]

Finished benchmarking commit (ff00ddf): comparison URL.

Overall result: ❌ regressions - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.3%	[0.2%, 0.6%]	19
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.7%, secondary 0.2%)

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)	4.1%	[4.1%, 4.1%]	1
Improvements ✅ (primary)	-3.7%	[-3.7%, -3.7%]	1
Improvements ✅ (secondary)	-0.8%	[-0.9%, -0.6%]	4
All ❌✅ (primary)	-3.7%	[-3.7%, -3.7%]	1

Cycles

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

Binary size

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

Bootstrap: 475.013s -> 474.082s (-0.20%)
Artifact size: 402.89 MiB -> 402.87 MiB (-0.00%)