mbe: Mark expansion errors cold_path

[tgross35]

r? @ghost

[tgross35]

Alternative to #142815

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit 58e6baf with merge 32a326a…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 32a326a (32a326a9588c40e19473150adca13381519e8cf6, parent: 28f1c807911c63f08d98e7b468cfcf15a441e34b)

[rust-timer]

Finished benchmarking commit (32a326a): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -2.0%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-3.3%, -1.4%]	4
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 689.458s -> 688.974s (-0.07%)
Artifact size: 372.01 MiB -> 371.97 MiB (-0.01%)

[tgross35]

No such luck