Update document for std::io::Error::downcast

[NobodyXu]

Resolve concern raised by @BurntSushi #99262 (comment)

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
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

[NobodyXu]

r? @BurntSushi

[BurntSushi]

@bors r+

[bors]

📌 Commit 05366ee has been approved by BurntSushi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 05366ee with merge 84e729a...

[bors]

☀️ Test successful - checks-actions
Approved by: BurntSushi
Pushing 84e729a to master...

[rust-timer]

Finished benchmarking commit (84e729a): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	1

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	4.7%	[4.7%, 4.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.7%	[4.7%, 4.7%]	1

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: 678.734s -> 677.082s (-0.24%)
Artifact size: 316.01 MiB -> 316.00 MiB (-0.00%)