Document read_line gotchas

[kornelski]

1. The "You do not need to clear the buffer before appending" advice is ambiguous, because it depends what you use this function for. For a rather common case of reading individual lines in a loop, it is necessary to clear the buffer.

2. The docs warn about a DoS risk. I've added a hint how to mitigate unbounded memory growth.

[rustbot]

r? @joshtriplett

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[scottmcm]

Thanks!

r? @scottmcm
@bors r+ rollup

[bors]

📌 Commit 3a6ceeb has been approved by scottmcm

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3a6ceeb with merge ee11bfd...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing ee11bfd to master...

[rust-timer]

Finished benchmarking commit (ee11bfd): comparison URL.

Overall result: ❌ regressions - 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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.4%]	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)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.5%	[1.5%, 1.5%]	1

Cycles

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