Avoid short writes in LineWriter

[ChrisDenton]

If the bytes written to LineWriter contains at least one new line but doesn't end in a new line (e.g. "abc\ndef") then we:

• write up to the last new line direct to the underlying Writer.
• copy as many of the remaining bytes as will fit into our internal buffer.

That last step is inefficient if the remaining bytes are larger than our buffer. It will needlessly split the bytes in two, requiring at least two writes to the underlying Writer (one to flush the buffer, one more to write the rest). This PR skips the extra buffering if the remaining bytes are larger than the buffer.

[rustbot]

r? @tgross35

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[tgross35]

Sorry this took so long but looks reasonable!

@bors r+

[bors]

📌 Commit fdb43ef has been approved by tgross35

It is now in the queue for this repository.

[bors]

⌛ Testing commit fdb43ef with merge b5f4319...

[rust-log-analyzer]

The job i686-msvc failed! Check out the build log: (web) (plain)

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

---- [run-make] tests\run-make\extern-flag-pathless stdout ----

error: rmake recipe failed to complete
status: exit code: 1
command: "C:\\a\\rust\\rust\\build\\i686-pc-windows-msvc\\test\\run-make\\extern-flag-pathless\\rmake.exe"
--- stderr -------------------------------
command failed at line 14
command failed at line 14
Command { cmd: "C:\\a\\rust\\rust\\build\\i686-pc-windows-msvc\\stage2\\bin\\rustc.exe" "-L" "C:\\a\\rust\\rust\\build\\i686-pc-windows-msvc\\test\\run-make\\extern-flag-pathless\\rmake_out" "bar.rs" "--crate-type" "rlib" "--crate-type" "dylib" "-Cprefer-dynamic", stdin_buf: None, stdin: None, stdout: None, stderr: None, drop_bomb: DropBomb { command: "C:\\a\\rust\\rust\\build\\i686-pc-windows-msvc\\stage2\\bin\\rustc.exe", defused: true, armed_location: Location { file: "C:\\a\\rust\\rust\\tests\\run-make\\extern-flag-pathless\\rmake.rs", line: 14, col: 5 } }, already_executed: true }
output status: `exit code: 1`
=== STDOUT ===


=== STDERR ===
=== STDERR ===
error: couldn't create a temp dir: Access is denied. (os error 5) at path "C:\\a\\_temp\\msys64\\tmp\\rustck4OIAb"
error: aborting due to 1 previous error
------------------------------------------


---
test result: FAILED. 283 passed; 1 failed; 98 ignored; 0 measured; 0 filtered out; finished in 110.57s

Some tests failed in compiletest suite=run-make mode=run-make host=i686-pc-windows-msvc target=i686-pc-windows-msvc
Build completed unsuccessfully in 1:42:36
make: *** [Makefile:106: ci-msvc-ps1] Error 1
  network time: Tue, 31 Dec 2024 10:34:07 GMT
##[error]Process completed with exit code 2.
Post job cleanup.
[command]"C:\Program Files\Git\bin\git.exe" version

[bors]

💔 Test failed - checks-actions

[ChrisDenton]

@bors retry spurious i686-msvc couldn't create a temp dir: Access is denied.

[jieyouxu]

(Actually this one is i686-msvc)

[ChrisDenton]

Edited for future reference. Too many platforms.

[bors]

⌛ Testing commit fdb43ef with merge 7a0cde9...

[bors]

☀️ Test successful - checks-actions
Approved by: tgross35
Pushing 7a0cde9 to master...

[rust-timer]

Finished benchmarking commit (7a0cde9): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -0.2%, secondary 3.5%)

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)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-2.8%, 2.5%]	2

Cycles

Results (secondary 4.8%)

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)	-	-	0
Regressions ❌ (secondary)	4.8%	[4.0%, 6.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 760.593s -> 759.631s (-0.13%)
Artifact size: 325.49 MiB -> 325.60 MiB (0.03%)