Avoid a few more allocations in write_shared.rs

[yotamofek]

Inspired by #141421 , avoids a few Vec, PathBuf and String allocations in write_shared.rs. I don't think these will show up on benchmarks, but are still worthwhile IMHO.
Also includes a few small cleanups.
r? nnethercote - if you'd like :)

[rustbot]

nnethercote is not on the review rotation at the moment.
They may take a while to respond.

[yotamofek]

Oh, maybe r? @GuillaumeGomez then?

[GuillaumeGomez]

Looks promising, thanks! Let's check perf impact.

@bors try @rust-timer queue

[bors]

⌛ Trying commit 56fe2f6 with merge 91cf0c3...

[bors]

☀️ Try build successful - checks-actions
Build commit: 91cf0c3 (91cf0c3afad8861a1760cf5bae082ee32c4375b5)

[nnethercote]

I like the cleanups in commits 1, 3, 4, and 5.

I'm ambivalent about the introduction of Cow in commits 2 and 6. Optimized code is often more complex that non-optimized code, and if the optimizations don't have measurable effects then they aren't worth it. I.e. I disagree with "I don't think these will show up on benchmarks, but are still worthwhile IMHO." I did #141421 by looking at memory profiles (from DHAT) and avoiding the hot allocation paths.

[rust-timer]

Finished benchmarking commit (91cf0c3): 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 2.7%)

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)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -2.5%)

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)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 692.01s -> 691.962s (-0.01%)
Artifact size: 372.13 MiB -> 372.12 MiB (-0.00%)

[nnethercote]

The perf CI run is neutral. r=me for commits 1, 3, 4, and 5.

[yotamofek]

@nnethercote thanks for reviewing! Rebased without those two commits.
Just wanted to say that I didn't mean that it's worthwhile to make these changes even if they don't improve perf (because like you said - they just add complexity), what I meant was that the benchmark suite doesn't necessarily cover all workloads, and that some improvements might be too minute to register, but will have a real-life impact when added up. i.e., benchmarks are never perfect. :)

[nnethercote]

Thanks!

@bors r+

[bors]

📌 Commit 2bf66e3 has been approved by nnethercote

It is now in the queue for this repository.