Introduce RawVec::reserve_for_push.

[nnethercote]

If Vec::push's capacity check fails it calls RawVec::reserve, which
then also does a capacity check.

This commit introduces reserve_for_push which skips the redundant
capacity check, for some slight compile time speed-ups.

I tried lots of minor variations on this, e.g. different inlining
attributes. This was the best one I could find.

r? @ghost

[nnethercote]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 0b2cf79825ef392a8afb6f97c4520ed96d839f9b with merge 7ab15d61caf7de62a33d9209692eefb792143ae8...

[bors]

☀️ Try build successful - checks-actions
Build commit: 7ab15d61caf7de62a33d9209692eefb792143ae8 (7ab15d61caf7de62a33d9209692eefb792143ae8)

[rust-timer]

Queued 7ab15d61caf7de62a33d9209692eefb792143ae8 with parent 44723c5, future comparison URL.

[rust-timer]

Finished benchmarking commit (7ab15d61caf7de62a33d9209692eefb792143ae8): comparison url.

Summary: This change led to large relevant mixed results 🤷 in compiler performance.

• Large improvement in instruction counts (up to -3.0% on incr-patched: println builds of webrender-wrench)
• Small regression in instruction counts (up to 0.4% on incr-patched: dummy fn builds of unify-linearly)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

[nnethercote]

Perf results look good, and the instruction count improvements are mostly among the real-world benchmarks, which is nice.

r? @dtolnay

[dtolnay]

Awesome!

[dtolnay]

@bors r+

[bors]

📌 Commit 6a83352 has been approved by dtolnay

[bors]

⌛ Testing commit 6a83352 with merge 207c80f...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 207c80f to master...

[rust-timer]

Finished benchmarking commit (207c80f): comparison url.

Summary: This change led to large relevant mixed results 🤷 in compiler performance.

• Large improvement in instruction counts (up to -3.0% on incr-patched: println builds of webrender-wrench)
• Small regression in instruction counts (up to 0.4% on incr-patched: println builds of style-servo)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

[nnethercote]

@rustbot label: +perf-regression-triaged

Because (a) the instruction counts improvement significantly outweigh the regressions in size, and (b) the improvements are all in real-world benchmarks, while the regressions are mostly in artificial benchmarks.