[PERF] directly manage Buffer memory without using Vec

[cyrgani]

Extracted from #157271. This seemed to affect some crates positively and others negatively.

[Kobzol]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: f1dae0f (f1dae0fe573f0fab65559646850dab268f90cb6f, parent: 688ab44c7439a226d792990f81cd8df0603747d8)

[rust-timer]

Finished benchmarking commit (f1dae0f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.1%, 0.9%]	26
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.7%	[-1.5%, -0.2%]	11
All ❌✅ (primary)	0.3%	[-0.2%, 0.9%]	27

Max RSS (memory usage)

Results (primary 1.3%, secondary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.4%	[4.4%, 4.4%]	1
Regressions ❌ (secondary)	1.9%	[1.7%, 2.2%]	2
Improvements ✅ (primary)	-1.7%	[-1.7%, -1.7%]	1
Improvements ✅ (secondary)	-2.4%	[-2.5%, -2.2%]	2
All ❌✅ (primary)	1.3%	[-1.7%, 4.4%]	2

Cycles

Results (secondary -3.9%)

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)	-3.9%	[-5.5%, -1.9%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.2%, secondary -0.5%)

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)	-0.2%	[-0.2%, -0.1%]	8
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	3
All ❌✅ (primary)	-0.2%	[-0.2%, -0.1%]	8

Bootstrap: 515.112s -> 515.793s (0.13%)
Artifact size: 400.60 MiB -> 400.65 MiB (0.01%)

[rust-bors]

☔ The latest upstream changes (presumably #158342) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.