BorrowedCursor: make init a boolean

[a1phyr]

This PR changes uninitialized bytes tracking in BorrowedBuf from being byte-wise to being buffer-wise.

I've put all the API around init a new unstable feature borrowed_buf_init, to split the part that needs it and the part that doesn't. It will avoids accidental stabilization of this part.

I'm not really convinced of the rename of advance_unchecked to advance, but I did it anyway. The old advance was kept as advance_checked.

Alternative of #148937

Cc #78485 #117693
Cc @joshtriplett

r? @Amanieu

[joshtriplett]

The old advance was kept as advance_checked.

As far as I can tell, if we keep that API in any form, that commits us to always having initialization tracking. I don't think there's any way to have an advance_checked operation if we eventually drop initialization tracking entirely.

EDIT: That's fully addressed by keeping advance_checked under the separate borrowed_buf_init, which I'm hoping we never stabilize, and eventually remove.

[joshtriplett]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 1aaa6a2 (1aaa6a2263b7828878aab80329e328a476e73beb, parent: 9044e98b66d074e7f88b1d4cea58bb0538f2eda6)

[rust-timer]

Finished benchmarking commit (1aaa6a2): comparison URL.

Overall result: ❌✅ regressions and improvements - 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

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.6%, -0.3%]	2

Max RSS (memory usage)

Results (primary -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)	-3.9%	[-4.0%, -3.9%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.9%	[-4.0%, -3.9%]	2

Cycles

Results (primary 2.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Binary size

Results (primary -0.2%, secondary -0.1%)

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%	[-0.5%, -0.0%]	6
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.2%	[-0.5%, -0.0%]	6

Bootstrap: 478.41s -> 476.787s (-0.34%)
Artifact size: 390.55 MiB -> 390.57 MiB (0.01%)