Fix performance regression with jemalloc

[madsmtm]

Fixes the performance regression introduced in #146627.

This effectively reverts 65f0b7a (second commit in that PR), and adds a comment explaining why extern crate is needed here instead of use (we need to load tikv_jemalloc_sys from the sysroot for some reason).

r? Kobzol

[madsmtm]

I could reproduce the performance regression locally, let's see if rustc-perf agrees:

@bors try
@rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: f30bc23 (f30bc23f4189bac695183835d28078b38cf28d74, parent: 2286e5d224b3413484cf4f398a9f078487e7b49d)

[rust-timer]

Finished benchmarking commit (f30bc23): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	1.2%	[0.2%, 1.5%]	8
Improvements ✅ (primary)	-2.1%	[-3.4%, -1.1%]	20
Improvements ✅ (secondary)	-1.4%	[-2.7%, -0.2%]	25
All ❌✅ (primary)	-1.9%	[-3.4%, 3.1%]	21

Max RSS (memory usage)

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

Cycles

Results (primary -1.7%, 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)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-2.5%	[-3.2%, -2.1%]	6
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-1.7%	[-3.2%, 2.6%]	7

Binary size

Results (primary 1.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Bootstrap: 474.605s -> 473.65s (-0.20%)
Artifact size: 388.41 MiB -> 388.64 MiB (0.06%)

[madsmtm]

@bors try
@rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[rust-bors]

☀️ Try build successful (CI)
Build commit: aef7be6 (aef7be6c425e18433bd4afc8a6ffcccfe9ffe796, parent: 7a72c5459dd58f81b0e1a0e5436d145485889375)

[rust-timer]

Queued aef7be6 with parent 7a72c54, future comparison URL.
There are currently 0 preceding artifacts in the queue.
It will probably take at least ~1.3 hours until the benchmark run finishes.