library: optimize truncate method

[xtqqczze]

Closes #76089

Slightly better code when the elements have drop glue, see #74172 (comment).

Slightly worse code for truncate(0), see #78884 (comment). However, this is mitigated by the new clippy::manual_clear lint which is warn-by-default.

[rustbot]

r? @Mark-Simulacrum

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[ZuseZ4]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 0b2c3b8 (0b2c3b8072bcdf1845525eef84dd5e719d76118f, parent: 6368fd52cb9f230dfb156097625993e7a8891800)

[rust-timer]

Finished benchmarking commit (0b2c3b8): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

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

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (primary 2.3%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

Results (primary -0.0%, secondary -0.0%)

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)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	14
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 511.654s -> 511.381s (-0.05%)
Artifact size: 400.70 MiB -> 400.67 MiB (-0.01%)

[mati865]

The same, slightly improved assembly is generated if you remove or equal from the original condition. I.e. turnif new_len <= self.len() { into if new_len < self.len() { which is equiva to your change.

[xtqqczze]

The same, slightly improved assembly is generated if you remove or equal from the original condition. I.e. turnif new_len <= self.len() { into if new_len < self.len() { which is equiva to your change.

You’re correct that changing <= to < in the original condition yields the same improvement. I opted for the early-return style instead, for consistency and to better align with the intent expressed in the comments.