rs-vec-no-capacity — Vec::new + push vs with_capacity
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Claim
On a large known-size fill (N=500000, rustc -C opt-level=3), Vec::new() + push is already amortized by geometric growth and measures ~1.0× vs with_capacity (noise). On repeated small builders (N=100 × 10000 reps) with_capacity wins ~2.07×. Useful as a hint for tight inner builders; not an escalate-to-error case.
How measured
- Harness:
packages/rustperf/examples/bench/evidence_rs003_rs005_rs006.rs - Build:
rustc -C opt-level=3 - Pair A: N=500000 items copy
- Pair B: N=100 × 10000 outer reps
- Raw:
rs-vec-no-capacity.jsonl
Runtime (this evidence file)
| Field | Value |
|---|---|
| When | 2026-10-22 Asia/Shanghai |
| rustc | 1.85.1 / opt-level=3 |
Median before (vec_new_push) | 0.544 ms |
Median after (with_capacity) | 0.545 ms |
| ratio hot large | ~1.00× |
Median before (small_vec_new_push) | 2.044 ms |
Median after (small_with_capacity) | 0.989 ms |
| ratio small builders | ~2.07× |
Lint mapping
Default stays hint. Escalate recommended: no.
Limits
- Microbench; capacity-only win is modest once growth amortizes.
Landed 2026-10-22
Scope note: wins show for many small builders or very large realloc spikes; a single large fill is already amortized by geometric growth. Keep hint; do not escalate.
Memory (2026-10-22)
Counting allocator live bytes N=500k u32: Vec::new 2097152 vs with_capacity 2000000 (1.049×). rs-vec-no-capacity.mem.jsonl.