rs-format-rebind-hot — format! rebind accumulation in loop

ClaimCopy Anchor Link

Hot-loop s = format!("{s}…") / s = format!("{}{}", s, …) accumulation on Rust is substantially slower than String::with_capacity + write! / push_str, because each format! allocates a fresh String and copies the growing prefix (quadratic-ish).

How measuredCopy Anchor Link

Same harness and run as the former rs-string-plus-in-loop format! pair (not re-invented):

• Harness: packages/rustperf/examples/bench/evidence_rs002.rs (Pair C)
• Build: rustc -C opt-level=3 evidence_rs002.rs -o out/evidence_rs002
• Timer: std::time::Instant wall clock; warm=3; median of 9; std::hint::black_box
• Pair C (matches fixtures rustperf-lint/fixtures/bad/rs-format-rebind-hot_format_rebind.rs / good rs-format-rebind-hot_write.rs): N=20000
  • before: s = format!("{s}{i}") in loop
  • after: with_capacity + write!(s, "{i}")
• Raw lines: rs-format-rebind-hot.jsonl — aliased from the 2026-10-22 rs-string-plus-in-loop G4 JSONL (rule field set to rs-format-rebind-hot; aliased_from=rs-string-plus-in-loop; same median_ms / ops / ts)

Runtime (this evidence file)Copy Anchor Link

Direction: format! rebind accumulation is a large, stable win for the flagged smell. Absolute ms and ratios are N/machine/opt-sensitive.

Lint mappingCopy Anchor Link

rustperf-lint rs-format-rebind-hot fires only when a loop line has ident = format!(…) (not let ident = format!(…)) and the format body/args reuse ident ({ident} or positional token). Bare format!(…) exprs and fresh let-bindings stay quiet. Default severity error under evidence-policy: the flagged smell itself is ~99×. Path skips mirror rs-string-plus-in-loop (shells / cold CLI formatters). Amortized += / push_str stays on rs-string-plus-in-loop (hint).

LimitsCopy Anchor Link

• Microbench; not an app profile.
• Line heuristic can still hit one-shot wraps that reuse the LHS once per outer iteration (see G3 residual note) — override with --severity rs-format-rebind-hot=off / hint if needed.
• Does not measure Write trait objects, itoa, or SSO.
• Historical note: written before r4 redesign; kit is now corpus-ready (see docs/corpus.md).