Profile before optimizing
A hot allocator frame often points to repeated temporary ownership rather than expensive business logic. Confirm the call path and workload first; then remove allocations at the narrowest responsible layer.
rust / LEVEL 4
The flamegraph made of tiny allocations
EST.40 MIN
01
THEORY / RETRIEVAL
What to restore
- Translate an allocation profile into a targeted code change
- Pre-size a String and append borrowed slices directly
Capacity is a performance contract, not correctness
`String::with_capacity` can avoid growth reallocations when the final byte size is cheap to compute. It must not change separators, Unicode bytes or empty-input behavior, and an allocation test should tolerate implementation details outside the measured region.
CHECKPOINT
A profile attributes most time to allocating temporary Strings inside a join loop. What is the most targeted first change?
02 / PROFILING
Read the profile and fix the cause
Rewrite `join_tags` to produce identical comma-separated output with at most one allocation for non-empty input. Do not use `join`, `format!`, or per-tag owned Strings.
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OUTPUT
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