Rust Performance

Rust Performance

No change lands here that no measurement named: the profile is the entry ticket, and the codegen flags are the last five percent, not the first move. Async throughput is /async-rust; unsafe for speed is /unsafe-rust.

Measure, then change

No optimisation lands without a measurement that named it. Skipping it costs twice: the change that made nothing faster, and the change that made the code worse in exchange for nothing. A profile finds the one function that holds the time — almost never the one that looked expensive while reading.

Benchmarks that mean something

criterion for a statistical comparison against a stored baseline; divan when a lighter harness is enough. Benchmark the operation, not the setup around it.

Without it, the profiler shows addresses instead of function names:

[profile.bench]
debug = 1

A benchmark run on a laptop under thermal throttling produces a number, just not a comparable one — the machine has to be quiet between the runs being compared.

Allocation is usually the answer

The highest-yield category, and the one a profile names most often:

• Vec::with_capacity when the size is known — the default grows and copies.
• Clear-and-reuse over allocate-per-iteration: buffer outside, clear() inside.
• Box<[T]> and Box<str> for an owned sequence that will never grow.
• mem::take to move a value out of a &mut without cloning.
• No format! in a hot loop — every call allocates a fresh String.
• Cow for the borrow-or-own case — copy only when the value changed.

One complete example, joining with a known total size:

fn joined(parts: &[String]) -> String {
    let total = parts.iter().map(|p| p.len() + 1).sum();
    let mut out = String::with_capacity(total);
    for (i, part) in parts.iter().enumerate() {
        if i > 0 {
            out.push(' ');
        }
        out.push_str(part);
    }
    out
}

The reuse patterns and the buffer-passing shapes are in ALLOCATION.md.

Pick the collection for the access pattern

Vec beats HashMap for small n because of cache behaviour, not asymptotics. BTreeMap when iteration order matters. A sorted Vec plus binary_search when built once and read many times. The crossover is a thing to measure, not a constant to memorise.

Hashing

The default hasher is DoS-resistant and correspondingly slow. Swapping it is safe and worthwhile only when the keys are not attacker-controlled — internal IDs, enum discriminants, interned strings. With attacker-controlled keys it is a denial-of-service surface, and no lookup speed buys that back. Say the risk plainly rather than presenting the swap as a free win.

Iterators and bounds checks

An iterator chain gives the optimiser the length invariant an index loop hides, which is how bounds checks disappear: two [i] reads in one loop become iter().zip(), and the optimiser can prove the accesses in range. This is the one place where the idiomatic form is also the fast form — no tradeoff to argue about.

The last five percent

LTO, codegen-units = 1, PGO, target-cpu. Per ADR 0007: application decisions, never library defaults, each with the portability cost stated alongside the gain — a knob is one line and a profile is an afternoon:

• target-cpu=native in committed config produces a binary that faults on an older machine.
• PGO needs a representative workload, or it optimises for the wrong path.
• LTO and codegen-units = 1 buy a percentage, not a factor, at the cost of build time.

panic = "abort" is not a performance default: it is an application-level choice, a library cannot assume unwinding, and it changes behaviour under #[should_panic].

Deferrals

Async throughput, executor stalls, and spawn_blocking are /async-rust — the runtime question, not the throughput one. Reaching for unsafe to skip a check is /unsafe-rust, and the answer there is that the block needs a soundness argument, not a benchmark. Whether the clone the profile found should exist at all is /ownership-not-clone.

Verification

cargo bench                  # against the recorded baseline where one exists
cargo clippy --all-targets   # at the level the repo configures; add -- -D warnings only if it configures none
cargo test --release

A benchmark with no stored baseline proves nothing about a change — record the before, then measure the after, and compare against the noise the benchmark reports. Clippy keeps the lint level the target repo configured; -D warnings is the fallback for a repo that configures no lints at all.