cargo-toml-optimization

Cargo.toml Optimization

Tune a Rust manifest (Cargo.toml) and its build config (.cargo/config.toml) toward one goal at a time. Runtime speed, binary size, compile time, dependency hygiene: these pull against each other, so name the target before editing. Then take a baseline measurement, change a single lever, re-measure.

Measure First

Advice you haven't measured is guessing. opt-level "z" is not always smaller than "s" or 3. Fat LTO can cost minutes of link time for a percent of runtime. The real bottleneck is often one crate on the critical path rather than the whole graph.

Question	Command
What is slow to compile?	cargo build --timings -> HTML report; read the critical path
Which deps are duplicated at incompatible versions?	cargo tree -d
What makes the binary big?	cargo bloat --release --crates (install cargo-bloat)
Why is this feature / crate here?	cargo tree -e features -i <crate>

Build cache, Cargo.lock rules, build.rs cost: references/how-cargo-works.md.

Two Files, Two Jobs

• Cargo.toml lists what to build: dependencies, features, profile definitions, workspace layout.
• .cargo/config.toml governs how it gets built: linker, rustflags, sccache, target, env. It can override any profile field; that override then wins over the Cargo.toml profile.

Trap: [profile.*] and [patch] are read only from the workspace root; the same tables in a member crate are ignored with a warning, not applied. ([workspace] in a member is different: it makes that crate its own workspace root.)

Profiles: The Core of It

Most Cargo.toml tuning happens here. A profile applies to every target (lib, bin, test, bench). Per-setting detail (every value, every default) is in references/profiles.md. Start from one of these:

Balanced release (near-fat-LTO quality, ~2x faster link than full fat LTO):

[profile.release]
opt-level = 3
lto = "thin"
codegen-units = 4
panic = "abort"          # smaller + faster panic path; drop it if you need unwinding or catch_unwind
strip = "debuginfo"      # trims size, keeps symbol names for crash reports

Max runtime speed (slow build, single-threaded codegen):

[profile.release]
opt-level = 3
lto = "fat"
codegen-units = 1
panic = "abort"
strip = "symbols"

Min binary size (benchmark "z" vs "s" vs 3; size is workload-dependent):

[profile.release]
opt-level = "z"
lto = "fat"
codegen-units = 1
panic = "abort"
strip = "symbols"

Fast iteration (keep dev unoptimized, give deps light passes so generics aren't dog-slow):

[profile.dev]
opt-level = 0
debug = 1                # line tables only: faster, still usable backtraces
incremental = true

[profile.dev.package."*"]
opt-level = 1            # deps get basic opt; your crate stays fast to recompile

[profile.dev.build-override]
opt-level = 3            # build scripts + proc macros run fast (they compile slower)

To raise opt-level on a single hot dependency without touching the rest: [profile.<name>.package.<crate>].

Dependencies + Features

Detail in references/dependencies.md, references/workspaces.md. Highest-value moves:

• default-features = false on heavy deps, then add back only the features you use. Single biggest key for cutting compile time plus binary size. Audit what a crate's defaults pull in with cargo tree -e features.
• resolver = "2" (edition 2021+) or "3" (edition 2024+). Stops dev-only, build-script, or off-target features from inflating the production build. Often the biggest correctness + size win. In a virtual workspace it must be set explicitly under [workspace].
• [workspace.dependencies] pins one version of each shared crate for every member. Duplicate compiles of the same crate at different semver-compatible versions go away.
• dep:<name> and crate?/feature (weak) to keep optional deps from leaking as public feature API.
• [target.'cfg(...)'.dependencies] so OS/arch-specific crates don't compile on irrelevant targets.

Build Speed (.cargo/config.toml)

Detail in references/config.md. The levers that move the needle:

# faster linking: the single biggest incremental-build win on Linux
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=mold"]   # or rust-lld; mold is Linux-only

[build]
rustc-wrapper = "sccache"   # cache compiled artifacts across builds + CI runs (cargo install sccache)

target-cpu=native (via rustflags) squeezes out runtime speed but produces a non-portable binary. Never ship it from CI or to other machines.

Trim Dead Weight

Unused or duplicate deps, plus license/security audits, with the tools to find them: references/dep-hygiene.md. Quick hits: cargo tree -d (duplicates), cargo machete (unused), cargo deny check (audit + version bans).

Routing

The references embed the actual data (all 104 crates.io category slugs, SPDX identifiers, cfg/target values, every rustc -C flag), so the skill answers without a web lookup. The doc links are only a fallback if upstream changes. Read the one reference matching the task:

Task	Reference
[package] fields, target tables, crate-type, publish-size trimming (include/exclude)	references/manifest.md
Full crates.io category slug list (104 slugs), keyword/badge rules, publish requirements	references/crates-io-metadata.md
license / license-file: SPDX expression grammar, common identifiers, the MIT OR Apache-2.0 convention	references/licenses.md
Version syntax, sources (git/path/registry), [patch] dedupe, default-features, renaming	references/dependencies.md
cfg(...) values (target_os/target_arch/target_env/target_feature…) + common target triples for [target.*] gating	references/cfg-targets.md
Every [profile.*] setting + the full speed / size / iteration cheat-sheets	references/profiles.md
rustc -C codegen flags behind profiles + rustflags (target-cpu, target-feature, PGO, link args)	references/rustc-flags.md
.cargo/config.toml: linker, rustflags, sccache, target-cpu, registries, vendoring, aliases, env	references/config.md
Workspaces + inheritance, resolver v1/v2/v3, feature unification + dep:/weak features	references/workspaces.md
Unused / duplicate / heavy dep trimming (cargo-machete, cargo-udeps, cargo-deny)	references/dep-hygiene.md
How cargo builds, Cargo.lock vs Cargo.toml, build cache, build.rs, editions, diagnostics	references/how-cargo-works.md

Pitfalls to Check Before Declaring Victory

• target-cpu=native: non-portable, keep out of shipped artifacts.
• Features must be additive; removing one or dropping it from default is a SemVer break.
• Missing cargo::rerun-if-changed in build.rs: re-runs every build, common slow-rebuild cause.
• Two semver-incompatible versions of one crate bloat everything; cargo tree -d finds them, [patch] or [workspace.dependencies] collapses them.
• panic = "abort" breaks code relying on unwinding (catch_unwind, some test setups).

Official Docs

Canonical entry points (each reference file links its exact pages inline):

• Manifest: https://doc.rust-lang.org/cargo/reference/manifest.html
• Specifying dependencies: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html
• Profiles: https://doc.rust-lang.org/cargo/reference/profiles.html
• Configuration: https://doc.rust-lang.org/cargo/reference/config.html
• Workspaces: https://doc.rust-lang.org/cargo/reference/workspaces.html
• Resolver: https://doc.rust-lang.org/cargo/reference/resolver.html
• Features: https://doc.rust-lang.org/cargo/reference/features.html
• Cargo Book: https://doc.rust-lang.org/cargo/

Maintenance

Reference data is captured against Rust/Cargo stable (edition 2024 / resolver v3 era). Every references/*.md carries its own capture date at the top, next to the upstream URL it came from; dates differ per file as they get refreshed. To refresh one file: re-fetch its source link, diff, update the file, bump its date.