| name | cargo-toml-optimization |
| description | Tunes a Rust `Cargo.toml` or `.cargo/config.toml`: profiles, features, workspaces, dependency hygiene. Use when cutting compile time or binary size, trimming deps, or configuring a workspace. |
| metadata | {"author":"uwuclxdy","version":"1.6"} |
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"
strip = "debuginfo"
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
incremental = true
[profile.dev.package."*"]
opt-level = 1
[profile.dev.build-override]
opt-level = 3
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:
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=mold"]
[build]
rustc-wrapper = "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).[paths] override (.cargo/config.toml) only works for crates already on crates.io; it cannot add a new dependency, change graph structure, or point at an unpublished internal crate. Use a normal path = "..." dependency in Cargo.toml or [patch] instead.{ workspace = true } dependency inheritance: a member can only add optional/extra features; it cannot override the workspace entry's version, default-features, git, path, or registry.
Official Docs
Canonical entry points (each reference file links its exact pages inline):
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.
