Dependency Management

Q: Does cargo update change Cargo.toml?

No, only Cargo.lock (unless you use tools that edit manifests). Bump requirements in Cargo.toml manually when adopting new major versions.

Q: How do I pin all dependencies exactly?

Rarely desirable. For apps, commit Cargo.lock. For exact pins, use = version requirements per crate.

Q: How do I allow duplicate versions temporarily?

Document in ADR, set cargo deny to warn, and schedule unification. Some proc-macro stacks temporarily need two syn versions.

Q: What licenses are common?

MIT OR Apache-2.0 dual license is standard. Run cargo deny check licenses before shipping commercial products.

Q: How do I remove a dependency?

Delete from Cargo.toml, cargo build to refresh lock, and grep codebase for imports.

Q: Can I automate updates?

Yes: Dependabot, Renovate, or scripted cargo update PRs with cargo test --workspace gate.

Q: How do advisories affect transitive deps?

You may need cargo update -p transitive or [patch] until upstream releases a fix. Track RustSec IDs in incident notes.

Healthy Rust projects pin reproducible dependency graphs, update deliberately within semver, and audit for known vulnerabilities.

Recipe

cargo update -p serde
cargo tree -i openssl
cargo audit
cargo deny check

When to reach for this: Weekly maintenance, before releases, or after a security advisory.

Working Example

Workspace maintenance workflow:

# See what would change
cargo update --dry-run
 
# Bump one crate within semver bounds in Cargo.lock
cargo update -p tokio
 
# Find why a crate is in the graph
cargo tree -i ring
 
# Security scan (install: cargo install cargo-audit)
cargo audit
 
# License + ban + duplicate detection
cargo deny check

# deny.toml (excerpt)
[bans]
multiple-versions = "warn"
 
[sources]
unknown-registry = "deny"

What this demonstrates:

cargo update refreshes Cargo.lock, not necessarily Cargo.toml version requirements
cargo tree -i finds reverse dependencies (who pulls in openssl?)
cargo audit matches lockfile against RustSec advisory DB
cargo-deny enforces license and dependency policies in CI

Deep Dive

Semver in Practice

Change	Bump
Bug fix, same API	PATCH
New backward-compatible API	MINOR
Breaking public API	MAJOR

serde = "1.0.210"   # allow compatible 1.0.x
my_lib = "=2.1.0"   # exact pin when needed

Minimizing the Graph

cargo tree --edges normal --depth 2
cargo udeps   # unused deps (nightly tool)

Remove unused features to drop transitive crates
Prefer crates with few dependencies for security surface
Workspace [workspace.dependencies] keeps versions aligned

Supply Chain Hygiene

Pin registry to crates.io (default)
Review new deps: maintenance, unsafe usage, bus factor
Enable Dependabot or cargo update automation with CI gates

Gotchas

cargo update without CI - breaks builds on fresh clones until lock committed. Fix: always commit lockfile changes with passing tests.
Ignoring duplicate versions - two syn majors inflate compile time. Fix: cargo tree -d and unify via dependency bumps or [patch].
Audit only on release - advisories land mid-sprint. Fix: weekly cargo audit job.
Yanked crate panic - old lockfiles may reference yanked versions. Fix: cargo update -p affected immediately.
Git deps without rev - floating branch breaks reproducibility. Fix: pin rev, tag, or branch with locked commit in lockfile.

Alternatives

Alternative	Use When	Don't Use When
`cargo-outdated`	See available newer versions	You need lockfile-only refresh
`[patch.crates-io]`	Emergency fork	Long-term dependency strategy
Vendor dir (`cargo vendor`)	Air-gapped builds	Normal internet-connected CI

FAQs

Does cargo update change Cargo.toml?

No, only Cargo.lock (unless you use tools that edit manifests). Bump requirements in Cargo.toml manually when adopting new major versions.

How do I pin all dependencies exactly?

Rarely desirable. For apps, commit Cargo.lock. For exact pins, use = version requirements per crate.

What is a yanked crate?

crates.io marks broken releases as yanked; new resolves avoid them but existing lockfiles may still reference until updated.

How do I allow duplicate versions temporarily?

Document in ADR, set cargo deny to warn, and schedule unification. Some proc-macro stacks temporarily need two syn versions.

Should I use git dependencies?

For unreleased patches only. Prefer crates.io releases with semver for production services.

How often should I update?

Weekly minor/patch refresh with CI; major upgrades planned with changelog review and dedicated PR.

What licenses are common?

MIT OR Apache-2.0 dual license is standard. Run cargo deny check licenses before shipping commercial products.

How do I remove a dependency?

Delete from Cargo.toml, cargo build to refresh lock, and grep codebase for imports.

Can I automate updates?

Yes: Dependabot, Renovate, or scripted cargo update PRs with cargo test --workspace gate.

How do advisories affect transitive deps?

You may need cargo update -p transitive or [patch] until upstream releases a fix. Track RustSec IDs in incident notes.

Cargo.toml Explained - version syntax
Workspaces - shared dependency tables
Supply Chain Security - broader policy
Cargo Best Practices - maintenance checklist

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

cargo update -p serde
cargo tree -i openssl
cargo audit
cargo deny check

When to reach for this: Weekly maintenance, before releases, or after a security advisory.

Working Example

Workspace maintenance workflow:

# See what would change
cargo update --dry-run
 
# Bump one crate within semver bounds in Cargo.lock
cargo update -p tokio
 
# Find why a crate is in the graph
cargo tree -i ring
 
# Security scan (install: cargo install cargo-audit)
cargo audit
 
# License + ban + duplicate detection
cargo deny check

# deny.toml (excerpt)
[bans]
multiple-versions = "warn"
 
[sources]
unknown-registry = "deny"

What this demonstrates:

cargo update refreshes Cargo.lock, not necessarily Cargo.toml version requirements
cargo tree -i finds reverse dependencies (who pulls in openssl?)
cargo audit matches lockfile against RustSec advisory DB
cargo-deny enforces license and dependency policies in CI

Deep Dive

Semver in Practice

Change	Bump
Bug fix, same API	PATCH
New backward-compatible API	MINOR
Breaking public API	MAJOR

serde = "1.0.210"   # allow compatible 1.0.x
my_lib = "=2.1.0"   # exact pin when needed

Minimizing the Graph

cargo tree --edges normal --depth 2
cargo udeps   # unused deps (nightly tool)

Remove unused features to drop transitive crates
Prefer crates with few dependencies for security surface
Workspace [workspace.dependencies] keeps versions aligned

Supply Chain Hygiene

Pin registry to crates.io (default)
Review new deps: maintenance, unsafe usage, bus factor
Enable Dependabot or cargo update automation with CI gates

Gotchas

cargo update without CI - breaks builds on fresh clones until lock committed. Fix: always commit lockfile changes with passing tests.
Ignoring duplicate versions - two syn majors inflate compile time. Fix: cargo tree -d and unify via dependency bumps or [patch].
Audit only on release - advisories land mid-sprint. Fix: weekly cargo audit job.
Yanked crate panic - old lockfiles may reference yanked versions. Fix: cargo update -p affected immediately.
Git deps without rev - floating branch breaks reproducibility. Fix: pin rev, tag, or branch with locked commit in lockfile.

Alternatives

Alternative	Use When	Don't Use When
`cargo-outdated`	See available newer versions	You need lockfile-only refresh
`[patch.crates-io]`	Emergency fork	Long-term dependency strategy
Vendor dir (`cargo vendor`)	Air-gapped builds	Normal internet-connected CI

FAQs

Does cargo update change Cargo.toml?

No, only Cargo.lock (unless you use tools that edit manifests). Bump requirements in Cargo.toml manually when adopting new major versions.

How do I pin all dependencies exactly?

Rarely desirable. For apps, commit Cargo.lock. For exact pins, use = version requirements per crate.

What is a yanked crate?

crates.io marks broken releases as yanked; new resolves avoid them but existing lockfiles may still reference until updated.

How do I allow duplicate versions temporarily?

Document in ADR, set cargo deny to warn, and schedule unification. Some proc-macro stacks temporarily need two syn versions.

Should I use git dependencies?

For unreleased patches only. Prefer crates.io releases with semver for production services.

How often should I update?

Weekly minor/patch refresh with CI; major upgrades planned with changelog review and dedicated PR.

What licenses are common?

MIT OR Apache-2.0 dual license is standard. Run cargo deny check licenses before shipping commercial products.

How do I remove a dependency?

Delete from Cargo.toml, cargo build to refresh lock, and grep codebase for imports.

Can I automate updates?

Yes: Dependabot, Renovate, or scripted cargo update PRs with cargo test --workspace gate.

How do advisories affect transitive deps?

You may need cargo update -p transitive or [patch] until upstream releases a fix. Track RustSec IDs in incident notes.

Cargo.toml Explained - version syntax
Workspaces - shared dependency tables
Supply Chain Security - broader policy
Cargo Best Practices - maintenance checklist

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Dependency Management

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Dependency Management