Lints

Note: Cargo’s linting system is unstable and can only be used on nightly toolchains

Allowed-by-default

These lints are all set to the ‘allow’ level by default.

implicit_features

Warn-by-default

These lints are all set to the ‘warn’ level by default.

unknown_lints
unused_optional_dependency

`implicit_features`

Set to allow by default

What it does

Checks for implicit features for optional dependencies

By default, cargo will treat any optional dependency as a feature. As of cargo 1.60, these can be disabled by declaring a feature that activates the optional dependency as dep:<name> (see RFC #3143).

In the 2024 edition, cargo will stop exposing optional dependencies as features implicitly, requiring users to add foo = ["dep:foo"] if they still want it exposed.

For more information, see RFC #3491

Example

edition = "2021"

[dependencies]
bar = { version = "0.1.0", optional = true }

[features]
# No explicit feature activation for `bar`

Instead, the dependency should have an explicit feature:

edition = "2021"

[dependencies]
bar = { version = "0.1.0", optional = true }

[features]
bar = ["dep:bar"]

`unknown_lints`

Set to warn by default

What it does

Checks for unknown lints in the [lints.cargo] table

Why it is bad

The lint name could be misspelled, leading to confusion as to why it is not working as expected
The unknown lint could end up causing an error if cargo decides to make a lint with the same name in the future

Example

[lints.cargo]
this-lint-does-not-exist = "warn"

`unused_optional_dependency`

Set to warn by default

What it does

Checks for optional dependencies that are not activated by any feature

Why it is bad

Starting in the 2024 edition, cargo no longer implicitly creates features for optional dependencies (see RFC #3491). This means that any optional dependency not specified with "dep:<name>" in some feature is now unused. This change may be surprising to users who have been using the implicit features cargo has been creating for optional dependencies.

Example

edition = "2024"

[dependencies]
bar = { version = "0.1.0", optional = true }

[features]
# No explicit feature activation for `bar`

Instead, the dependency should be removed or activated in a feature:

edition = "2024"

[dependencies]
bar = { version = "0.1.0", optional = true }

[features]
bar = ["dep:bar"]

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