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_minimum_version_req

Warn-by-default

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

• blanket_hint_mostly_unused
• unknown_lints

blanket_hint_mostly_unused

Set to warn by default

What it does

Checks if hint-mostly-unused being applied to all dependencies.

Why it is bad

hint-mostly-unused indicates that most of a crate’s API surface will go unused by anything depending on it; this hint can speed up the build by attempting to minimize compilation time for items that aren’t used at all. Misapplication to crates that don’t fit that criteria will slow down the build rather than speeding it up. It should be selectively applied to dependencies that meet these criteria. Applying it globally is always a misapplication and will likely slow down the build.

Example

[profile.dev.package."*"]
hint-mostly-unused = true

Should instead be:

[profile.dev.package.huge-mostly-unused-dependency]
hint-mostly-unused = true

implicit_minimum_version_req

Set to allow by default

What it does

Checks for dependency version requirements that do not explicitly specify a full major.minor.patch version requirement, such as serde = "1" or serde = "1.0".

This lint currently only applies to caret requirements (the default requirements).

Why it is bad

Version requirements without an explicit full version can be misleading about the actual minimum supported version. For example, serde = "1" has an implicit minimum bound of 1.0.0. If your code actually requires features from 1.0.219, the implicit minimum bound of 1.0.0 gives a false impression about compatibility.

Specifying the full version helps with:

• Accurate minimum version documentation
• Better compatibility with -Z minimal-versions
• Clearer dependency constraints for consumers

Drawbacks

Even with a fully specified version, the minimum bound might still be incorrect if untested. This lint helps make the minimum version requirement explicit but doesn’t guarantee correctness.

Example

[dependencies]
serde = "1"

Should be written as a full specific version:

[dependencies]
serde = "1.0.219"

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"