Lints - The Cargo Book

## Keyboard shortcuts

Press `←` or `→` to navigate between chapters

Press `S` or `/` to search in the book

Press `?` to show this help

Press `Esc` to hide this help

# Lints

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

Group | Description | Default level
--- | --- | ---
`cargo::complexity` | code that does something simple but in a complex way | warn
`cargo::correctness` | code that is outright wrong or useless | deny
`cargo::nursery` | new lints that are still under development | allow
`cargo::pedantic` | lints which are rather strict or have occasional false positives | allow
`cargo::perf` | code that can be written to run faster | warn
`cargo::restriction` | lints which prevent the use of Cargo features | allow
`cargo::style` | code that should be written in a more idiomatic way | warn
`cargo::suspicious` | code that is most likely wrong or useless | warn

## 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`

Group: `suspicious`

Level: `warn`

### 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`

Group: `pedantic`

Level: `allow`

### 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`

Group: `suspicious`

Level: `warn`

### 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"