Lint Levels

In rustc, lints are divided into six levels:

allow
expect
warn
force-warn
deny
forbid

Each lint has a default level (explained in the lint listing later in this chapter), and the compiler has a default warning level. First, let's explain what these levels mean, and then we'll talk about configuration.

allow

These lints exist, but by default, do nothing. For example, consider this source:

pub fn foo() {}

Compiling this file produces no warnings:

$ rustc lib.rs --crate-type=lib
$

But this code violates the missing_docs lint.

These lints exist mostly to be manually turned on via configuration, as we'll talk about later in this section.

expect

Sometimes, it can be helpful to suppress lints, but at the same time ensure that the code in question still emits them. The 'expect' level does exactly this. If the lint in question is not emitted, the unfulfilled_lint_expectation lint triggers on the expect attribute, notifying you that the expectation is no longer fulfilled.

fn main() {
    #[expect(unused_variables)]
    let unused = "Everyone ignores me";

    #[expect(unused_variables)] // `unused_variables` lint is not emitted
    let used = "I'm useful";    // the expectation is therefore unfulfilled
    println!("The `used` value is equal to: {:?}", used);
}

This will produce the following warning:

warning: this lint expectation is unfulfilled
 --> src/main.rs:7:14
  |
7 |     #[expect(unused_variables)]
  |              ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

This level can only be defined via the #[expect] attribute, there is no equivalent flag. Lints with the special 'force-warn' level will still be emitted as usual.

warn

The 'warn' lint level will produce a warning if you violate the lint. For example, this code runs afoul of the unused_variables lint:

pub fn foo() {
    let x = 5;
}

This will produce this warning:

$ rustc lib.rs --crate-type=lib
warning: unused variable: `x`
 --> lib.rs:2:9
  |
2 |     let x = 5;
  |         ^
  |
  = note: `#[warn(unused_variables)]` on by default
  = note: to avoid this warning, consider using `_x` instead

force-warn

'force-warn' is a special lint level. It's the same as 'warn' in that a lint at this level will produce a warning, but unlike the 'warn' level, the 'force-warn' level cannot be overridden. If a lint is set to 'force-warn', it is guaranteed to warn: no more, no less. This is true even if the overall lint level is capped via cap-lints.

deny

A 'deny' lint produces an error if you violate it. For example, this code runs into the exceeding_bitshifts lint.

fn main() {
    100u8 << 10;
}

$ rustc main.rs
error: bitshift exceeds the type's number of bits
 --> main.rs:2:13
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^
  |
  = note: `#[deny(exceeding_bitshifts)]` on by default

What's the difference between an error from a lint and a regular old error? Lints are configurable via levels, so in a similar way to 'allow' lints, warnings that are 'deny' by default let you allow them. Similarly, you may wish to set up a lint that is warn by default to produce an error instead. This lint level gives you that.

forbid

'forbid' is a special lint level that fills the same role for 'deny' that 'force-warn' does for 'warn'. It's the same as 'deny' in that a lint at this level will produce an error, but unlike the 'deny' level, the 'forbid' level can not be overridden to be anything lower than an error. However, lint levels may still be capped with --cap-lints (see below) so rustc --cap-lints warn will make lints set to 'forbid' just warn.

Configuring warning levels

Remember our missing_docs example from the 'allow' lint level?

$ cat lib.rs
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
$

We can configure this lint to operate at a higher level, both with compiler flags, as well as with an attribute in the source code.

You can also "cap" lints so that the compiler can choose to ignore certain lint levels. We'll talk about that last.

Via compiler flag

The -A, -W, --force-warn -D, and -F flags let you turn one or more lints into allowed, warning, force-warn, deny, or forbid levels, like this:

$ rustc lib.rs --crate-type=lib -W missing-docs
warning: missing documentation for crate
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^
  |
  = note: requested on the command line with `-W missing-docs`

warning: missing documentation for a function
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^

$ rustc lib.rs --crate-type=lib -D missing-docs
error: missing documentation for crate
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^
  |
  = note: requested on the command line with `-D missing-docs`

error: missing documentation for a function
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^

error: aborting due to 2 previous errors

You can also pass each flag more than once for changing multiple lints:

$ rustc lib.rs --crate-type=lib -D missing-docs -D unused-variables

And of course, you can mix these five flags together:

$ rustc lib.rs --crate-type=lib -D missing-docs -A unused-variables

The order of these command line arguments is taken into account. The following allows the unused-variables lint, because it is the last argument for that lint:

$ rustc lib.rs --crate-type=lib -D unused-variables -A unused-variables

You can make use of this behavior by overriding the level of one specific lint out of a group of lints. The following example denies all the lints in the unused group, but explicitly allows the unused-variables lint in that group (forbid still trumps everything regardless of ordering):

$ rustc lib.rs --crate-type=lib -D unused -A unused-variables

Since force-warn and forbid cannot be overridden, setting one of them will prevent any later level for the same lint from taking effect.

Via an attribute

You can also modify the lint level with a crate-wide attribute:

$ cat lib.rs
#![warn(missing_docs)]

pub fn foo() {}
$ rustc lib.rs --crate-type=lib
warning: missing documentation for crate
 --> lib.rs:1:1
  |
1 | / #![warn(missing_docs)]
2 | |
3 | | pub fn foo() {}
  | |_______________^
  |
note: lint level defined here
 --> lib.rs:1:9
  |
1 | #![warn(missing_docs)]
  |         ^^^^^^^^^^^^

warning: missing documentation for a function
 --> lib.rs:3:1
  |
3 | pub fn foo() {}
  | ^^^^^^^^^^^^

warn, allow, deny, and forbid all work this way. There is no way to set a lint to force-warn using an attribute.

You can also pass in multiple lints per attribute:

#![warn(missing_docs, unused_variables)]

pub fn foo() {}

And use multiple attributes together:

#![warn(missing_docs)]
#![deny(unused_variables)]

pub fn foo() {}

All lint attributes support an additional reason parameter, to give context why a certain attribute was added. This reason will be displayed as part of the lint message, if the lint is emitted at the defined level.

use std::path::PathBuf;
pub fn get_path() -> PathBuf {
    #[allow(unused_mut, reason = "this is only modified on some platforms")]
    let mut file_name = PathBuf::from("git");
    #[cfg(target_os = "windows")]
    file_name.set_extension("exe");
    file_name
}

Capping lints

rustc supports a flag, --cap-lints LEVEL that sets the "lint cap level." This is the maximum level for all lints. So for example, if we take our code sample from the "deny" lint level above:

fn main() {
    100u8 << 10;
}

And we compile it, capping lints to warn:

$ rustc lib.rs --cap-lints warn
warning: bitshift exceeds the type's number of bits
 --> lib.rs:2:5
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^
  |
  = note: `#[warn(exceeding_bitshifts)]` on by default

warning: this expression will panic at run-time
 --> lib.rs:2:5
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^ attempt to shift left with overflow

It now only warns, rather than errors. We can go further and allow all lints:

$ rustc lib.rs --cap-lints allow
$

This feature is used heavily by Cargo; it will pass --cap-lints allow when compiling your dependencies, so that if they have any warnings, they do not pollute the output of your build.

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