Diagnostic attributes

The following attributes are used for controlling or generating diagnostic messages during compilation.

Lint check attributes

A lint check names a potentially undesirable coding pattern, such as unreachable code or omitted documentation. The lint attributes allow, warn, deny, and forbid use the MetaListPaths syntax to specify a list of lint names to change the lint level for the entity to which the attribute applies.

For any lint check C:

allow(C) overrides the check for C so that violations will go unreported,
warn(C) warns about violations of C but continues compilation.
deny(C) signals an error after encountering a violation of C,
forbid(C) is the same as deny(C), but also forbids changing the lint level afterwards,

Note: The lint checks supported by rustc can be found via rustc -W help, along with their default settings and are documented in the rustc book.

#![allow(unused)]
fn main() {
pub mod m1 {
    // Missing documentation is ignored here
    #[allow(missing_docs)]
    pub fn undocumented_one() -> i32 { 1 }

    // Missing documentation signals a warning here
    #[warn(missing_docs)]
    pub fn undocumented_too() -> i32 { 2 }

    // Missing documentation signals an error here
    #[deny(missing_docs)]
    pub fn undocumented_end() -> i32 { 3 }
}
}

Lint attributes can override the level specified from a previous attribute, as long as the level does not attempt to change a forbidden lint. Previous attributes are those from a higher level in the syntax tree, or from a previous attribute on the same entity as listed in left-to-right source order.

This example shows how one can use allow and warn to toggle a particular check on and off:

#![allow(unused)]
fn main() {
#[warn(missing_docs)]
pub mod m2{
    #[allow(missing_docs)]
    pub mod nested {
        // Missing documentation is ignored here
        pub fn undocumented_one() -> i32 { 1 }

        // Missing documentation signals a warning here,
        // despite the allow above.
        #[warn(missing_docs)]
        pub fn undocumented_two() -> i32 { 2 }
    }

    // Missing documentation signals a warning here
    pub fn undocumented_too() -> i32 { 3 }
}
}

This example shows how one can use forbid to disallow uses of allow for that lint check:

#![allow(unused)]
fn main() {
#[forbid(missing_docs)]
pub mod m3 {
    // Attempting to toggle warning signals an error here
    #[allow(missing_docs)]
    /// Returns 2.
    pub fn undocumented_too() -> i32 { 2 }
}
}

Note: rustc allows setting lint levels on the command-line, and also supports setting caps on the lints that are reported.

Lint groups

Lints may be organized into named groups so that the level of related lints can be adjusted together. Using a named group is equivalent to listing out the lints within that group.

#![allow(unused)]
fn main() {
// This allows all lints in the "unused" group.
#[allow(unused)]
// This overrides the "unused_must_use" lint from the "unused"
// group to deny.
#[deny(unused_must_use)]
fn example() {
    // This does not generate a warning because the "unused_variables"
    // lint is in the "unused" group.
    let x = 1;
    // This generates an error because the result is unused and
    // "unused_must_use" is marked as "deny".
    std::fs::remove_file("some_file"); // ERROR: unused `Result` that must be used
}
}

There is a special group named "warnings" which includes all lints at the "warn" level. The "warnings" group ignores attribute order and applies to all lints that would otherwise warn within the entity.

#![allow(unused)]
fn main() {
unsafe fn an_unsafe_fn() {}
// The order of these two attributes does not matter.
#[deny(warnings)]
// The unsafe_code lint is normally "allow" by default.
#[warn(unsafe_code)]
fn example_err() {
    // This is an error because the `unsafe_code` warning has
    // been lifted to "deny".
    unsafe { an_unsafe_fn() } // ERROR: usage of `unsafe` block
}
}

Tool lint attributes

Tool lints allows using scoped lints, to allow, warn, deny or forbid lints of certain tools.

Tool lints only get checked when the associated tool is active. If a lint attribute, such as allow, references a nonexistent tool lint, the compiler will not warn about the nonexistent lint until you use the tool.

Otherwise, they work just like regular lint attributes:

// set the entire `pedantic` clippy lint group to warn
#![warn(clippy::pedantic)]
// silence warnings from the `filter_map` clippy lint
#![allow(clippy::filter_map)]

fn main() {
    // ...
}

// silence the `cmp_nan` clippy lint just for this function
#[allow(clippy::cmp_nan)]
fn foo() {
    // ...
}

Note: rustc currently recognizes the tool lints for "clippy" and "rustdoc".

The `deprecated` attribute

The deprecated attribute marks an item as deprecated. rustc will issue warnings on usage of #[deprecated] items. rustdoc will show item deprecation, including the since version and note, if available.

The deprecated attribute has several forms:

deprecated — Issues a generic message.
deprecated = "message" — Includes the given string in the deprecation message.
MetaListNameValueStr syntax with two optional fields:
- since — Specifies a version number when the item was deprecated. rustc does not currently interpret the string, but external tools like Clippy may check the validity of the value.
- note — Specifies a string that should be included in the deprecation message. This is typically used to provide an explanation about the deprecation and preferred alternatives.

The deprecated attribute may be applied to any item, trait item, enum variant, struct field, external block item, or macro definition. It cannot be applied to trait implementation items. When applied to an item containing other items, such as a module or implementation, all child items inherit the deprecation attribute.

Here is an example:

#![allow(unused)]
fn main() {
#[deprecated(since = "5.2.0", note = "foo was rarely used. Users should instead use bar")]
pub fn foo() {}

pub fn bar() {}
}

The RFC contains motivations and more details.

The `must_use` attribute

The must_use attribute is used to issue a diagnostic warning when a value is not "used". It can be applied to user-defined composite types (structs, enums, and unions), functions, and traits.

The must_use attribute may include a message by using the MetaNameValueStr syntax such as #[must_use = "example message"]. The message will be given alongside the warning.

When used on user-defined composite types, if the expression of an expression statement has that type, then the unused_must_use lint is violated.

#![allow(unused)]
fn main() {
#[must_use]
struct MustUse {
    // some fields
}

impl MustUse {
  fn new() -> MustUse { MustUse {} }
}

// Violates the `unused_must_use` lint.
MustUse::new();
}

When used on a function, if the expression of an expression statement is a call expression to that function, then the unused_must_use lint is violated.

#![allow(unused)]
fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// Violates the unused_must_use lint.
five();
}

When used on a trait declaration, a call expression of an expression statement to a function that returns an impl trait or a dyn trait of that trait violates the unused_must_use lint.

#![allow(unused)]
fn main() {
#[must_use]
trait Critical {}
impl Critical for i32 {}

fn get_critical() -> impl Critical {
    4i32
}

// Violates the `unused_must_use` lint.
get_critical();
}

When used on a function in a trait declaration, then the behavior also applies when the call expression is a function from an implementation of the trait.

#![allow(unused)]
fn main() {
trait Trait {
    #[must_use]
    fn use_me(&self) -> i32;
}

impl Trait for i32 {
    fn use_me(&self) -> i32 { 0i32 }
}

// Violates the `unused_must_use` lint.
5i32.use_me();
}

When used on a function in a trait implementation, the attribute does nothing.

Note: Trivial no-op expressions containing the value will not violate the lint. Examples include wrapping the value in a type that does not implement Drop and then not using that type and being the final expression of a block expression that is not used.

#![allow(unused)]
fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// None of these violate the unused_must_use lint.
(five(),);
Some(five());
{ five() };
if true { five() } else { 0i32 };
match true {
    _ => five()
};
}

Note: It is idiomatic to use a let statement with a pattern of _ when a must-used value is purposely discarded.

#![allow(unused)]
fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// Does not violate the unused_must_use lint.
let _ = five();
}

The Rust Reference