Diagnostics - The Rust Reference

# 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_variables)]
    #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 }
    }
    #}

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

    # #![allow(unused_variables)]
    #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_variables)]
    #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 }
    }
    #}

### Tool lint attributes

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

Currently `clippy` is the only available lint tool.

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() {
        // ...
    }

## 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, or external block item. 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_variables)]
    #fn main() {
    #[deprecated(since = "5.2", 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 (`struct`s, `enum`s, and `union`s), 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_variables)]
    #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_variables)]
    #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 of that trait violates the `unused_must_use` lint.

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

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