never - Rust
Description: API documentation for the Rust `never` primitive in crate `std`.
Keywords: rust, rustlang, rust-lang, never

# Primitive Type never [−]

🔬 This is a nightly-only experimental API. (`never_type` #35121)

The `!` type, also called "never".

`!` represents the type of computations which never resolve to any value at all. For example, the `exit` function `fn exit(code: i32) -> !` exits the process without ever returning, and so returns `!`.

`break`, `continue` and `return` expressions also have type `!`. For example we are allowed to write:

    #![feature(never_type)]
    let x: ! = {
        return 123
    };Run

Although the `let` is pointless here, it illustrates the meaning of `!`. Since `x` is never assigned a value (because `return` returns from the entire function), `x` can be given type `!`. We could also replace `return 123` with a `panic!` or a never-ending `loop` and this code would still be valid.

A more realistic usage of `!` is in this code:

    let num: u32 = match get_a_number() {
        Some(num) => num,
        None => break,
    };Run

Both match arms must produce values of type `u32`, but since `break` never produces a value at all we know it can never produce a value which isn't a `u32`. This illustrates another behaviour of the `!` type - expressions with type `!` will coerce into any other type.

# `!` and generics

The main place you'll see `!` used explicitly is in generic code. Consider the `FromStr` trait:

    trait FromStr: Sized {
        type Err;
        fn from_str(s: &str) -> Result<Self, Self::Err>;
    }Run

When implementing this trait for `String` we need to pick a type for `Err`. And since converting a string into a string will never result in an error, the appropriate type is `!`. (Currently the type actually used is an enum with no variants, though this is only because `!` was added to Rust at a later date and it may change in the future). With an `Err` type of `!`, if we have to call `String::from_str` for some reason the result will be a `Result<String, !>` which we can unpack like this:

ⓘThis example is not tested

    // NOTE: This does not work today!
    let Ok(s) = String::from_str("hello");Run

Since the `Err` variant contains a `!`, it can never occur. So we can exhaustively match on `Result<T, !>` by just taking the `Ok` variant. This illustrates another behaviour of `!` - it can be used to "delete" certain enum variants from generic types like `Result`.

# `!` and traits

When writing your own traits, `!` should have an `impl` whenever there is an obvious `impl` which doesn't `panic!`. As is turns out, most traits can have an `impl` for `!`. Take `Debug` for example:

    impl Debug for ! {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            *self
        }
    }Run

Once again we're using `!`'s ability to coerce into any other type, in this case `fmt::Result`. Since this method takes a `&!` as an argument we know that it can never be called (because there is no value of type `!` for it to be called with). Writing `*self` essentially tells the compiler "We know that this code can never be run, so just treat the entire function body has having type `fmt::Result`". This pattern can be used a lot when implementing traits for `!`. Generally, any trait which only has methods which take a `self` parameter should have such as impl.

On the other hand, one trait which would not be appropriate to implement is `Default`:

    trait Default {
        fn default() -> Self;
    }Run

Since `!` has no values, it has no default value either. It's true that we could write an `impl` for this which simply panics, but the same is true for any type (we could `impl Default` for (eg.) `File` by just making `default()` panic.)

## Trait Implementations

### `impl PartialOrd<!> for !`
[src]

#### `fn partial_cmp(&self, &!) -> Option<Ordering>`
[src]

This method returns an ordering between `self` and `other` values if one exists. Read more

#### `fn lt(&self, other: &Rhs) -> bool`
1.0.0
[src]

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

#### `fn le(&self, other: &Rhs) -> bool`
1.0.0
[src]

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more

#### `fn gt(&self, other: &Rhs) -> bool`
1.0.0
[src]

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more

#### `fn ge(&self, other: &Rhs) -> bool`
1.0.0
[src]

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

### `impl Eq for !`
[src]

### `impl Display for !`
[src]

#### `fn fmt(&self, &mut Formatter) -> Result<(), Error>`
[src]

Formats the value using the given formatter. Read more

### `impl Ord for !`
[src]

#### `fn cmp(&self, &!) -> Ordering`
[src]

This method returns an `Ordering` between `self` and `other`. Read more

#### `fn max(self, other: Self) -> Self`
1.21.0
[src]

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`
1.21.0
[src]

Compares and returns the minimum of two values. Read more

### `impl PartialEq<!> for !`
[src]

#### `fn eq(&self, &!) -> bool`
[src]

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `fn ne(&self, other: &Rhs) -> bool`
1.0.0
[src]

This method tests for `!=`.

### `impl Debug for !`
[src]

#### `fn fmt(&self, &mut Formatter) -> Result<(), Error>`
[src]

Formats the value using the given formatter. Read more

### `impl Error for !`
[src]

#### `fn description(&self) -> &str`
[src]

A short description of the error. Read more

#### `fn cause(&self) -> Option<&Error>`
1.0.0
[src]

The lower-level cause of this error, if any. Read more

### `impl Termination for !`
[src]

#### `fn report(self) -> i32`
[src]

🔬 This is a nightly-only experimental API. (`termination_trait` #43301)

Is called to get the representation of the value as status code. This status code is returned to the operating system. Read more

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