Enum std::convert::Infallible

pub enum Infallible {}

The error type for errors that can never happen.

Since this enum has no variant, a value of this type can never actually exist. This can be useful for generic APIs that use Result and parameterize the error type, to indicate that the result is always Ok.

For example, the TryFrom trait (conversion that returns a Result) has a blanket implementation for all types where a reverse Into implementation exists.

impl<T, U> TryFrom<U> for T where U: Into<T> {
    type Error = Infallible;

    fn try_from(value: U) -> Result<Self, Infallible> {
        Ok(U::into(value))  // Never returns `Err`
    }
}

Future compatibility

This enum has the same role as the ! “never” type, which is unstable in this version of Rust. When ! is stabilized, we plan to make Infallible a type alias to it:

pub type Infallible = !;

… and eventually deprecate Infallible.

However there is one case where ! syntax can be used before ! is stabilized as a full-fleged type: in the position of a function’s return type. Specifically, it is possible implementations for two different function pointer types:

trait MyTrait {}
impl MyTrait for fn() -> ! {}
impl MyTrait for fn() -> std::convert::Infallible {}

With Infallible being an enum, this code is valid. However when Infallible becomes an alias for the never type, the two impls will start to overlap and therefore will be disallowed by the language’s trait coherence rules.

Trait Implementations

impl From<!> for Infallible

fn from(x: !) -> Infallible

Performs the conversion.

impl From<Infallible> for TryFromIntError

fn from(x: Infallible) -> TryFromIntError

Performs the conversion.

impl From<Infallible> for TryFromSliceError

fn from(x: Infallible) -> TryFromSliceError

Performs the conversion.

impl PartialOrd<Infallible> for Infallible

fn partial_cmp(&self, _other: &Infallible) -> Option<Ordering>

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

#[must_use]

fn lt(&self, other: &Rhs) -> bool

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

#[must_use]

fn le(&self, other: &Rhs) -> bool

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

#[must_use]

fn gt(&self, other: &Rhs) -> bool

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

#[must_use]

fn ge(&self, other: &Rhs) -> bool

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

impl Eq for Infallible

impl Clone for Infallible

fn clone(&self) -> Infallible

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Infallible

fn fmt(&self, &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl Copy for Infallible

impl Ord for Infallible

fn cmp(&self, _other: &Infallible) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp #44095)

Restrict a value to a certain interval.

impl PartialEq<Infallible> for Infallible

fn eq(&self, &Infallible) -> bool

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

#[must_use]

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl Display for Infallible

fn fmt(&self, &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.