Enum never::Never

pub enum Never {}

A type that can never be constructed.

Never can never be constructed (in type theory parlance, it is "uninhabited"). It represents any computation which never resolves to a particular value (because it runs forever, panics, aborts the process, etc). Because the Never type can never be constructed, the existence of a Never proves that a piece of code can never be reached.

For example, we could write a function like:

fn result_into_ok<T>(res: Result<T, Never>) -> T {
    match res {
        Ok(t) => t,
        // This branch can never be taken, and so the
        // compiler is happy to treat it as evaluating
        // to whatever type we wish - in this case, `T`.
        Err(never) => match never {},
    }
}

Generalizing, it is always valid to convert a Never into a value of any other type. We provide the into_any and to_any methods for this purpose.

Never is a stable version of the currently-unstable ! type from the standard library.

Methods

impl Never

pub fn into_any<T>(self) -> T

Convert this Never into a value of a different type.

Since a Never can never be constructed, this is valid for any Sized type.

pub fn to_any<T>(&self) -> T

Convert this Never into a value of a different type.

Since a Never can never be constructed, this is valid for any Sized type.

Trait Implementations

impl<T: ?Sized> AsMut<T> for Never

fn as_mut(&mut self) -> &mut T

Performs the conversion.

impl<T: ?Sized> AsRef<T> for Never

fn as_ref(&self) -> &T

Performs the conversion.

impl BufRead for Never

fn fill_buf(&mut self) -> Result<&[u8]>

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty.

fn consume(&mut self, _amt: usize)

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read.

fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize, Error>

Read all bytes into buf until the delimiter byte or EOF is reached.

fn read_line(&mut self, buf: &mut String) -> Result<usize, Error>

Read all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer.

fn split(self, byte: u8) -> Split<Self>

Returns an iterator over the contents of this reader split on the byte byte.

fn lines(self) -> Lines<Self>

Returns an iterator over the lines of this reader.

impl Clone for Never

fn clone(&self) -> Never

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Copy for Never

impl Debug for Never

fn fmt(&self, f: &mut Formatter) -> Result

Formats the value using the given formatter.

impl Display for Never

fn fmt(&self, _: &mut Formatter) -> Result

Formats the value using the given formatter.

impl Eq for Never

impl Error for Never

fn source(&self) -> Option<&(dyn Error + 'static)>

The lower-level source of this error, if any.

fn backtrace(&self) -> Option<&Backtrace>

🔬 This is a nightly-only experimental API. (backtrace)

Returns a stack backtrace, if available, of where this error occurred.

fn description(&self) -> &str

Deprecated since 1.42.0:

use the Display impl or to_string()

if let Err(e) = "xc".parse::<u32>() { // Print e itself, no need for description(). eprintln!("Error: {}", e); }

fn cause(&self) -> Option<&dyn Error>

Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

impl Hash for Never

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

impl Ord for Never

fn cmp(&self, other: &Never) -> Ordering

This method returns an [Ordering] between self and other.

#[must_use]fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

#[must_use]fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

#[must_use]fn clamp(self, min: Self, max: Self) -> Self

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

Restrict a value to a certain interval.

impl PartialEq<Never> for Never

fn eq(&self, other: &Never) -> 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 PartialOrd<Never> for Never

fn partial_cmp(&self, other: &Never) -> 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 Read for Never

fn read(&mut self, _buf: &mut [u8]) -> Result<usize>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut]) -> Result<usize, Error>

Like read, except that it reads into a slice of buffers.

unsafe fn initializer(&self) -> Initializer

🔬 This is a nightly-only experimental API. (read_initializer)

Determines if this Reader can work with buffers of uninitialized memory.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize, Error>

Read all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize, Error>

Read all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<(), Error>

Read the exact number of bytes required to fill buf.

fn by_ref(&mut self) -> &mut Self

Creates a "by reference" adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

Transforms this Read instance to an [Iterator] over its bytes.

fn chain<R>(self, next: R) -> Chain<Self, R> where
    R: Read, 

Creates an adaptor which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

Creates an adaptor which will read at most limit bytes from it.

impl Seek for Never

fn seek(&mut self, _pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in a stream.

fn stream_len(&mut self) -> Result<u64, Error>

🔬 This is a nightly-only experimental API. (seek_convenience)

Returns the length of this stream (in bytes).

fn stream_position(&mut self) -> Result<u64, Error>

🔬 This is a nightly-only experimental API. (seek_convenience)

Returns the current seek position from the start of the stream.

impl StructuralEq for Never

impl StructuralPartialEq for Never

impl Write for Never

fn write(&mut self, _buf: &[u8]) -> Result<usize>

Write a buffer into this writer, returning how many bytes were written.

fn flush(&mut self) -> Result<()>

Flush this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_vectored(&mut self, bufs: &[IoSlice]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>

Attempts to write an entire buffer into this writer.

fn write_fmt(&mut self, fmt: Arguments) -> Result<(), Error>

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

Creates a "by reference" adaptor for this instance of Write.