Primitive Type never
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 };
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, };
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
Infallible errors
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>; }
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:
#![feature(exhaustive_patterns)] use std::str::FromStr; let Ok(s) = String::from_str("hello");
Since the Err
variant contains a !
, it can never occur. If the exhaustive_patterns
feature is present this means 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
.
Infinite loops
While Result<T, !>
is very useful for removing errors, !
can also be used to remove successes as well. If we think of Result<T, !>
as “if this function returns, it has not errored,” we get a very intuitive idea of Result<!, E>
as well: if the function returns, it has errored.
For example, consider the case of a simple web server, which can be simplified to:
loop { let (client, request) = get_request().expect("disconnected"); let response = request.process(); response.send(client); }
Currently, this isn’t ideal, because we simply panic whenever we fail to get a new connection. Instead, we’d like to keep track of this error, like this:
loop { match get_request() { Err(err) => break err, Ok((client, request)) => { let response = request.process(); response.send(client); }, } }
Now, when the server disconnects, we exit the loop with an error instead of panicking. While it might be intuitive to simply return the error, we might want to wrap it in a Result<!, E>
instead:
fn server_loop() -> Result<!, ConnectionError> { loop { let (client, request) = get_request()?; let response = request.process(); response.send(client); } }
Now, we can use ?
instead of match
, and the return type makes a lot more sense: if the loop ever stops, it means that an error occurred. We don’t even have to wrap the loop in an Ok
because !
coerces to Result<!, ConnectionError>
automatically.
!
and traits
When writing your own traits, !
should have an impl
whenever there is an obvious impl
which doesn’t panic!
. The reason is that functions returning an impl Trait
where !
does not have an impl
of Trait
cannot diverge as their only possible code path. In other words, they can’t return !
from every code path. As an example, this code doesn’t compile:
use std::ops::Add; fn foo() -> impl Add<u32> { unimplemented!() }
But this code does:
use std::ops::Add; fn foo() -> impl Add<u32> { if true { unimplemented!() } else { 0 } }
The reason is that, in the first example, there are many possible types that !
could coerce to, because many types implement Add<u32>
. However, in the second example, the else
branch returns a 0
, which the compiler infers from the return type to be of type u32
. Since u32
is a concrete type, !
can and will be coerced to it. See issue #36375 for more information on this quirk of !
.
As it turns out, though, most traits can have an impl
for !
. Take Debug
for example:
#![feature(never_type)] impl Debug for ! { fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result { *self } }
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 as 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 an impl.
On the other hand, one trait which would not be appropriate to implement is Default
:
trait Default { fn default() -> Self; }
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 Clone for !
pub fn clone(&self) -> !
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source
. Read more
impl Debug for !
pub fn fmt(&self, &mut Formatter<'_>) -> Result<(), Error>
Formats the value using the given formatter. Read more
impl Display for !
pub fn fmt(&self, &mut Formatter<'_>) -> Result<(), Error>
Formats the value using the given formatter. Read more
impl Error for !
fn source(&self) -> Option<&(dyn Error + 'static)>
The lower-level source of this error, if any. Read more
fn backtrace(&self) -> Option<&Backtrace>
Returns a stack backtrace, if available, of where this error occurred. Read more
fn description(&self) -> &str
use the Display impl or to_string()
fn cause(&self) -> Option<&dyn Error>
replaced by Error::source, which can support downcasting
impl Hash for !
impl Ord for !
pub fn cmp(&self, &!) -> Ordering
fn max(self, other: Self) -> Self
Compares and returns the maximum of two values. Read more
fn min(self, other: Self) -> Self
Compares and returns the minimum of two values. Read more
fn clamp(self, min: Self, max: Self) -> Self
Restrict a value to a certain interval. Read more
impl PartialEq<!> for !
pub fn eq(&self, &!) -> bool
This method tests for self
and other
values to be equal, and is used by ==
. Read more
fn ne(&self, other: &Rhs) -> bool
This method tests for !=
.
impl PartialOrd<!> for !
pub fn partial_cmp(&self, &!) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
fn lt(&self, other: &Rhs) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
fn le(&self, other: &Rhs) -> bool
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
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
fn ge(&self, other: &Rhs) -> bool
This method tests greater than or equal to (for self
and other
) and is used by the >=
operator. Read more
impl Termination for !
fn report(self) -> i32
Is called to get the representation of the value as status code. This status code is returned to the operating system. Read more
impl Copy for !
impl Eq for !
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/primitive.never.html