never - Rust (original) (raw)
Primitive Type never
🔬This is a nightly-only experimental API. (never_type #35121)
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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 behavior 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 FromStrtrait:
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 aResult<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_patternsfeature is present this means we can exhaustively match on Result<T, !> by just taking theOk variant. This illustrates another behavior 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 Okbecause ! coerces to Result<!, ConnectionError> automatically.
§! and traits
When writing your own traits, ! should have an impl whenever there is an obvious implwhich 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 typeu32. Since u32 is a concrete type, ! can and will be coerced to it. See issue #36375for more information on this quirk of !.
As it turns out, though, most traits can have an impl for !. Take Debugfor 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 casefmt::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 *selfessentially 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 selfparameter 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 animpl 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.)
§Never type fallback
When the compiler sees a value of type ! in a coercion site, it implicitly inserts a coercion to allow the type checker to infer any type:
// this
let x: u8 = panic!();
// is (essentially) turned by the compiler into
let x: u8 = absurd(panic!());
// where absurd is a function with the following signature
// (it's sound, because `!` always marks unreachable code):
fn absurd<T>(_: !) -> T { ... }This can lead to compilation errors if the type cannot be inferred:
// this
{ panic!() };
// gets turned into this
{ absurd(panic!()) }; // error: can't infer the type of `absurd`To prevent such errors, the compiler remembers where it inserted absurd calls, and if it can’t infer the type, it uses the fallback type instead:
type Fallback = /* An arbitrarily selected type! */;
{ absurd::<Fallback>(panic!()) }This is what is known as “never type fallback”.
Historically, the fallback type was (), causing confusing behavior where ! spontaneously coerced to (), even when it would not infer () without the fallback. There are plans to change it in the 2024 edition (and possibly in all editions on a later date); seeTracking Issue for making ! fall back to !.
Returns the lower-level source of this error, if any. Read more
👎Deprecated since 1.42.0: use the Display impl or to_string()
👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting
🔬This is a nightly-only experimental API. (error_generic_member_access #99301)
Provides type-based access to context intended for error reports. Read more
Converts to this type from the input type.
Converts to this type from the input type.
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
This method returns an ordering between self and other values if one exists. Read more
Tests less than (for self and other) and is used by the < operator. Read more
Tests less than or equal to (for self and other) and is used by the<= operator. Read more
Tests greater than (for self and other) and is used by the >operator. Read more
Tests greater than or equal to (for self and other) and is used by the >= operator. Read more
Is called to get the representation of the value as status code. This status code is returned to the operating system.