> After this change (and on the 2024 edition), the compiler assumes that T should be !, which doesn't implement Default, and therefore causes a compilation error.
If ! can coerce to every type, why not treat it as if it implemented every trait too?
The Default trait provides a function that actually constructs the type in question. But here the ! type can never be constructed, so the only way to implement Default would be to have it panic, loop infinitely, or otherwise fail at runtime.
So this would risk turning a compile-time error into a runtime error.
Moreover, Rust traits' associated constants/types get in the way of having a proper bottom type. What would <! as Iterator>::Item be? (In Scala I think it just doesn't compile?)
Yep, they can have static methods, as it were (in Rust lingo called "associated functions"; "methods" in Rust always take a `self` receiver). Traits can also have associated types and associated constants, which (naturally) also relate to the type, not any particular instance.
Is it obvious to rust developers that "!" would be the never type? I frequently use "never" in typescript. I could imagine using the never type frequently in rust too. I feel like a longer more human-understandable name would've been a good decision here. (feels like more rust jargon that makes the language harder to learn)
> Is it obvious to rust developers that "!" would be the never type?
Prior to this change most Rust developers would never have cause to ever use `!` for any reason. The only stable way to do so would be to specify the quote-unquote "return type" of divergent functions, which Rust has supported via this special-cased syntax since prehistoric days, before even Mozilla got involved. You can see it in the oldest capture of the tutorial from Jan 2012: https://web.archive.org/web/20120109041112/http://www.rust-l...
So when it came time to elevate `!` from being a special-cased return type to being a fully-fledged type, it was only natural to reuse this syntax. However, I tend to agree that, because we call it "the never type" in casual conversation, the most natural thing to do would be to just have a type alias called `Never` that we could encourage people to use instead. But that would be a perfectly backwards-compatible change that could be made at any point (as proven by the fact that the stopgap and long-stable `Infallible` type is becoming just such a type).
The never type seems very useful in various languages to either signal that a branch can never happen (the example of string -> bytestring never erroring) or to mark that a function will never return a value (and thus control) to the caller.
A simple TypeScript example:
const forever = (): never => {
while (true) {
// whatever
}
}
If ! can coerce to every type, why not treat it as if it implemented every trait too?
So this would risk turning a compile-time error into a runtime error.
"When is never?"
https://youtube.com/watch?v=3jM4cnEVrLc
Prior to this change most Rust developers would never have cause to ever use `!` for any reason. The only stable way to do so would be to specify the quote-unquote "return type" of divergent functions, which Rust has supported via this special-cased syntax since prehistoric days, before even Mozilla got involved. You can see it in the oldest capture of the tutorial from Jan 2012: https://web.archive.org/web/20120109041112/http://www.rust-l...
So when it came time to elevate `!` from being a special-cased return type to being a fully-fledged type, it was only natural to reuse this syntax. However, I tend to agree that, because we call it "the never type" in casual conversation, the most natural thing to do would be to just have a type alias called `Never` that we could encourage people to use instead. But that would be a perfectly backwards-compatible change that could be made at any point (as proven by the fact that the stopgap and long-stable `Infallible` type is becoming just such a type).
A simple TypeScript example:
const forever = (): never => { while (true) { // whatever } }