type_alias_impl_trait is unusable

[Sherlock-Holo]

#![feature(type_alias_impl_trait)]

fn main() {}

type Adder = impl Fn(usize) -> usize;

this codes should be right like #2515 (comment)
but now cargo check report error

error: could not find defining uses
 --> src/main.rs:5:14
  |
5 | type Adder = impl Fn(usize) -> usize;
  |              ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: could not compile `learn`

To learn more, run the command again with --verbose.

the cargo version is cargo 1.51.0-nightly (ab64d1393 2021-02-10)

[RustyYato]

That isn't a complete snippet, you also need a defining use, i.e. a function return type.

#![feature(type_alias_impl_trait)]

type Adder = impl Fn(usize) -> usize;

fn adder() -> Adder { /* snip */ }

[Sherlock-Holo]

I try this

#![feature(type_alias_impl_trait)]

fn main() {}

type Adder = impl Fn(usize) -> usize;

fn doit() -> Adder {
    func
}

fn func(n: usize) -> usize {
    n
}

now cargo check doesn't report errors, but why I have to use it?
If I write type RR<T> = Result<T, Infallible>; cargo check won't force me to use it

[RustyYato]

I think this will be easier to understand via analogy.

1 - does not compile type_alias_impl_trait

#![feature(type_alias_impl_trait)]

fn main() {}

type Adder = impl Fn(usize) -> usize;

is similar to

let Adder: impl Fn(usize) -> usize;

Note how Adder is declared, but has no value. Now, Rust enforces that all top-level declarations must have a value. So leaving it how doesn't work.

2 - compiles type_alias_impl_trait

#![feature(type_alias_impl_trait)]

fn main() {}

type Adder = impl Fn(usize) -> usize;

fn doit() -> Adder {
    func
}

fn func(n: usize) -> usize {
    n
}

is similar to

let Adder: impl Fn(usize) -> usize;

Adder = func;

// snip

Note how Adder now has a value. So this is fine, even though the value is provided in a different spot from the declaration.

3 - compiles type alias

type RR<T> = Result<T, Infallible>;

is similar to

let Adder: impl Fn(usize) -> usize = func;

Note how the declaration and definition are done at the same time. So this is also fine. There are two big differences between normal variables, and top-level declarations:

• Top-level declarations can be in an arbitrary order, where as normal variables must be declared before or at the same time as the they are defined/used.
• Top-level declarations must have a defined value (even if unused), whereas normal variables are allowed to be undefined (as long as they aren't used).

[scottmcm]

It looks like this is working as expected, then, so I'll close this issue.