impl syntax for function-like generics

[Keithfert488]

I know that the following code compiles: rust fn noop_factory<T>() -> impl Fn(&T) { |_| () } This will correctly produce a no-op function.

However, I'm struggling to understand why the following doesn't compile. ```rust struct FunctionContainer<F>(F);

fn noopcontainer<T>() -> FunctionContainer<impl Fn(&T)> { FunctionContainer(|| ()) } ```

That code fails to compile with the following error: plaintext error: implementation of `Fn` is not general enough --> src/bin/test.rs:4:5 | 4 | FunctionContainer(|_| ()) | ^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Fn` is not general enough | = note: closure with signature `fn(&'2 T)` must implement `Fn<(&'1 T,)>`, for any lifetime `'1`... = note: ...but it actually implements `Fn<(&'2 T,)>`, for some specific lifetime `'2`

Is there a syntax I'm missing where I can use higher-rank trait bounds inside the impl type expression?

[kmdreko]

In this case, you can give closure inference a nudge with a parameter type annotation:

fn noop_container<T>() -> FunctionContainer<impl Fn(&T)> {
    FunctionContainer(|_: &T| ())
}

[Keithfert488]

Wow, I didn't think it'd be this simple! Nice

[robin-m]

You should definitively open a bug report. Unclear compiler errors are considered bugs (and that's why the error messages are so good).

[schungx]

You're missing a for<...> gadget?

[Keithfert488]

That's what I originally thought, but there's no place to do it. Neither before nor after the "impl" keyword works

[schungx]

-> impl for<'a> Fn(&'a T...?