Confusing error message with reference to boxed trait object

[varkor]

fn main() {
    let _: Box<dyn '_ + Fn()> = Box::new(|| {});
    let _: &'_ Box<dyn '_ + Fn()> = &Box::new(|| {}); //~ ERROR
}

Results in:

error[E0308]: mismatched types
 --> src/main.rs:3:37
  |
3 |     let _: &'_ Box<dyn '_ + Fn()> = &Box::new(|| {});
  |                                     ^^^^^^^^^^^^^^^^ expected trait std::ops::Fn, found closure
  |
  = note: expected type `&std::boxed::Box<dyn std::ops::Fn()>`
             found type `&std::boxed::Box<[closure@src/main.rs:3:47: 3:52]>`

This error message is confusing, because without the reference the types match up, so it seems odd that there's a type error as soon as a reference is introduced.

[varkor]

It could also be useful to add a note mentioning that 'static is the default for trait objects, as the sudden appearance of 'static otherwise can be quite confusing.

[matthewjasper]

The problem isn't the 'static bound, the message is correct as is. You can't do an unsize coercion behind two pointers.

edit: and that you're returning a reference to a temporary, but all the lifetime annotations in the world won't let you do that

[varkor]

@matthewjasper: you're right, I misdiagnosed the problem. I've updated the example, which I think is still confusing: we should add an explanation to the error message in this case.

[estebank]

Triage: no change.