Suggest creating a `const` when calling a `const` function in an array initializer

[Aaron1011]

The following code:

fn main() {
    let strs: [String; 5] = [String::new(); 5];
}

gives the following error:

error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
 --> src/main.rs:2:29
  |
2 |     let strs: [String; 5] = [String::new(); 5];
  |                             ^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::string::String`
  |
  = note: the `Copy` trait is required because the repeated element will be copied

However, String::new() is a const fn, so this code can be made to compile by creating a new const item:

fn main() {
    const EMPTY: String = String::new();
    let strs: [String; 5] = [EMPTY; 5];
}

We should suggest this to user when a const function is being called in the array initializer.

Assuming that rust-lang/rfcs#2920 is accepted, we could eventually suggest an inline const block.
On Nightly, we could suggest an inline const block.

[Aaron1011]

I think this would make a good first issue for someone looking to get started contributing. I'd be happy to mentor anyone who wants to work on this.

[DavidHusicka]

@rustbot claim

[henryboisdequin]

@Aaron1011 Is this still an issue that I can work on?

[Aaron1011]

@henryboisdequin Yes, assuming that @DavidHusicka is no longer working on this.

[henryboisdequin]

Should I claim it?