Improve help for const-able values used in array initialization

[ilyvion]

Code

fn main() {
    let x = [None::<String>(); 10];
}

Current output

error[E0277]: the trait bound `String: Copy` is not satisfied
 --> src/main.rs:2:14
  |
2 |     let x = [None::<String>; 10];
  |              ^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
  |
  = note: required for `Option<String>` to implement `Copy`
  = note: the `Copy` trait is required because this value will be copied for each element of the array

Desired output

error[E0277]: the trait bound `String: Copy` is not satisfied
 --> src/main.rs:2:14
  |
2 |     let x = [None::<String>; 10];
  |              ^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
  |
  = note: required for `Option<String>` to implement `Copy`
  = note: the `Copy` trait is required because this value will be copied for each element of the array
  = help: consider creating a new `const` item and initializing it with the value to be used in the repeat position, like `const VAL: Type = value;` and `let x = [VAL; 42];`

Rationale and extra context

I'm basing this suggestion on two things. One, that if you follow it, the code compiles:

fn main() {
    const NONE: Option<String> = None;
    let x = [NONE; 10];
}

and two, that the diagnostic help you get when you try to use a const fn in the repeat position suggests something very similar, see "other cases." It would require the compiler to know that a value that is attempted to be repeated is constable though, which I don't know whether it can, but I figured I'd suggest this, and you can reject it if it isn't possible to do.

Alternatively, the help text could say something like "if the value can be made const, consider creating [...]"

Other cases

I was inspired to make this issue based on the output you get when you try to use a const fn as an array initializer. For instance, the code

fn main() {
    let x = [none::<String>(); 10];
}

const fn none<T>() -> Option<T> {
    None
}

produces the output

error[E0277]: the trait bound `String: Copy` is not satisfied
 --> src/main.rs:2:14
  |
2 |     let x = [none::<String>(); 10];
  |              ^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
  |
  = note: required for `Option<String>` to implement `Copy`
  = note: the `Copy` trait is required because this value will be copied for each element of the array
  = help: consider creating a new `const` item and initializing it with the result of the function call to be used in the repeat position, like `const VAL: Type = const_fn();` and `let x = [VAL; 42];`

Anything else?

No response

[lcnr]

@AnuragNishad12 I don't think your comment is related to this issue? If this is a problem you've encountered while trying to fix this issue, please reach out on zulip instead.