Suggest const block, instead of const item, for array repeat of non-Copy element

[dtolnay]

Code

struct Thing;

fn main() {
    let _ = [Thing; 2];
}

Current output

error[E0277]: the trait bound `Thing: Copy` is not satisfied
 --> src/main.rs:4:14
  |
4 |     let _ = [Thing; 2];
  |              ^^^^^ the trait `Copy` is not implemented for `Thing`
  |
  = note: the `Copy` trait is required because this value will be copied for each element of the array
  = help: create an inline `const` block, see RFC #2920 <https://github.com/rust-lang/rfcs/pull/2920> for more information
help: consider annotating `Thing` with `#[derive(Copy)]`
  |
1 + #[derive(Copy)]
2 | struct Thing;
  |
help: consider creating a new `const` item and initializing it with the result of the constructor to be used in the repeat position
  |
4 ~     const ARRAY_REPEAT_VALUE: Thing = Thing;
5 ~     let _ = [ARRAY_REPEAT_VALUE; 2];
  |

Rationale and extra context

Rustc's suggestion to make this code compile with a non-Copy type is:

fn main() {
    const ARRAY_REPEAT_VALUE: Thing = Thing;
    let _ = [ARRAY_REPEAT_VALUE; 2];
}

Instead it should be:

fn main() {
    let _ = [const { Thing }; 2];
}

Relatedly, now that const blocks are a stable part of the language since Rust 1.79 (https://blog.rust-lang.org/2024/06/13/Rust-1.79.0.html#inline-const-expressions), I don't think the diagnostic should be linking to the initial const block RFC.

Rust Version

rustc 1.81.0-nightly (bcf94dec5 2024-06-23)
binary: rustc
commit-hash: bcf94dec5ba6838e435902120c0384c360126a26
commit-date: 2024-06-23
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

[GrigorenkoPV]

@rustbot claim

[GrigorenkoPV]

rust/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

Line 2953 in 2c243d9

// FIXME: we may suggest array::repeat instead

Is that a thing? I am not sure array::repeat even exists.

[fmease]

Is that a thing? I am not sure array::repeat even exists.

https://doc.rust-lang.org/nightly/std/array/fn.repeat.html

[GrigorenkoPV]

Is that a thing? I am not sure array::repeat even exists.

https://doc.rust-lang.org/nightly/std/array/fn.repeat.html

Ah, thank you! Completely missed it.

Should I make the array::from_fn suggestion machine-applicable too and recommend array::from_fn::<N>(|_| ...) ?

Should I add the array::repeat suggestion on nightly?

[chenyukang]

Close it since #126899 is merged.