derive(Clone) has trouble correctly understanding generics.

[dawid2193487]

When attempting to derive Clone on a struct, Clone requires itself to implemented on all generic type arguments, even though that's not needed

I tried this code:

use std::sync::{ Arc, Mutex };

trait Entity { }

struct Tile { }
impl Entity for Tile { }

#[derive(Clone)]
struct EntityStore<T: Entity> {
    entities: Arc<Mutex<Vec<Arc<T>>>>
}

#[derive(Clone)]
struct World {
    tiles: EntityStore<Tile>
}

I expected to see this happen: EntityStore wraps T in an Arc, so no matter what T is, it should be cloneable.

Instead, this happened: Rust complains that Tile isn't Clone, even though it being Clone is not needed.

error[E0277]: the trait bound `Tile: std::clone::Clone` is not satisfied
  --> src/main.rs:15:5
   |
15 |     tiles: EntityStore<Tile>
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` is not implemented for `Tile`
   |
   = note: required because of the requirements on the impl of `std::clone::Clone` for `EntityStore<Tile>`
   = note: required by `std::clone::Clone::clone`

When I don't make EntityStore generic, it works with no issue

use std::sync::{ Arc, Mutex };

trait Entity { }

struct Tile { }
impl Entity for Tile { }

#[derive(Clone)]
struct EntityStore {
    entities: Arc<Mutex<Vec<Arc<Tile>>>>
}

#[derive(Clone)]
struct World {
    tiles: EntityStore
}

Meta

rustc --version --verbose: n/a, using current playgrounds stable

Backtrace: n/a

[durka]

Duplicate of #26925. Known issue for quite a long time and there are serious roadblocks to fixing it. In the meantime, a #[derive(Clone)] with customizable bounds can be implemented as a custom derive (or maybe someone has done that already).