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Using associated constants for generics is not considered using a type generic #80977

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clarfonthey opened this issue Jan 13, 2021 · 0 comments · Fixed by #89829
Closed

Using associated constants for generics is not considered using a type generic #80977

clarfonthey opened this issue Jan 13, 2021 · 0 comments · Fixed by #89829
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A-const-generics Area: const generics (parameters and arguments) C-bug Category: This is a bug. F-generic_const_exprs `#![feature(generic_const_exprs)]`

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@clarfonthey
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Filing in addition to #80976 because this seems to be a separate issue. Includes the same example below. Essentially, even though the associated constant To::DIM is being used in the definition of the struct, the compiler marks To as being unused since no types related to it are being used.

#![feature(const_generics, const_evaluatable_checked)]

trait Vector {
    type Scalar;
    const DIM: usize;
}

struct Transform<From, To>
where
    From: Vector,
    To: Vector<Scalar = From::Scalar>,
{
    columns: [[From::Scalar; From::DIM]; To::DIM],
}

(Playground)
@clarfonthey clarfonthey added the C-bug Category: This is a bug. label Jan 13, 2021
@clarfonthey clarfonthey mentioned this issue Jan 13, 2021
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@voidc voidc mentioned this issue Oct 12, 2021
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@BoxyUwU BoxyUwU added A-const-generics Area: const generics (parameters and arguments) F-generic_const_exprs `#![feature(generic_const_exprs)]` labels Oct 13, 2021
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Oct 23, 2021
@matthiaskrgr
Rollup merge of rust-lang#89829 - voidc:assoc-const-variance, r=lcnr
809e5c8 
Consider types appearing in const expressions to be invariant

This is an approach to fix rust-lang#80977.
Currently, a type parameter which is only used in a constant expression is considered bivariant and will trigger error E0392 *"parameter T is never used"*.
Here is a short example:

```rust
pub trait Foo {
    const N: usize;
}

struct Bar<T: Foo>([u8; T::N])
where [(); T::N]:;
```
([playgound](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=b51a272853f75925e72efc1597478aa5))

While it is possible to silence this error by adding a `PhantomData<T>` field, I think the better solution would be to make `T` invariant.
This would be analogous to the invariance constraints added for associated types.
However, I'm quite new to the compiler and unsure whether this is the right approach.

r? `@varkor` (since you authored rust-lang#60058)
@bors bors closed this as completed in 2b874f0 Oct 23, 2021
@voidc voidc mentioned this issue Oct 26, 2021
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A-const-generics Area: const generics (parameters and arguments) C-bug Category: This is a bug. F-generic_const_exprs `#![feature(generic_const_exprs)]`
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Consider types appearing in const expressions to be invariant voidc/rust
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@clarfonthey @BoxyUwU