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Consider types appearing in const expressions to be invariant This is an approach to fix #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 #60058) |
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For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.