Make const parameters enforce no variance constraints #60058
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⌛ Testing commit 318a10e with merge 9622c488405c6688f3868b24967a4cdd0394666a... |
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Apr 18, 2019
Make const parameters enforce no variance constraints Fixes #60047. Also includes some minor const refactoring for convenience.
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Tested on commit rust-lang/rust@e577e49. Direct link to PR: <rust-lang/rust#60058> 🎉 rls on windows: test-fail → test-pass (cc @Xanewok, @rust-lang/infra).
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| // Const parameters are always invariant. | ||
| for (idx, param) in generics.params.iter().enumerate() { | ||
| if let ty::GenericParamDefKind::Const = param.kind { | ||
| variances[idx] = ty::Invariant; |
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Shouldn't this be variances[generics.parent_count + idx] = ty::Invariant? Since count also include parent generics while params do not.
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Not sure if you'd also want to change the variance of parent generics here. Maybe this code should be elsewhere?
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Ah, you're absolutely right. Thanks for spotting that!
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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)
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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)
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Fixes #60047. Also includes some minor const refactoring for convenience.