70 lines
2.3 KiB
Rust
70 lines
2.3 KiB
Rust
// compile-flags: -Znext-solver
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#![feature(rustc_attrs)]
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// This test is incredibly subtle. At its core the goal is to get a coinductive cycle,
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// which, depending on its root goal, either holds or errors. We achieve this by getting
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// incomplete inference via a `ParamEnv` candidate in the `A<T>` impl and required
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// inference from an `Impl` candidate in the `B<T>` impl.
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//
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// To make global cache accesses stronger than the guidance from the where-bounds, we add
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// another coinductive cycle from `A<T>: Trait<U, V, D>` to `A<T>: Trait<U, D, V>` and only
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// constrain `D` directly. This means that any candidates which rely on `V` only make
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// progress in the second iteration, allowing a cache access in the first iteration to take
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// precedence.
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//
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// tl;dr: our caching of coinductive cycles was broken and this is a regression
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// test for that.
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#[rustc_coinductive]
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trait Trait<T: ?Sized, V: ?Sized, D: ?Sized> {}
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struct A<T: ?Sized>(*const T);
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struct B<T: ?Sized>(*const T);
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trait IncompleteGuidance<T: ?Sized, V: ?Sized> {}
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impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, u8> for T {}
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impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i8> for T {}
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impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i16> for T {}
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trait ImplGuidance<T: ?Sized, V: ?Sized> {}
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impl<T: ?Sized> ImplGuidance<u32, u8> for T {}
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impl<T: ?Sized> ImplGuidance<i32, i8> for T {}
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impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
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where
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T: IncompleteGuidance<U, V>,
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A<T>: Trait<U, D, V>,
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B<T>: Trait<U, V, D>,
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(): ToU8<D>,
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{
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}
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trait ToU8<T: ?Sized> {}
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impl ToU8<u8> for () {}
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impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for B<T>
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where
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T: ImplGuidance<U, V>,
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A<T>: Trait<U, V, D>,
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{
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}
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fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
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fn with_bound<X>()
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where
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X: IncompleteGuidance<i32, u8>,
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X: IncompleteGuidance<u32, i8>,
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X: IncompleteGuidance<u32, i16>,
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{
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impls_trait::<B<X>, _, _, _>(); // entering the cycle from `B` works
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// entering the cycle from `A` fails, but would work if we were to use the cache
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// result of `B<X>`.
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impls_trait::<A<X>, _, _, _>();
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//~^ ERROR the trait bound `A<X>: Trait<_, _, _>` is not satisfied
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}
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fn main() {
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with_bound::<u32>();
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}
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