Rollup merge of #108033 - lcnr:coinductive-attr, r=compiler-errors

add an unstable `#[rustc_coinductive]` attribute

useful to test coinduction, especially in the new solver.

as this attribute should remain permanently unstable I don't think this needs any official approval. cc ``@rust-lang/types``

had to weaken the check for stable query results in the solver to prevent an ICE if there's a coinductive cycle with constraints.

r? ``@compiler-errors``

1 files changed, 54 insertions, 0 deletions

diff --git a/tests/ui/coinduction/canonicalization-rerun.rs b/tests/ui/coinduction/canonicalization-rerun.rs
new file mode 100644
index 00000000000..b10ba3a810f
--- /dev/null
+++ b/tests/ui/coinduction/canonicalization-rerun.rs
@@ -0,0 +1,54 @@
+// check-pass
+// revisions: old new
+//[new] compile-flags: -Ztrait-solver=next
+
+// If we use canonical goals during trait solving we have to reevaluate
+// the root goal of a cycle until we hit a fixpoint.
+//
+// Here `main` has a goal `(?0, ?1): Trait` which is canonicalized to
+// `exists<^0, ^1> (^0, ^1): Trait`.
+//
+// - `exists<^0, ^1> (^0, ^1): Trait` -instantiate-> `(?0, ?1): Trait`
+//   -`(?1, ?0): Trait` -canonicalize-> `exists<^0, ^1> (^0, ^1): Trait`
+//     - COINDUCTIVE CYCLE OK (no constraints)
+//   - `(): ConstrainToU32<?0>` -canonicalize-> `exists<^0> (): ConstrainToU32<^0>`
+//     - OK (^0 = u32 -apply-> ?0 = u32)
+//   - OK (?0 = u32 -canonicalize-> ^0 = u32)
+//   - coinductive cycle with provisional result != final result, rerun
+//
+// - `exists<^0, ^1> (^0, ^1): Trait` -instantiate-> `(?0, ?1): Trait`
+//   -`(?1, ?0): Trait` -canonicalize-> `exists<^0, ^1> (^0, ^1): Trait`
+//     - COINDUCTIVE CYCLE OK (^0 = u32 -apply-> ?1 = u32)
+//   - `(): ConstrainToU32<?0>` -canonicalize-> `exists<^0> (): ConstrainToU32<^0>`
+//     - OK (^0 = u32 -apply-> ?1 = u32)
+//   - OK (?0 = u32, ?1 = u32 -canonicalize-> ^0 = u32, ^1 = u32)
+//   - coinductive cycle with provisional result != final result, rerun
+//
+// - `exists<^0, ^1> (^0, ^1): Trait` -instantiate-> `(?0, ?1): Trait`
+//   -`(?1, ?0): Trait` -canonicalize-> `exists<^0, ^1> (^0, ^1): Trait`
+//     - COINDUCTIVE CYCLE OK (^0 = u32, ^1 = u32 -apply-> ?1 = u32, ?0 = u32)
+//   - `(): ConstrainToU32<?0>` -canonicalize-> `exists<^0> (): ConstrainToU32<^0>`
+//     - OK (^0 = u32 -apply-> ?1 = u32)
+//   - OK (?0 = u32, ?1 = u32 -canonicalize-> ^0 = u32, ^1 = u32)
+//   - coinductive cycle with provisional result == final result, DONE
+#![feature(rustc_attrs)]
+#[rustc_coinductive]
+trait Trait {}
+
+impl<T, U> Trait for (T, U)
+where
+    (U, T): Trait,
+    (): ConstrainToU32<T>,
+{}
+
+trait ConstrainToU32<T> {}
+impl ConstrainToU32<u32> for () {}
+
+fn impls_trait<T, U>()
+where
+    (T, U): Trait,
+{}
+
+fn main() {
+    impls_trait::<_, _>();
+}