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``

Author: matthiaskrgr

core/src/marker.rs

@@ -97,6 +97,7 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 #[fundamental] // for Default, for example, which requires that `[T]: !Default` be evaluatable
 #[rustc_specialization_trait]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_coinductive)]
 pub trait Sized {
     // Empty.
 }