rework cycle handling

A cycle was previously coinductive if all steps were coinductive.
Change this to instead considerm cycles to be coinductive if they
step through at least one where-bound of an impl of a coinductive
trait goal.

2 files changed, 6 insertions, 4 deletions

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 00993c40dea..38a3f722ca2 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -594,6 +594,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.trait_is_auto(trait_def_id)
     }
 
+    fn trait_is_coinductive(self, trait_def_id: DefId) -> bool {
+        self.trait_is_coinductive(trait_def_id)
+    }
+
     fn trait_is_alias(self, trait_def_id: DefId) -> bool {
         self.trait_is_alias(trait_def_id)
     }
diff --git a/compiler/rustc_middle/src/ty/predicate.rs b/compiler/rustc_middle/src/ty/predicate.rs
index de6d30a89d4..089855bfb61 100644
--- a/compiler/rustc_middle/src/ty/predicate.rs
+++ b/compiler/rustc_middle/src/ty/predicate.rs
@@ -51,10 +51,6 @@ impl<'tcx> rustc_type_ir::inherent::Predicate<TyCtxt<'tcx>> for Predicate<'tcx>
         self.as_clause()
     }
 
-    fn is_coinductive(self, interner: TyCtxt<'tcx>) -> bool {
-        self.is_coinductive(interner)
-    }
-
     fn allow_normalization(self) -> bool {
         self.allow_normalization()
     }
@@ -119,6 +115,8 @@ impl<'tcx> Predicate<'tcx> {
         Some(tcx.mk_predicate(kind))
     }
 
+    /// Only used by the old solver to decide whether a predicate is accepted
+    /// in a coinductive trait solver cycle.
     #[instrument(level = "debug", skip(tcx), ret)]
     pub fn is_coinductive(self, tcx: TyCtxt<'tcx>) -> bool {
         match self.kind().skip_binder() {