Rollup merge of #147061 - lcnr:provisional-cache-woops, r=BoxyUwU

fix rebasing cycle heads when not reaching a fixpoint

fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/232

annoyingly subtle, imagine the following proof tree

- A (no cycle head usages, final result Y)
  - *ignored* B (depends on A with provisional result X)
    - A (cycle, provisional result X)
- B (using the cache entry here incorrectly assumes A has final result X)

r? ``@BoxyUwU``

1 files changed, 17 insertions, 9 deletions

diff --git a/compiler/rustc_next_trait_solver/src/solve/search_graph.rs b/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
index aa9dfc9a9a2..109c8476ccb 100644
--- a/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/search_graph.rs
@@ -74,20 +74,28 @@ where
         }
     }
 
-    fn is_initial_provisional_result(
-        cx: Self::Cx,
-        kind: PathKind,
-        input: CanonicalInput<I>,
-        result: QueryResult<I>,
-    ) -> bool {
-        Self::initial_provisional_result(cx, kind, input) == result
+    fn is_initial_provisional_result(result: QueryResult<I>) -> Option<PathKind> {
+        match result {
+            Ok(response) => {
+                if has_no_inference_or_external_constraints(response) {
+                    if response.value.certainty == Certainty::Yes {
+                        return Some(PathKind::Coinductive);
+                    } else if response.value.certainty == Certainty::overflow(false) {
+                        return Some(PathKind::Unknown);
+                    }
+                }
+
+                None
+            }
+            Err(NoSolution) => Some(PathKind::Inductive),
+        }
     }
 
-    fn on_stack_overflow(cx: I, input: CanonicalInput<I>) -> QueryResult<I> {
+    fn stack_overflow_result(cx: I, input: CanonicalInput<I>) -> QueryResult<I> {
         response_no_constraints(cx, input, Certainty::overflow(true))
     }
 
-    fn on_fixpoint_overflow(cx: I, input: CanonicalInput<I>) -> QueryResult<I> {
+    fn fixpoint_overflow_result(cx: I, input: CanonicalInput<I>) -> QueryResult<I> {
         response_no_constraints(cx, input, Certainty::overflow(false))
     }