Simplify break_query_cycles

compiler/rustc_middle/src/query/plumbing.rs

@@ -87,6 +87,19 @@ pub struct CycleError<I = QueryStackFrameExtra> {
 }
 
 impl<'tcx> CycleError<QueryStackDeferred<'tcx>> {
+    pub fn is_similar_to(&self, other: &Self) -> bool {
+        (match (&self.usage, &other.usage) {
+            (None, None) => true,
+            (None, Some(_)) | (Some(_), None) => false,
+            (Some((s1, f1)), Some((s2, f2))) => s1 == s2 && f1.is_similar_to(&f2),
+        }) && self.cycle.len() == other.cycle.len()
+            && self
+                .cycle
+                .iter()
+                .zip(&other.cycle)
+                .all(|(q1, q2)| q1.span == q2.span && q1.frame.is_similar_to(&q2.frame))
+    }
+
     pub fn lift(&self) -> CycleError<QueryStackFrameExtra> {
         CycleError {
             usage: self.usage.as_ref().map(|(span, frame)| (*span, frame.lift())),

compiler/rustc_middle/src/query/stack.rs

@@ -46,6 +46,12 @@ impl<'tcx> QueryStackFrame<QueryStackDeferred<'tcx>> {
             def_id_for_ty_in_cycle: self.def_id_for_ty_in_cycle,
         }
     }
+
+    pub fn is_similar_to(&self, other: &Self) -> bool {
+        self.dep_kind == other.dep_kind
+            && self.def_id == other.def_id
+            && self.def_id_for_ty_in_cycle == other.def_id_for_ty_in_cycle
+    }
 }
 
 #[derive(Clone, Debug)]

compiler/rustc_query_impl/src/execution.rs

@@ -220,7 +220,8 @@ fn cycle_error<'tcx, C: QueryCache>(
         .ok()
         .expect("failed to collect active queries");
 
-    let error = find_cycle_in_stack(try_execute, job_map, &current_query_job(tcx), span);
+    let error = find_cycle_in_stack(try_execute, &job_map, current_query_job(tcx), span)
+        .expect("did not find a cycle");
     (mk_cycle(query, tcx, error.lift()), None)
 }