Assert that global cache values are stable on insertion

compiler/rustc_middle/src/traits/mod.rs

@@ -569,7 +569,7 @@ pub struct DerivedObligationCause<'tcx> {
     pub parent_code: InternedObligationCauseCode<'tcx>,
 }
 
-#[derive(Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
+#[derive(PartialEq, Eq, Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
 pub enum SelectionError<'tcx> {
     /// The trait is not implemented.
     Unimplemented,

compiler/rustc_query_system/src/cache.rs

@@ -5,6 +5,7 @@ use crate::dep_graph::{DepContext, DepNodeIndex};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lock;
 
+use std::fmt::Debug;
 use std::hash::Hash;
 
 pub struct Cache<Key, Value> {
@@ -30,13 +31,22 @@ impl<Key, Value> Cache<Key, Value> {
     }
 }
 
-impl<Key: Eq + Hash, Value: Clone> Cache<Key, Value> {
+impl<Key: Eq + Hash, Value: Eq + Clone + Debug> Cache<Key, Value> {
     pub fn get<Tcx: DepContext>(&self, key: &Key, tcx: Tcx) -> Option<Value> {
         Some(self.hashmap.borrow().get(key)?.get(tcx))
     }
 
     pub fn insert(&self, key: Key, dep_node: DepNodeIndex, value: Value) {
-        self.hashmap.borrow_mut().insert(key, WithDepNode::new(dep_node, value));
+        // FIXME(#50507): For some reason we're getting different `DepNodeIndex`es
+        // for the same evaluation in the parallel compiler. Once that's fixed
+        // we can just use `HashMap::insert_same` here instead of doing all this.
+        self.hashmap
+            .borrow_mut()
+            .entry(key)
+            .and_modify(|old_value| {
+                assert_eq!(old_value.cached_value, value, "unstable cached value")
+            })
+            .or_insert(WithDepNode::new(dep_node, value));
     }
 }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1333,10 +1333,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if self.can_use_global_caches(param_env) {
             if !trait_pred.needs_infer() {
                 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
-                // This may overwrite the cache with the same value
-                // FIXME: Due to #50507 this overwrites the different values
-                // This should be changed to use HashMapExt::insert_same
-                // when that is fixed
                 self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
                 return;
             }