create remove_stale_output method on ingredients

but we are not doing anything in it, yet

components/salsa-2022-macros/src/db.rs

@@ -140,6 +140,12 @@ fn has_jars_dyn_impl(input: &syn::ItemStruct, storage: &syn::Ident) -> syn::Item
                 let ingredient = self.#storage.ingredient(index.ingredient_index());
                 ingredient.inputs(index.key_index())
             }
+
+            fn remove_stale_output(&self, executor: salsa::DatabaseKeyIndex, stale_output: salsa::key::DependencyIndex) {
+                let ingredient = self.#storage.ingredient(stale_output.ingredient_index());
+                ingredient.remove_stale_output(executor, stale_output.key_index());
+            }
+
         }
     }
 }

components/salsa-2022/src/accumulator.rs

@@ -81,6 +81,10 @@ where
     fn inputs(&self, _key_index: crate::Id) -> Option<QueryEdges> {
         None
     }
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<crate::Id>) {
+        // FIXME
+    }
 }
 
 impl<DB: ?Sized, Data> MutIngredient<DB> for AccumulatorIngredient<Data>

components/salsa-2022/src/function.rs

@@ -203,6 +203,11 @@ where
         let key = C::key_from_id(key_index);
         self.inputs(key)
     }
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<crate::Id>) {
+        let stale_output_key = C::key_from_id(stale_output_key.unwrap());
+        // FIXME
+    }
 }
 
 impl<DB, C> MutIngredient<DB> for FunctionIngredient<C>

components/salsa-2022/src/function/diff_outputs.rs

@@ -1,6 +1,6 @@
 use crate::{
-    key::DependencyIndex, runtime::local_state::QueryRevisions, Database, DatabaseKeyIndex, Event,
-    EventKind,
+    key::DependencyIndex, runtime::local_state::QueryRevisions, storage::HasJarsDyn, Database,
+    DatabaseKeyIndex, Event, EventKind,
 };
 
 use super::{memo::Memo, Configuration, DynDb, FunctionIngredient};
@@ -59,5 +59,7 @@ where
                 output_key: output,
             },
         });
+
+        db.remove_stale_output(key, output);
     }
 }

components/salsa-2022/src/ingredient.rs

@@ -1,5 +1,6 @@
 use crate::{
-    cycle::CycleRecoveryStrategy, key::DependencyIndex, runtime::local_state::QueryEdges, Id,
+    cycle::CycleRecoveryStrategy, key::DependencyIndex, runtime::local_state::QueryEdges,
+    DatabaseKeyIndex, Id,
 };
 
 use super::Revision;
@@ -22,6 +23,12 @@ pub trait Ingredient<DB: ?Sized> {
 
     /// What were the inputs (if any) that were used to create the value at `key_index`.
     fn inputs(&self, key_index: Id) -> Option<QueryEdges>;
+
+    /// Invoked when the value `stale_output` was output by `executor` in a previous
+    /// revision, but was NOT output in the current revision.
+    ///
+    /// This hook is used to clear out the stale value so others cannot read it.
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<Id>);
 }
 
 /// Optional trait for ingredients that wish to be notified when new revisions are

components/salsa-2022/src/input.rs

@@ -61,4 +61,8 @@ where
     fn inputs(&self, _key_index: crate::Id) -> Option<QueryEdges> {
         None
     }
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<crate::Id>) {
+        unreachable!("input cannot be the output of a tracked function");
+    }
 }

components/salsa-2022/src/interned.rs

@@ -8,6 +8,7 @@ use crate::id::AsId;
 use crate::key::DependencyIndex;
 use crate::runtime::local_state::QueryEdges;
 use crate::runtime::Runtime;
+use crate::DatabaseKeyIndex;
 
 use super::hash::FxDashMap;
 use super::ingredient::Ingredient;
@@ -197,6 +198,10 @@ where
     fn inputs(&self, _key_index: crate::Id) -> Option<QueryEdges> {
         None
     }
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<crate::Id>) {
+        unreachable!("interned ids are not outputs");
+    }
 }
 
 pub struct IdentityInterner<Id: AsId> {

components/salsa-2022/src/storage.rs

@@ -179,6 +179,8 @@ pub trait HasJarsDyn {
     fn cycle_recovery_strategy(&self, input: IngredientIndex) -> CycleRecoveryStrategy;
 
     fn inputs(&self, input: DatabaseKeyIndex) -> Option<QueryEdges>;
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output: DependencyIndex);
 }
 
 pub trait HasIngredientsFor<I>

components/salsa-2022/src/tracked_struct.rs

@@ -117,6 +117,11 @@ where
     fn inputs(&self, _key_index: crate::Id) -> Option<QueryEdges> {
         None
     }
+
+    fn remove_stale_output(&self, executor: DatabaseKeyIndex, stale_output_key: Option<crate::Id>) {
+        let key: Id = Id::from_id(stale_output_key.unwrap());
+        // FIXME -- we can delete this entity
+    }
 }
 
 impl<DB: ?Sized, Id, Data> MutIngredient<DB> for TrackedStructIngredient<Id, Data>