Add associated_ty_from_impl to Chalk db to avoid computing associated types eagerly

book/src/clauses/coherence.md

@@ -19,7 +19,7 @@ To check for coherence, the Rust compiler completes two separate but related che
 - [Coherence - talk by withoutboats](https://www.youtube.com/watch?v=AI7SLCubTnk&t=43m19s)
 - [Little Orphan Impls](https://smallcultfollowing.com/babysteps/blog/2015/01/14/little-orphan-impls/)
 - [RFC 1023 Rebalancing Coherence](https://rust-lang.github.io/rfcs/1023-rebalancing-coherence.html)
-- [Type classes: confluence, coherence and global uniqueness](http://blog.ezyang.com/2014/07/type-classes-confluence-coherence-global-uniqueness/)
+- [Type classes: confluence, coherence and global uniqueness](https://web.archive.org/web/20250308110404/https://blog.ezyang.com/2014/07/type-classes-confluence-coherence-global-uniqueness/)
 ## Axioms & Properties of Coherence
 > Historical Note: We used to use the term “external” instead of “upstream”.
 

chalk-integration/src/db.rs

@@ -99,6 +99,19 @@ impl RustIrDatabase<ChalkIr> for ChalkDatabase {
         self.program_ir().unwrap().impl_datum(id)
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: ImplId<ChalkIr>,
+        assoc_type_id: AssocTypeId<ChalkIr>,
+    ) -> Option<AssociatedTyValueId<ChalkIr>> {
+        let ir = self.program_ir().unwrap();
+        ir.impl_data[&impl_id]
+            .associated_ty_value_ids
+            .iter()
+            .copied()
+            .find(|id| ir.associated_ty_values[id].associated_ty_id == assoc_type_id)
+    }
+
     fn associated_ty_value(
         &self,
         id: AssociatedTyValueId<ChalkIr>,

chalk-integration/src/program.rs

@@ -398,6 +398,18 @@ impl RustIrDatabase<ChalkIr> for Program {
         self.impl_data[&id].clone()
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: ImplId<ChalkIr>,
+        assoc_type_id: AssocTypeId<ChalkIr>,
+    ) -> Option<AssociatedTyValueId<ChalkIr>> {
+        self.impl_data[&impl_id]
+            .associated_ty_value_ids
+            .iter()
+            .copied()
+            .find(|id| self.associated_ty_values[id].associated_ty_id == assoc_type_id)
+    }
+
     fn associated_ty_value(
         &self,
         id: AssociatedTyValueId<ChalkIr>,

chalk-solve/src/clauses.rs

@@ -639,6 +639,7 @@ pub fn program_clauses_that_could_match<I: Interner>(
                     builder,
                     environment,
                     trait_id,
+                    proj.associated_ty_id,
                     trait_parameters,
                     binders,
                 );
@@ -749,6 +750,7 @@ fn push_program_clauses_for_associated_type_values_in_impls_of<I: Interner>(
     builder: &mut ClauseBuilder<'_, I>,
     environment: &Environment<I>,
     trait_id: TraitId<I>,
+    assoc_id: AssocTypeId<I>,
     trait_parameters: &[GenericArg<I>],
     binders: &CanonicalVarKinds<I>,
 ) {
@@ -763,7 +765,7 @@ fn push_program_clauses_for_associated_type_values_in_impls_of<I: Interner>(
 
         debug!(?impl_id);
 
-        for &atv_id in &impl_datum.associated_ty_value_ids {
+        if let Some(atv_id) = builder.db.associated_ty_from_impl(impl_id, assoc_id) {
             let atv = builder.db.associated_ty_value(atv_id);
             debug!(?atv_id, ?atv);
             atv.to_program_clauses(builder, environment);

chalk-solve/src/display/stub.rs

@@ -116,6 +116,14 @@ impl<I: Interner, DB: RustIrDatabase<I>> RustIrDatabase<I> for StubWrapper<'_, D
         unreachable!("impl items should never be stubbed")
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        _impl_id: chalk_ir::ImplId<I>,
+        _assoc_type_id: chalk_ir::AssocTypeId<I>,
+    ) -> Option<crate::rust_ir::AssociatedTyValueId<I>> {
+        unreachable!("should never reach projection if impl datum is not stubbed")
+    }
+
     fn associated_ty_value(
         &self,
         _id: crate::rust_ir::AssociatedTyValueId<I>,

chalk-solve/src/lib.rs

@@ -76,6 +76,12 @@ pub trait RustIrDatabase<I: Interner>: Debug {
     /// Returns the datum for the impl with the given id.
     fn impl_datum(&self, impl_id: ImplId<I>) -> Arc<ImplDatum<I>>;
 
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: ImplId<I>,
+        assoc_type_id: AssocTypeId<I>,
+    ) -> Option<AssociatedTyValueId<I>>;
+
     /// Returns the `AssociatedTyValue` with the given id.
     fn associated_ty_value(&self, id: AssociatedTyValueId<I>) -> Arc<AssociatedTyValue<I>>;
 

chalk-solve/src/logging_db.rs

@@ -171,6 +171,14 @@ where
         self.ws.db().hidden_opaque_type(id)
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: ImplId<I>,
+        assoc_type_id: AssocTypeId<I>,
+    ) -> Option<AssociatedTyValueId<I>> {
+        self.ws.db().associated_ty_from_impl(impl_id, assoc_type_id)
+    }
+
     fn associated_ty_value(
         &self,
         id: crate::rust_ir::AssociatedTyValueId<I>,
@@ -434,6 +442,14 @@ where
         self.db.impl_datum(impl_id)
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: ImplId<I>,
+        assoc_type_id: AssocTypeId<I>,
+    ) -> Option<AssociatedTyValueId<I>> {
+        self.db.associated_ty_from_impl(impl_id, assoc_type_id)
+    }
+
     fn associated_ty_value(
         &self,
         id: crate::rust_ir::AssociatedTyValueId<I>,

tests/display/unique_names.rs

@@ -94,6 +94,13 @@ where
     ) -> std::sync::Arc<chalk_solve::rust_ir::ImplDatum<I>> {
         self.db.impl_datum(impl_id)
     }
+    fn associated_ty_from_impl(
+        &self,
+        impl_id: chalk_ir::ImplId<I>,
+        assoc_type_id: chalk_ir::AssocTypeId<I>,
+    ) -> Option<chalk_solve::rust_ir::AssociatedTyValueId<I>> {
+        self.db.associated_ty_from_impl(impl_id, assoc_type_id)
+    }
     fn associated_ty_value(
         &self,
         id: chalk_solve::rust_ir::AssociatedTyValueId<I>,

tests/integration/panic.rs

@@ -118,6 +118,14 @@ impl RustIrDatabase<ChalkIr> for MockDatabase {
         })
     }
 
+    fn associated_ty_from_impl(
+        &self,
+        _impl_id: ImplId<ChalkIr>,
+        _assoc_type_id: AssocTypeId<ChalkIr>,
+    ) -> Option<AssociatedTyValueId<ChalkIr>> {
+        unimplemented!()
+    }
+
     fn associated_ty_value(
         &self,
         id: AssociatedTyValueId<ChalkIr>,