Add stopgap solution for builtin traits

Author: jackh726

chalk-solve/src/clauses.rs

@@ -238,7 +238,7 @@ fn program_clauses_that_could_match<I: Interner>(
             }
 
             if let Some(well_known) = trait_datum.well_known {
-                builtin_traits::add_builtin_program_clauses(well_known, trait_ref, builder);
+                builtin_traits::add_builtin_program_clauses(db, builder, well_known, trait_ref, self_ty.data(interner));
             }
         }
         DomainGoal::Holds(WhereClause::AliasEq(alias_predicate)) => {

chalk-solve/src/clauses/builtin_traits.rs

@@ -1,14 +1,20 @@
+use chalk_ir::TyData;
 use super::builder::ClauseBuilder;
-use crate::Interner;
-use crate::{TraitRef, WellKnownTrait};
+use crate::{Interner, RustIrDatabase, TraitRef, WellKnownTrait};
 
 /// For well known traits we have special hard-coded impls, either as an
 /// optimization or to enforce special rules for correctness.
 pub fn add_builtin_program_clauses<I: Interner>(
+    db: &dyn RustIrDatabase<I>,
+    builder: &mut ClauseBuilder<'_, I>,
     well_known: WellKnownTrait,
-    _trait_ref: &TraitRef<I>,
-    _builder: &mut ClauseBuilder<'_, I>,
+    trait_ref: &TraitRef<I>,
+    ty: &TyData<I>,
 ) {
+    if db.force_impl_for(well_known, ty) {
+        builder.push_fact(trait_ref.clone());
+    }
+
     match well_known {
         WellKnownTrait::SizedTrait => { /* TODO */ }
     }

chalk-solve/src/lib.rs

@@ -68,6 +68,13 @@ pub trait RustIrDatabase<I: Interner>: Debug {
     /// user gave).
     fn impl_provided_for(&self, auto_trait_id: TraitId<I>, struct_id: StructId<I>) -> bool;
 
+    /// A stop-gap solution to force an impl for a given well-known trait.
+    /// Useful when the logic for a given trait is absent or incomplete.
+    #[allow(unused_variables)]
+    fn force_impl_for(&self, well_known: WellKnownTrait, ty: &TyData<I>) -> bool {
+        false
+    }
+
     fn interner(&self) -> &I;
 }