Consider subst in prioritising placeholder inherent methods

crates/hir-ty/src/chalk_ext.rs

@@ -43,6 +43,7 @@ pub trait TyExt {
 
     /// If this is a `dyn Trait`, returns that trait.
     fn dyn_trait(&self) -> Option<TraitId>;
+    fn dyn_trait_ref(&self) -> Option<TraitRef>;
 
     fn impl_trait_bounds(&self, db: &dyn HirDatabase) -> Option<Vec<QuantifiedWhereClause>>;
     fn associated_type_parent_trait(&self, db: &dyn HirDatabase) -> Option<TraitId>;
@@ -193,6 +194,31 @@ impl TyExt for Ty {
         }
     }
 
+    fn dyn_trait_ref(&self) -> Option<TraitRef> {
+        let trait_ref = match self.kind(Interner) {
+            // The principal trait bound should be the first element of the bounds. This is an
+            // invariant ensured by `TyLoweringContext::lower_dyn_trait()`.
+            // FIXME: dyn types may not have principal trait and we don't want to return auto trait
+            // here.
+            TyKind::Dyn(dyn_ty) => dyn_ty.bounds.as_ref().filter_map(|x| {
+                x.interned().get(0).and_then(|b| {
+                    b.as_ref().filter_map(|b| match b {
+                        WhereClause::Implemented(trait_ref) => Some(trait_ref.clone()),
+                        _ => None,
+                    })
+                })
+            }),
+            _ => None,
+        }?;
+        // FIXME: qualified bounds not handled (is not valid here I think?)
+        Some(
+            trait_ref
+                .substitute(Interner, &Substitution::from1(Interner, self.clone()))
+                .into_value_and_skipped_binders()
+                .0,
+        )
+    }
+
     fn dyn_trait(&self) -> Option<TraitId> {
         let trait_ref = match self.kind(Interner) {
             // The principal trait bound should be the first element of the bounds. This is an

crates/hir-ty/src/infer/expr.rs

@@ -1483,10 +1483,44 @@ impl<'a> InferenceContext<'a> {
             method_name,
         );
         let (receiver_ty, method_ty, substs) = match resolved {
-            Some((adjust, func, visible)) => {
+            Some((adjust, func, visible, subst)) => {
                 let (ty, adjustments) = adjust.apply(&mut self.table, receiver_ty);
                 let generics = generics(self.db.upcast(), func.into());
-                let substs = self.substs_for_method_call(generics, generic_args);
+                let substs = match subst {
+                    Some(s) => {
+                        // FIXME: here we need the method subst, but result of `lookup_method` incorrectly returns the trait
+                        // subst. For some traits (which their methods are not generic, and are not `dyn Trait`) this
+                        // happens to be the same, so I didn't bothered myself to correctly implement it. Instead, I added
+                        // this check and we ignore the returned subst if it doesn't match.
+                        //
+                        // It might be a good idea to keep this check even after fixing the above, and just add a `never!` to
+                        // it, as mistakes will make chalk panic. But if you are sure about the implementation, you can remove
+                        // it to save some performance.
+                        let default_subst = self.substs_for_method_call(generics, generic_args);
+                        let is_valid = 'b: {
+                            if default_subst.len(Interner) != s.len(Interner) {
+                                break 'b false;
+                            }
+                            default_subst.iter(Interner).zip(s.iter(Interner)).all(|(x, y)| {
+                                matches!(
+                                    (x.data(Interner), y.data(Interner),),
+                                    (GenericArgData::Ty(_), GenericArgData::Ty(_))
+                                        | (
+                                            GenericArgData::Lifetime(_),
+                                            GenericArgData::Lifetime(_)
+                                        )
+                                        | (GenericArgData::Const(_), GenericArgData::Const(_))
+                                )
+                            })
+                        };
+                        if is_valid {
+                            s
+                        } else {
+                            default_subst
+                        }
+                    }
+                    None => self.substs_for_method_call(generics, generic_args),
+                };
                 self.write_expr_adj(receiver, adjustments);
                 self.write_method_resolution(tgt_expr, func, substs.clone());
                 if !visible {

crates/hir-ty/src/infer/path.rs

@@ -237,19 +237,19 @@ impl<'a> InferenceContext<'a> {
             VisibleFromModule::Filter(self.resolver.module()),
             Some(name),
             method_resolution::LookupMode::Path,
-            |_ty, item, visible| {
+            |_ty, item, visible, subst| {
                 if visible {
-                    Some((item, true))
+                    Some((item, true, subst))
                 } else {
                     if not_visible.is_none() {
-                        not_visible = Some((item, false));
+                        not_visible = Some((item, false, subst));
                     }
                     None
                 }
             },
         );
         let res = res.or(not_visible);
-        let (item, visible) = res?;
+        let (item, visible, substs) = res?;
 
         let (def, container) = match item {
             AssocItemId::FunctionId(f) => {
@@ -258,7 +258,7 @@ impl<'a> InferenceContext<'a> {
             AssocItemId::ConstId(c) => (ValueNs::ConstId(c), c.lookup(self.db.upcast()).container),
             AssocItemId::TypeAliasId(_) => unreachable!(),
         };
-        let substs = match container {
+        let substs = substs.unwrap_or_else(|| match container {
             ItemContainerId::ImplId(impl_id) => {
                 let impl_substs = TyBuilder::subst_for_def(self.db, impl_id, None)
                     .fill_with_inference_vars(&mut self.table)
@@ -278,9 +278,9 @@ impl<'a> InferenceContext<'a> {
             }
             ItemContainerId::ModuleId(_) | ItemContainerId::ExternBlockId(_) => {
                 never!("assoc item contained in module/extern block");
-                return None;
+                return Substitution::empty(Interner);
             }
-        };
+        });
 
         self.write_assoc_resolution(id, item, substs.clone());
         if !visible {

crates/hir-ty/src/lower.rs

@@ -1405,9 +1405,11 @@ pub(crate) fn generic_predicates_for_param_query(
                             Some(TypeNs::TraitId(tr)) => tr,
                             _ => return false,
                         };
-
-                        all_super_traits(db.upcast(), tr).iter().any(|tr| {
-                            db.trait_data(*tr).items.iter().any(|(name, item)| {
+                        let substs = TyBuilder::placeholder_subst(db, tr);
+                        let tr_ref =
+                            TraitRef { trait_id: to_chalk_trait_id(tr), substitution: substs };
+                        all_super_traits(db, tr_ref).iter().any(|tr| {
+                            db.trait_data(tr.hir_trait_id()).items.iter().any(|(name, item)| {
                                 matches!(item, AssocItemId::TypeAliasId(_)) && name == assoc_name
                             })
                         })
@@ -1464,7 +1466,7 @@ pub(crate) fn trait_environment_query(
     for pred in resolver.where_predicates_in_scope() {
         for pred in ctx.lower_where_predicate(pred, false) {
             if let WhereClause::Implemented(tr) = &pred.skip_binders() {
-                traits_in_scope.push((tr.self_type_parameter(Interner).clone(), tr.hir_trait_id()));
+                traits_in_scope.push((tr.self_type_parameter(Interner).clone(), tr.clone()));
             }
             let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(Interner);
             clauses.push(program_clause.into_from_env_clause(Interner));