Ignore type bindings in generic_predicates_for_param

This allows us to handle more cases without a query cycle, which
includes certain cases that rustc accepted. That in turn means we avoid
triggering salsa-rs/salsa#257 on valid code (it will still happen if the
user writes an actual cycle).

We actually accept more definitions than rustc now; that's because rustc
only ignores bindings when looking up super traits, whereas we now also
ignore them when looking for predicates to disambiguate associated type
shorthand. We could introduce a separate query for super traits if
necessary, but for now I think this should be fine.

6 files changed, 52 insertions, 19 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index e34be7e42..568cb7eb9 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -2068,7 +2068,10 @@ impl Type {
                 match pred {
                     WhereClause::Implemented(trait_ref) => {
                         cb(type_.clone());
-                        walk_substs(db, type_, &trait_ref.substitution, cb);
+                        // skip the self type. it's likely the type we just got the bounds from
+                        for ty in trait_ref.substitution.iter().skip(1) {
+                            walk_type(db, &type_.derived(ty.clone()), cb);
+                        }
                     }
                     _ => (),
                 }
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index 3845009ae..9d3b79be3 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -571,13 +571,22 @@ impl HirDisplay for Ty {
                         write!(f, "{}", param_data.name.clone().unwrap_or_else(Name::missing))?
                     }
                     TypeParamProvenance::ArgumentImplTrait => {
-                        let bounds = f.db.generic_predicates_for_param(id);
                         let substs = Substitution::type_params_for_generics(f.db, &generics);
-                        write_bounds_like_dyn_trait_with_prefix(
-                            "impl",
-                            &bounds.iter().map(|b| b.clone().subst(&substs)).collect::<Vec<_>>(),
-                            f,
-                        )?;
+                        let bounds = f
+                            .db
+                            .generic_predicates(id.parent)
+                            .into_iter()
+                            .map(|pred| pred.clone().subst(&substs))
+                            .filter(|wc| match &wc {
+                                WhereClause::Implemented(tr) => tr.self_type_parameter() == self,
+                                WhereClause::AliasEq(AliasEq {
+                                    alias: AliasTy::Projection(proj),
+                                    ty: _,
+                                }) => proj.self_type_parameter() == self,
+                                _ => false,
+                            })
+                            .collect::<Vec<_>>();
+                        write_bounds_like_dyn_trait_with_prefix("impl", &bounds, f)?;
                     }
                 }
             }
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index 815bb8418..ad908f957 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -940,10 +940,19 @@ impl Ty {
                 let param_data = &generic_params.types[id.local_id];
                 match param_data.provenance {
                     hir_def::generics::TypeParamProvenance::ArgumentImplTrait => {
+                        let substs = Substitution::type_params(db, id.parent);
                         let predicates = db
-                            .generic_predicates_for_param(id)
+                            .generic_predicates(id.parent)
                             .into_iter()
-                            .map(|pred| pred.value.clone())
+                            .map(|pred| pred.clone().subst(&substs))
+                            .filter(|wc| match &wc {
+                                WhereClause::Implemented(tr) => tr.self_type_parameter() == self,
+                                WhereClause::AliasEq(AliasEq {
+                                    alias: AliasTy::Projection(proj),
+                                    ty: _,
+                                }) => proj.self_type_parameter() == self,
+                                _ => false,
+                            })
                             .collect_vec();
 
                         Some(predicates)
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index cbbb535e5..2bdfcd310 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -189,7 +189,10 @@ impl<'a> TyLoweringContext<'a> {
                 let self_ty =
                     TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(&Interner);
                 let predicates = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
-                    bounds.iter().flat_map(|b| ctx.lower_type_bound(b, self_ty.clone())).collect()
+                    bounds
+                        .iter()
+                        .flat_map(|b| ctx.lower_type_bound(b, self_ty.clone(), false))
+                        .collect()
                 });
                 TyKind::Dyn(predicates).intern(&Interner)
             }
@@ -666,6 +669,7 @@ impl<'a> TyLoweringContext<'a> {
     pub(crate) fn lower_where_predicate(
         &'a self,
         where_predicate: &'a WherePredicate,
+        ignore_bindings: bool,
     ) -> impl Iterator<Item = WhereClause> + 'a {
         match where_predicate {
             WherePredicate::ForLifetime { target, bound, .. }
@@ -688,7 +692,9 @@ impl<'a> TyLoweringContext<'a> {
                         .intern(&Interner)
                     }
                 };
-                self.lower_type_bound(bound, self_ty).collect::<Vec<_>>().into_iter()
+                self.lower_type_bound(bound, self_ty, ignore_bindings)
+                    .collect::<Vec<_>>()
+                    .into_iter()
             }
             WherePredicate::Lifetime { .. } => vec![].into_iter(),
         }
@@ -698,6 +704,7 @@ impl<'a> TyLoweringContext<'a> {
         &'a self,
         bound: &'a TypeBound,
         self_ty: Ty,
+        ignore_bindings: bool,
     ) -> impl Iterator<Item = WhereClause> + 'a {
         let mut bindings = None;
         let trait_ref = match bound {
@@ -711,6 +718,7 @@ impl<'a> TyLoweringContext<'a> {
         trait_ref.into_iter().chain(
             bindings
                 .into_iter()
+                .filter(move |_| !ignore_bindings)
                 .flat_map(move |tr| self.assoc_type_bindings_from_type_bound(bound, tr)),
         )
     }
@@ -755,6 +763,7 @@ impl<'a> TyLoweringContext<'a> {
                     preds.extend(self.lower_type_bound(
                         bound,
                         TyKind::Alias(AliasTy::Projection(projection_ty.clone())).intern(&Interner),
+                        false,
                     ));
                 }
                 preds
@@ -766,7 +775,7 @@ impl<'a> TyLoweringContext<'a> {
         let self_ty =
             TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(&Interner);
         let predicates = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
-            bounds.iter().flat_map(|b| ctx.lower_type_bound(b, self_ty.clone())).collect()
+            bounds.iter().flat_map(|b| ctx.lower_type_bound(b, self_ty.clone(), false)).collect()
         });
         ReturnTypeImplTrait { bounds: Binders::new(1, predicates) }
     }
@@ -896,7 +905,9 @@ pub(crate) fn generic_predicates_for_param_query(
             },
             WherePredicate::Lifetime { .. } => false,
         })
-        .flat_map(|pred| ctx.lower_where_predicate(pred).map(|p| Binders::new(generics.len(), p)))
+        .flat_map(|pred| {
+            ctx.lower_where_predicate(pred, true).map(|p| Binders::new(generics.len(), p))
+        })
         .collect()
 }
 
@@ -918,7 +929,7 @@ pub(crate) fn trait_environment_query(
     let mut traits_in_scope = Vec::new();
     let mut clauses = Vec::new();
     for pred in resolver.where_predicates_in_scope() {
-        for pred in ctx.lower_where_predicate(pred) {
+        for pred in ctx.lower_where_predicate(pred, false) {
             if let WhereClause::Implemented(tr) = &pred {
                 traits_in_scope.push((tr.self_type_parameter().clone(), tr.hir_trait_id()));
             }
@@ -967,7 +978,9 @@ pub(crate) fn generic_predicates_query(
     let generics = generics(db.upcast(), def);
     resolver
         .where_predicates_in_scope()
-        .flat_map(|pred| ctx.lower_where_predicate(pred).map(|p| Binders::new(generics.len(), p)))
+        .flat_map(|pred| {
+            ctx.lower_where_predicate(pred, false).map(|p| Binders::new(generics.len(), p))
+        })
         .collect()
 }
 
diff --git a/crates/hir_ty/src/tests/traits.rs b/crates/hir_ty/src/tests/traits.rs
index 1bb6dff95..37cd04c6f 100644
--- a/crates/hir_ty/src/tests/traits.rs
+++ b/crates/hir_ty/src/tests/traits.rs
@@ -2273,8 +2273,7 @@ fn test<T, U>() where T: Trait<U::Item>, U: Trait<T::Item> {
 
 #[test]
 fn unselected_projection_in_trait_env_cycle_3() {
-    // this is a cycle, although it would be possible to handle if we didn't go
-    // into bindings when looking for traits
+    // this is a cycle for rustc; we currently accept it
     check_types(
         r#"
 //- /main.rs
@@ -2285,7 +2284,7 @@ trait Trait {
 
 fn test<T>() where T: Trait<OtherItem = T::Item> {
     let x: T::Item = no_matter;
-}                   //^ {unknown}
+}                   //^ Trait::Item<T>
 "#,
     );
 }
diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index 734679414..944145603 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -395,7 +395,7 @@ pub(crate) fn associated_ty_data_query(
     let bounds = type_alias_data
         .bounds
         .iter()
-        .flat_map(|bound| ctx.lower_type_bound(bound, self_ty.clone()))
+        .flat_map(|bound| ctx.lower_type_bound(bound, self_ty.clone(), false))
         .filter_map(|pred| generic_predicate_to_inline_bound(db, &pred, &self_ty))
         .map(|bound| make_binders(bound.shifted_in(&Interner), 0))
         .collect();