9 files changed, 104 insertions, 84 deletions

diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 8169b759f..db278d0db 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -245,7 +245,8 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                     Some(callee) => callee,
                     None => return,
                 };
-                let sig = db.callable_item_signature(callee.into()).value;
+                let sig =
+                    db.callable_item_signature(callee.into()).into_value_and_skipped_binders().0;
 
                 (sig, args)
             }
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index 1108e5a10..2e178e5a8 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -353,7 +353,7 @@ impl HirDisplay for Ty {
                                 .as_ref()
                                 .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
                             let bounds = data.subst(parameters);
-                            bounds.value
+                            bounds.into_value_and_skipped_binders().0
                         } else {
                             Vec::new()
                         }
@@ -543,7 +543,7 @@ impl HirDisplay for Ty {
                             .as_ref()
                             .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
                         let bounds = data.subst(&parameters);
-                        write_bounds_like_dyn_trait_with_prefix("impl", &bounds.value, f)?;
+                        write_bounds_like_dyn_trait_with_prefix("impl", bounds.skip_binders(), f)?;
                         // FIXME: it would maybe be good to distinguish this from the alias type (when debug printing), and to show the substitution
                     }
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => {
@@ -630,7 +630,7 @@ impl HirDisplay for Ty {
                             .as_ref()
                             .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
                         let bounds = data.subst(&opaque_ty.substitution);
-                        write_bounds_like_dyn_trait_with_prefix("impl", &bounds.value, f)?;
+                        write_bounds_like_dyn_trait_with_prefix("impl", bounds.skip_binders(), f)?;
                     }
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => {
                         write!(f, "{{async block}}")?;
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index adfdcaa37..61b5cf269 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -118,49 +118,27 @@ pub fn param_idx(db: &dyn HirDatabase, id: TypeParamId) -> Option<usize> {
 }
 
 impl<T> Binders<T> {
-    pub fn new(num_binders: usize, value: T) -> Self {
-        Self { num_binders, value }
-    }
-
     pub fn wrap_empty(value: T) -> Self
     where
         T: TypeWalk,
     {
-        Self { num_binders: 0, value: value.shift_bound_vars(DebruijnIndex::ONE) }
-    }
-
-    pub fn as_ref(&self) -> Binders<&T> {
-        Binders { num_binders: self.num_binders, value: &self.value }
-    }
-
-    pub fn map<U>(self, f: impl FnOnce(T) -> U) -> Binders<U> {
-        Binders { num_binders: self.num_binders, value: f(self.value) }
-    }
-
-    pub fn filter_map<U>(self, f: impl FnOnce(T) -> Option<U>) -> Option<Binders<U>> {
-        Some(Binders { num_binders: self.num_binders, value: f(self.value)? })
-    }
-
-    pub fn skip_binders(&self) -> &T {
-        &self.value
-    }
-
-    pub fn into_value_and_skipped_binders(self) -> (T, usize) {
-        (self.value, self.num_binders)
+        Binders::empty(&Interner, value.shift_bound_vars(DebruijnIndex::ONE))
     }
 }
 
 impl<T: Clone> Binders<&T> {
     pub fn cloned(&self) -> Binders<T> {
-        Binders { num_binders: self.num_binders, value: self.value.clone() }
+        let (value, binders) = self.into_value_and_skipped_binders();
+        Binders::new(binders, value.clone())
     }
 }
 
 impl<T: TypeWalk> Binders<T> {
     /// Substitutes all variables.
     pub fn subst(self, subst: &Substitution) -> T {
-        assert_eq!(subst.len(&Interner), self.num_binders);
-        self.value.subst_bound_vars(subst)
+        let (value, binders) = self.into_value_and_skipped_binders();
+        assert_eq!(subst.len(&Interner), binders);
+        value.subst_bound_vars(subst)
     }
 }
 
@@ -334,12 +312,12 @@ impl Ty {
     /// If this is a `dyn Trait` type, this returns the `Trait` part.
     fn dyn_trait_ref(&self) -> Option<&TraitRef> {
         match self.kind(&Interner) {
-            TyKind::Dyn(dyn_ty) => {
-                dyn_ty.bounds.value.interned().get(0).and_then(|b| match b.skip_binders() {
+            TyKind::Dyn(dyn_ty) => dyn_ty.bounds.skip_binders().interned().get(0).and_then(|b| {
+                match b.skip_binders() {
                     WhereClause::Implemented(trait_ref) => Some(trait_ref),
                     _ => None,
-                })
-            }
+                }
+            }),
             _ => None,
         }
     }
@@ -459,7 +437,7 @@ impl Ty {
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => unreachable!(),
                 };
 
-                predicates.map(|it| it.value)
+                predicates.map(|it| it.into_value_and_skipped_binders().0)
             }
             TyKind::Placeholder(idx) => {
                 let id = from_placeholder_idx(db, *idx);
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index e9e4e69ad..19465b2ed 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -832,12 +832,15 @@ pub fn associated_type_shorthand_candidates<R>(
 
     match res {
         // FIXME: how to correctly handle higher-ranked bounds here?
-        TypeNs::SelfType(impl_id) => {
-            search(db.impl_trait(impl_id)?.value.shift_bound_vars_out(DebruijnIndex::ONE))
-        }
+        TypeNs::SelfType(impl_id) => search(
+            db.impl_trait(impl_id)?
+                .into_value_and_skipped_binders()
+                .0
+                .shift_bound_vars_out(DebruijnIndex::ONE),
+        ),
         TypeNs::GenericParam(param_id) => {
             let predicates = db.generic_predicates_for_param(param_id);
-            let res = predicates.iter().find_map(|pred| match &pred.value.value {
+            let res = predicates.iter().find_map(|pred| match pred.skip_binders().skip_binders() {
                 // FIXME: how to correctly handle higher-ranked bounds here?
                 WhereClause::Implemented(tr) => {
                     search(tr.clone().shift_bound_vars_out(DebruijnIndex::ONE))
@@ -1088,8 +1091,8 @@ fn fn_sig_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> PolyFnS
     let ctx =
         TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
     let params = fields.iter().map(|(_, field)| ctx.lower_ty(&field.type_ref)).collect::<Vec<_>>();
-    let ret = type_for_adt(db, def.into());
-    Binders::new(ret.num_binders, CallableSig::from_params_and_return(params, ret.value, false))
+    let (ret, binders) = type_for_adt(db, def.into()).into_value_and_skipped_binders();
+    Binders::new(binders, CallableSig::from_params_and_return(params, ret, false))
 }
 
 /// Build the type of a tuple struct constructor.
@@ -1114,8 +1117,8 @@ fn fn_sig_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId)
     let ctx =
         TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
     let params = fields.iter().map(|(_, field)| ctx.lower_ty(&field.type_ref)).collect::<Vec<_>>();
-    let ret = type_for_adt(db, def.parent.into());
-    Binders::new(ret.num_binders, CallableSig::from_params_and_return(params, ret.value, false))
+    let (ret, binders) = type_for_adt(db, def.parent.into()).into_value_and_skipped_binders();
+    Binders::new(binders, CallableSig::from_params_and_return(params, ret, false))
 }
 
 /// Build the type of a tuple enum variant constructor.
@@ -1267,9 +1270,9 @@ pub(crate) fn impl_trait_query(db: &dyn HirDatabase, impl_id: ImplId) -> Option<
     let resolver = impl_id.resolver(db.upcast());
     let ctx =
         TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
-    let self_ty = db.impl_self_ty(impl_id);
+    let (self_ty, binders) = db.impl_self_ty(impl_id).into_value_and_skipped_binders();
     let target_trait = impl_data.target_trait.as_ref()?;
-    Some(Binders::new(self_ty.num_binders, ctx.lower_trait_ref(target_trait, Some(self_ty.value))?))
+    Some(Binders::new(binders, ctx.lower_trait_ref(target_trait, Some(self_ty))?))
 }
 
 pub(crate) fn return_type_impl_traits(
diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index 6ace970d1..ad5022808 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -102,11 +102,11 @@ impl TraitImpls {
         for (_module_id, module_data) in crate_def_map.modules() {
             for impl_id in module_data.scope.impls() {
                 let target_trait = match db.impl_trait(impl_id) {
-                    Some(tr) => tr.value.hir_trait_id(),
+                    Some(tr) => tr.skip_binders().hir_trait_id(),
                     None => continue,
                 };
                 let self_ty = db.impl_self_ty(impl_id);
-                let self_ty_fp = TyFingerprint::for_impl(&self_ty.value);
+                let self_ty_fp = TyFingerprint::for_impl(self_ty.skip_binders());
                 impls
                     .map
                     .entry(target_trait)
@@ -201,7 +201,7 @@ impl InherentImpls {
                 }
 
                 let self_ty = db.impl_self_ty(impl_id);
-                if let Some(fp) = TyFingerprint::for_impl(&self_ty.value) {
+                if let Some(fp) = TyFingerprint::for_impl(self_ty.skip_binders()) {
                     map.entry(fp).or_default().push(impl_id);
                 }
             }
@@ -774,7 +774,7 @@ fn transform_receiver_ty(
         AssocContainerId::ModuleId(_) => unreachable!(),
     };
     let sig = db.callable_item_signature(function_id.into());
-    Some(sig.value.params()[0].clone().subst_bound_vars(&substs))
+    Some(sig.map(|s| s.params()[0].clone()).subst(&substs))
 }
 
 pub fn implements_trait(
diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index b7388b98c..47867f77e 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -184,10 +184,15 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
                     .db
                     .return_type_impl_traits(func)
                     .expect("impl trait id without impl traits");
-                let data = &datas.value.impl_traits[idx as usize];
+                let data = &datas.skip_binders().impl_traits[idx as usize];
                 let bound = OpaqueTyDatumBound {
                     bounds: make_binders(
-                        data.bounds.value.iter().cloned().map(|b| b.to_chalk(self.db)).collect(),
+                        data.bounds
+                            .skip_binders()
+                            .iter()
+                            .cloned()
+                            .map(|b| b.to_chalk(self.db))
+                            .collect(),
                         1,
                     ),
                     where_clauses: make_binders(vec![], 0),
@@ -535,7 +540,8 @@ fn impl_def_datum(
         .impl_trait(impl_id)
         // ImplIds for impls where the trait ref can't be resolved should never reach Chalk
         .expect("invalid impl passed to Chalk")
-        .value;
+        .into_value_and_skipped_binders()
+        .0;
     let impl_data = db.impl_data(impl_id);
 
     let generic_params = generics(db.upcast(), impl_id.into());
@@ -605,18 +611,22 @@ fn type_alias_associated_ty_value(
         _ => panic!("assoc ty value should be in impl"),
     };
 
-    let trait_ref = db.impl_trait(impl_id).expect("assoc ty value should not exist").value; // we don't return any assoc ty values if the impl'd trait can't be resolved
+    let trait_ref = db
+        .impl_trait(impl_id)
+        .expect("assoc ty value should not exist")
+        .into_value_and_skipped_binders()
+        .0; // we don't return any assoc ty values if the impl'd trait can't be resolved
 
     let assoc_ty = db
         .trait_data(trait_ref.hir_trait_id())
         .associated_type_by_name(&type_alias_data.name)
         .expect("assoc ty value should not exist"); // validated when building the impl data as well
-    let ty = db.ty(type_alias.into());
-    let value_bound = rust_ir::AssociatedTyValueBound { ty: ty.value.to_chalk(db) };
+    let (ty, binders) = db.ty(type_alias.into()).into_value_and_skipped_binders();
+    let value_bound = rust_ir::AssociatedTyValueBound { ty: ty.to_chalk(db) };
     let value = rust_ir::AssociatedTyValue {
         impl_id: impl_id.to_chalk(db),
         associated_ty_id: to_assoc_type_id(assoc_ty),
-        value: make_binders(value_bound, ty.num_binders),
+        value: make_binders(value_bound, binders),
     };
     Arc::new(value)
 }
@@ -628,20 +638,15 @@ pub(crate) fn fn_def_datum_query(
 ) -> Arc<FnDefDatum> {
     let callable_def: CallableDefId = from_chalk(db, fn_def_id);
     let generic_params = generics(db.upcast(), callable_def.into());
-    let sig = db.callable_item_signature(callable_def);
+    let (sig, binders) = db.callable_item_signature(callable_def).into_value_and_skipped_binders();
     let bound_vars = generic_params.bound_vars_subst(DebruijnIndex::INNERMOST);
     let where_clauses = convert_where_clauses(db, callable_def.into(), &bound_vars);
     let bound = rust_ir::FnDefDatumBound {
         // Note: Chalk doesn't actually use this information yet as far as I am aware, but we provide it anyway
         inputs_and_output: make_binders(
             rust_ir::FnDefInputsAndOutputDatum {
-                argument_types: sig
-                    .value
-                    .params()
-                    .iter()
-                    .map(|ty| ty.clone().to_chalk(db))
-                    .collect(),
-                return_type: sig.value.ret().clone().to_chalk(db),
+                argument_types: sig.params().iter().map(|ty| ty.clone().to_chalk(db)).collect(),
+                return_type: sig.ret().clone().to_chalk(db),
             }
             .shifted_in(&Interner),
             0,
@@ -650,12 +655,8 @@ pub(crate) fn fn_def_datum_query(
     };
     let datum = FnDefDatum {
         id: fn_def_id,
-        sig: chalk_ir::FnSig {
-            abi: (),
-            safety: chalk_ir::Safety::Safe,
-            variadic: sig.value.is_varargs,
-        },
-        binders: make_binders(bound, sig.num_binders),
+        sig: chalk_ir::FnSig { abi: (), safety: chalk_ir::Safety::Safe, variadic: sig.is_varargs },
+        binders: make_binders(bound, binders),
     };
     Arc::new(datum)
 }
diff --git a/crates/hir_ty/src/traits/chalk/mapping.rs b/crates/hir_ty/src/traits/chalk/mapping.rs
index 67e88ebf4..72458f367 100644
--- a/crates/hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/hir_ty/src/traits/chalk/mapping.rs
@@ -93,12 +93,13 @@ impl ToChalk for Ty {
             TyKind::BoundVar(idx) => chalk_ir::TyKind::BoundVar(idx).intern(&Interner),
             TyKind::InferenceVar(..) => panic!("uncanonicalized infer ty"),
             TyKind::Dyn(dyn_ty) => {
+                let (bounds, binders) = dyn_ty.bounds.into_value_and_skipped_binders();
                 let where_clauses = chalk_ir::QuantifiedWhereClauses::from_iter(
                     &Interner,
-                    dyn_ty.bounds.value.interned().iter().cloned().map(|p| p.to_chalk(db)),
+                    bounds.interned().iter().cloned().map(|p| p.to_chalk(db)),
                 );
                 let bounded_ty = chalk_ir::DynTy {
-                    bounds: make_binders(where_clauses, 1),
+                    bounds: make_binders(where_clauses, binders),
                     lifetime: LifetimeData::Static.intern(&Interner),
                 };
                 chalk_ir::TyKind::Dyn(bounded_ty).intern(&Interner)
@@ -486,13 +487,14 @@ where
     type Chalk = chalk_ir::Binders<T::Chalk>;
 
     fn to_chalk(self, db: &dyn HirDatabase) -> chalk_ir::Binders<T::Chalk> {
+        let (value, binders) = self.into_value_and_skipped_binders();
         chalk_ir::Binders::new(
             chalk_ir::VariableKinds::from_iter(
                 &Interner,
                 std::iter::repeat(chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General))
-                    .take(self.num_binders),
+                    .take(binders),
             ),
-            self.value.to_chalk(db),
+            value.to_chalk(db),
         )
     }
 
@@ -537,7 +539,8 @@ pub(super) fn generic_predicate_to_inline_bound(
     // An InlineBound is like a GenericPredicate, except the self type is left out.
     // We don't have a special type for this, but Chalk does.
     let self_ty_shifted_in = self_ty.clone().shift_bound_vars(DebruijnIndex::ONE);
-    match &pred.value {
+    let (pred, binders) = pred.as_ref().into_value_and_skipped_binders();
+    match pred {
         WhereClause::Implemented(trait_ref) => {
             if trait_ref.self_type_parameter(&Interner) != &self_ty_shifted_in {
                 // we can only convert predicates back to type bounds if they
@@ -549,7 +552,7 @@ pub(super) fn generic_predicate_to_inline_bound(
                 .map(|ty| ty.clone().to_chalk(db).cast(&Interner))
                 .collect();
             let trait_bound = rust_ir::TraitBound { trait_id: trait_ref.trait_id, args_no_self };
-            Some(make_binders(rust_ir::InlineBound::TraitBound(trait_bound), pred.num_binders))
+            Some(make_binders(rust_ir::InlineBound::TraitBound(trait_bound), binders))
         }
         WhereClause::AliasEq(AliasEq { alias: AliasTy::Projection(projection_ty), ty }) => {
             if projection_ty.self_type_parameter(&Interner) != &self_ty_shifted_in {
@@ -566,7 +569,7 @@ pub(super) fn generic_predicate_to_inline_bound(
                 associated_ty_id: projection_ty.associated_ty_id,
                 parameters: Vec::new(), // FIXME we don't support generic associated types yet
             };
-            Some(make_binders(rust_ir::InlineBound::AliasEqBound(alias_eq_bound), pred.num_binders))
+            Some(make_binders(rust_ir::InlineBound::AliasEqBound(alias_eq_bound), binders))
         }
         _ => None,
     }
diff --git a/crates/hir_ty/src/types.rs b/crates/hir_ty/src/types.rs
index bac086318..0e3dd511f 100644
--- a/crates/hir_ty/src/types.rs
+++ b/crates/hir_ty/src/types.rs
@@ -299,7 +299,41 @@ impl Substitution {
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub struct Binders<T> {
     pub num_binders: usize,
-    pub value: T,
+    value: T,
+}
+
+impl<T> Binders<T> {
+    pub fn new(num_binders: usize, value: T) -> Self {
+        Self { num_binders, value }
+    }
+
+    pub fn empty(_interner: &Interner, value: T) -> Self {
+        Self { num_binders: 0, value }
+    }
+
+    pub fn as_ref(&self) -> Binders<&T> {
+        Binders { num_binders: self.num_binders, value: &self.value }
+    }
+
+    pub fn map<U>(self, f: impl FnOnce(T) -> U) -> Binders<U> {
+        Binders { num_binders: self.num_binders, value: f(self.value) }
+    }
+
+    pub fn filter_map<U>(self, f: impl FnOnce(T) -> Option<U>) -> Option<Binders<U>> {
+        Some(Binders { num_binders: self.num_binders, value: f(self.value)? })
+    }
+
+    pub fn skip_binders(&self) -> &T {
+        &self.value
+    }
+
+    pub fn into_value_and_skipped_binders(self) -> (T, usize) {
+        (self.value, self.num_binders)
+    }
+
+    pub fn skip_binders_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
 }
 
 /// A trait with type parameters. This includes the `Self`, so this represents a concrete type implementing the trait.
diff --git a/crates/hir_ty/src/walk.rs b/crates/hir_ty/src/walk.rs
index 5dfd59746..b1e22da0a 100644
--- a/crates/hir_ty/src/walk.rs
+++ b/crates/hir_ty/src/walk.rs
@@ -139,7 +139,7 @@ impl TypeWalk for Ty {
                 }
             }
             TyKind::Dyn(dyn_ty) => {
-                for p in dyn_ty.bounds.value.interned().iter() {
+                for p in dyn_ty.bounds.skip_binders().interned().iter() {
                     p.walk(f);
                 }
             }
@@ -167,7 +167,7 @@ impl TypeWalk for Ty {
                 p_ty.substitution.walk_mut_binders(f, binders);
             }
             TyKind::Dyn(dyn_ty) => {
-                for p in make_mut_slice(dyn_ty.bounds.value.interned_mut()) {
+                for p in make_mut_slice(dyn_ty.bounds.skip_binders_mut().interned_mut()) {
                     p.walk_mut_binders(f, binders.shifted_in());
                 }
             }
@@ -294,7 +294,7 @@ impl TypeWalk for Substitution {
 
 impl<T: TypeWalk> TypeWalk for Binders<T> {
     fn walk(&self, f: &mut impl FnMut(&Ty)) {
-        self.value.walk(f);
+        self.skip_binders().walk(f);
     }
 
     fn walk_mut_binders(
@@ -302,7 +302,7 @@ impl<T: TypeWalk> TypeWalk for Binders<T> {
         f: &mut impl FnMut(&mut Ty, DebruijnIndex),
         binders: DebruijnIndex,
     ) {
-        self.value.walk_mut_binders(f, binders.shifted_in())
+        self.skip_binders_mut().walk_mut_binders(f, binders.shifted_in())
     }
 }