16 files changed, 301 insertions, 152 deletions

diff --git a/crates/hir_ty/src/autoderef.rs b/crates/hir_ty/src/autoderef.rs
index dc5fc759a..5e3c018a8 100644
--- a/crates/hir_ty/src/autoderef.rs
+++ b/crates/hir_ty/src/autoderef.rs
@@ -131,7 +131,7 @@ fn deref_by_trait(
             // new variables in that case
 
             for i in 1..vars.0.binders.len(&Interner) {
-                if vars.0.value[i - 1].interned(&Interner)
+                if vars.0.value.at(&Interner, i - 1).assert_ty_ref(&Interner).interned(&Interner)
                     != &TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
                 {
                     warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.goal, solution);
@@ -139,7 +139,12 @@ fn deref_by_trait(
                 }
             }
             Some(Canonical {
-                value: vars.0.value[vars.0.value.len() - 1].clone(),
+                value: vars
+                    .0
+                    .value
+                    .at(&Interner, vars.0.value.len(&Interner) - 1)
+                    .assert_ty_ref(&Interner)
+                    .clone(),
                 binders: vars.0.binders.clone(),
             })
         }
diff --git a/crates/hir_ty/src/chalk_cast.rs b/crates/hir_ty/src/chalk_cast.rs
index bf884ae15..df6492113 100644
--- a/crates/hir_ty/src/chalk_cast.rs
+++ b/crates/hir_ty/src/chalk_cast.rs
@@ -5,7 +5,7 @@ use chalk_ir::{
     interner::HasInterner,
 };
 
-use crate::{AliasEq, DomainGoal, Interner, TraitRef, WhereClause};
+use crate::{AliasEq, DomainGoal, GenericArg, GenericArgData, Interner, TraitRef, Ty, WhereClause};
 
 macro_rules! has_interner {
     ($t:ty) => {
@@ -17,6 +17,8 @@ macro_rules! has_interner {
 
 has_interner!(WhereClause);
 has_interner!(DomainGoal);
+has_interner!(GenericArg);
+has_interner!(Ty);
 
 impl CastTo<WhereClause> for TraitRef {
     fn cast_to(self, _interner: &Interner) -> WhereClause {
@@ -36,6 +38,12 @@ impl CastTo<DomainGoal> for WhereClause {
     }
 }
 
+impl CastTo<GenericArg> for Ty {
+    fn cast_to(self, interner: &Interner) -> GenericArg {
+        GenericArg::new(interner, GenericArgData::Ty(self))
+    }
+}
+
 macro_rules! transitive_impl {
     ($a:ty, $b:ty, $c:ty) => {
         impl CastTo<$c> for $a {
@@ -51,3 +59,15 @@ macro_rules! transitive_impl {
 
 transitive_impl!(TraitRef, WhereClause, DomainGoal);
 transitive_impl!(AliasEq, WhereClause, DomainGoal);
+
+macro_rules! reflexive_impl {
+    ($a:ty) => {
+        impl CastTo<$a> for $a {
+            fn cast_to(self, _interner: &Interner) -> $a {
+                self
+            }
+        }
+    };
+}
+
+reflexive_impl!(GenericArg);
diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 3909ad354..373d1cb74 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -392,7 +392,9 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             _ => return,
         };
 
-        if params.len() > 0 && params[0] == mismatch.actual {
+        if params.len(&Interner) > 0
+            && params.at(&Interner, 0).ty(&Interner) == Some(&mismatch.actual)
+        {
             let (_, source_map) = db.body_with_source_map(self.owner);
 
             if let Ok(source_ptr) = source_map.expr_syntax(id) {
diff --git a/crates/hir_ty/src/diagnostics/match_check.rs b/crates/hir_ty/src/diagnostics/match_check.rs
index 9cb472b51..85ba58c44 100644
--- a/crates/hir_ty/src/diagnostics/match_check.rs
+++ b/crates/hir_ty/src/diagnostics/match_check.rs
@@ -792,7 +792,10 @@ fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Opt
         Pat::Tuple { .. } => {
             let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
             Some(Constructor::Tuple {
-                arity: cx.infer.type_of_pat[pat_id].as_tuple().ok_or(MatchCheckErr::Unknown)?.len(),
+                arity: cx.infer.type_of_pat[pat_id]
+                    .as_tuple()
+                    .ok_or(MatchCheckErr::Unknown)?
+                    .len(&Interner),
             })
         }
         Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index 51480304b..59fd18c2a 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -8,7 +8,7 @@ use hir_def::{
     find_path,
     generics::TypeParamProvenance,
     item_scope::ItemInNs,
-    path::{GenericArg, Path, PathKind},
+    path::{Path, PathKind},
     type_ref::{TypeBound, TypeRef},
     visibility::Visibility,
     AssocContainerId, Lookup, ModuleId, TraitId,
@@ -18,7 +18,7 @@ use hir_expand::name::Name;
 use crate::{
     db::HirDatabase, from_assoc_type_id, from_foreign_def_id, from_placeholder_idx, primitive,
     to_assoc_type_id, traits::chalk::from_chalk, utils::generics, AdtId, AliasEq, AliasTy,
-    CallableDefId, CallableSig, DomainGoal, ImplTraitId, Interner, Lifetime, OpaqueTy,
+    CallableDefId, CallableSig, DomainGoal, GenericArg, ImplTraitId, Interner, Lifetime, OpaqueTy,
     ProjectionTy, QuantifiedWhereClause, Scalar, Substitution, TraitRef, Ty, TyKind, WhereClause,
 };
 
@@ -251,16 +251,16 @@ impl HirDisplay for ProjectionTy {
         }
 
         let trait_ = f.db.trait_data(self.trait_(f.db));
-        let first_parameter = self.substitution[0].into_displayable(
+        let first_parameter = self.self_type_parameter().into_displayable(
             f.db,
             f.max_size,
             f.omit_verbose_types,
             f.display_target,
         );
         write!(f, "<{} as {}", first_parameter, trait_.name)?;
-        if self.substitution.len() > 1 {
+        if self.substitution.len(&Interner) > 1 {
             write!(f, "<")?;
-            f.write_joined(&self.substitution[1..], ", ")?;
+            f.write_joined(&self.substitution.interned(&Interner)[1..], ", ")?;
             write!(f, ">")?;
         }
         write!(f, ">::{}", f.db.type_alias_data(from_assoc_type_id(self.associated_ty_id)).name)?;
@@ -274,7 +274,15 @@ impl HirDisplay for OpaqueTy {
             return write!(f, "{}", TYPE_HINT_TRUNCATION);
         }
 
-        self.substitution[0].hir_fmt(f)
+        self.substitution.at(&Interner, 0).hir_fmt(f)
+    }
+}
+
+impl HirDisplay for GenericArg {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        match self.interned() {
+            crate::GenericArgData::Ty(ty) => ty.hir_fmt(f),
+        }
     }
 }
 
@@ -373,9 +381,9 @@ impl HirDisplay for Ty {
                 }
             }
             TyKind::Tuple(_, substs) => {
-                if substs.len() == 1 {
+                if substs.len(&Interner) == 1 {
                     write!(f, "(")?;
-                    substs[0].hir_fmt(f)?;
+                    substs.at(&Interner, 0).hir_fmt(f)?;
                     write!(f, ",)")?;
                 } else {
                     write!(f, "(")?;
@@ -399,7 +407,7 @@ impl HirDisplay for Ty {
                         write!(f, "{}", f.db.enum_data(e.parent).variants[e.local_id].name)?
                     }
                 };
-                if parameters.len() > 0 {
+                if parameters.len(&Interner) > 0 {
                     let generics = generics(f.db.upcast(), def.into());
                     let (parent_params, self_param, type_params, _impl_trait_params) =
                         generics.provenance_split();
@@ -451,7 +459,7 @@ impl HirDisplay for Ty {
                     }
                 }
 
-                if parameters.len() > 0 {
+                if parameters.len(&Interner) > 0 {
                     let parameters_to_write = if f.display_target.is_source_code()
                         || f.omit_verbose_types()
                     {
@@ -463,9 +471,11 @@ impl HirDisplay for Ty {
                             None => parameters.0.as_ref(),
                             Some(default_parameters) => {
                                 let mut default_from = 0;
-                                for (i, parameter) in parameters.iter().enumerate() {
-                                    match (parameter.interned(&Interner), default_parameters.get(i))
-                                    {
+                                for (i, parameter) in parameters.iter(&Interner).enumerate() {
+                                    match (
+                                        parameter.assert_ty_ref(&Interner).interned(&Interner),
+                                        default_parameters.get(i),
+                                    ) {
                                         (&TyKind::Unknown, _) | (_, None) => {
                                             default_from = i + 1;
                                         }
@@ -473,7 +483,8 @@ impl HirDisplay for Ty {
                                             let actual_default = default_parameter
                                                 .clone()
                                                 .subst(&parameters.prefix(i));
-                                            if parameter != &actual_default {
+                                            if parameter.assert_ty_ref(&Interner) != &actual_default
+                                            {
                                                 default_from = i + 1;
                                             }
                                         }
@@ -504,7 +515,7 @@ impl HirDisplay for Ty {
                 // Use placeholder associated types when the target is test (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
                 if f.display_target.is_test() {
                     write!(f, "{}::{}", trait_.name, type_alias_data.name)?;
-                    if parameters.len() > 0 {
+                    if parameters.len(&Interner) > 0 {
                         write!(f, "<")?;
                         f.write_joined(&*parameters.0, ", ")?;
                         write!(f, ">")?;
@@ -537,7 +548,7 @@ impl HirDisplay for Ty {
                     }
                     ImplTraitId::AsyncBlockTypeImplTrait(..) => {
                         write!(f, "impl Future<Output = ")?;
-                        parameters[0].hir_fmt(f)?;
+                        parameters.at(&Interner, 0).hir_fmt(f)?;
                         write!(f, ">")?;
                     }
                 }
@@ -548,7 +559,7 @@ impl HirDisplay for Ty {
                         DisplaySourceCodeError::Closure,
                     ));
                 }
-                let sig = substs[0].callable_sig(f.db);
+                let sig = substs.at(&Interner, 0).assert_ty_ref(&Interner).callable_sig(f.db);
                 if let Some(sig) = sig {
                     if sig.params().is_empty() {
                         write!(f, "||")?;
@@ -718,7 +729,9 @@ fn write_bounds_like_dyn_trait(
                 write!(f, "{}", f.db.trait_data(trait_).name)?;
                 if let [_, params @ ..] = &*trait_ref.substitution.0 {
                     if is_fn_trait {
-                        if let Some(args) = params.first().and_then(|it| it.as_tuple()) {
+                        if let Some(args) =
+                            params.first().and_then(|it| it.assert_ty_ref(&Interner).as_tuple())
+                        {
                             write!(f, "(")?;
                             f.write_joined(&*args.0, ", ")?;
                             write!(f, ")")?;
@@ -767,16 +780,16 @@ impl TraitRef {
             return write!(f, "{}", TYPE_HINT_TRUNCATION);
         }
 
-        self.substitution[0].hir_fmt(f)?;
+        self.self_type_parameter().hir_fmt(f)?;
         if use_as {
             write!(f, " as ")?;
         } else {
             write!(f, ": ")?;
         }
         write!(f, "{}", f.db.trait_data(self.hir_trait_id()).name)?;
-        if self.substitution.len() > 1 {
+        if self.substitution.len(&Interner) > 1 {
             write!(f, "<")?;
-            f.write_joined(&self.substitution[1..], ", ")?;
+            f.write_joined(&self.substitution.interned(&Interner)[1..], ", ")?;
             write!(f, ">")?;
         }
         Ok(())
@@ -1016,11 +1029,11 @@ impl HirDisplay for Path {
     }
 }
 
-impl HirDisplay for GenericArg {
+impl HirDisplay for hir_def::path::GenericArg {
     fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
         match self {
-            GenericArg::Type(ty) => ty.hir_fmt(f),
-            GenericArg::Lifetime(lifetime) => write!(f, "{}", lifetime.name),
+            hir_def::path::GenericArg::Type(ty) => ty.hir_fmt(f),
+            hir_def::path::GenericArg::Lifetime(lifetime) => write!(f, "{}", lifetime.name),
         }
     }
 }
diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index 9c62932b1..8f7322b36 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -100,7 +100,7 @@ impl<'a> InferenceContext<'a> {
             },
 
             (TyKind::Closure(.., substs), TyKind::Function { .. }) => {
-                from_ty = substs[0].clone();
+                from_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner).clone();
             }
 
             _ => {}
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 25ab3ea4c..b99b6cd21 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -266,7 +266,7 @@ impl<'a> InferenceContext<'a> {
                 let sig_ty = TyKind::Function(FnPointer {
                     num_args: sig_tys.len() - 1,
                     sig: FnSig { abi: (), safety: chalk_ir::Safety::Safe, variadic: false },
-                    substs: Substitution(sig_tys.clone().into()),
+                    substs: Substitution::from_iter(&Interner, sig_tys.clone()),
                 })
                 .intern(&Interner);
                 let closure_id = self.db.intern_closure((self.owner, tgt_expr)).into();
@@ -406,7 +406,7 @@ impl<'a> InferenceContext<'a> {
 
                 self.unify(&ty, &expected.ty);
 
-                let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+                let substs = ty.substs().cloned().unwrap_or_else(|| Substitution::empty(&Interner));
                 let field_types = def_id.map(|it| self.db.field_types(it)).unwrap_or_default();
                 let variant_data = def_id.map(|it| variant_data(self.db.upcast(), it));
                 for field in fields.iter() {
@@ -456,9 +456,13 @@ impl<'a> InferenceContext<'a> {
                             .unwrap_or(true)
                     };
                     match canonicalized.decanonicalize_ty(derefed_ty.value).interned(&Interner) {
-                        TyKind::Tuple(_, substs) => {
-                            name.as_tuple_index().and_then(|idx| substs.0.get(idx).cloned())
-                        }
+                        TyKind::Tuple(_, substs) => name.as_tuple_index().and_then(|idx| {
+                            substs
+                                .interned(&Interner)
+                                .get(idx)
+                                .map(|a| a.assert_ty_ref(&Interner))
+                                .cloned()
+                        }),
                         TyKind::Adt(AdtId(hir_def::AdtId::StructId(s)), parameters) => {
                             let local_id = self.db.struct_data(*s).variant_data.field(name)?;
                             let field = FieldId { parent: (*s).into(), local_id };
@@ -635,7 +639,7 @@ impl<'a> InferenceContext<'a> {
                 let rhs_ty = rhs.map(|e| self.infer_expr(e, &rhs_expect));
                 match (range_type, lhs_ty, rhs_ty) {
                     (RangeOp::Exclusive, None, None) => match self.resolve_range_full() {
-                        Some(adt) => Ty::adt_ty(adt, Substitution::empty()),
+                        Some(adt) => Ty::adt_ty(adt, Substitution::empty(&Interner)),
                         None => self.err_ty(),
                     },
                     (RangeOp::Exclusive, None, Some(ty)) => match self.resolve_range_to() {
@@ -694,8 +698,8 @@ impl<'a> InferenceContext<'a> {
             Expr::Tuple { exprs } => {
                 let mut tys = match expected.ty.interned(&Interner) {
                     TyKind::Tuple(_, substs) => substs
-                        .iter()
-                        .cloned()
+                        .iter(&Interner)
+                        .map(|a| a.assert_ty_ref(&Interner).clone())
                         .chain(repeat_with(|| self.table.new_type_var()))
                         .take(exprs.len())
                         .collect::<Vec<_>>(),
@@ -706,7 +710,7 @@ impl<'a> InferenceContext<'a> {
                     self.infer_expr_coerce(*expr, &Expectation::has_type(ty.clone()));
                 }
 
-                TyKind::Tuple(tys.len(), Substitution(tys.into())).intern(&Interner)
+                TyKind::Tuple(tys.len(), Substitution::from_iter(&Interner, tys)).intern(&Interner)
             }
             Expr::Array(array) => {
                 let elem_ty = match expected.ty.interned(&Interner) {
@@ -953,7 +957,7 @@ impl<'a> InferenceContext<'a> {
             substs.push(self.err_ty());
         }
         assert_eq!(substs.len(), total_len);
-        Substitution(substs.into())
+        Substitution::from_iter(&Interner, substs)
     }
 
     fn register_obligations_for_call(&mut self, callable_ty: &Ty) {
diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
index 474363709..f1316415f 100644
--- a/crates/hir_ty/src/infer/pat.rs
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -35,7 +35,7 @@ impl<'a> InferenceContext<'a> {
         }
         self.unify(&ty, expected);
 
-        let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+        let substs = ty.substs().cloned().unwrap_or_else(|| Substitution::empty(&Interner));
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
         let (pre, post) = match ellipsis {
@@ -74,7 +74,7 @@ impl<'a> InferenceContext<'a> {
 
         self.unify(&ty, expected);
 
-        let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+        let substs = ty.substs().cloned().unwrap_or_else(|| Substitution::empty(&Interner));
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
         for subpat in subpats {
@@ -134,7 +134,8 @@ impl<'a> InferenceContext<'a> {
                 };
                 let n_uncovered_patterns = expectations.len().saturating_sub(args.len());
                 let err_ty = self.err_ty();
-                let mut expectations_iter = expectations.iter().chain(repeat(&err_ty));
+                let mut expectations_iter =
+                    expectations.iter().map(|a| a.assert_ty_ref(&Interner)).chain(repeat(&err_ty));
                 let mut infer_pat = |(&pat, ty)| self.infer_pat(pat, ty, default_bm);
 
                 let mut inner_tys = Vec::with_capacity(n_uncovered_patterns + args.len());
@@ -142,7 +143,8 @@ impl<'a> InferenceContext<'a> {
                 inner_tys.extend(expectations_iter.by_ref().take(n_uncovered_patterns).cloned());
                 inner_tys.extend(post.iter().zip(expectations_iter).map(infer_pat));
 
-                TyKind::Tuple(inner_tys.len(), Substitution(inner_tys.into())).intern(&Interner)
+                TyKind::Tuple(inner_tys.len(), Substitution::from_iter(&Interner, inner_tys))
+                    .intern(&Interner)
             }
             Pat::Or(ref pats) => {
                 if let Some((first_pat, rest)) = pats.split_first() {
@@ -236,9 +238,10 @@ impl<'a> InferenceContext<'a> {
             Pat::Box { inner } => match self.resolve_boxed_box() {
                 Some(box_adt) => {
                     let (inner_ty, alloc_ty) = match expected.as_adt() {
-                        Some((adt, subst)) if adt == box_adt => {
-                            (subst[0].clone(), subst.get(1).cloned())
-                        }
+                        Some((adt, subst)) if adt == box_adt => (
+                            subst.at(&Interner, 0).assert_ty_ref(&Interner).clone(),
+                            subst.interned(&Interner).get(1).and_then(|a| a.ty(&Interner).cloned()),
+                        ),
                         _ => (self.result.standard_types.unknown.clone(), None),
                     };
 
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
index 717738789..b96391776 100644
--- a/crates/hir_ty/src/infer/path.rs
+++ b/crates/hir_ty/src/infer/path.rs
@@ -97,12 +97,12 @@ impl<'a> InferenceContext<'a> {
 
         let ty = self.db.value_ty(typable);
         // self_subst is just for the parent
-        let parent_substs = self_subst.unwrap_or_else(Substitution::empty);
+        let parent_substs = self_subst.unwrap_or_else(|| Substitution::empty(&Interner));
         let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
         let substs = ctx.substs_from_path(path, typable, true);
-        let full_substs = Substitution::builder(substs.len())
+        let full_substs = Substitution::builder(substs.len(&Interner))
             .use_parent_substs(&parent_substs)
-            .fill(substs.0[parent_substs.len()..].iter().cloned())
+            .fill(substs.interned(&Interner)[parent_substs.len(&Interner)..].iter().cloned())
             .build();
         let ty = ty.subst(&full_substs);
         Some(ty)
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index 5ea4b7481..0efc62e53 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -129,29 +129,28 @@ impl<T> Canonicalized<T> {
         solution: Canonical<Substitution>,
     ) {
         // the solution may contain new variables, which we need to convert to new inference vars
-        let new_vars = Substitution(
-            solution
-                .binders
-                .iter(&Interner)
-                .map(|k| match k.kind {
-                    VariableKind::Ty(TyVariableKind::General) => ctx.table.new_type_var(),
-                    VariableKind::Ty(TyVariableKind::Integer) => ctx.table.new_integer_var(),
-                    VariableKind::Ty(TyVariableKind::Float) => ctx.table.new_float_var(),
-                    // HACK: Chalk can sometimes return new lifetime variables. We
-                    // want to just skip them, but to not mess up the indices of
-                    // other variables, we'll just create a new type variable in
-                    // their place instead. This should not matter (we never see the
-                    // actual *uses* of the lifetime variable).
-                    VariableKind::Lifetime => ctx.table.new_type_var(),
-                    _ => panic!("const variable in solution"),
-                })
-                .collect(),
+        let new_vars = Substitution::from_iter(
+            &Interner,
+            solution.binders.iter(&Interner).map(|k| match k.kind {
+                VariableKind::Ty(TyVariableKind::General) => ctx.table.new_type_var(),
+                VariableKind::Ty(TyVariableKind::Integer) => ctx.table.new_integer_var(),
+                VariableKind::Ty(TyVariableKind::Float) => ctx.table.new_float_var(),
+                // HACK: Chalk can sometimes return new lifetime variables. We
+                // want to just skip them, but to not mess up the indices of
+                // other variables, we'll just create a new type variable in
+                // their place instead. This should not matter (we never see the
+                // actual *uses* of the lifetime variable).
+                VariableKind::Lifetime => ctx.table.new_type_var(),
+                _ => panic!("const variable in solution"),
+            }),
         );
-        for (i, ty) in solution.value.into_iter().enumerate() {
+        for (i, ty) in solution.value.iter(&Interner).enumerate() {
             let (v, k) = self.free_vars[i];
             // eagerly replace projections in the type; we may be getting types
             // e.g. from where clauses where this hasn't happened yet
-            let ty = ctx.normalize_associated_types_in(ty.clone().subst_bound_vars(&new_vars));
+            let ty = ctx.normalize_associated_types_in(
+                ty.assert_ty_ref(&Interner).clone().subst_bound_vars(&new_vars),
+            );
             ctx.table.unify(&TyKind::InferenceVar(v, k).intern(&Interner), &ty);
         }
     }
@@ -163,13 +162,13 @@ pub fn could_unify(t1: &Ty, t2: &Ty) -> bool {
 
 pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
     let mut table = InferenceTable::new();
-    let vars = Substitution(
+    let vars = Substitution::from_iter(
+        &Interner,
         tys.binders
             .iter(&Interner)
             // we always use type vars here because we want everything to
             // fallback to Unknown in the end (kind of hacky, as below)
-            .map(|_| table.new_type_var())
-            .collect(),
+            .map(|_| table.new_type_var()),
     );
     let ty1_with_vars = tys.value.0.clone().subst_bound_vars(&vars);
     let ty2_with_vars = tys.value.1.clone().subst_bound_vars(&vars);
@@ -178,7 +177,8 @@ pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
     }
     // default any type vars that weren't unified back to their original bound vars
     // (kind of hacky)
-    for (i, var) in vars.iter().enumerate() {
+    for (i, var) in vars.iter(&Interner).enumerate() {
+        let var = var.assert_ty_ref(&Interner);
         if &*table.resolve_ty_shallow(var) == var {
             table.unify(
                 var,
@@ -188,7 +188,10 @@ pub(crate) fn unify(tys: &Canonical<(Ty, Ty)>) -> Option<Substitution> {
     }
     Some(
         Substitution::builder(tys.binders.len(&Interner))
-            .fill(vars.iter().map(|v| table.resolve_ty_completely(v.clone())))
+            .fill(
+                vars.iter(&Interner)
+                    .map(|v| table.resolve_ty_completely(v.assert_ty_ref(&Interner).clone())),
+            )
             .build(),
     )
 }
@@ -284,7 +287,9 @@ impl InferenceTable {
         substs2: &Substitution,
         depth: usize,
     ) -> bool {
-        substs1.0.iter().zip(substs2.0.iter()).all(|(t1, t2)| self.unify_inner(t1, t2, depth))
+        substs1.0.iter().zip(substs2.0.iter()).all(|(t1, t2)| {
+            self.unify_inner(t1.assert_ty_ref(&Interner), t2.assert_ty_ref(&Interner), depth)
+        })
     }
 
     fn unify_inner(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index 6f9c698e6..c927ed973 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -24,9 +24,10 @@ mod tests;
 #[cfg(test)]
 mod test_db;
 
-use std::{iter, mem, ops::Deref, sync::Arc};
+use std::{iter, mem, sync::Arc};
 
 use base_db::salsa;
+use chalk_ir::cast::{CastTo, Caster};
 use hir_def::{
     builtin_type::BuiltinType, expr::ExprId, type_ref::Rawness, AssocContainerId, FunctionId,
     GenericDefId, HasModule, LifetimeParamId, Lookup, TraitId, TypeAliasId, TypeParamId,
@@ -109,7 +110,7 @@ impl ProjectionTy {
     }
 
     pub fn self_type_parameter(&self) -> &Ty {
-        &self.substitution[0]
+        &self.substitution.interned(&Interner)[0].assert_ty_ref(&Interner)
     }
 
     fn trait_(&self, db: &dyn HirDatabase) -> TraitId {
@@ -324,9 +325,72 @@ impl Ty {
     }
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct GenericArg {
+    interned: GenericArgData,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub enum GenericArgData {
+    Ty(Ty),
+}
+
+impl GenericArg {
+    /// Constructs a generic argument using `GenericArgData`.
+    pub fn new(_interner: &Interner, data: GenericArgData) -> Self {
+        GenericArg { interned: data }
+    }
+
+    /// Gets the interned value.
+    pub fn interned(&self) -> &GenericArgData {
+        &self.interned
+    }
+
+    /// Asserts that this is a type argument.
+    pub fn assert_ty_ref(&self, interner: &Interner) -> &Ty {
+        self.ty(interner).unwrap()
+    }
+
+    /// Checks whether the generic argument is a type.
+    pub fn is_ty(&self, _interner: &Interner) -> bool {
+        match self.interned() {
+            GenericArgData::Ty(_) => true,
+        }
+    }
+
+    /// Returns the type if it is one, `None` otherwise.
+    pub fn ty(&self, _interner: &Interner) -> Option<&Ty> {
+        match self.interned() {
+            GenericArgData::Ty(t) => Some(t),
+        }
+    }
+}
+
+impl TypeWalk for GenericArg {
+    fn walk(&self, f: &mut impl FnMut(&Ty)) {
+        match &self.interned {
+            GenericArgData::Ty(ty) => {
+                ty.walk(f);
+            }
+        }
+    }
+
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
+        match &mut self.interned {
+            GenericArgData::Ty(ty) => {
+                ty.walk_mut_binders(f, binders);
+            }
+        }
+    }
+}
+
 /// A list of substitutions for generic parameters.
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
-pub struct Substitution(SmallVec<[Ty; 2]>);
+pub struct Substitution(SmallVec<[GenericArg; 2]>);
 
 impl TypeWalk for Substitution {
     fn walk(&self, f: &mut impl FnMut(&Ty)) {
@@ -347,18 +411,34 @@ impl TypeWalk for Substitution {
 }
 
 impl Substitution {
-    pub fn interned(&self, _: &Interner) -> &[Ty] {
+    pub fn interned(&self, _: &Interner) -> &[GenericArg] {
         &self.0
     }
 
-    pub fn empty() -> Substitution {
+    pub fn len(&self, _: &Interner) -> usize {
+        self.0.len()
+    }
+
+    pub fn is_empty(&self, _: &Interner) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn at(&self, _: &Interner, i: usize) -> &GenericArg {
+        &self.0[i]
+    }
+
+    pub fn empty(_: &Interner) -> Substitution {
         Substitution(SmallVec::new())
     }
 
+    pub fn iter(&self, _: &Interner) -> std::slice::Iter<'_, GenericArg> {
+        self.0.iter()
+    }
+
     pub fn single(ty: Ty) -> Substitution {
         Substitution({
             let mut v = SmallVec::new();
-            v.push(ty);
+            v.push(ty.cast(&Interner));
             v
         })
     }
@@ -371,15 +451,11 @@ impl Substitution {
         Substitution(self.0[self.0.len() - std::cmp::min(self.0.len(), n)..].into())
     }
 
-    pub fn as_single(&self) -> &Ty {
-        if self.0.len() != 1 {
-            panic!("expected substs of len 1, got {:?}", self);
-        }
-        &self.0[0]
-    }
-
-    pub fn from_iter(_interner: &Interner, elements: impl IntoIterator<Item = Ty>) -> Self {
-        Substitution(elements.into_iter().collect())
+    pub fn from_iter(
+        interner: &Interner,
+        elements: impl IntoIterator<Item = impl CastTo<GenericArg>>,
+    ) -> Self {
+        Substitution(elements.into_iter().casted(interner).collect())
     }
 
     /// Return Substs that replace each parameter by itself (i.e. `Ty::Param`).
@@ -387,11 +463,11 @@ impl Substitution {
         db: &dyn HirDatabase,
         generic_params: &Generics,
     ) -> Substitution {
-        Substitution(
+        Substitution::from_iter(
+            &Interner,
             generic_params
                 .iter()
-                .map(|(id, _)| TyKind::Placeholder(to_placeholder_idx(db, id)).intern(&Interner))
-                .collect(),
+                .map(|(id, _)| TyKind::Placeholder(to_placeholder_idx(db, id)).intern(&Interner)),
         )
     }
 
@@ -403,12 +479,12 @@ impl Substitution {
 
     /// Return Substs that replace each parameter by a bound variable.
     pub(crate) fn bound_vars(generic_params: &Generics, debruijn: DebruijnIndex) -> Substitution {
-        Substitution(
+        Substitution::from_iter(
+            &Interner,
             generic_params
                 .iter()
                 .enumerate()
-                .map(|(idx, _)| TyKind::BoundVar(BoundVar::new(debruijn, idx)).intern(&Interner))
-                .collect(),
+                .map(|(idx, _)| TyKind::BoundVar(BoundVar::new(debruijn, idx)).intern(&Interner)),
         )
     }
 
@@ -435,18 +511,18 @@ pub fn param_idx(db: &dyn HirDatabase, id: TypeParamId) -> Option<usize> {
 
 #[derive(Debug, Clone)]
 pub struct SubstsBuilder {
-    vec: Vec<Ty>,
+    vec: Vec<GenericArg>,
     param_count: usize,
 }
 
 impl SubstsBuilder {
     pub fn build(self) -> Substitution {
         assert_eq!(self.vec.len(), self.param_count);
-        Substitution(self.vec.into())
+        Substitution::from_iter(&Interner, self.vec)
     }
 
-    pub fn push(mut self, ty: Ty) -> Self {
-        self.vec.push(ty);
+    pub fn push(mut self, ty: impl CastTo<GenericArg>) -> Self {
+        self.vec.push(ty.cast(&Interner));
         self
     }
 
@@ -465,28 +541,20 @@ impl SubstsBuilder {
         self.fill(iter::repeat(TyKind::Unknown.intern(&Interner)))
     }
 
-    pub fn fill(mut self, filler: impl Iterator<Item = Ty>) -> Self {
-        self.vec.extend(filler.take(self.remaining()));
+    pub fn fill(mut self, filler: impl Iterator<Item = impl CastTo<GenericArg>>) -> Self {
+        self.vec.extend(filler.take(self.remaining()).casted(&Interner));
         assert_eq!(self.remaining(), 0);
         self
     }
 
     pub fn use_parent_substs(mut self, parent_substs: &Substitution) -> Self {
         assert!(self.vec.is_empty());
-        assert!(parent_substs.len() <= self.param_count);
-        self.vec.extend(parent_substs.iter().cloned());
+        assert!(parent_substs.len(&Interner) <= self.param_count);
+        self.vec.extend(parent_substs.iter(&Interner).cloned());
         self
     }
 }
 
-impl Deref for Substitution {
-    type Target = [Ty];
-
-    fn deref(&self) -> &[Ty] {
-        &self.0
-    }
-}
-
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub struct Binders<T> {
     pub num_binders: usize,
@@ -535,7 +603,7 @@ impl<T: Clone> Binders<&T> {
 impl<T: TypeWalk> Binders<T> {
     /// Substitutes all variables.
     pub fn subst(self, subst: &Substitution) -> T {
-        assert_eq!(subst.len(), self.num_binders);
+        assert_eq!(subst.len(&Interner), self.num_binders);
         self.value.subst_bound_vars(subst)
     }
 }
@@ -563,7 +631,7 @@ pub struct TraitRef {
 
 impl TraitRef {
     pub fn self_type_parameter(&self) -> &Ty {
-        &self.substitution[0]
+        &self.substitution.at(&Interner, 0).assert_ty_ref(&Interner)
     }
 
     pub fn hir_trait_id(&self) -> TraitId {
@@ -699,14 +767,20 @@ impl CallableSig {
                 .shift_bound_vars_out(DebruijnIndex::ONE)
                 .interned(&Interner)
                 .iter()
-                .cloned()
+                .map(|arg| arg.assert_ty_ref(&Interner).clone())
                 .collect(),
             is_varargs: fn_ptr.sig.variadic,
         }
     }
 
     pub fn from_substs(substs: &Substitution) -> CallableSig {
-        CallableSig { params_and_return: substs.iter().cloned().collect(), is_varargs: false }
+        CallableSig {
+            params_and_return: substs
+                .iter(&Interner)
+                .map(|arg| arg.assert_ty_ref(&Interner).clone())
+                .collect(),
+            is_varargs: false,
+        }
     }
 
     pub fn params(&self) -> &[Ty] {
@@ -738,7 +812,7 @@ impl TypeWalk for CallableSig {
 
 impl Ty {
     pub fn unit() -> Self {
-        TyKind::Tuple(0, Substitution::empty()).intern(&Interner)
+        TyKind::Tuple(0, Substitution::empty(&Interner)).intern(&Interner)
     }
 
     pub fn adt_ty(adt: hir_def::AdtId, substs: Substitution) -> Ty {
@@ -908,7 +982,7 @@ impl Ty {
                 Some(sig.subst(&parameters))
             }
             TyKind::Closure(.., substs) => {
-                let sig_param = &substs[0];
+                let sig_param = substs.at(&Interner, 0).assert_ty_ref(&Interner);
                 sig_param.callable_sig(db)
             }
             _ => None,
@@ -960,7 +1034,7 @@ impl Ty {
                                 0,
                                 WhereClause::Implemented(TraitRef {
                                     trait_id: to_chalk_trait_id(future_trait),
-                                    substitution: Substitution::empty(),
+                                    substitution: Substitution::empty(&Interner),
                                 }),
                             );
                             Some(vec![impl_bound])
@@ -1109,7 +1183,10 @@ pub trait TypeWalk {
             &mut |ty, binders| {
                 if let &mut TyKind::BoundVar(bound) = ty.interned_mut() {
                     if bound.debruijn >= binders {
-                        *ty = substs.0[bound.index].clone().shift_bound_vars(binders);
+                        *ty = substs.0[bound.index]
+                            .assert_ty_ref(&Interner)
+                            .clone()
+                            .shift_bound_vars(binders);
                     }
                 }
             },
@@ -1156,12 +1233,12 @@ impl TypeWalk for Ty {
     fn walk(&self, f: &mut impl FnMut(&Ty)) {
         match self.interned(&Interner) {
             TyKind::Alias(AliasTy::Projection(p_ty)) => {
-                for t in p_ty.substitution.iter() {
+                for t in p_ty.substitution.iter(&Interner) {
                     t.walk(f);
                 }
             }
             TyKind::Alias(AliasTy::Opaque(o_ty)) => {
-                for t in o_ty.substitution.iter() {
+                for t in o_ty.substitution.iter(&Interner) {
                     t.walk(f);
                 }
             }
@@ -1175,7 +1252,7 @@ impl TypeWalk for Ty {
             }
             _ => {
                 if let Some(substs) = self.substs() {
-                    for t in substs.iter() {
+                    for t in substs.iter(&Interner) {
                         t.walk(f);
                     }
                 }
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index 72ee060e0..f595683e5 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -178,9 +178,10 @@ impl<'a> TyLoweringContext<'a> {
             }
             TypeRef::Placeholder => TyKind::Unknown.intern(&Interner),
             TypeRef::Fn(params, is_varargs) => {
-                let substs = Substitution(params.iter().map(|tr| self.lower_ty(tr)).collect());
+                let substs =
+                    Substitution::from_iter(&Interner, params.iter().map(|tr| self.lower_ty(tr)));
                 TyKind::Function(FnPointer {
-                    num_args: substs.len() - 1,
+                    num_args: substs.len(&Interner) - 1,
                     sig: FnSig { abi: (), safety: Safety::Safe, variadic: *is_varargs },
                     substs,
                 })
@@ -625,7 +626,7 @@ impl<'a> TyLoweringContext<'a> {
 
                 for default_ty in defaults.iter().skip(substs.len()) {
                     // each default can depend on the previous parameters
-                    let substs_so_far = Substitution(substs.clone().into());
+                    let substs_so_far = Substitution::from_iter(&Interner, substs.clone());
                     substs.push(default_ty.clone().subst(&substs_so_far));
                 }
             }
@@ -638,7 +639,7 @@ impl<'a> TyLoweringContext<'a> {
         }
         assert_eq!(substs.len(), total_len);
 
-        Substitution(substs.into())
+        Substitution::from_iter(&Interner, substs)
     }
 
     fn lower_trait_ref_from_path(
@@ -1062,7 +1063,7 @@ fn type_for_fn(db: &dyn HirDatabase, def: FunctionId) -> Binders<Ty> {
     let generics = generics(db.upcast(), def.into());
     let substs = Substitution::bound_vars(&generics, DebruijnIndex::INNERMOST);
     Binders::new(
-        substs.len(),
+        substs.len(&Interner),
         TyKind::FnDef(CallableDefId::FunctionId(def).to_chalk(db), substs).intern(&Interner),
     )
 }
@@ -1107,7 +1108,7 @@ fn type_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> Binders<T
     let generics = generics(db.upcast(), def.into());
     let substs = Substitution::bound_vars(&generics, DebruijnIndex::INNERMOST);
     Binders::new(
-        substs.len(),
+        substs.len(&Interner),
         TyKind::FnDef(CallableDefId::StructId(def).to_chalk(db), substs).intern(&Interner),
     )
 }
@@ -1134,7 +1135,7 @@ fn type_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId) -
     let generics = generics(db.upcast(), def.parent.into());
     let substs = Substitution::bound_vars(&generics, DebruijnIndex::INNERMOST);
     Binders::new(
-        substs.len(),
+        substs.len(&Interner),
         TyKind::FnDef(CallableDefId::EnumVariantId(def).to_chalk(db), substs).intern(&Interner),
     )
 }
@@ -1142,7 +1143,7 @@ fn type_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId) -
 fn type_for_adt(db: &dyn HirDatabase, adt: AdtId) -> Binders<Ty> {
     let generics = generics(db.upcast(), adt.into());
     let substs = Substitution::bound_vars(&generics, DebruijnIndex::INNERMOST);
-    Binders::new(substs.len(), Ty::adt_ty(adt, substs))
+    Binders::new(substs.len(&Interner), Ty::adt_ty(adt, substs))
 }
 
 fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index bf7d5eded..054896475 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -720,7 +720,7 @@ pub(crate) fn inherent_impl_substs(
             chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General),
             UniverseIndex::ROOT,
         ))
-        .take(vars.len()),
+        .take(vars.len(&Interner)),
     );
     let tys = Canonical {
         binders: CanonicalVarKinds::from_iter(&Interner, kinds),
@@ -732,7 +732,8 @@ pub(crate) fn inherent_impl_substs(
     // Unknown. I think this can only really happen if self_ty contained
     // Unknown, and in that case we want the result to contain Unknown in those
     // places again.
-    substs.map(|s| fallback_bound_vars(s.suffix(vars.len()), self_ty.binders.len(&Interner)))
+    substs
+        .map(|s| fallback_bound_vars(s.suffix(vars.len(&Interner)), self_ty.binders.len(&Interner)))
 }
 
 /// This replaces any 'free' Bound vars in `s` (i.e. those with indices past
@@ -821,7 +822,7 @@ fn generic_implements_goal(
             chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General),
             UniverseIndex::ROOT,
         ))
-        .take(substs.len() - 1),
+        .take(substs.len(&Interner) - 1),
     );
     let trait_ref = TraitRef { trait_id: to_chalk_trait_id(trait_), substitution: substs };
     let obligation = trait_ref.cast(&Interner);
diff --git a/crates/hir_ty/src/traits.rs b/crates/hir_ty/src/traits.rs
index ccee0e5ad..a15b6486e 100644
--- a/crates/hir_ty/src/traits.rs
+++ b/crates/hir_ty/src/traits.rs
@@ -138,7 +138,7 @@ pub(crate) fn trait_solve_query(
         ..
     })) = &goal.value.goal
     {
-        if let TyKind::BoundVar(_) = &projection_ty.substitution[0].interned(&Interner) {
+        if let TyKind::BoundVar(_) = projection_ty.self_type_parameter().interned(&Interner) {
             // Hack: don't ask Chalk to normalize with an unknown self type, it'll say that's impossible
             return Some(Solution::Ambig(Guidance::Unknown));
         }
diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index 011bef6f6..cf7ed1e11 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -3,7 +3,7 @@ use std::sync::Arc;
 
 use log::debug;
 
-use chalk_ir::{fold::shift::Shift, CanonicalVarKinds, GenericArg};
+use chalk_ir::{fold::shift::Shift, CanonicalVarKinds};
 use chalk_solve::rust_ir::{self, OpaqueTyDatumBound, WellKnownTrait};
 
 use base_db::{salsa::InternKey, CrateId};
@@ -80,7 +80,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
     fn impls_for_trait(
         &self,
         trait_id: TraitId,
-        parameters: &[GenericArg<Interner>],
+        parameters: &[chalk_ir::GenericArg<Interner>],
         binders: &CanonicalVarKinds<Interner>,
     ) -> Vec<ImplId> {
         debug!("impls_for_trait {:?}", trait_id);
@@ -308,7 +308,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         _closure_id: chalk_ir::ClosureId<Interner>,
         _substs: &chalk_ir::Substitution<Interner>,
     ) -> chalk_ir::Substitution<Interner> {
-        Substitution::empty().to_chalk(self.db)
+        Substitution::empty(&Interner).to_chalk(self.db)
     }
 
     fn trait_name(&self, trait_id: chalk_ir::TraitId<Interner>) -> String {
@@ -439,7 +439,7 @@ pub(crate) fn trait_datum_query(
         lang_attr(db.upcast(), trait_).and_then(|name| well_known_trait_from_lang_attr(&name));
     let trait_datum = TraitDatum {
         id: trait_id,
-        binders: make_binders(trait_datum_bound, bound_vars.len()),
+        binders: make_binders(trait_datum_bound, bound_vars.len(&Interner)),
         flags,
         associated_ty_ids,
         well_known,
@@ -577,7 +577,7 @@ fn impl_def_datum(
         .collect();
     debug!("impl_datum: {:?}", impl_datum_bound);
     let impl_datum = ImplDatum {
-        binders: make_binders(impl_datum_bound, bound_vars.len()),
+        binders: make_binders(impl_datum_bound, bound_vars.len(&Interner)),
         impl_type,
         polarity,
         associated_ty_value_ids,
diff --git a/crates/hir_ty/src/traits/chalk/mapping.rs b/crates/hir_ty/src/traits/chalk/mapping.rs
index aef6b8a15..452b357e8 100644
--- a/crates/hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/hir_ty/src/traits/chalk/mapping.rs
@@ -13,7 +13,7 @@ use crate::{
     db::HirDatabase,
     primitive::UintTy,
     traits::{Canonical, DomainGoal},
-    AliasTy, CallableDefId, FnPointer, InEnvironment, OpaqueTy, ProjectionTy,
+    AliasTy, CallableDefId, FnPointer, GenericArg, InEnvironment, OpaqueTy, ProjectionTy,
     QuantifiedWhereClause, Scalar, Substitution, TraitRef, Ty, TypeWalk, WhereClause,
 };
 
@@ -137,7 +137,7 @@ impl ToChalk for Ty {
                     db,
                     substitution.0.shifted_out(&Interner).expect("fn ptr should have no binders"),
                 );
-                TyKind::Function(FnPointer { num_args: (substs.len() - 1), sig, substs })
+                TyKind::Function(FnPointer { num_args: (substs.len(&Interner) - 1), sig, substs })
             }
             chalk_ir::TyKind::BoundVar(idx) => TyKind::BoundVar(idx),
             chalk_ir::TyKind::InferenceVar(_iv, _kind) => TyKind::Unknown,
@@ -216,24 +216,39 @@ fn array_to_chalk(db: &dyn HirDatabase, ty: Ty) -> chalk_ir::Ty<Interner> {
     chalk_ir::TyKind::Array(arg, const_).intern(&Interner)
 }
 
+impl ToChalk for GenericArg {
+    type Chalk = chalk_ir::GenericArg<Interner>;
+
+    fn to_chalk(self, db: &dyn HirDatabase) -> Self::Chalk {
+        match self.interned {
+            crate::GenericArgData::Ty(ty) => ty.to_chalk(db).cast(&Interner),
+        }
+    }
+
+    fn from_chalk(db: &dyn HirDatabase, chalk: Self::Chalk) -> Self {
+        match chalk.interned() {
+            chalk_ir::GenericArgData::Ty(ty) => Ty::from_chalk(db, ty.clone()).cast(&Interner),
+            chalk_ir::GenericArgData::Lifetime(_) => unimplemented!(),
+            chalk_ir::GenericArgData::Const(_) => unimplemented!(),
+        }
+    }
+}
+
 impl ToChalk for Substitution {
     type Chalk = chalk_ir::Substitution<Interner>;
 
     fn to_chalk(self, db: &dyn HirDatabase) -> chalk_ir::Substitution<Interner> {
-        chalk_ir::Substitution::from_iter(&Interner, self.iter().map(|ty| ty.clone().to_chalk(db)))
+        chalk_ir::Substitution::from_iter(
+            &Interner,
+            self.iter(&Interner).map(|ty| ty.clone().to_chalk(db)),
+        )
     }
 
     fn from_chalk(
         db: &dyn HirDatabase,
         parameters: chalk_ir::Substitution<Interner>,
     ) -> Substitution {
-        let tys = parameters
-            .iter(&Interner)
-            .map(|p| match p.ty(&Interner) {
-                Some(ty) => from_chalk(db, ty.clone()),
-                None => unimplemented!(),
-            })
-            .collect();
+        let tys = parameters.iter(&Interner).map(|p| from_chalk(db, p.clone())).collect();
         Substitution(tys)
     }
 }
@@ -531,7 +546,7 @@ pub(super) fn generic_predicate_to_inline_bound(
                 // have the expected self type
                 return None;
             }
-            let args_no_self = trait_ref.substitution[1..]
+            let args_no_self = trait_ref.substitution.interned(&Interner)[1..]
                 .iter()
                 .map(|ty| ty.clone().to_chalk(db).cast(&Interner))
                 .collect();
@@ -543,7 +558,7 @@ pub(super) fn generic_predicate_to_inline_bound(
                 return None;
             }
             let trait_ = projection_ty.trait_(db);
-            let args_no_self = projection_ty.substitution[1..]
+            let args_no_self = projection_ty.substitution.interned(&Interner)[1..]
                 .iter()
                 .map(|ty| ty.clone().to_chalk(db).cast(&Interner))
                 .collect();