5 files changed, 364 insertions, 318 deletions

diff --git a/crates/hir_ty/src/infer/coerce.rs b/crates/hir_ty/src/infer/coerce.rs
index 9c62932b1..1f463a425 100644
--- a/crates/hir_ty/src/infer/coerce.rs
+++ b/crates/hir_ty/src/infer/coerce.rs
@@ -7,9 +7,7 @@
 use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
 use hir_def::lang_item::LangItemTarget;
 
-use crate::{
-    autoderef, to_chalk_trait_id, traits::Solution, Interner, Substitution, TraitRef, Ty, TyKind,
-};
+use crate::{autoderef, Canonical, Interner, Solution, Ty, TyBuilder, TyExt, TyKind};
 
 use super::{InEnvironment, InferenceContext};
 
@@ -36,7 +34,7 @@ impl<'a> InferenceContext<'a> {
             ty1.clone()
         } else {
             if let (TyKind::FnDef(..), TyKind::FnDef(..)) =
-                (ty1.interned(&Interner), ty2.interned(&Interner))
+                (ty1.kind(&Interner), ty2.kind(&Interner))
             {
                 cov_mark::hit!(coerce_fn_reification);
                 // Special case: two function types. Try to coerce both to
@@ -44,8 +42,8 @@ impl<'a> InferenceContext<'a> {
                 // https://github.com/rust-lang/rust/blob/7b805396bf46dce972692a6846ce2ad8481c5f85/src/librustc_typeck/check/coercion.rs#L877-L916
                 let sig1 = ty1.callable_sig(self.db).expect("FnDef without callable sig");
                 let sig2 = ty2.callable_sig(self.db).expect("FnDef without callable sig");
-                let ptr_ty1 = Ty::fn_ptr(sig1);
-                let ptr_ty2 = Ty::fn_ptr(sig2);
+                let ptr_ty1 = TyBuilder::fn_ptr(sig1);
+                let ptr_ty2 = TyBuilder::fn_ptr(sig2);
                 self.coerce_merge_branch(&ptr_ty1, &ptr_ty2)
             } else {
                 cov_mark::hit!(coerce_merge_fail_fallback);
@@ -55,7 +53,7 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
-        match (from_ty.interned(&Interner), to_ty.interned(&Interner)) {
+        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
             // Never type will make type variable to fallback to Never Type instead of Unknown.
             (TyKind::Never, TyKind::InferenceVar(tv, TyVariableKind::General)) => {
                 self.table.type_variable_table.set_diverging(*tv, true);
@@ -73,17 +71,19 @@ impl<'a> InferenceContext<'a> {
         }
 
         // Pointer weakening and function to pointer
-        match (from_ty.interned_mut(), to_ty.interned(&Interner)) {
+        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
             // `*mut T` -> `*const T`
+            (TyKind::Raw(_, inner), TyKind::Raw(m2 @ Mutability::Not, ..)) => {
+                from_ty = TyKind::Raw(*m2, inner.clone()).intern(&Interner);
+            }
             // `&mut T` -> `&T`
-            (TyKind::Raw(m1, ..), TyKind::Raw(m2 @ Mutability::Not, ..))
-            | (TyKind::Ref(m1, ..), TyKind::Ref(m2 @ Mutability::Not, ..)) => {
-                *m1 = *m2;
+            (TyKind::Ref(_, lt, inner), TyKind::Ref(m2 @ Mutability::Not, ..)) => {
+                from_ty = TyKind::Ref(*m2, lt.clone(), inner.clone()).intern(&Interner);
             }
             // `&T` -> `*const T`
             // `&mut T` -> `*mut T`/`*const T`
             (TyKind::Ref(.., substs), &TyKind::Raw(m2 @ Mutability::Not, ..))
-            | (TyKind::Ref(Mutability::Mut, substs), &TyKind::Raw(m2, ..)) => {
+            | (TyKind::Ref(Mutability::Mut, _, substs), &TyKind::Raw(m2, ..)) => {
                 from_ty = TyKind::Raw(m2, substs.clone()).intern(&Interner);
             }
 
@@ -95,12 +95,12 @@ impl<'a> InferenceContext<'a> {
             (TyKind::FnDef(..), TyKind::Function { .. }) => match from_ty.callable_sig(self.db) {
                 None => return false,
                 Some(sig) => {
-                    from_ty = Ty::fn_ptr(sig);
+                    from_ty = TyBuilder::fn_ptr(sig);
                 }
             },
 
             (TyKind::Closure(.., substs), TyKind::Function { .. }) => {
-                from_ty = substs[0].clone();
+                from_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner).clone();
             }
 
             _ => {}
@@ -111,9 +111,11 @@ impl<'a> InferenceContext<'a> {
         }
 
         // Auto Deref if cannot coerce
-        match (from_ty.interned(&Interner), to_ty.interned(&Interner)) {
+        match (from_ty.kind(&Interner), to_ty.kind(&Interner)) {
             // FIXME: DerefMut
-            (TyKind::Ref(_, st1), TyKind::Ref(_, st2)) => self.unify_autoderef_behind_ref(st1, st2),
+            (TyKind::Ref(.., st1), TyKind::Ref(.., st2)) => {
+                self.unify_autoderef_behind_ref(st1, st2)
+            }
 
             // Otherwise, normal unify
             _ => self.unify(&from_ty, to_ty),
@@ -130,19 +132,16 @@ impl<'a> InferenceContext<'a> {
             _ => return None,
         };
 
-        let generic_params = crate::utils::generics(self.db.upcast(), coerce_unsized_trait.into());
-        if generic_params.len() != 2 {
-            // The CoerceUnsized trait should have two generic params: Self and T.
-            return None;
-        }
+        let trait_ref = {
+            let b = TyBuilder::trait_ref(self.db, coerce_unsized_trait);
+            if b.remaining() != 2 {
+                // The CoerceUnsized trait should have two generic params: Self and T.
+                return None;
+            }
+            b.push(from_ty.clone()).push(to_ty.clone()).build()
+        };
 
-        let substs = Substitution::build_for_generics(&generic_params)
-            .push(from_ty.clone())
-            .push(to_ty.clone())
-            .build();
-        let trait_ref =
-            TraitRef { trait_id: to_chalk_trait_id(coerce_unsized_trait), substitution: substs };
-        let goal = InEnvironment::new(self.trait_env.env.clone(), trait_ref.cast(&Interner));
+        let goal = InEnvironment::new(&self.trait_env.env, trait_ref.cast(&Interner));
 
         let canonicalizer = self.canonicalizer();
         let canonicalized = canonicalizer.canonicalize_obligation(goal);
@@ -151,7 +150,14 @@ impl<'a> InferenceContext<'a> {
 
         match solution {
             Solution::Unique(v) => {
-                canonicalized.apply_solution(self, v.0);
+                canonicalized.apply_solution(
+                    self,
+                    Canonical {
+                        binders: v.binders,
+                        // FIXME handle constraints
+                        value: v.value.subst,
+                    },
+                );
             }
             _ => return None,
         };
diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index e6ede05ca..50497eecb 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -3,7 +3,7 @@
 use std::iter::{repeat, repeat_with};
 use std::{mem, sync::Arc};
 
-use chalk_ir::{cast::Cast, Mutability, TyVariableKind};
+use chalk_ir::{cast::Cast, fold::Shift, Mutability, TyVariableKind};
 use hir_def::{
     expr::{Array, BinaryOp, Expr, ExprId, Literal, Statement, UnaryOp},
     path::{GenericArg, GenericArgs},
@@ -15,15 +15,16 @@ use stdx::always;
 use syntax::ast::RangeOp;
 
 use crate::{
-    autoderef,
+    autoderef, dummy_usize_const,
     lower::lower_to_chalk_mutability,
+    mapping::from_chalk,
     method_resolution, op,
     primitive::{self, UintTy},
-    to_assoc_type_id, to_chalk_trait_id,
-    traits::{chalk::from_chalk, FnTrait, InEnvironment},
-    utils::{generics, variant_data, Generics},
-    AdtId, Binders, CallableDefId, DomainGoal, FnPointer, FnSig, Interner, Rawness, Scalar,
-    Substitution, TraitRef, Ty, TyKind,
+    static_lifetime, to_chalk_trait_id,
+    traits::FnTrait,
+    utils::{generics, Generics},
+    AdtId, Binders, CallableDefId, FnPointer, FnSig, FnSubst, InEnvironment, Interner,
+    ProjectionTyExt, Rawness, Scalar, Substitution, TraitRef, Ty, TyBuilder, TyExt, TyKind,
 };
 
 use super::{
@@ -73,38 +74,33 @@ impl<'a> InferenceContext<'a> {
         let fn_once_trait = FnTrait::FnOnce.get_id(self.db, krate)?;
         let output_assoc_type =
             self.db.trait_data(fn_once_trait).associated_type_by_name(&name![Output])?;
-        let generic_params = generics(self.db.upcast(), fn_once_trait.into());
-        if generic_params.len() != 2 {
-            return None;
-        }
 
-        let mut param_builder = Substitution::builder(num_args);
         let mut arg_tys = vec![];
-        for _ in 0..num_args {
-            let arg = self.table.new_type_var();
-            param_builder = param_builder.push(arg.clone());
-            arg_tys.push(arg);
-        }
-        let parameters = param_builder.build();
-        let arg_ty = TyKind::Tuple(num_args, parameters).intern(&Interner);
-        let substs =
-            Substitution::build_for_generics(&generic_params).push(ty.clone()).push(arg_ty).build();
+        let arg_ty = TyBuilder::tuple(num_args)
+            .fill(repeat_with(|| {
+                let arg = self.table.new_type_var();
+                arg_tys.push(arg.clone());
+                arg
+            }))
+            .build();
+
+        let projection = {
+            let b = TyBuilder::assoc_type_projection(self.db, output_assoc_type);
+            if b.remaining() != 2 {
+                return None;
+            }
+            b.push(ty.clone()).push(arg_ty).build()
+        };
 
         let trait_env = self.trait_env.env.clone();
-        let implements_fn_trait: DomainGoal =
-            TraitRef { trait_id: to_chalk_trait_id(fn_once_trait), substitution: substs.clone() }
-                .cast(&Interner);
-        let goal = self.canonicalizer().canonicalize_obligation(InEnvironment {
-            goal: implements_fn_trait.clone(),
+        let obligation = InEnvironment {
+            goal: projection.trait_ref(self.db).cast(&Interner),
             environment: trait_env,
-        });
-        if self.db.trait_solve(krate, goal.value).is_some() {
-            self.obligations.push(implements_fn_trait);
-            let output_proj_ty = crate::ProjectionTy {
-                associated_ty_id: to_assoc_type_id(output_assoc_type),
-                substitution: substs,
-            };
-            let return_ty = self.normalize_projection_ty(output_proj_ty);
+        };
+        let canonical = self.canonicalizer().canonicalize_obligation(obligation.clone());
+        if self.db.trait_solve(krate, canonical.value).is_some() {
+            self.push_obligation(obligation.goal);
+            let return_ty = self.normalize_projection_ty(projection);
             Some((arg_tys, return_ty))
         } else {
             None
@@ -119,6 +115,8 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
+        self.db.check_canceled();
+
         let body = Arc::clone(&self.body); // avoid borrow checker problem
         let ty = match &body[tgt_expr] {
             Expr::Missing => self.err_ty(),
@@ -136,7 +134,7 @@ impl<'a> InferenceContext<'a> {
                 both_arms_diverge &= mem::replace(&mut self.diverges, Diverges::Maybe);
                 let else_ty = match else_branch {
                     Some(else_branch) => self.infer_expr_inner(*else_branch, &expected),
-                    None => Ty::unit(),
+                    None => TyBuilder::unit(),
                 };
                 both_arms_diverge &= self.diverges;
 
@@ -183,7 +181,8 @@ impl<'a> InferenceContext<'a> {
                 let inner_ty = self.infer_expr(*body, &Expectation::none());
                 let impl_trait_id = crate::ImplTraitId::AsyncBlockTypeImplTrait(self.owner, *body);
                 let opaque_ty_id = self.db.intern_impl_trait_id(impl_trait_id).into();
-                TyKind::OpaqueType(opaque_ty_id, Substitution::single(inner_ty)).intern(&Interner)
+                TyKind::OpaqueType(opaque_ty_id, Substitution::from1(&Interner, inner_ty))
+                    .intern(&Interner)
             }
             Expr::Loop { body, label } => {
                 self.breakables.push(BreakableContext {
@@ -191,7 +190,7 @@ impl<'a> InferenceContext<'a> {
                     break_ty: self.table.new_type_var(),
                     label: label.map(|label| self.body[label].name.clone()),
                 });
-                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+                self.infer_expr(*body, &Expectation::has_type(TyBuilder::unit()));
 
                 let ctxt = self.breakables.pop().expect("breakable stack broken");
                 if ctxt.may_break {
@@ -215,11 +214,11 @@ impl<'a> InferenceContext<'a> {
                     *condition,
                     &Expectation::has_type(TyKind::Scalar(Scalar::Bool).intern(&Interner)),
                 );
-                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+                self.infer_expr(*body, &Expectation::has_type(TyBuilder::unit()));
                 let _ctxt = self.breakables.pop().expect("breakable stack broken");
                 // the body may not run, so it diverging doesn't mean we diverge
                 self.diverges = Diverges::Maybe;
-                Ty::unit()
+                TyBuilder::unit()
             }
             Expr::For { iterable, body, pat, label } => {
                 let iterable_ty = self.infer_expr(*iterable, &Expectation::none());
@@ -234,11 +233,11 @@ impl<'a> InferenceContext<'a> {
 
                 self.infer_pat(*pat, &pat_ty, BindingMode::default());
 
-                self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
+                self.infer_expr(*body, &Expectation::has_type(TyBuilder::unit()));
                 let _ctxt = self.breakables.pop().expect("breakable stack broken");
                 // the body may not run, so it diverging doesn't mean we diverge
                 self.diverges = Diverges::Maybe;
-                Ty::unit()
+                TyBuilder::unit()
             }
             Expr::Lambda { body, args, ret_type, arg_types } => {
                 assert_eq!(args.len(), arg_types.len());
@@ -262,14 +261,17 @@ impl<'a> InferenceContext<'a> {
                 };
                 sig_tys.push(ret_ty.clone());
                 let sig_ty = TyKind::Function(FnPointer {
-                    num_args: sig_tys.len() - 1,
+                    num_binders: 0,
                     sig: FnSig { abi: (), safety: chalk_ir::Safety::Safe, variadic: false },
-                    substs: Substitution(sig_tys.clone().into()),
+                    substitution: FnSubst(
+                        Substitution::from_iter(&Interner, sig_tys.clone()).shifted_in(&Interner),
+                    ),
                 })
                 .intern(&Interner);
                 let closure_id = self.db.intern_closure((self.owner, tgt_expr)).into();
                 let closure_ty =
-                    TyKind::Closure(closure_id, Substitution::single(sig_ty)).intern(&Interner);
+                    TyKind::Closure(closure_id, Substitution::from1(&Interner, sig_ty))
+                        .intern(&Interner);
 
                 // Eagerly try to relate the closure type with the expected
                 // type, otherwise we often won't have enough information to
@@ -316,7 +318,13 @@ impl<'a> InferenceContext<'a> {
                 self.normalize_associated_types_in(ret_ty)
             }
             Expr::MethodCall { receiver, args, method_name, generic_args } => self
-                .infer_method_call(tgt_expr, *receiver, &args, &method_name, generic_args.as_ref()),
+                .infer_method_call(
+                    tgt_expr,
+                    *receiver,
+                    &args,
+                    &method_name,
+                    generic_args.as_deref(),
+                ),
             Expr::Match { expr, arms } => {
                 let input_ty = self.infer_expr(*expr, &Expectation::none());
 
@@ -358,7 +366,7 @@ impl<'a> InferenceContext<'a> {
                 let val_ty = if let Some(expr) = expr {
                     self.infer_expr(*expr, &Expectation::none())
                 } else {
-                    Ty::unit()
+                    TyBuilder::unit()
                 };
 
                 let last_ty =
@@ -384,7 +392,7 @@ impl<'a> InferenceContext<'a> {
                 if let Some(expr) = expr {
                     self.infer_expr_coerce(*expr, &Expectation::has_type(self.return_ty.clone()));
                 } else {
-                    let unit = Ty::unit();
+                    let unit = TyBuilder::unit();
                     self.coerce(&unit, &self.return_ty.clone());
                 }
                 TyKind::Never.intern(&Interner)
@@ -397,16 +405,19 @@ impl<'a> InferenceContext<'a> {
                 TyKind::Never.intern(&Interner)
             }
             Expr::RecordLit { path, fields, spread } => {
-                let (ty, def_id) = self.resolve_variant(path.as_ref());
+                let (ty, def_id) = self.resolve_variant(path.as_deref());
                 if let Some(variant) = def_id {
                     self.write_variant_resolution(tgt_expr.into(), variant);
                 }
 
                 self.unify(&ty, &expected.ty);
 
-                let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+                let substs = ty
+                    .as_adt()
+                    .map(|(_, s)| s.clone())
+                    .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));
+                let variant_data = def_id.map(|it| it.variant_data(self.db.upcast()));
                 for field in fields.iter() {
                     let field_def =
                         variant_data.as_ref().and_then(|it| match it.field(&field.name) {
@@ -418,11 +429,8 @@ impl<'a> InferenceContext<'a> {
                                 None
                             }
                         });
-                    if let Some(field_def) = field_def {
-                        self.result.record_field_resolutions.insert(field.expr, field_def);
-                    }
                     let field_ty = field_def.map_or(self.err_ty(), |it| {
-                        field_types[it.local_id].clone().subst(&substs)
+                        field_types[it.local_id].clone().substitute(&Interner, &substs)
                     });
                     self.infer_expr_coerce(field.expr, &Expectation::has_type(field_ty));
                 }
@@ -453,10 +461,14 @@ 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())
-                        }
+                    match canonicalized.decanonicalize_ty(derefed_ty.value).kind(&Interner) {
+                        TyKind::Tuple(_, substs) => name.as_tuple_index().and_then(|idx| {
+                            substs
+                                .as_slice(&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 };
@@ -465,7 +477,7 @@ impl<'a> InferenceContext<'a> {
                                 Some(
                                     self.db.field_types((*s).into())[field.local_id]
                                         .clone()
-                                        .subst(&parameters),
+                                        .substitute(&Interner, &parameters),
                                 )
                             } else {
                                 None
@@ -479,7 +491,7 @@ impl<'a> InferenceContext<'a> {
                                 Some(
                                     self.db.field_types((*u).into())[field.local_id]
                                         .clone()
-                                        .subst(&parameters),
+                                        .substitute(&Interner, &parameters),
                                 )
                             } else {
                                 None
@@ -526,24 +538,17 @@ impl<'a> InferenceContext<'a> {
                 let inner_ty = self.infer_expr_inner(*expr, &expectation);
                 match rawness {
                     Rawness::RawPtr => TyKind::Raw(mutability, inner_ty),
-                    Rawness::Ref => TyKind::Ref(mutability, inner_ty),
+                    Rawness::Ref => TyKind::Ref(mutability, static_lifetime(), inner_ty),
                 }
                 .intern(&Interner)
             }
             Expr::Box { expr } => {
                 let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
                 if let Some(box_) = self.resolve_boxed_box() {
-                    let mut sb =
-                        Substitution::build_for_generics(&generics(self.db.upcast(), box_.into()));
-                    sb = sb.push(inner_ty);
-                    match self.db.generic_defaults(box_.into()).get(1) {
-                        Some(alloc_ty) if !alloc_ty.value.is_unknown() && sb.remaining() > 0 => {
-                            sb = sb.push(alloc_ty.value.clone());
-                        }
-                        _ => (),
-                    }
-                    sb = sb.fill(repeat_with(|| self.table.new_type_var()));
-                    Ty::adt_ty(box_, sb.build())
+                    TyBuilder::adt(self.db, box_)
+                        .push(inner_ty)
+                        .fill_with_defaults(self.db, || self.table.new_type_var())
+                        .build()
                 } else {
                     self.err_ty()
                 }
@@ -571,7 +576,7 @@ impl<'a> InferenceContext<'a> {
                         None => self.err_ty(),
                     },
                     UnaryOp::Neg => {
-                        match inner_ty.interned(&Interner) {
+                        match inner_ty.kind(&Interner) {
                             // Fast path for builtins
                             TyKind::Scalar(Scalar::Int(_))
                             | TyKind::Scalar(Scalar::Uint(_))
@@ -584,7 +589,7 @@ impl<'a> InferenceContext<'a> {
                         }
                     }
                     UnaryOp::Not => {
-                        match inner_ty.interned(&Interner) {
+                        match inner_ty.kind(&Interner) {
                             // Fast path for builtins
                             TyKind::Scalar(Scalar::Bool)
                             | TyKind::Scalar(Scalar::Int(_))
@@ -633,31 +638,31 @@ 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) => TyBuilder::adt(self.db, adt).build(),
                         None => self.err_ty(),
                     },
                     (RangeOp::Exclusive, None, Some(ty)) => match self.resolve_range_to() {
-                        Some(adt) => Ty::adt_ty(adt, Substitution::single(ty)),
+                        Some(adt) => TyBuilder::adt(self.db, adt).push(ty).build(),
                         None => self.err_ty(),
                     },
                     (RangeOp::Inclusive, None, Some(ty)) => {
                         match self.resolve_range_to_inclusive() {
-                            Some(adt) => Ty::adt_ty(adt, Substitution::single(ty)),
+                            Some(adt) => TyBuilder::adt(self.db, adt).push(ty).build(),
                             None => self.err_ty(),
                         }
                     }
                     (RangeOp::Exclusive, Some(_), Some(ty)) => match self.resolve_range() {
-                        Some(adt) => Ty::adt_ty(adt, Substitution::single(ty)),
+                        Some(adt) => TyBuilder::adt(self.db, adt).push(ty).build(),
                         None => self.err_ty(),
                     },
                     (RangeOp::Inclusive, Some(_), Some(ty)) => {
                         match self.resolve_range_inclusive() {
-                            Some(adt) => Ty::adt_ty(adt, Substitution::single(ty)),
+                            Some(adt) => TyBuilder::adt(self.db, adt).push(ty).build(),
                             None => self.err_ty(),
                         }
                     }
                     (RangeOp::Exclusive, Some(ty), None) => match self.resolve_range_from() {
-                        Some(adt) => Ty::adt_ty(adt, Substitution::single(ty)),
+                        Some(adt) => TyBuilder::adt(self.db, adt).push(ty).build(),
                         None => self.err_ty(),
                     },
                     (RangeOp::Inclusive, _, None) => self.err_ty(),
@@ -690,10 +695,10 @@ impl<'a> InferenceContext<'a> {
                 }
             }
             Expr::Tuple { exprs } => {
-                let mut tys = match expected.ty.interned(&Interner) {
+                let mut tys = match expected.ty.kind(&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<_>>(),
@@ -704,11 +709,11 @@ 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) {
-                    TyKind::Array(st) | TyKind::Slice(st) => st.clone(),
+                let elem_ty = match expected.ty.kind(&Interner) {
+                    TyKind::Array(st, _) | TyKind::Slice(st) => st.clone(),
                     _ => self.table.new_type_var(),
                 };
 
@@ -732,17 +737,19 @@ impl<'a> InferenceContext<'a> {
                     }
                 }
 
-                TyKind::Array(elem_ty).intern(&Interner)
+                TyKind::Array(elem_ty, dummy_usize_const()).intern(&Interner)
             }
             Expr::Literal(lit) => match lit {
                 Literal::Bool(..) => TyKind::Scalar(Scalar::Bool).intern(&Interner),
                 Literal::String(..) => {
-                    TyKind::Ref(Mutability::Not, TyKind::Str.intern(&Interner)).intern(&Interner)
+                    TyKind::Ref(Mutability::Not, static_lifetime(), TyKind::Str.intern(&Interner))
+                        .intern(&Interner)
                 }
                 Literal::ByteString(..) => {
                     let byte_type = TyKind::Scalar(Scalar::Uint(UintTy::U8)).intern(&Interner);
-                    let array_type = TyKind::Array(byte_type).intern(&Interner);
-                    TyKind::Ref(Mutability::Not, array_type).intern(&Interner)
+                    let array_type =
+                        TyKind::Array(byte_type, dummy_usize_const()).intern(&Interner);
+                    TyKind::Ref(Mutability::Not, static_lifetime(), array_type).intern(&Interner)
                 }
                 Literal::Char(..) => TyKind::Scalar(Scalar::Char).intern(&Interner),
                 Literal::Int(_v, ty) => match ty {
@@ -822,8 +829,8 @@ impl<'a> InferenceContext<'a> {
                 // we don't even make an attempt at coercion
                 self.table.new_maybe_never_var()
             } else {
-                self.coerce(&Ty::unit(), &expected.coercion_target());
-                Ty::unit()
+                self.coerce(&TyBuilder::unit(), &expected.coercion_target());
+                TyBuilder::unit()
             }
         };
         ty
@@ -859,10 +866,10 @@ impl<'a> InferenceContext<'a> {
                 self.write_method_resolution(tgt_expr, func);
                 (ty, self.db.value_ty(func.into()), Some(generics(self.db.upcast(), func.into())))
             }
-            None => (receiver_ty, Binders::new(0, self.err_ty()), None),
+            None => (receiver_ty, Binders::empty(&Interner, self.err_ty()), None),
         };
         let substs = self.substs_for_method_call(def_generics, generic_args, &derefed_receiver_ty);
-        let method_ty = method_ty.subst(&substs);
+        let method_ty = method_ty.substitute(&Interner, &substs);
         let method_ty = self.insert_type_vars(method_ty);
         self.register_obligations_for_call(&method_ty);
         let (expected_receiver_ty, param_tys, ret_ty) = match method_ty.callable_sig(self.db) {
@@ -878,7 +885,9 @@ impl<'a> InferenceContext<'a> {
         // Apply autoref so the below unification works correctly
         // FIXME: return correct autorefs from lookup_method
         let actual_receiver_ty = match expected_receiver_ty.as_reference() {
-            Some((_, mutability)) => TyKind::Ref(mutability, derefed_receiver_ty).intern(&Interner),
+            Some((_, lifetime, mutability)) => {
+                TyKind::Ref(mutability, lifetime, derefed_receiver_ty).intern(&Interner)
+            }
             _ => derefed_receiver_ty,
         };
         self.unify(&expected_receiver_ty, &actual_receiver_ty);
@@ -951,18 +960,20 @@ 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) {
-        if let TyKind::FnDef(fn_def, parameters) = callable_ty.interned(&Interner) {
+        if let TyKind::FnDef(fn_def, parameters) = callable_ty.kind(&Interner) {
             let def: CallableDefId = from_chalk(self.db, *fn_def);
             let generic_predicates = self.db.generic_predicates(def.into());
             for predicate in generic_predicates.iter() {
-                let (predicate, binders) =
-                    predicate.clone().subst(parameters).into_value_and_skipped_binders();
-                always!(binders == 0); // quantified where clauses not yet handled
-                self.obligations.push(predicate.cast(&Interner));
+                let (predicate, binders) = predicate
+                    .clone()
+                    .substitute(&Interner, parameters)
+                    .into_value_and_skipped_binders();
+                always!(binders.len(&Interner) == 0); // quantified where clauses not yet handled
+                self.push_obligation(predicate.cast(&Interner));
             }
             // add obligation for trait implementation, if this is a trait method
             match def {
@@ -970,9 +981,11 @@ impl<'a> InferenceContext<'a> {
                     if let AssocContainerId::TraitId(trait_) = f.lookup(self.db.upcast()).container
                     {
                         // construct a TraitRef
-                        let substs =
-                            parameters.prefix(generics(self.db.upcast(), trait_.into()).len());
-                        self.obligations.push(
+                        let substs = crate::subst_prefix(
+                            &*parameters,
+                            generics(self.db.upcast(), trait_.into()).len(),
+                        );
+                        self.push_obligation(
                             TraitRef { trait_id: to_chalk_trait_id(trait_), substitution: substs }
                                 .cast(&Interner),
                         );
diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
index 474363709..aea354cde 100644
--- a/crates/hir_ty/src/infer/pat.rs
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -7,15 +7,13 @@ use chalk_ir::Mutability;
 use hir_def::{
     expr::{BindingAnnotation, Expr, Literal, Pat, PatId, RecordFieldPat},
     path::Path,
-    FieldId,
 };
 use hir_expand::name::Name;
 
 use super::{BindingMode, Expectation, InferenceContext};
 use crate::{
-    lower::lower_to_chalk_mutability,
-    utils::{generics, variant_data},
-    Interner, Substitution, Ty, TyKind,
+    lower::lower_to_chalk_mutability, static_lifetime, Interner, Substitution, Ty, TyBuilder,
+    TyExt, TyKind,
 };
 
 impl<'a> InferenceContext<'a> {
@@ -29,13 +27,14 @@ impl<'a> InferenceContext<'a> {
         ellipsis: Option<usize>,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
-        let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        let var_data = def.map(|it| it.variant_data(self.db.upcast()));
         if let Some(variant) = def {
             self.write_variant_resolution(id.into(), variant);
         }
         self.unify(&ty, expected);
 
-        let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+        let substs =
+            ty.as_adt().map(|(_, s)| s.clone()).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 {
@@ -50,7 +49,9 @@ impl<'a> InferenceContext<'a> {
             let expected_ty = var_data
                 .as_ref()
                 .and_then(|d| d.field(&Name::new_tuple_field(i)))
-                .map_or(self.err_ty(), |field| field_tys[field].clone().subst(&substs));
+                .map_or(self.err_ty(), |field| {
+                    field_tys[field].clone().substitute(&Interner, &substs)
+                });
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat, &expected_ty, default_bm);
         }
@@ -67,25 +68,22 @@ impl<'a> InferenceContext<'a> {
         id: PatId,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
-        let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        let var_data = def.map(|it| it.variant_data(self.db.upcast()));
         if let Some(variant) = def {
             self.write_variant_resolution(id.into(), variant);
         }
 
         self.unify(&ty, expected);
 
-        let substs = ty.substs().cloned().unwrap_or_else(Substitution::empty);
+        let substs =
+            ty.as_adt().map(|(_, s)| s.clone()).unwrap_or_else(|| Substitution::empty(&Interner));
 
         let field_tys = def.map(|it| self.db.field_types(it)).unwrap_or_default();
         for subpat in subpats {
             let matching_field = var_data.as_ref().and_then(|it| it.field(&subpat.name));
-            if let Some(local_id) = matching_field {
-                let field_def = FieldId { parent: def.unwrap(), local_id };
-                self.result.record_pat_field_resolutions.insert(subpat.pat, field_def);
-            }
-
-            let expected_ty = matching_field
-                .map_or(self.err_ty(), |field| field_tys[field].clone().subst(&substs));
+            let expected_ty = matching_field.map_or(self.err_ty(), |field| {
+                field_tys[field].clone().substitute(&Interner, &substs)
+            });
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat.pat, &expected_ty, default_bm);
         }
@@ -102,7 +100,7 @@ impl<'a> InferenceContext<'a> {
         let body = Arc::clone(&self.body); // avoid borrow checker problem
 
         if is_non_ref_pat(&body, pat) {
-            while let Some((inner, mutability)) = expected.as_reference() {
+            while let Some((inner, _lifetime, mutability)) = expected.as_reference() {
                 expected = inner;
                 default_bm = match default_bm {
                     BindingMode::Move => BindingMode::Ref(mutability),
@@ -124,7 +122,7 @@ impl<'a> InferenceContext<'a> {
         let ty = match &body[pat] {
             &Pat::Tuple { ref args, ellipsis } => {
                 let expectations = match expected.as_tuple() {
-                    Some(parameters) => &*parameters.0,
+                    Some(parameters) => &*parameters.as_slice(&Interner),
                     _ => &[],
                 };
 
@@ -134,7 +132,8 @@ 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 +141,8 @@ 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() {
@@ -158,7 +158,7 @@ impl<'a> InferenceContext<'a> {
             Pat::Ref { pat, mutability } => {
                 let mutability = lower_to_chalk_mutability(*mutability);
                 let expectation = match expected.as_reference() {
-                    Some((inner_ty, exp_mut)) => {
+                    Some((inner_ty, _lifetime, exp_mut)) => {
                         if mutability != exp_mut {
                             // FIXME: emit type error?
                         }
@@ -167,10 +167,10 @@ impl<'a> InferenceContext<'a> {
                     _ => self.result.standard_types.unknown.clone(),
                 };
                 let subty = self.infer_pat(*pat, &expectation, default_bm);
-                TyKind::Ref(mutability, subty).intern(&Interner)
+                TyKind::Ref(mutability, static_lifetime(), subty).intern(&Interner)
             }
             Pat::TupleStruct { path: p, args: subpats, ellipsis } => self.infer_tuple_struct_pat(
-                p.as_ref(),
+                p.as_deref(),
                 subpats,
                 expected,
                 default_bm,
@@ -178,7 +178,7 @@ impl<'a> InferenceContext<'a> {
                 *ellipsis,
             ),
             Pat::Record { path: p, args: fields, ellipsis: _ } => {
-                self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
+                self.infer_record_pat(p.as_deref(), fields, expected, default_bm, pat)
             }
             Pat::Path(path) => {
                 // FIXME use correct resolver for the surrounding expression
@@ -200,7 +200,8 @@ impl<'a> InferenceContext<'a> {
 
                 let bound_ty = match mode {
                     BindingMode::Ref(mutability) => {
-                        TyKind::Ref(mutability, inner_ty.clone()).intern(&Interner)
+                        TyKind::Ref(mutability, static_lifetime(), inner_ty.clone())
+                            .intern(&Interner)
                     }
                     BindingMode::Move => inner_ty.clone(),
                 };
@@ -209,17 +210,20 @@ impl<'a> InferenceContext<'a> {
                 return inner_ty;
             }
             Pat::Slice { prefix, slice, suffix } => {
-                let (container_ty, elem_ty): (fn(_) -> _, _) = match expected.interned(&Interner) {
-                    TyKind::Array(st) => (TyKind::Array, st.clone()),
-                    TyKind::Slice(st) => (TyKind::Slice, st.clone()),
-                    _ => (TyKind::Slice, self.err_ty()),
+                let elem_ty = match expected.kind(&Interner) {
+                    TyKind::Array(st, _) | TyKind::Slice(st) => st.clone(),
+                    _ => self.err_ty(),
                 };
 
                 for pat_id in prefix.iter().chain(suffix) {
                     self.infer_pat(*pat_id, &elem_ty, default_bm);
                 }
 
-                let pat_ty = container_ty(elem_ty).intern(&Interner);
+                let pat_ty = match expected.kind(&Interner) {
+                    TyKind::Array(_, const_) => TyKind::Array(elem_ty, const_.clone()),
+                    _ => TyKind::Slice(elem_ty),
+                }
+                .intern(&Interner);
                 if let Some(slice_pat_id) = slice {
                     self.infer_pat(*slice_pat_id, &pat_ty, default_bm);
                 }
@@ -236,30 +240,20 @@ 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.as_slice(&Interner).get(1).and_then(|a| a.ty(&Interner).cloned()),
+                        ),
                         _ => (self.result.standard_types.unknown.clone(), None),
                     };
 
                     let inner_ty = self.infer_pat(*inner, &inner_ty, default_bm);
-                    let mut sb = Substitution::build_for_generics(&generics(
-                        self.db.upcast(),
-                        box_adt.into(),
-                    ));
-                    sb = sb.push(inner_ty);
-                    if sb.remaining() == 1 {
-                        sb = sb.push(match alloc_ty {
-                            Some(alloc_ty) if !alloc_ty.is_unknown() => alloc_ty,
-                            _ => match self.db.generic_defaults(box_adt.into()).get(1) {
-                                Some(alloc_ty) if !alloc_ty.value.is_unknown() => {
-                                    alloc_ty.value.clone()
-                                }
-                                _ => self.table.new_type_var(),
-                            },
-                        });
+                    let mut b = TyBuilder::adt(self.db, box_adt).push(inner_ty);
+
+                    if let Some(alloc_ty) = alloc_ty {
+                        b = b.push(alloc_ty);
                     }
-                    Ty::adt_ty(box_adt, sb.build())
+                    b.fill_with_defaults(self.db, || self.table.new_type_var()).build()
                 }
                 None => self.err_ty(),
             },
diff --git a/crates/hir_ty/src/infer/path.rs b/crates/hir_ty/src/infer/path.rs
index cefa38509..495282eba 100644
--- a/crates/hir_ty/src/infer/path.rs
+++ b/crates/hir_ty/src/infer/path.rs
@@ -11,7 +11,8 @@ use hir_def::{
 use hir_expand::name::Name;
 
 use crate::{
-    method_resolution, to_chalk_trait_id, Interner, Substitution, Ty, TyKind, ValueTyDefId,
+    method_resolution, Interner, Substitution, TraitRefExt, Ty, TyBuilder, TyExt, TyKind,
+    ValueTyDefId,
 };
 
 use super::{ExprOrPatId, InferenceContext, TraitRef};
@@ -82,10 +83,10 @@ impl<'a> InferenceContext<'a> {
             }
             ValueNs::ImplSelf(impl_id) => {
                 let generics = crate::utils::generics(self.db.upcast(), impl_id.into());
-                let substs = Substitution::type_params_for_generics(self.db, &generics);
-                let ty = self.db.impl_self_ty(impl_id).subst(&substs);
+                let substs = generics.type_params_subst(self.db);
+                let ty = self.db.impl_self_ty(impl_id).substitute(&Interner, &substs);
                 if let Some((AdtId::StructId(struct_id), substs)) = ty.as_adt() {
-                    let ty = self.db.value_ty(struct_id.into()).subst(&substs);
+                    let ty = self.db.value_ty(struct_id.into()).substitute(&Interner, &substs);
                     return Some(ty);
                 } else {
                     // FIXME: diagnostic, invalid Self reference
@@ -95,16 +96,13 @@ impl<'a> InferenceContext<'a> {
             ValueNs::GenericParam(it) => return Some(self.db.const_param_ty(it)),
         };
 
-        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 ty = TyBuilder::value_ty(self.db, typable)
             .use_parent_substs(&parent_substs)
-            .fill(substs.0[parent_substs.len()..].iter().cloned())
+            .fill(substs.as_slice(&Interner)[parent_substs.len(&Interner)..].iter().cloned())
             .build();
-        let ty = ty.subst(&full_substs);
         Some(ty)
     }
 
@@ -147,7 +145,7 @@ impl<'a> InferenceContext<'a> {
                     remaining_segments_for_ty,
                     true,
                 );
-                if let TyKind::Unknown = ty.interned(&Interner) {
+                if let TyKind::Error = ty.kind(&Interner) {
                     return None;
                 }
 
@@ -212,7 +210,7 @@ impl<'a> InferenceContext<'a> {
         name: &Name,
         id: ExprOrPatId,
     ) -> Option<(ValueNs, Option<Substitution>)> {
-        if let TyKind::Unknown = ty.interned(&Interner) {
+        if let TyKind::Error = ty.kind(&Interner) {
             return None;
         }
 
@@ -245,27 +243,22 @@ impl<'a> InferenceContext<'a> {
                 };
                 let substs = match container {
                     AssocContainerId::ImplId(impl_id) => {
-                        let impl_substs = Substitution::build_for_def(self.db, impl_id)
+                        let impl_substs = TyBuilder::subst_for_def(self.db, impl_id)
                             .fill(iter::repeat_with(|| self.table.new_type_var()))
                             .build();
-                        let impl_self_ty = self.db.impl_self_ty(impl_id).subst(&impl_substs);
+                        let impl_self_ty =
+                            self.db.impl_self_ty(impl_id).substitute(&Interner, &impl_substs);
                         self.unify(&impl_self_ty, &ty);
                         Some(impl_substs)
                     }
                     AssocContainerId::TraitId(trait_) => {
                         // we're picking this method
-                        let trait_substs = Substitution::build_for_def(self.db, trait_)
+                        let trait_ref = TyBuilder::trait_ref(self.db, trait_)
                             .push(ty.clone())
                             .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                             .build();
-                        self.obligations.push(
-                            TraitRef {
-                                trait_id: to_chalk_trait_id(trait_),
-                                substitution: trait_substs.clone(),
-                            }
-                            .cast(&Interner),
-                        );
-                        Some(trait_substs)
+                        self.push_obligation(trait_ref.clone().cast(&Interner));
+                        Some(trait_ref.substitution)
                     }
                     AssocContainerId::ModuleId(_) => None,
                 };
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index 6e7b0f5a6..a887e20b0 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -2,13 +2,17 @@
 
 use std::borrow::Cow;
 
-use chalk_ir::{FloatTy, IntTy, TyVariableKind, UniverseIndex, VariableKind};
+use chalk_ir::{
+    cast::Cast, fold::Fold, interner::HasInterner, FloatTy, IntTy, TyVariableKind, UniverseIndex,
+    VariableKind,
+};
 use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
 
 use super::{DomainGoal, InferenceContext};
 use crate::{
-    AliasEq, AliasTy, BoundVar, Canonical, CanonicalVarKinds, DebruijnIndex, FnPointer,
-    InEnvironment, InferenceVar, Interner, Scalar, Substitution, Ty, TyKind, TypeWalk, WhereClause,
+    fold_tys, static_lifetime, AliasEq, AliasTy, BoundVar, Canonical, CanonicalVarKinds,
+    DebruijnIndex, FnPointer, FnSubst, InEnvironment, InferenceVar, Interner, Scalar, Substitution,
+    Ty, TyExt, TyKind, WhereClause,
 };
 
 impl<'a> InferenceContext<'a> {
@@ -33,7 +37,10 @@ where
 }
 
 #[derive(Debug)]
-pub(super) struct Canonicalized<T> {
+pub(super) struct Canonicalized<T>
+where
+    T: HasInterner<Interner = Interner>,
+{
     pub(super) value: Canonical<T>,
     free_vars: Vec<(InferenceVar, TyVariableKind)>,
 }
@@ -47,11 +54,16 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
         })
     }
 
-    fn do_canonicalize<T: TypeWalk>(&mut self, t: T, binders: DebruijnIndex) -> T {
-        t.fold_binders(
-            &mut |ty, binders| match ty.interned(&Interner) {
+    fn do_canonicalize<T: Fold<Interner, Result = T> + HasInterner<Interner = Interner>>(
+        &mut self,
+        t: T,
+        binders: DebruijnIndex,
+    ) -> T {
+        fold_tys(
+            t,
+            |ty, binders| match ty.kind(&Interner) {
                 &TyKind::InferenceVar(var, kind) => {
-                    let inner = var.to_inner();
+                    let inner = from_inference_var(var);
                     if self.var_stack.contains(&inner) {
                         // recursive type
                         return self.ctx.table.type_variable_table.fallback_value(var, kind);
@@ -65,7 +77,7 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
                         result
                     } else {
                         let root = self.ctx.table.var_unification_table.find(inner);
-                        let position = self.add(InferenceVar::from_inner(root), kind);
+                        let position = self.add(to_inference_var(root), kind);
                         TyKind::BoundVar(BoundVar::new(binders, position)).intern(&Interner)
                     }
                 }
@@ -75,7 +87,10 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
         )
     }
 
-    fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
+    fn into_canonicalized<T: HasInterner<Interner = Interner>>(
+        self,
+        result: T,
+    ) -> Canonicalized<T> {
         let kinds = self
             .free_vars
             .iter()
@@ -102,25 +117,18 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
             DomainGoal::Holds(wc) => {
                 DomainGoal::Holds(self.do_canonicalize(wc, DebruijnIndex::INNERMOST))
             }
+            _ => unimplemented!(),
         };
         self.into_canonicalized(InEnvironment { goal: result, environment: obligation.environment })
     }
 }
 
-impl<T> Canonicalized<T> {
-    pub(super) fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
-        ty.walk_mut_binders(
-            &mut |ty, binders| {
-                if let &mut TyKind::BoundVar(bound) = ty.interned_mut() {
-                    if bound.debruijn >= binders {
-                        let (v, k) = self.free_vars[bound.index];
-                        *ty = TyKind::InferenceVar(v, k).intern(&Interner);
-                    }
-                }
-            },
-            DebruijnIndex::INNERMOST,
-        );
-        ty
+impl<T: HasInterner<Interner = Interner>> Canonicalized<T> {
+    pub(super) fn decanonicalize_ty(&self, ty: Ty) -> Ty {
+        crate::fold_free_vars(ty, |bound, _binders| {
+            let (v, k) = self.free_vars[bound.index];
+            TyKind::InferenceVar(v, k).intern(&Interner)
+        })
     }
 
     pub(super) fn apply_solution(
@@ -129,29 +137,30 @@ 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().cast(&Interner)
+                }
+                VariableKind::Ty(TyVariableKind::Integer) => {
+                    ctx.table.new_integer_var().cast(&Interner)
+                }
+                VariableKind::Ty(TyVariableKind::Float) => {
+                    ctx.table.new_float_var().cast(&Interner)
+                }
+                // Chalk can sometimes return new lifetime variables. We just use the static lifetime everywhere
+                VariableKind::Lifetime => static_lifetime().cast(&Interner),
+                _ => 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(
+                new_vars.apply(ty.assert_ty_ref(&Interner).clone(), &Interner),
+            );
             ctx.table.unify(&TyKind::InferenceVar(v, k).intern(&Interner), &ty);
         }
     }
@@ -163,22 +172,23 @@ 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);
+    let ty1_with_vars = vars.apply(tys.value.0.clone(), &Interner);
+    let ty2_with_vars = vars.apply(tys.value.1.clone(), &Interner);
     if !table.unify(&ty1_with_vars, &ty2_with_vars) {
         return None;
     }
     // 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,
@@ -186,11 +196,11 @@ 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())))
-            .build(),
-    )
+    Some(Substitution::from_iter(
+        &Interner,
+        vars.iter(&Interner)
+            .map(|v| table.resolve_ty_completely(v.assert_ty_ref(&Interner).clone())),
+    ))
 }
 
 #[derive(Clone, Debug)]
@@ -204,17 +214,17 @@ impl TypeVariableTable {
     }
 
     pub(super) fn set_diverging(&mut self, iv: InferenceVar, diverging: bool) {
-        self.inner[iv.to_inner().0 as usize].diverging = diverging;
+        self.inner[from_inference_var(iv).0 as usize].diverging = diverging;
     }
 
     fn is_diverging(&mut self, iv: InferenceVar) -> bool {
-        self.inner[iv.to_inner().0 as usize].diverging
+        self.inner[from_inference_var(iv).0 as usize].diverging
     }
 
     fn fallback_value(&self, iv: InferenceVar, kind: TyVariableKind) -> Ty {
         match kind {
-            _ if self.inner[iv.to_inner().0 as usize].diverging => TyKind::Never,
-            TyVariableKind::General => TyKind::Unknown,
+            _ if self.inner[from_inference_var(iv).0 as usize].diverging => TyKind::Never,
+            TyVariableKind::General => TyKind::Error,
             TyVariableKind::Integer => TyKind::Scalar(Scalar::Int(IntTy::I32)),
             TyVariableKind::Float => TyKind::Scalar(Scalar::Float(FloatTy::F64)),
         }
@@ -231,6 +241,7 @@ pub(crate) struct TypeVariableData {
 pub(crate) struct InferenceTable {
     pub(super) var_unification_table: InPlaceUnificationTable<TypeVarId>,
     pub(super) type_variable_table: TypeVariableTable,
+    pub(super) revision: u32,
 }
 
 impl InferenceTable {
@@ -238,6 +249,7 @@ impl InferenceTable {
         InferenceTable {
             var_unification_table: InPlaceUnificationTable::new(),
             type_variable_table: TypeVariableTable { inner: Vec::new() },
+            revision: 0,
         }
     }
 
@@ -245,7 +257,7 @@ impl InferenceTable {
         self.type_variable_table.push(TypeVariableData { diverging });
         let key = self.var_unification_table.new_key(TypeVarValue::Unknown);
         assert_eq!(key.0 as usize, self.type_variable_table.inner.len() - 1);
-        TyKind::InferenceVar(InferenceVar::from_inner(key), kind).intern(&Interner)
+        TyKind::InferenceVar(to_inference_var(key), kind).intern(&Interner)
     }
 
     pub(crate) fn new_type_var(&mut self) -> Ty {
@@ -282,7 +294,9 @@ impl InferenceTable {
         substs2: &Substitution,
         depth: usize,
     ) -> bool {
-        substs1.0.iter().zip(substs2.0.iter()).all(|(t1, t2)| self.unify_inner(t1, t2, depth))
+        substs1.iter(&Interner).zip(substs2.iter(&Interner)).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 {
@@ -297,12 +311,12 @@ impl InferenceTable {
         let ty1 = self.resolve_ty_shallow(ty1);
         let ty2 = self.resolve_ty_shallow(ty2);
         if ty1.equals_ctor(&ty2) {
-            match (ty1.interned(&Interner), ty2.interned(&Interner)) {
+            match (ty1.kind(&Interner), ty2.kind(&Interner)) {
                 (TyKind::Adt(_, substs1), TyKind::Adt(_, substs2))
                 | (TyKind::FnDef(_, substs1), TyKind::FnDef(_, substs2))
                 | (
-                    TyKind::Function(FnPointer { substs: substs1, .. }),
-                    TyKind::Function(FnPointer { substs: substs2, .. }),
+                    TyKind::Function(FnPointer { substitution: FnSubst(substs1), .. }),
+                    TyKind::Function(FnPointer { substitution: FnSubst(substs2), .. }),
                 )
                 | (TyKind::Tuple(_, substs1), TyKind::Tuple(_, substs2))
                 | (TyKind::OpaqueType(_, substs1), TyKind::OpaqueType(_, substs2))
@@ -310,9 +324,11 @@ impl InferenceTable {
                 | (TyKind::Closure(.., substs1), TyKind::Closure(.., substs2)) => {
                     self.unify_substs(substs1, substs2, depth + 1)
                 }
-                (TyKind::Ref(_, ty1), TyKind::Ref(_, ty2))
+                (TyKind::Array(ty1, c1), TyKind::Array(ty2, c2)) if c1 == c2 => {
+                    self.unify_inner(ty1, ty2, depth + 1)
+                }
+                (TyKind::Ref(_, _, ty1), TyKind::Ref(_, _, ty2))
                 | (TyKind::Raw(_, ty1), TyKind::Raw(_, ty2))
-                | (TyKind::Array(ty1), TyKind::Array(ty2))
                 | (TyKind::Slice(ty1), TyKind::Slice(ty2)) => self.unify_inner(ty1, ty2, depth + 1),
                 _ => true, /* we checked equals_ctor already */
             }
@@ -322,8 +338,8 @@ impl InferenceTable {
     }
 
     pub(super) fn unify_inner_trivial(&mut self, ty1: &Ty, ty2: &Ty, depth: usize) -> bool {
-        match (ty1.interned(&Interner), ty2.interned(&Interner)) {
-            (TyKind::Unknown, _) | (_, TyKind::Unknown) => true,
+        match (ty1.kind(&Interner), ty2.kind(&Interner)) {
+            (TyKind::Error, _) | (_, TyKind::Error) => true,
 
             (TyKind::Placeholder(p1), TyKind::Placeholder(p2)) if *p1 == *p2 => true,
 
@@ -360,7 +376,14 @@ impl InferenceTable {
                 == self.type_variable_table.is_diverging(*tv2) =>
             {
                 // both type vars are unknown since we tried to resolve them
-                self.var_unification_table.union(tv1.to_inner(), tv2.to_inner());
+                if !self
+                    .var_unification_table
+                    .unioned(from_inference_var(*tv1), from_inference_var(*tv2))
+                {
+                    self.var_unification_table
+                        .union(from_inference_var(*tv1), from_inference_var(*tv2));
+                    self.revision += 1;
+                }
                 true
             }
 
@@ -395,9 +418,10 @@ impl InferenceTable {
             ) => {
                 // the type var is unknown since we tried to resolve it
                 self.var_unification_table.union_value(
-                    tv.to_inner(),
+                    from_inference_var(*tv),
                     TypeVarValue::Known(other.clone().intern(&Interner)),
                 );
+                self.revision += 1;
                 true
             }
 
@@ -447,9 +471,9 @@ impl InferenceTable {
             if i > 0 {
                 cov_mark::hit!(type_var_resolves_to_int_var);
             }
-            match ty.interned(&Interner) {
+            match ty.kind(&Interner) {
                 TyKind::InferenceVar(tv, _) => {
-                    let inner = tv.to_inner();
+                    let inner = from_inference_var(*tv);
                     match self.var_unification_table.inlined_probe_value(inner).known() {
                         Some(known_ty) => {
                             // The known_ty can't be a type var itself
@@ -470,55 +494,63 @@ impl InferenceTable {
     /// be resolved as far as possible, i.e. contain no type variables with
     /// known type.
     fn resolve_ty_as_possible_inner(&mut self, tv_stack: &mut Vec<TypeVarId>, ty: Ty) -> Ty {
-        ty.fold(&mut |ty| match ty.interned(&Interner) {
-            &TyKind::InferenceVar(tv, kind) => {
-                let inner = tv.to_inner();
-                if tv_stack.contains(&inner) {
-                    cov_mark::hit!(type_var_cycles_resolve_as_possible);
-                    // recursive type
-                    return self.type_variable_table.fallback_value(tv, kind);
-                }
-                if let Some(known_ty) =
-                    self.var_unification_table.inlined_probe_value(inner).known()
-                {
-                    // known_ty may contain other variables that are known by now
-                    tv_stack.push(inner);
-                    let result = self.resolve_ty_as_possible_inner(tv_stack, known_ty.clone());
-                    tv_stack.pop();
-                    result
-                } else {
-                    ty
+        fold_tys(
+            ty,
+            |ty, _| match ty.kind(&Interner) {
+                &TyKind::InferenceVar(tv, kind) => {
+                    let inner = from_inference_var(tv);
+                    if tv_stack.contains(&inner) {
+                        cov_mark::hit!(type_var_cycles_resolve_as_possible);
+                        // recursive type
+                        return self.type_variable_table.fallback_value(tv, kind);
+                    }
+                    if let Some(known_ty) =
+                        self.var_unification_table.inlined_probe_value(inner).known()
+                    {
+                        // known_ty may contain other variables that are known by now
+                        tv_stack.push(inner);
+                        let result = self.resolve_ty_as_possible_inner(tv_stack, known_ty.clone());
+                        tv_stack.pop();
+                        result
+                    } else {
+                        ty
+                    }
                 }
-            }
-            _ => ty,
-        })
+                _ => ty,
+            },
+            DebruijnIndex::INNERMOST,
+        )
     }
 
     /// Resolves the type completely; type variables without known type are
     /// replaced by TyKind::Unknown.
     fn resolve_ty_completely_inner(&mut self, tv_stack: &mut Vec<TypeVarId>, ty: Ty) -> Ty {
-        ty.fold(&mut |ty| match ty.interned(&Interner) {
-            &TyKind::InferenceVar(tv, kind) => {
-                let inner = tv.to_inner();
-                if tv_stack.contains(&inner) {
-                    cov_mark::hit!(type_var_cycles_resolve_completely);
-                    // recursive type
-                    return self.type_variable_table.fallback_value(tv, kind);
-                }
-                if let Some(known_ty) =
-                    self.var_unification_table.inlined_probe_value(inner).known()
-                {
-                    // known_ty may contain other variables that are known by now
-                    tv_stack.push(inner);
-                    let result = self.resolve_ty_completely_inner(tv_stack, known_ty.clone());
-                    tv_stack.pop();
-                    result
-                } else {
-                    self.type_variable_table.fallback_value(tv, kind)
+        fold_tys(
+            ty,
+            |ty, _| match ty.kind(&Interner) {
+                &TyKind::InferenceVar(tv, kind) => {
+                    let inner = from_inference_var(tv);
+                    if tv_stack.contains(&inner) {
+                        cov_mark::hit!(type_var_cycles_resolve_completely);
+                        // recursive type
+                        return self.type_variable_table.fallback_value(tv, kind);
+                    }
+                    if let Some(known_ty) =
+                        self.var_unification_table.inlined_probe_value(inner).known()
+                    {
+                        // known_ty may contain other variables that are known by now
+                        tv_stack.push(inner);
+                        let result = self.resolve_ty_completely_inner(tv_stack, known_ty.clone());
+                        tv_stack.pop();
+                        result
+                    } else {
+                        self.type_variable_table.fallback_value(tv, kind)
+                    }
                 }
-            }
-            _ => ty,
-        })
+                _ => ty,
+            },
+            DebruijnIndex::INNERMOST,
+        )
     }
 }
 
@@ -542,6 +574,14 @@ impl UnifyKey for TypeVarId {
     }
 }
 
+fn from_inference_var(var: InferenceVar) -> TypeVarId {
+    TypeVarId(var.index())
+}
+
+fn to_inference_var(TypeVarId(index): TypeVarId) -> InferenceVar {
+    index.into()
+}
+
 /// The value of a type variable: either we already know the type, or we don't
 /// know it yet.
 #[derive(Clone, PartialEq, Eq, Debug)]