diff --git a/src/librustc/infer/opaque_types/mod.rs b/src/librustc/infer/opaque_types/mod.rs
index e7205dd47a61..d45869ed160c 100644
--- a/src/librustc/infer/opaque_types/mod.rs
+++ b/src/librustc/infer/opaque_types/mod.rs
@@ -733,12 +733,12 @@ where
                 // Skip lifetime parameters of the enclosing item(s)
                 // Also skip the witness type, because that has no free regions.
 
-                for upvar_ty in substs.upvar_tys(def_id, self.tcx) {
+                for upvar_ty in substs.as_generator().upvar_tys(def_id, self.tcx) {
                     upvar_ty.visit_with(self);
                 }
 
-                substs.return_ty(def_id, self.tcx).visit_with(self);
-                substs.yield_ty(def_id, self.tcx).visit_with(self);
+                substs.as_generator().return_ty(def_id, self.tcx).visit_with(self);
+                substs.as_generator().yield_ty(def_id, self.tcx).visit_with(self);
             }
             _ => {
                 ty.super_visit_with(self);
@@ -902,7 +902,7 @@ impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
             ty::Generator(def_id, substs, movability) => {
                 let generics = self.tcx.generics_of(def_id);
                 let substs =
-                    self.tcx.mk_substs(substs.substs.iter().enumerate().map(|(index, &kind)| {
+                    self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
                         if index < generics.parent_count {
                             // Accommodate missing regions in the parent kinds...
                             self.fold_kind_mapping_missing_regions_to_empty(kind)
diff --git a/src/librustc/mir/tcx.rs b/src/librustc/mir/tcx.rs
index 26f718e858da..e87aabf9a056 100644
--- a/src/librustc/mir/tcx.rs
+++ b/src/librustc/mir/tcx.rs
@@ -197,7 +197,7 @@ impl<'tcx> Rvalue<'tcx> {
                 let ty = place.ty(local_decls, tcx).ty;
                 match ty.kind {
                     ty::Adt(adt_def, _) => adt_def.repr.discr_type().to_ty(tcx),
-                    ty::Generator(_, substs, _) => substs.discr_ty(tcx),
+                    ty::Generator(_, substs, _) => substs.as_generator().discr_ty(tcx),
                     _ => {
                         // This can only be `0`, for now, so `u8` will suffice.
                         tcx.types.u8
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index 1fae2a2fe8db..4ee5700f43fe 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -2761,8 +2761,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .collect(),
 
             ty::Generator(def_id, ref substs, _) => {
-                let witness = substs.witness(def_id, self.tcx());
+                let witness = substs.as_generator().witness(def_id, self.tcx());
                 substs
+                    .as_generator()
                     .upvar_tys(def_id, self.tcx())
                     .chain(iter::once(witness))
                     .collect()
@@ -3324,8 +3325,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         )?);
 
         Ok(VtableGeneratorData {
-            generator_def_id: generator_def_id,
-            substs: substs.clone(),
+            generator_def_id,
+            substs,
             nested: obligations,
         })
     }
@@ -3911,9 +3912,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         obligation: &TraitObligation<'tcx>,
         closure_def_id: DefId,
-        substs: ty::GeneratorSubsts<'tcx>,
+        substs: SubstsRef<'tcx>,
     ) -> ty::PolyTraitRef<'tcx> {
-        let gen_sig = substs.poly_sig(closure_def_id, self.tcx());
+        let gen_sig = substs.as_generator().poly_sig(closure_def_id, self.tcx());
 
         // (1) Feels icky to skip the binder here, but OTOH we know
         // that the self-type is an generator type and hence is
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 5aad6c1dc183..48820540ebeb 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -2510,7 +2510,7 @@ impl<'tcx> TyCtxt<'tcx> {
     #[inline]
     pub fn mk_generator(self,
                         id: DefId,
-                        generator_substs: GeneratorSubsts<'tcx>,
+                        generator_substs: SubstsRef<'tcx>,
                         movability: hir::GeneratorMovability)
                         -> Ty<'tcx> {
         self.mk_ty(Generator(id, generator_substs, movability))
diff --git a/src/librustc/ty/flags.rs b/src/librustc/ty/flags.rs
index b513ef5a9667..4ff5e1a7b8de 100644
--- a/src/librustc/ty/flags.rs
+++ b/src/librustc/ty/flags.rs
@@ -94,7 +94,7 @@ impl FlagComputation {
             &ty::Generator(_, ref substs, _) => {
                 self.add_flags(TypeFlags::HAS_TY_CLOSURE);
                 self.add_flags(TypeFlags::HAS_FREE_LOCAL_NAMES);
-                self.add_substs(&substs.substs);
+                self.add_substs(substs);
             }
 
             &ty::GeneratorWitness(ref ts) => {
diff --git a/src/librustc/ty/instance.rs b/src/librustc/ty/instance.rs
index 34f806b15c0c..241b00a2dc40 100644
--- a/src/librustc/ty/instance.rs
+++ b/src/librustc/ty/instance.rs
@@ -71,7 +71,7 @@ impl<'tcx> Instance<'tcx> {
                 ))
             }
             ty::Generator(def_id, substs, _) => {
-                let sig = substs.poly_sig(def_id, tcx);
+                let sig = substs.as_generator().poly_sig(def_id, tcx);
 
                 let env_region = ty::ReLateBound(ty::INNERMOST, ty::BrEnv);
                 let env_ty = tcx.mk_mut_ref(tcx.mk_region(env_region), ty);
diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index 6e01e1bf26a5..2e6e2f012de9 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -1,5 +1,5 @@
 use crate::session::{self, DataTypeKind};
-use crate::ty::{self, Ty, TyCtxt, TypeFoldable, ReprOptions};
+use crate::ty::{self, Ty, TyCtxt, TypeFoldable, ReprOptions, subst::SubstsRef};
 
 use syntax::ast::{self, Ident, IntTy, UintTy};
 use syntax::attr;
@@ -15,7 +15,6 @@ use std::ops::Bound;
 use crate::hir;
 use crate::ich::StableHashingContext;
 use crate::mir::{GeneratorLayout, GeneratorSavedLocal};
-use crate::ty::GeneratorSubsts;
 use crate::ty::subst::Subst;
 use rustc_index::bit_set::BitSet;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -671,7 +670,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                 tcx.intern_layout(unit)
             }
 
-            ty::Generator(def_id, substs, _) => self.generator_layout(ty, def_id, &substs)?,
+            ty::Generator(def_id, substs, _) => self.generator_layout(ty, def_id, substs)?,
 
             ty::Closure(def_id, ref substs) => {
                 let tys = substs.as_closure().upvar_tys(def_id, tcx);
@@ -1406,7 +1405,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
         &self,
         ty: Ty<'tcx>,
         def_id: hir::def_id::DefId,
-        substs: &GeneratorSubsts<'tcx>,
+        substs: SubstsRef<'tcx>,
     ) -> Result<&'tcx LayoutDetails, LayoutError<'tcx>> {
         use SavedLocalEligibility::*;
         let tcx = self.tcx;
@@ -1419,9 +1418,9 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
         // Build a prefix layout, including "promoting" all ineligible
         // locals as part of the prefix. We compute the layout of all of
         // these fields at once to get optimal packing.
-        let discr_index = substs.prefix_tys(def_id, tcx).count();
+        let discr_index = substs.as_generator().prefix_tys(def_id, tcx).count();
         // FIXME(eddyb) set the correct vaidity range for the discriminant.
-        let discr_layout = self.layout_of(substs.discr_ty(tcx))?;
+        let discr_layout = self.layout_of(substs.as_generator().discr_ty(tcx))?;
         let discr = match &discr_layout.abi {
             Abi::Scalar(s) => s.clone(),
             _ => bug!(),
@@ -2153,7 +2152,7 @@ where
             ty::Generator(def_id, ref substs, _) => {
                 match this.variants {
                     Variants::Single { index } => {
-                        substs.state_tys(def_id, tcx)
+                        substs.as_generator().state_tys(def_id, tcx)
                             .nth(index.as_usize()).unwrap()
                             .nth(i).unwrap()
                     }
@@ -2161,7 +2160,7 @@ where
                         if i == discr_index {
                             return discr_layout(discr);
                         }
-                        substs.prefix_tys(def_id, tcx).nth(i).unwrap()
+                        substs.as_generator().prefix_tys(def_id, tcx).nth(i).unwrap()
                     }
                 }
             }
diff --git a/src/librustc/ty/outlives.rs b/src/librustc/ty/outlives.rs
index 3ea767d51159..80e77cdfad0b 100644
--- a/src/librustc/ty/outlives.rs
+++ b/src/librustc/ty/outlives.rs
@@ -69,7 +69,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
             ty::Generator(def_id, ref substs, _) => {
                 // Same as the closure case
-                for upvar_ty in substs.upvar_tys(def_id, *self) {
+                for upvar_ty in substs.as_generator().upvar_tys(def_id, *self) {
                     self.compute_components(upvar_ty, out);
                 }
 
diff --git a/src/librustc/ty/print/obsolete.rs b/src/librustc/ty/print/obsolete.rs
index 21c018d9ee6d..3855e9e9b8a5 100644
--- a/src/librustc/ty/print/obsolete.rs
+++ b/src/librustc/ty/print/obsolete.rs
@@ -154,7 +154,7 @@ impl DefPathBasedNames<'tcx> {
                     self.push_type_name(sig.output(), output, debug);
                 }
             }
-            ty::Generator(def_id, GeneratorSubsts { substs }, _)
+            ty::Generator(def_id,  substs, _)
             | ty::Closure(def_id, substs) => {
                 self.push_def_path(def_id, output);
                 let generics = self.tcx.generics_of(self.tcx.closure_base_def_id(def_id));
diff --git a/src/librustc/ty/print/pretty.rs b/src/librustc/ty/print/pretty.rs
index ad4be788dae4..732638929963 100644
--- a/src/librustc/ty/print/pretty.rs
+++ b/src/librustc/ty/print/pretty.rs
@@ -605,8 +605,8 @@ pub trait PrettyPrinter<'tcx>:
             }
             ty::Str => p!(write("str")),
             ty::Generator(did, substs, movability) => {
-                let upvar_tys = substs.upvar_tys(did, self.tcx());
-                let witness = substs.witness(did, self.tcx());
+                let upvar_tys = substs.as_generator().upvar_tys(did, self.tcx());
+                let witness = substs.as_generator().witness(did, self.tcx());
                 if movability == hir::GeneratorMovability::Movable {
                     p!(write("[generator"));
                 } else {
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index c1589b31de78..ff20996df56f 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -2109,7 +2109,8 @@ impl<'tcx> TyS<'tcx> {
     pub fn variant_range(&self, tcx: TyCtxt<'tcx>) -> Option<Range<VariantIdx>> {
         match self.kind {
             TyKind::Adt(adt, _) => Some(adt.variant_range()),
-            TyKind::Generator(def_id, substs, _) => Some(substs.variant_range(def_id, tcx)),
+            TyKind::Generator(def_id, substs, _) =>
+                Some(substs.assert_generator().variant_range(def_id, tcx)),
             _ => None,
         }
     }
@@ -2126,7 +2127,7 @@ impl<'tcx> TyS<'tcx> {
         match self.kind {
             TyKind::Adt(adt, _) => Some(adt.discriminant_for_variant(tcx, variant_index)),
             TyKind::Generator(def_id, substs, _) =>
-                Some(substs.discriminant_for_variant(def_id, tcx, variant_index)),
+                Some(substs.as_generator().discriminant_for_variant(def_id, tcx, variant_index)),
             _ => None,
         }
     }
@@ -2149,7 +2150,7 @@ impl<'tcx> TyS<'tcx> {
                 out.extend(substs.regions())
             }
             Closure(_, ref substs ) |
-            Generator(_, GeneratorSubsts { ref substs }, _) => {
+            Generator(_, ref substs, _) => {
                 out.extend(substs.regions())
             }
             Projection(ref data) | UnnormalizedProjection(ref data) => {
diff --git a/src/librustc/ty/walk.rs b/src/librustc/ty/walk.rs
index 1895ab83674e..f5b1902e3cc8 100644
--- a/src/librustc/ty/walk.rs
+++ b/src/librustc/ty/walk.rs
@@ -110,12 +110,10 @@ fn push_subtypes<'tcx>(stack: &mut TypeWalkerStack<'tcx>, parent_ty: Ty<'tcx>) {
         ty::Adt(_, substs) | ty::Opaque(_, substs) => {
             stack.extend(substs.types().rev());
         }
-        ty::Closure(_, ref substs) => {
+        ty::Closure(_, ref substs)
+        | ty::Generator(_, ref substs, _) => {
             stack.extend(substs.types().rev());
         }
-        ty::Generator(_, ref substs, _) => {
-            stack.extend(substs.substs.types().rev());
-        }
         ty::GeneratorWitness(ts) => {
             stack.extend(ts.skip_binder().iter().cloned().rev());
         }