Rollup merge of #63750 - eddyb:lazy-seq, r=Zoxc

rustc_metadata: replace LazySeq<T> with Lazy<[T]>.

Part of #59953, split out for separate landing (see https://github.com/rust-lang/rust/pull/59953#issuecomment-502460092).

r? @Zoxc

5 files changed, 301 insertions, 296 deletions

diff --git a/src/librustc_metadata/cstore.rs b/src/librustc_metadata/cstore.rs
index 792922a1837..efc77699313 100644
--- a/src/librustc_metadata/cstore.rs
+++ b/src/librustc_metadata/cstore.rs
@@ -68,9 +68,9 @@ pub struct CrateMetadata {
     pub alloc_decoding_state: AllocDecodingState,
 
     // NOTE(eddyb) we pass `'static` to a `'tcx` parameter because this
-    // lifetime is only used behind `Lazy` / `LazySeq`, and therefore
-    // acts like an universal (`for<'tcx>`), that is paired up with
-    // whichever `TyCtxt` is being used to decode those values.
+    // lifetime is only used behind `Lazy`, and therefore acts like an
+    // universal (`for<'tcx>`), that is paired up with whichever `TyCtxt`
+    // is being used to decode those values.
     pub root: schema::CrateRoot<'static>,
 
     /// For each definition in this crate, we encode a key. When the
@@ -80,7 +80,7 @@ pub struct CrateMetadata {
     /// compilation support.
     pub def_path_table: Lrc<DefPathTable>,
 
-    pub trait_impls: FxHashMap<(u32, DefIndex), schema::LazySeq<DefIndex>>,
+    pub trait_impls: FxHashMap<(u32, DefIndex), schema::Lazy<[DefIndex]>>,
 
     pub dep_kind: Lock<DepKind>,
     pub source: CrateSource,
diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index d29592a5d68..da96728d2de 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -134,14 +134,14 @@ impl<'a, 'tcx, T: Decodable> Lazy<T> {
     }
 }
 
-impl<'a: 'x, 'tcx: 'x, 'x, T: Decodable> LazySeq<T> {
+impl<'a: 'x, 'tcx: 'x, 'x, T: Decodable> Lazy<[T]> {
     pub fn decode<M: Metadata<'a, 'tcx>>(
         self,
         meta: M,
     ) -> impl ExactSizeIterator<Item = T> + Captures<'a> + Captures<'tcx> + 'x {
         let mut dcx = meta.decoder(self.position);
         dcx.lazy_state = LazyState::NodeStart(self.position);
-        (0..self.len).map(move |_| T::decode(&mut dcx).unwrap())
+        (0..self.meta).map(move |_| T::decode(&mut dcx).unwrap())
     }
 }
 
@@ -154,10 +154,14 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
         self.cdata.expect("missing CrateMetadata in DecodeContext")
     }
 
-    fn read_lazy_distance(&mut self, min_size: usize) -> Result<usize, <Self as Decoder>::Error> {
+    fn read_lazy_with_meta<T: ?Sized + LazyMeta>(
+        &mut self,
+        meta: T::Meta,
+    ) -> Result<Lazy<T>, <Self as Decoder>::Error> {
+        let min_size = T::min_size(meta);
         let distance = self.read_usize()?;
         let position = match self.lazy_state {
-            LazyState::NoNode => bug!("read_lazy_distance: outside of a metadata node"),
+            LazyState::NoNode => bug!("read_lazy_with_meta: outside of a metadata node"),
             LazyState::NodeStart(start) => {
                 assert!(distance + min_size <= start);
                 start - distance - min_size
@@ -165,7 +169,7 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
             LazyState::Previous(last_min_end) => last_min_end + distance,
         };
         self.lazy_state = LazyState::Previous(position + min_size);
-        Ok(position)
+        Ok(Lazy::from_position_and_meta(position, meta))
     }
 }
 
@@ -230,19 +234,18 @@ impl<'a, 'tcx> TyDecoder<'tcx> for DecodeContext<'a, 'tcx> {
 
 impl<'a, 'tcx, T> SpecializedDecoder<Lazy<T>> for DecodeContext<'a, 'tcx> {
     fn specialized_decode(&mut self) -> Result<Lazy<T>, Self::Error> {
-        Ok(Lazy::with_position(self.read_lazy_distance(Lazy::<T>::min_size())?))
+        self.read_lazy_with_meta(())
     }
 }
 
-impl<'a, 'tcx, T> SpecializedDecoder<LazySeq<T>> for DecodeContext<'a, 'tcx> {
-    fn specialized_decode(&mut self) -> Result<LazySeq<T>, Self::Error> {
+impl<'a, 'tcx, T> SpecializedDecoder<Lazy<[T]>> for DecodeContext<'a, 'tcx> {
+    fn specialized_decode(&mut self) -> Result<Lazy<[T]>, Self::Error> {
         let len = self.read_usize()?;
-        let position = if len == 0 {
-            0
+        if len == 0 {
+            Ok(Lazy::empty())
         } else {
-            self.read_lazy_distance(LazySeq::<T>::min_size(len))?
-        };
-        Ok(LazySeq::with_position_and_length(position, len))
+            self.read_lazy_with_meta(len)
+        }
     }
 }
 
@@ -378,7 +381,7 @@ impl<'tcx> MetadataBlob {
     }
 
     pub fn get_rustc_version(&self) -> String {
-        Lazy::with_position(METADATA_HEADER.len() + 4).decode(self)
+        Lazy::<String>::from_position(METADATA_HEADER.len() + 4).decode(self)
     }
 
     pub fn get_root(&self) -> CrateRoot<'tcx> {
@@ -387,7 +390,7 @@ impl<'tcx> MetadataBlob {
         let pos = (((slice[offset + 0] as u32) << 24) | ((slice[offset + 1] as u32) << 16) |
                    ((slice[offset + 2] as u32) << 8) |
                    ((slice[offset + 3] as u32) << 0)) as usize;
-        Lazy::with_position(pos).decode(self)
+        Lazy::<CrateRoot<'tcx>>::from_position(pos).decode(self)
     }
 
     pub fn list_crate_metadata(&self,
@@ -1140,7 +1143,7 @@ impl<'a, 'tcx> CrateMetadata {
             EntryKind::Fn(data) |
             EntryKind::ForeignFn(data) => data.decode(self).arg_names,
             EntryKind::Method(data) => data.decode(self).fn_data.arg_names,
-            _ => LazySeq::empty(),
+            _ => Lazy::empty(),
         };
         arg_names.decode(self).collect()
     }
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 6058ae99cf4..df3320c64a9 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -98,17 +98,17 @@ impl<'tcx> Encoder for EncodeContext<'tcx> {
 
 impl<'tcx, T> SpecializedEncoder<Lazy<T>> for EncodeContext<'tcx> {
     fn specialized_encode(&mut self, lazy: &Lazy<T>) -> Result<(), Self::Error> {
-        self.emit_lazy_distance(lazy.position, Lazy::<T>::min_size())
+        self.emit_lazy_distance(*lazy)
     }
 }
 
-impl<'tcx, T> SpecializedEncoder<LazySeq<T>> for EncodeContext<'tcx> {
-    fn specialized_encode(&mut self, seq: &LazySeq<T>) -> Result<(), Self::Error> {
-        self.emit_usize(seq.len)?;
-        if seq.len == 0 {
+impl<'tcx, T> SpecializedEncoder<Lazy<[T]>> for EncodeContext<'tcx> {
+    fn specialized_encode(&mut self, lazy: &Lazy<[T]>) -> Result<(), Self::Error> {
+        self.emit_usize(lazy.meta)?;
+        if lazy.meta == 0 {
             return Ok(());
         }
-        self.emit_lazy_distance(seq.position, LazySeq::<T>::min_size(seq.len))
+        self.emit_lazy_distance(*lazy)
     }
 }
 
@@ -239,21 +239,38 @@ impl<'tcx> TyEncoder for EncodeContext<'tcx> {
     }
 }
 
-impl<'tcx> EncodeContext<'tcx> {
-    fn emit_node<F: FnOnce(&mut Self, usize) -> R, R>(&mut self, f: F) -> R {
-        assert_eq!(self.lazy_state, LazyState::NoNode);
-        let pos = self.position();
-        self.lazy_state = LazyState::NodeStart(pos);
-        let r = f(self, pos);
-        self.lazy_state = LazyState::NoNode;
-        r
+/// Helper trait to allow overloading `EncodeContext::lazy` for iterators.
+trait EncodeContentsForLazy<T: ?Sized + LazyMeta> {
+    fn encode_contents_for_lazy(self, ecx: &mut EncodeContext<'tcx>) -> T::Meta;
+}
+
+impl<T: Encodable> EncodeContentsForLazy<T> for &T {
+    fn encode_contents_for_lazy(self, ecx: &mut EncodeContext<'tcx>) {
+        self.encode(ecx).unwrap()
+    }
+}
+
+impl<T: Encodable> EncodeContentsForLazy<T> for T {
+    fn encode_contents_for_lazy(self, ecx: &mut EncodeContext<'tcx>) {
+        self.encode(ecx).unwrap()
+    }
+}
+
+impl<I, T> EncodeContentsForLazy<[T]> for I
+    where I: IntoIterator,
+          I::Item: EncodeContentsForLazy<T>,
+{
+    fn encode_contents_for_lazy(self, ecx: &mut EncodeContext<'tcx>) -> usize {
+        self.into_iter().map(|value| value.encode_contents_for_lazy(ecx)).count()
     }
+}
 
-    fn emit_lazy_distance(&mut self,
-                          position: usize,
-                          min_size: usize)
-                          -> Result<(), <Self as Encoder>::Error> {
-        let min_end = position + min_size;
+impl<'tcx> EncodeContext<'tcx> {
+    fn emit_lazy_distance<T: ?Sized + LazyMeta>(
+        &mut self,
+        lazy: Lazy<T>,
+    ) -> Result<(), <Self as Encoder>::Error> {
+        let min_end = lazy.position + T::min_size(lazy.meta);
         let distance = match self.lazy_state {
             LazyState::NoNode => bug!("emit_lazy_distance: outside of a metadata node"),
             LazyState::NodeStart(start) => {
@@ -262,48 +279,31 @@ impl<'tcx> EncodeContext<'tcx> {
             }
             LazyState::Previous(last_min_end) => {
                 assert!(
-                    last_min_end <= position,
+                    last_min_end <= lazy.position,
                     "make sure that the calls to `lazy*` \
                     are in the same order as the metadata fields",
                 );
-                position - last_min_end
+                lazy.position - last_min_end
             }
         };
         self.lazy_state = LazyState::Previous(min_end);
         self.emit_usize(distance)
     }
 
-    pub fn lazy<T: Encodable>(&mut self, value: &T) -> Lazy<T> {
-        self.emit_node(|ecx, pos| {
-            value.encode(ecx).unwrap();
-
-            assert!(pos + Lazy::<T>::min_size() <= ecx.position());
-            Lazy::with_position(pos)
-        })
-    }
-
-    pub fn lazy_seq<I, T>(&mut self, iter: I) -> LazySeq<T>
-        where I: IntoIterator<Item = T>,
-              T: Encodable
-    {
-        self.emit_node(|ecx, pos| {
-            let len = iter.into_iter().map(|value| value.encode(ecx).unwrap()).count();
+    fn lazy<T: ?Sized + LazyMeta>(
+        &mut self,
+        value: impl EncodeContentsForLazy<T>,
+    ) -> Lazy<T> {
+        let pos = self.position();
 
-            assert!(pos + LazySeq::<T>::min_size(len) <= ecx.position());
-            LazySeq::with_position_and_length(pos, len)
-        })
-    }
+        assert_eq!(self.lazy_state, LazyState::NoNode);
+        self.lazy_state = LazyState::NodeStart(pos);
+        let meta = value.encode_contents_for_lazy(self);
+        self.lazy_state = LazyState::NoNode;
 
-    pub fn lazy_seq_ref<'b, I, T>(&mut self, iter: I) -> LazySeq<T>
-        where I: IntoIterator<Item = &'b T>,
-              T: 'b + Encodable
-    {
-        self.emit_node(|ecx, pos| {
-            let len = iter.into_iter().map(|value| value.encode(ecx).unwrap()).count();
+        assert!(pos + <T>::min_size(meta) <= self.position());
 
-            assert!(pos + LazySeq::<T>::min_size(len) <= ecx.position());
-            LazySeq::with_position_and_length(pos, len)
-        })
+        Lazy::from_position_and_meta(pos, meta)
     }
 
     /// Emit the data for a `DefId` to the metadata. The function to
@@ -320,7 +320,7 @@ impl<'tcx> EncodeContext<'tcx> {
         assert!(id.is_local());
 
         let entry = op(self, data);
-        let entry = self.lazy(&entry);
+        let entry = self.lazy(entry);
         self.entries_index.record(id, entry);
     }
 
@@ -341,7 +341,7 @@ impl<'tcx> EncodeContext<'tcx> {
         self.lazy(definitions.def_path_table())
     }
 
-    fn encode_source_map(&mut self) -> LazySeq<syntax_pos::SourceFile> {
+    fn encode_source_map(&mut self) -> Lazy<[syntax_pos::SourceFile]> {
         let source_map = self.tcx.sess.source_map();
         let all_source_files = source_map.files();
 
@@ -380,7 +380,7 @@ impl<'tcx> EncodeContext<'tcx> {
             })
             .collect::<Vec<_>>();
 
-        self.lazy_seq_ref(adapted.iter().map(|rc| &**rc))
+        self.lazy(adapted.iter().map(|rc| &**rc))
     }
 
     fn encode_crate_root(&mut self) -> Lazy<CrateRoot<'tcx>> {
@@ -463,7 +463,7 @@ impl<'tcx> EncodeContext<'tcx> {
                 }
                 n = new_n;
             }
-            self.lazy_seq(interpret_alloc_index)
+            self.lazy(interpret_alloc_index)
         };
 
 
@@ -482,8 +482,7 @@ impl<'tcx> EncodeContext<'tcx> {
         let has_global_allocator = *tcx.sess.has_global_allocator.get();
         let has_panic_handler = *tcx.sess.has_panic_handler.try_get().unwrap_or(&false);
 
-
-        let root = self.lazy(&CrateRoot {
+        let root = self.lazy(CrateRoot {
             name: tcx.crate_name(LOCAL_CRATE),
             extra_filename: tcx.sess.opts.cg.extra_filename.clone(),
             triple: tcx.sess.opts.target_triple.clone(),
@@ -562,17 +561,17 @@ impl<'tcx> EncodeContext<'tcx> {
 }
 
 impl EncodeContext<'tcx> {
-    fn encode_variances_of(&mut self, def_id: DefId) -> LazySeq<ty::Variance> {
+    fn encode_variances_of(&mut self, def_id: DefId) -> Lazy<[ty::Variance]> {
         debug!("EncodeContext::encode_variances_of({:?})", def_id);
         let tcx = self.tcx;
-        self.lazy_seq_ref(&tcx.variances_of(def_id)[..])
+        self.lazy(&tcx.variances_of(def_id)[..])
     }
 
     fn encode_item_type(&mut self, def_id: DefId) -> Lazy<Ty<'tcx>> {
         let tcx = self.tcx;
         let ty = tcx.type_of(def_id);
         debug!("EncodeContext::encode_item_type({:?}) => {:?}", def_id, ty);
-        self.lazy(&ty)
+        self.lazy(ty)
     }
 
     fn encode_enum_variant_info(
@@ -601,11 +600,11 @@ impl EncodeContext<'tcx> {
         let enum_vis = &tcx.hir().expect_item(enum_id).vis;
 
         Entry {
-            kind: EntryKind::Variant(self.lazy(&data)),
-            visibility: self.lazy(&ty::Visibility::from_hir(enum_vis, enum_id, tcx)),
-            span: self.lazy(&tcx.def_span(def_id)),
+            kind: EntryKind::Variant(self.lazy(data)),
+            visibility: self.lazy(ty::Visibility::from_hir(enum_vis, enum_id, tcx)),
+            span: self.lazy(tcx.def_span(def_id)),
             attributes: self.encode_attributes(&tcx.get_attrs(def_id)),
-            children: self.lazy_seq(variant.fields.iter().map(|f| {
+            children: self.lazy(variant.fields.iter().map(|f| {
                 assert!(f.did.is_local());
                 f.did.index
             })),
@@ -613,11 +612,11 @@ impl EncodeContext<'tcx> {
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: if variant.ctor_kind == CtorKind::Fn {
                 self.encode_variances_of(def_id)
             } else {
-                LazySeq::empty()
+                Lazy::empty()
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
@@ -642,7 +641,7 @@ impl EncodeContext<'tcx> {
             discr: variant.discr,
             ctor: Some(def_id.index),
             ctor_sig: if variant.ctor_kind == CtorKind::Fn {
-                Some(self.lazy(&tcx.fn_sig(def_id)))
+                Some(self.lazy(tcx.fn_sig(def_id)))
             } else {
                 None
             }
@@ -658,20 +657,20 @@ impl EncodeContext<'tcx> {
         }
 
         Entry {
-            kind: EntryKind::Variant(self.lazy(&data)),
-            visibility: self.lazy(&ctor_vis),
-            span: self.lazy(&tcx.def_span(def_id)),
-            attributes: LazySeq::empty(),
-            children: LazySeq::empty(),
+            kind: EntryKind::Variant(self.lazy(data)),
+            visibility: self.lazy(ctor_vis),
+            span: self.lazy(tcx.def_span(def_id)),
+            attributes: Lazy::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: if variant.ctor_kind == CtorKind::Fn {
                 self.encode_variances_of(def_id)
             } else {
-                LazySeq::empty()
+                Lazy::empty()
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
@@ -691,25 +690,25 @@ impl EncodeContext<'tcx> {
 
         let data = ModData {
             reexports: match tcx.module_exports(def_id) {
-                Some(exports) => self.lazy_seq_ref(exports),
-                _ => LazySeq::empty(),
+                Some(exports) => self.lazy(exports),
+                _ => Lazy::empty(),
             },
         };
 
         Entry {
-            kind: EntryKind::Mod(self.lazy(&data)),
-            visibility: self.lazy(&ty::Visibility::from_hir(vis, id, tcx)),
-            span: self.lazy(&tcx.def_span(def_id)),
+            kind: EntryKind::Mod(self.lazy(data)),
+            visibility: self.lazy(ty::Visibility::from_hir(vis, id, tcx)),
+            span: self.lazy(tcx.def_span(def_id)),
             attributes: self.encode_attributes(attrs),
-            children: self.lazy_seq(md.item_ids.iter().map(|item_id| {
+            children: self.lazy(md.item_ids.iter().map(|item_id| {
                 tcx.hir().local_def_id(item_id.id).index
             })),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: None,
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: None,
             predicates: None,
             predicates_defined_on: None,
@@ -734,16 +733,16 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind: EntryKind::Field,
-            visibility: self.lazy(&field.vis),
-            span: self.lazy(&tcx.def_span(def_id)),
+            visibility: self.lazy(field.vis),
+            span: self.lazy(tcx.def_span(def_id)),
             attributes: self.encode_attributes(&variant_data.fields()[field_index].attrs),
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
             predicates_defined_on: None,
@@ -763,7 +762,7 @@ impl EncodeContext<'tcx> {
             discr: variant.discr,
             ctor: Some(def_id.index),
             ctor_sig: if variant.ctor_kind == CtorKind::Fn {
-                Some(self.lazy(&tcx.fn_sig(def_id)))
+                Some(self.lazy(tcx.fn_sig(def_id)))
             } else {
                 None
             }
@@ -789,20 +788,20 @@ impl EncodeContext<'tcx> {
         let repr_options = get_repr_options(tcx, adt_def_id);
 
         Entry {
-            kind: EntryKind::Struct(self.lazy(&data), repr_options),
-            visibility: self.lazy(&ctor_vis),
-            span: self.lazy(&tcx.def_span(def_id)),
-            attributes: LazySeq::empty(),
-            children: LazySeq::empty(),
+            kind: EntryKind::Struct(self.lazy(data), repr_options),
+            visibility: self.lazy(ctor_vis),
+            span: self.lazy(tcx.def_span(def_id)),
+            attributes: Lazy::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: if variant.ctor_kind == CtorKind::Fn {
                 self.encode_variances_of(def_id)
             } else {
-                LazySeq::empty()
+                Lazy::empty()
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
@@ -821,13 +820,13 @@ impl EncodeContext<'tcx> {
     fn encode_predicates(&mut self, def_id: DefId) -> Lazy<ty::GenericPredicates<'tcx>> {
         debug!("EncodeContext::encode_predicates({:?})", def_id);
         let tcx = self.tcx;
-        self.lazy(&tcx.predicates_of(def_id))
+        self.lazy(&*tcx.predicates_of(def_id))
     }
 
     fn encode_predicates_defined_on(&mut self, def_id: DefId) -> Lazy<ty::GenericPredicates<'tcx>> {
         debug!("EncodeContext::encode_predicates_defined_on({:?})", def_id);
         let tcx = self.tcx;
-        self.lazy(&tcx.predicates_defined_on(def_id))
+        self.lazy(&*tcx.predicates_defined_on(def_id))
     }
 
     fn encode_info_for_trait_item(&mut self, def_id: DefId) -> Entry<'tcx> {
@@ -858,7 +857,7 @@ impl EncodeContext<'tcx> {
 
                 let rendered =
                     hir::print::to_string(self.tcx.hir(), |s| s.print_trait_item(ast_item));
-                let rendered_const = self.lazy(&RenderedConst(rendered));
+                let rendered_const = self.lazy(RenderedConst(rendered));
 
                 EntryKind::AssocConst(container, const_qualif, rendered_const)
             }
@@ -875,12 +874,12 @@ impl EncodeContext<'tcx> {
                     FnData {
                         constness: hir::Constness::NotConst,
                         arg_names,
-                        sig: self.lazy(&tcx.fn_sig(def_id)),
+                        sig: self.lazy(tcx.fn_sig(def_id)),
                     }
                 } else {
                     bug!()
                 };
-                EntryKind::Method(self.lazy(&MethodData {
+                EntryKind::Method(self.lazy(MethodData {
                     fn_data,
                     container,
                     has_self: trait_item.method_has_self_argument,
@@ -892,10 +891,10 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind,
-            visibility: self.lazy(&trait_item.vis),
-            span: self.lazy(&ast_item.span),
+            visibility: self.lazy(trait_item.vis),
+            span: self.lazy(ast_item.span),
             attributes: self.encode_attributes(&ast_item.attrs),
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
@@ -913,11 +912,11 @@ impl EncodeContext<'tcx> {
                 }
                 ty::AssocKind::OpaqueTy => unreachable!(),
             },
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: if trait_item.kind == ty::AssocKind::Method {
                 self.encode_variances_of(def_id)
             } else {
-                LazySeq::empty()
+                Lazy::empty()
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
@@ -971,12 +970,12 @@ impl EncodeContext<'tcx> {
                     FnData {
                         constness: sig.header.constness,
                         arg_names: self.encode_fn_arg_names_for_body(body),
-                        sig: self.lazy(&tcx.fn_sig(def_id)),
+                        sig: self.lazy(tcx.fn_sig(def_id)),
                     }
                 } else {
                     bug!()
                 };
-                EntryKind::Method(self.lazy(&MethodData {
+                EntryKind::Method(self.lazy(MethodData {
                     fn_data,
                     container,
                     has_self: impl_item.method_has_self_argument,
@@ -1004,19 +1003,19 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind,
-            visibility: self.lazy(&impl_item.vis),
-            span: self.lazy(&ast_item.span),
+            visibility: self.lazy(impl_item.vis),
+            span: self.lazy(ast_item.span),
             attributes: self.encode_attributes(&ast_item.attrs),
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: if impl_item.kind == ty::AssocKind::Method {
                 self.encode_variances_of(def_id)
             } else {
-                LazySeq::empty()
+                Lazy::empty()
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
@@ -1027,10 +1026,10 @@ impl EncodeContext<'tcx> {
     }
 
     fn encode_fn_arg_names_for_body(&mut self, body_id: hir::BodyId)
-                                    -> LazySeq<ast::Name> {
+                                    -> Lazy<[ast::Name]> {
         self.tcx.dep_graph.with_ignore(|| {
             let body = self.tcx.hir().body(body_id);
-            self.lazy_seq(body.arguments.iter().map(|arg| {
+            self.lazy(body.arguments.iter().map(|arg| {
                 match arg.pat.node {
                     PatKind::Binding(_, _, ident, _) => ident.name,
                     _ => kw::Invalid,
@@ -1039,28 +1038,28 @@ impl EncodeContext<'tcx> {
         })
     }
 
-    fn encode_fn_arg_names(&mut self, param_names: &[ast::Ident]) -> LazySeq<ast::Name> {
-        self.lazy_seq(param_names.iter().map(|ident| ident.name))
+    fn encode_fn_arg_names(&mut self, param_names: &[ast::Ident]) -> Lazy<[ast::Name]> {
+        self.lazy(param_names.iter().map(|ident| ident.name))
     }
 
     fn encode_optimized_mir(&mut self, def_id: DefId) -> Option<Lazy<mir::Body<'tcx>>> {
         debug!("EntryBuilder::encode_mir({:?})", def_id);
         if self.tcx.mir_keys(LOCAL_CRATE).contains(&def_id) {
             let mir = self.tcx.optimized_mir(def_id);
-            Some(self.lazy(&mir))
+            Some(self.lazy(mir))
         } else {
             None
         }
     }
 
     // Encodes the inherent implementations of a structure, enumeration, or trait.
-    fn encode_inherent_implementations(&mut self, def_id: DefId) -> LazySeq<DefIndex> {
+    fn encode_inherent_implementations(&mut self, def_id: DefId) -> Lazy<[DefIndex]> {
         debug!("EncodeContext::encode_inherent_implementations({:?})", def_id);
         let implementations = self.tcx.inherent_impls(def_id);
         if implementations.is_empty() {
-            LazySeq::empty()
+            Lazy::empty()
         } else {
-            self.lazy_seq(implementations.iter().map(|&def_id| {
+            self.lazy(implementations.iter().map(|&def_id| {
                 assert!(def_id.is_local());
                 def_id.index
             }))
@@ -1074,7 +1073,7 @@ impl EncodeContext<'tcx> {
 
     fn encode_deprecation(&mut self, def_id: DefId) -> Option<Lazy<attr::Deprecation>> {
         debug!("EncodeContext::encode_deprecation({:?})", def_id);
-        self.tcx.lookup_deprecation(def_id).map(|depr| self.lazy(&depr))
+        self.tcx.lookup_deprecation(def_id).map(|depr| self.lazy(depr))
     }
 
     fn encode_rendered_const_for_body(&mut self, body_id: hir::BodyId) -> Lazy<RenderedConst> {
@@ -1103,10 +1102,10 @@ impl EncodeContext<'tcx> {
                 let data = FnData {
                     constness: header.constness,
                     arg_names: self.encode_fn_arg_names_for_body(body),
-                    sig: self.lazy(&tcx.fn_sig(def_id)),
+                    sig: self.lazy(tcx.fn_sig(def_id)),
                 };
 
-                EntryKind::Fn(self.lazy(&data))
+                EntryKind::Fn(self.lazy(data))
             }
             hir::ItemKind::Mod(ref m) => {
                 return self.encode_info_for_mod((item.hir_id, m, &item.attrs, &item.vis));
@@ -1127,7 +1126,7 @@ impl EncodeContext<'tcx> {
 
                 let repr_options = get_repr_options(tcx, def_id);
 
-                EntryKind::Struct(self.lazy(&VariantData {
+                EntryKind::Struct(self.lazy(VariantData {
                     ctor_kind: variant.ctor_kind,
                     discr: variant.discr,
                     ctor,
@@ -1138,7 +1137,7 @@ impl EncodeContext<'tcx> {
                 let variant = tcx.adt_def(def_id).non_enum_variant();
                 let repr_options = get_repr_options(tcx, def_id);
 
-                EntryKind::Union(self.lazy(&VariantData {
+                EntryKind::Union(self.lazy(VariantData {
                     ctor_kind: variant.ctor_kind,
                     discr: variant.discr,
                     ctor: None,
@@ -1175,10 +1174,10 @@ impl EncodeContext<'tcx> {
                     defaultness,
                     parent_impl: parent,
                     coerce_unsized_info,
-                    trait_ref: trait_ref.map(|trait_ref| self.lazy(&trait_ref)),
+                    trait_ref: trait_ref.map(|trait_ref| self.lazy(trait_ref)),
                 };
 
-                EntryKind::Impl(self.lazy(&data))
+                EntryKind::Impl(self.lazy(data))
             }
             hir::ItemKind::Trait(..) => {
                 let trait_def = tcx.trait_def(def_id);
@@ -1187,17 +1186,17 @@ impl EncodeContext<'tcx> {
                     paren_sugar: trait_def.paren_sugar,
                     has_auto_impl: tcx.trait_is_auto(def_id),
                     is_marker: trait_def.is_marker,
-                    super_predicates: self.lazy(&tcx.super_predicates_of(def_id)),
+                    super_predicates: self.lazy(&*tcx.super_predicates_of(def_id)),
                 };
 
-                EntryKind::Trait(self.lazy(&data))
+                EntryKind::Trait(self.lazy(data))
             }
             hir::ItemKind::TraitAlias(..) => {
                 let data = TraitAliasData {
-                    super_predicates: self.lazy(&tcx.super_predicates_of(def_id)),
+                    super_predicates: self.lazy(&*tcx.super_predicates_of(def_id)),
                 };
 
-                EntryKind::TraitAlias(self.lazy(&data))
+                EntryKind::TraitAlias(self.lazy(data))
             }
             hir::ItemKind::ExternCrate(_) |
             hir::ItemKind::Use(..) => bug!("cannot encode info for item {:?}", item),
@@ -1205,19 +1204,19 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind,
-            visibility: self.lazy(&ty::Visibility::from_hir(&item.vis, item.hir_id, tcx)),
-            span: self.lazy(&item.span),
+            visibility: self.lazy(ty::Visibility::from_hir(&item.vis, item.hir_id, tcx)),
+            span: self.lazy(item.span),
             attributes: self.encode_attributes(&item.attrs),
             children: match item.node {
                 hir::ItemKind::ForeignMod(ref fm) => {
-                    self.lazy_seq(fm.items
+                    self.lazy(fm.items
                         .iter()
                         .map(|foreign_item| tcx.hir().local_def_id(
                             foreign_item.hir_id).index))
                 }
                 hir::ItemKind::Enum(..) => {
                     let def = self.tcx.adt_def(def_id);
-                    self.lazy_seq(def.variants.iter().map(|v| {
+                    self.lazy(def.variants.iter().map(|v| {
                         assert!(v.def_id.is_local());
                         v.def_id.index
                     }))
@@ -1225,19 +1224,19 @@ impl EncodeContext<'tcx> {
                 hir::ItemKind::Struct(..) |
                 hir::ItemKind::Union(..) => {
                     let def = self.tcx.adt_def(def_id);
-                    self.lazy_seq(def.non_enum_variant().fields.iter().map(|f| {
+                    self.lazy(def.non_enum_variant().fields.iter().map(|f| {
                         assert!(f.did.is_local());
                         f.did.index
                     }))
                 }
                 hir::ItemKind::Impl(..) |
                 hir::ItemKind::Trait(..) => {
-                    self.lazy_seq(tcx.associated_item_def_ids(def_id).iter().map(|&def_id| {
+                    self.lazy(tcx.associated_item_def_ids(def_id).iter().map(|&def_id| {
                         assert!(def_id.is_local());
                         def_id.index
                     }))
                 }
-                _ => LazySeq::empty(),
+                _ => Lazy::empty(),
             },
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
@@ -1260,7 +1259,7 @@ impl EncodeContext<'tcx> {
                 hir::ItemKind::Struct(..) |
                 hir::ItemKind::Union(..) |
                 hir::ItemKind::Fn(..) => self.encode_variances_of(def_id),
-                _ => LazySeq::empty(),
+                _ => Lazy::empty(),
             },
             generics: match item.node {
                 hir::ItemKind::Static(..) |
@@ -1333,20 +1332,20 @@ impl EncodeContext<'tcx> {
         use syntax::print::pprust;
         let def_id = self.tcx.hir().local_def_id(macro_def.hir_id);
         Entry {
-            kind: EntryKind::MacroDef(self.lazy(&MacroDef {
+            kind: EntryKind::MacroDef(self.lazy(MacroDef {
                 body: pprust::tokens_to_string(macro_def.body.clone()),
                 legacy: macro_def.legacy,
             })),
-            visibility: self.lazy(&ty::Visibility::Public),
-            span: self.lazy(&macro_def.span),
+            visibility: self.lazy(ty::Visibility::Public),
+            span: self.lazy(macro_def.span),
             attributes: self.encode_attributes(&macro_def.attrs),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             ty: None,
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: None,
             predicates: None,
             predicates_defined_on: None,
@@ -1363,15 +1362,15 @@ impl EncodeContext<'tcx> {
         let tcx = self.tcx;
         Entry {
             kind: entry_kind,
-            visibility: self.lazy(&ty::Visibility::Public),
-            span: self.lazy(&tcx.def_span(def_id)),
-            attributes: LazySeq::empty(),
-            children: LazySeq::empty(),
+            visibility: self.lazy(ty::Visibility::Public),
+            span: self.lazy(tcx.def_span(def_id)),
+            attributes: Lazy::empty(),
+            children: Lazy::empty(),
             stability: None,
             deprecation: None,
             ty: if encode_type { Some(self.encode_item_type(def_id)) } else { None },
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: None,
             predicates: None,
             predicates_defined_on: None,
@@ -1408,13 +1407,13 @@ impl EncodeContext<'tcx> {
                 let data = GeneratorData {
                     layout: layout.clone(),
                 };
-                EntryKind::Generator(self.lazy(&data))
+                EntryKind::Generator(self.lazy(data))
             }
 
             ty::Closure(def_id, substs) => {
                 let sig = substs.closure_sig(def_id, self.tcx);
-                let data = ClosureData { sig: self.lazy(&sig) };
-                EntryKind::Closure(self.lazy(&data))
+                let data = ClosureData { sig: self.lazy(sig) };
+                EntryKind::Closure(self.lazy(data))
             }
 
             _ => bug!("closure that is neither generator nor closure")
@@ -1422,16 +1421,16 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind,
-            visibility: self.lazy(&ty::Visibility::Public),
-            span: self.lazy(&tcx.def_span(def_id)),
+            visibility: self.lazy(ty::Visibility::Public),
+            span: self.lazy(tcx.def_span(def_id)),
             attributes: self.encode_attributes(&tcx.get_attrs(def_id)),
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             stability: None,
             deprecation: None,
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: None,
             predicates_defined_on: None,
@@ -1450,16 +1449,16 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind: EntryKind::Const(self.const_qualif(mir, body_id), const_data),
-            visibility: self.lazy(&ty::Visibility::Public),
-            span: self.lazy(&tcx.def_span(def_id)),
-            attributes: LazySeq::empty(),
-            children: LazySeq::empty(),
+            visibility: self.lazy(ty::Visibility::Public),
+            span: self.lazy(tcx.def_span(def_id)),
+            attributes: Lazy::empty(),
+            children: Lazy::empty(),
             stability: None,
             deprecation: None,
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
-            variances: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
+            variances: Lazy::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
             predicates_defined_on: None,
@@ -1468,37 +1467,37 @@ impl EncodeContext<'tcx> {
         }
     }
 
-    fn encode_attributes(&mut self, attrs: &[ast::Attribute]) -> LazySeq<ast::Attribute> {
-        self.lazy_seq_ref(attrs)
+    fn encode_attributes(&mut self, attrs: &[ast::Attribute]) -> Lazy<[ast::Attribute]> {
+        self.lazy(attrs)
     }
 
-    fn encode_native_libraries(&mut self) -> LazySeq<NativeLibrary> {
+    fn encode_native_libraries(&mut self) -> Lazy<[NativeLibrary]> {
         let used_libraries = self.tcx.native_libraries(LOCAL_CRATE);
-        self.lazy_seq(used_libraries.iter().cloned())
+        self.lazy(used_libraries.iter().cloned())
     }
 
-    fn encode_foreign_modules(&mut self) -> LazySeq<ForeignModule> {
+    fn encode_foreign_modules(&mut self) -> Lazy<[ForeignModule]> {
         let foreign_modules = self.tcx.foreign_modules(LOCAL_CRATE);
-        self.lazy_seq(foreign_modules.iter().cloned())
+        self.lazy(foreign_modules.iter().cloned())
     }
 
-    fn encode_proc_macros(&mut self) -> Option<LazySeq<DefIndex>> {
+    fn encode_proc_macros(&mut self) -> Option<Lazy<[DefIndex]>> {
         let is_proc_macro = self.tcx.sess.crate_types.borrow().contains(&CrateType::ProcMacro);
         if is_proc_macro {
-            let proc_macros: Vec<_> = self.tcx.hir().krate().items.values().filter_map(|item| {
+            let tcx = self.tcx;
+            Some(self.lazy(tcx.hir().krate().items.values().filter_map(|item| {
                 if item.attrs.iter().any(|attr| is_proc_macro_attr(attr)) {
                     Some(item.hir_id.owner)
                 } else {
                     None
                 }
-            }).collect();
-            Some(self.lazy_seq(proc_macros))
+            })))
         } else {
             None
         }
     }
 
-    fn encode_crate_deps(&mut self) -> LazySeq<CrateDep> {
+    fn encode_crate_deps(&mut self) -> Lazy<[CrateDep]> {
         let crates = self.tcx.crates();
 
         let mut deps = crates
@@ -1529,20 +1528,20 @@ impl EncodeContext<'tcx> {
         // the assumption that they are numbered 1 to n.
         // FIXME (#2166): This is not nearly enough to support correct versioning
         // but is enough to get transitive crate dependencies working.
-        self.lazy_seq_ref(deps.iter().map(|&(_, ref dep)| dep))
+        self.lazy(deps.iter().map(|&(_, ref dep)| dep))
     }
 
-    fn encode_lib_features(&mut self) -> LazySeq<(ast::Name, Option<ast::Name>)> {
+    fn encode_lib_features(&mut self) -> Lazy<[(ast::Name, Option<ast::Name>)]> {
         let tcx = self.tcx;
         let lib_features = tcx.lib_features();
-        self.lazy_seq(lib_features.to_vec())
+        self.lazy(lib_features.to_vec())
     }
 
-    fn encode_lang_items(&mut self) -> LazySeq<(DefIndex, usize)> {
+    fn encode_lang_items(&mut self) -> Lazy<[(DefIndex, usize)]> {
         let tcx = self.tcx;
         let lang_items = tcx.lang_items();
         let lang_items = lang_items.items().iter();
-        self.lazy_seq(lang_items.enumerate().filter_map(|(i, &opt_def_id)| {
+        self.lazy(lang_items.enumerate().filter_map(|(i, &opt_def_id)| {
             if let Some(def_id) = opt_def_id {
                 if def_id.is_local() {
                     return Some((def_id.index, i));
@@ -1552,13 +1551,13 @@ impl EncodeContext<'tcx> {
         }))
     }
 
-    fn encode_lang_items_missing(&mut self) -> LazySeq<lang_items::LangItem> {
+    fn encode_lang_items_missing(&mut self) -> Lazy<[lang_items::LangItem]> {
         let tcx = self.tcx;
-        self.lazy_seq_ref(&tcx.lang_items().missing)
+        self.lazy(&tcx.lang_items().missing)
     }
 
     /// Encodes an index, mapping each trait to its (local) implementations.
-    fn encode_impls(&mut self) -> LazySeq<TraitImpls> {
+    fn encode_impls(&mut self) -> Lazy<[TraitImpls]> {
         debug!("EncodeContext::encode_impls()");
         let tcx = self.tcx;
         let mut visitor = ImplVisitor {
@@ -1584,12 +1583,12 @@ impl EncodeContext<'tcx> {
 
                 TraitImpls {
                     trait_id: (trait_def_id.krate.as_u32(), trait_def_id.index),
-                    impls: self.lazy_seq_ref(&impls),
+                    impls: self.lazy(&impls),
                 }
             })
             .collect();
 
-        self.lazy_seq_ref(&all_impls)
+        self.lazy(&all_impls)
     }
 
     // Encodes all symbols exported from this crate into the metadata.
@@ -1600,12 +1599,12 @@ impl EncodeContext<'tcx> {
     // definition (as that's not defined in this crate).
     fn encode_exported_symbols(&mut self,
                                exported_symbols: &[(ExportedSymbol<'tcx>, SymbolExportLevel)])
-                               -> LazySeq<(ExportedSymbol<'tcx>, SymbolExportLevel)> {
+                               -> Lazy<[(ExportedSymbol<'tcx>, SymbolExportLevel)]> {
         // The metadata symbol name is special. It should not show up in
         // downstream crates.
         let metadata_symbol_name = SymbolName::new(&metadata_symbol_name(self.tcx));
 
-        self.lazy_seq(exported_symbols
+        self.lazy(exported_symbols
             .iter()
             .filter(|&&(ref exported_symbol, _)| {
                 match *exported_symbol {
@@ -1618,10 +1617,10 @@ impl EncodeContext<'tcx> {
             .cloned())
     }
 
-    fn encode_dylib_dependency_formats(&mut self) -> LazySeq<Option<LinkagePreference>> {
+    fn encode_dylib_dependency_formats(&mut self) -> Lazy<[Option<LinkagePreference>]> {
         match self.tcx.sess.dependency_formats.borrow().get(&config::CrateType::Dylib) {
             Some(arr) => {
-                self.lazy_seq(arr.iter().map(|slot| {
+                self.lazy(arr.iter().map(|slot| {
                     match *slot {
                         Linkage::NotLinked |
                         Linkage::IncludedFromDylib => None,
@@ -1631,7 +1630,7 @@ impl EncodeContext<'tcx> {
                     }
                 }))
             }
-            None => LazySeq::empty(),
+            None => Lazy::empty(),
         }
     }
 
@@ -1647,9 +1646,9 @@ impl EncodeContext<'tcx> {
                 let data = FnData {
                     constness: hir::Constness::NotConst,
                     arg_names: self.encode_fn_arg_names(names),
-                    sig: self.lazy(&tcx.fn_sig(def_id)),
+                    sig: self.lazy(tcx.fn_sig(def_id)),
                 };
-                EntryKind::ForeignFn(self.lazy(&data))
+                EntryKind::ForeignFn(self.lazy(data))
             }
             hir::ForeignItemKind::Static(_, hir::MutMutable) => EntryKind::ForeignMutStatic,
             hir::ForeignItemKind::Static(_, hir::MutImmutable) => EntryKind::ForeignImmStatic,
@@ -1658,18 +1657,18 @@ impl EncodeContext<'tcx> {
 
         Entry {
             kind,
-            visibility: self.lazy(&ty::Visibility::from_hir(&nitem.vis, nitem.hir_id, tcx)),
-            span: self.lazy(&nitem.span),
+            visibility: self.lazy(ty::Visibility::from_hir(&nitem.vis, nitem.hir_id, tcx)),
+            span: self.lazy(nitem.span),
             attributes: self.encode_attributes(&nitem.attrs),
-            children: LazySeq::empty(),
+            children: Lazy::empty(),
             stability: self.encode_stability(def_id),
             deprecation: self.encode_deprecation(def_id),
 
             ty: Some(self.encode_item_type(def_id)),
-            inherent_impls: LazySeq::empty(),
+            inherent_impls: Lazy::empty(),
             variances: match nitem.node {
                 hir::ForeignItemKind::Fn(..) => self.encode_variances_of(def_id),
-                _ => LazySeq::empty(),
+                _ => Lazy::empty(),
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
diff --git a/src/librustc_metadata/index.rs b/src/librustc_metadata/index.rs
index dd2f59922ef..6f248f22cf2 100644
--- a/src/librustc_metadata/index.rs
+++ b/src/librustc_metadata/index.rs
@@ -108,18 +108,18 @@ impl Index<'tcx> {
         position.write_to_bytes_at(positions, array_index)
     }
 
-    pub fn write_index(&self, buf: &mut Encoder) -> LazySeq<Self> {
+    pub fn write_index(&self, buf: &mut Encoder) -> Lazy<[Self]> {
         let pos = buf.position();
 
         // First we write the length of the lower range ...
         buf.emit_raw_bytes(&(self.positions.len() as u32 / 4).to_le_bytes());
         // ... then the values.
         buf.emit_raw_bytes(&self.positions);
-        LazySeq::with_position_and_length(pos as usize, self.positions.len() / 4 + 1)
+        Lazy::from_position_and_meta(pos as usize, self.positions.len() / 4 + 1)
     }
 }
 
-impl LazySeq<Index<'tcx>> {
+impl Lazy<[Index<'tcx>]> {
     /// Given the metadata, extract out the offset of a particular
     /// DefIndex (if any).
     #[inline(never)]
@@ -127,7 +127,7 @@ impl LazySeq<Index<'tcx>> {
         let bytes = &bytes[self.position..];
         debug!("Index::lookup: index={:?} len={:?}",
                def_index,
-               self.len);
+               self.meta);
 
         let position = u32::read_from_bytes_at(bytes, 1 + def_index.index());
         if position == u32::MAX {
@@ -135,7 +135,7 @@ impl LazySeq<Index<'tcx>> {
             None
         } else {
             debug!("Index::lookup: position={:?}", position);
-            Some(Lazy::with_position(position as usize))
+            Some(Lazy::from_position(position as usize))
         }
     }
 }
diff --git a/src/librustc_metadata/schema.rs b/src/librustc_metadata/schema.rs
index 13c599cf997..f37877b437e 100644
--- a/src/librustc_metadata/schema.rs
+++ b/src/librustc_metadata/schema.rs
@@ -41,6 +41,33 @@ pub const METADATA_VERSION: u8 = 4;
 pub const METADATA_HEADER: &[u8; 12] =
     &[0, 0, 0, 0, b'r', b'u', b's', b't', 0, 0, 0, METADATA_VERSION];
 
+/// Additional metadata for a `Lazy<T>` where `T` may not be `Sized`,
+/// e.g. for `Lazy<[T]>`, this is the length (count of `T` values).
+pub trait LazyMeta {
+    type Meta: Copy + 'static;
+
+    /// Returns the minimum encoded size.
+    // FIXME(eddyb) Give better estimates for certain types.
+    fn min_size(meta: Self::Meta) -> usize;
+}
+
+impl<T> LazyMeta for T {
+    type Meta = ();
+
+    fn min_size(_: ()) -> usize {
+        assert_ne!(std::mem::size_of::<T>(), 0);
+        1
+    }
+}
+
+impl<T> LazyMeta for [T] {
+    type Meta = usize;
+
+    fn min_size(len: usize) -> usize {
+        len * T::min_size(())
+    }
+}
+
 /// A value of type T referred to by its absolute position
 /// in the metadata, and which can be decoded lazily.
 ///
@@ -56,40 +83,8 @@ pub const METADATA_HEADER: &[u8; 12] =
 /// Distances start at 1, as 0-byte nodes are invalid.
 /// Also invalid are nodes being referred in a different
 /// order than they were encoded in.
-#[must_use]
-pub struct Lazy<T> {
-    pub position: usize,
-    _marker: PhantomData<T>,
-}
-
-impl<T> Lazy<T> {
-    pub fn with_position(position: usize) -> Lazy<T> {
-        Lazy {
-            position,
-            _marker: PhantomData,
-        }
-    }
-
-    /// Returns the minimum encoded size of a value of type `T`.
-    // FIXME(eddyb) Give better estimates for certain types.
-    pub fn min_size() -> usize {
-        1
-    }
-}
-
-impl<T> Copy for Lazy<T> {}
-impl<T> Clone for Lazy<T> {
-    fn clone(&self) -> Self {
-        *self
-    }
-}
-
-impl<T> rustc_serialize::UseSpecializedEncodable for Lazy<T> {}
-impl<T> rustc_serialize::UseSpecializedDecodable for Lazy<T> {}
-
-/// A sequence of type T referred to by its absolute position
-/// in the metadata and length, and which can be decoded lazily.
-/// The sequence is a single node for the purposes of `Lazy`.
+///
+/// # Sequences (`Lazy<[T]>`)
 ///
 /// Unlike `Lazy<Vec<T>>`, the length is encoded next to the
 /// position, not at the position, which means that the length
@@ -100,54 +95,62 @@ impl<T> rustc_serialize::UseSpecializedDecodable for Lazy<T> {}
 /// the minimal distance the length of the sequence, i.e.
 /// it's assumed there's no 0-byte element in the sequence.
 #[must_use]
-pub struct LazySeq<T> {
-    pub len: usize,
+// FIXME(#59875) the `Meta` parameter only exists to dodge
+// invariance wrt `T` (coming from the `meta: T::Meta` field).
+pub struct Lazy<T, Meta = <T as LazyMeta>::Meta>
+    where T: ?Sized + LazyMeta<Meta = Meta>,
+          Meta: 'static + Copy,
+{
     pub position: usize,
+    pub meta: Meta,
     _marker: PhantomData<T>,
 }
 
-impl<T> LazySeq<T> {
-    pub fn empty() -> LazySeq<T> {
-        LazySeq::with_position_and_length(0, 0)
-    }
-
-    pub fn with_position_and_length(position: usize, len: usize) -> LazySeq<T> {
-        LazySeq {
-            len,
+impl<T: ?Sized + LazyMeta> Lazy<T> {
+    pub fn from_position_and_meta(position: usize, meta: T::Meta) -> Lazy<T> {
+        Lazy {
             position,
+            meta,
             _marker: PhantomData,
         }
     }
+}
+
+impl<T> Lazy<T> {
+    pub fn from_position(position: usize) -> Lazy<T> {
+        Lazy::from_position_and_meta(position, ())
+    }
+}
 
-    /// Returns the minimum encoded size of `length` values of type `T`.
-    pub fn min_size(length: usize) -> usize {
-        length
+impl<T> Lazy<[T]> {
+    pub fn empty() -> Lazy<[T]> {
+        Lazy::from_position_and_meta(0, 0)
     }
 }
 
-impl<T> Copy for LazySeq<T> {}
-impl<T> Clone for LazySeq<T> {
+impl<T: ?Sized + LazyMeta> Copy for Lazy<T> {}
+impl<T: ?Sized + LazyMeta> Clone for Lazy<T> {
     fn clone(&self) -> Self {
         *self
     }
 }
 
-impl<T> rustc_serialize::UseSpecializedEncodable for LazySeq<T> {}
-impl<T> rustc_serialize::UseSpecializedDecodable for LazySeq<T> {}
+impl<T: ?Sized + LazyMeta> rustc_serialize::UseSpecializedEncodable for Lazy<T> {}
+impl<T: ?Sized + LazyMeta> rustc_serialize::UseSpecializedDecodable for Lazy<T> {}
 
-/// Encoding / decoding state for `Lazy` and `LazySeq`.
+/// Encoding / decoding state for `Lazy`.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum LazyState {
     /// Outside of a metadata node.
     NoNode,
 
-    /// Inside a metadata node, and before any `Lazy` or `LazySeq`.
+    /// Inside a metadata node, and before any `Lazy`.
     /// The position is that of the node itself.
     NodeStart(usize),
 
-    /// Inside a metadata node, with a previous `Lazy` or `LazySeq`.
+    /// Inside a metadata node, with a previous `Lazy`.
     /// The position is a conservative estimate of where that
-    /// previous `Lazy` / `LazySeq` would end (see their comments).
+    /// previous `Lazy` would end (see their comments).
     Previous(usize),
 }
 
@@ -167,24 +170,24 @@ pub struct CrateRoot<'tcx> {
     pub proc_macro_decls_static: Option<DefIndex>,
     pub proc_macro_stability: Option<attr::Stability>,
 
-    pub crate_deps: LazySeq<CrateDep>,
-    pub dylib_dependency_formats: LazySeq<Option<LinkagePreference>>,
-    pub lib_features: LazySeq<(Symbol, Option<Symbol>)>,
-    pub lang_items: LazySeq<(DefIndex, usize)>,
-    pub lang_items_missing: LazySeq<lang_items::LangItem>,
-    pub native_libraries: LazySeq<NativeLibrary>,
-    pub foreign_modules: LazySeq<ForeignModule>,
-    pub source_map: LazySeq<syntax_pos::SourceFile>,
+    pub crate_deps: Lazy<[CrateDep]>,
+    pub dylib_dependency_formats: Lazy<[Option<LinkagePreference>]>,
+    pub lib_features: Lazy<[(Symbol, Option<Symbol>)]>,
+    pub lang_items: Lazy<[(DefIndex, usize)]>,
+    pub lang_items_missing: Lazy<[lang_items::LangItem]>,
+    pub native_libraries: Lazy<[NativeLibrary]>,
+    pub foreign_modules: Lazy<[ForeignModule]>,
+    pub source_map: Lazy<[syntax_pos::SourceFile]>,
     pub def_path_table: Lazy<hir::map::definitions::DefPathTable>,
-    pub impls: LazySeq<TraitImpls>,
-    pub exported_symbols: LazySeq<(ExportedSymbol<'tcx>, SymbolExportLevel)>,
-    pub interpret_alloc_index: LazySeq<u32>,
+    pub impls: Lazy<[TraitImpls]>,
+    pub exported_symbols: Lazy<[(ExportedSymbol<'tcx>, SymbolExportLevel)]>,
+    pub interpret_alloc_index: Lazy<[u32]>,
 
-    pub entries_index: LazySeq<index::Index<'tcx>>,
+    pub entries_index: Lazy<[index::Index<'tcx>]>,
 
     /// The DefIndex's of any proc macros delcared by
     /// this crate
-    pub proc_macro_data: Option<LazySeq<DefIndex>>,
+    pub proc_macro_data: Option<Lazy<[DefIndex]>>,
 
     pub compiler_builtins: bool,
     pub needs_allocator: bool,
@@ -207,7 +210,7 @@ pub struct CrateDep {
 #[derive(RustcEncodable, RustcDecodable)]
 pub struct TraitImpls {
     pub trait_id: (u32, DefIndex),
-    pub impls: LazySeq<DefIndex>,
+    pub impls: Lazy<[DefIndex]>,
 }
 
 #[derive(RustcEncodable, RustcDecodable)]
@@ -215,14 +218,14 @@ pub struct Entry<'tcx> {
     pub kind: EntryKind<'tcx>,
     pub visibility: Lazy<ty::Visibility>,
     pub span: Lazy<Span>,
-    pub attributes: LazySeq<ast::Attribute>,
-    pub children: LazySeq<DefIndex>,
+    pub attributes: Lazy<[ast::Attribute]>,
+    pub children: Lazy<[DefIndex]>,
     pub stability: Option<Lazy<attr::Stability>>,
     pub deprecation: Option<Lazy<attr::Deprecation>>,
 
     pub ty: Option<Lazy<Ty<'tcx>>>,
-    pub inherent_impls: LazySeq<DefIndex>,
-    pub variances: LazySeq<ty::Variance>,
+    pub inherent_impls: Lazy<[DefIndex]>,
+    pub variances: Lazy<[ty::Variance]>,
     pub generics: Option<Lazy<ty::Generics>>,
     pub predicates: Option<Lazy<ty::GenericPredicates<'tcx>>>,
     pub predicates_defined_on: Option<Lazy<ty::GenericPredicates<'tcx>>>,
@@ -278,7 +281,7 @@ pub struct RenderedConst(pub String);
 
 #[derive(RustcEncodable, RustcDecodable)]
 pub struct ModData {
-    pub reexports: LazySeq<def::Export<hir::HirId>>,
+    pub reexports: Lazy<[def::Export<hir::HirId>]>,
 }
 
 #[derive(RustcEncodable, RustcDecodable)]
@@ -290,7 +293,7 @@ pub struct MacroDef {
 #[derive(RustcEncodable, RustcDecodable)]
 pub struct FnData<'tcx> {
     pub constness: hir::Constness,
-    pub arg_names: LazySeq<ast::Name>,
+    pub arg_names: Lazy<[ast::Name]>,
     pub sig: Lazy<ty::PolyFnSig<'tcx>>,
 }