diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index 9173d128274..c69ee180dc9 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -59,6 +59,9 @@ pub struct DecodeContext<'a, 'tcx: 'a> {
 
     // interpreter allocation cache
     interpret_alloc_cache: FxHashMap<usize, interpret::AllocId>,
+
+    // Read from the LazySeq CrateRoot::inpterpret_alloc_index on demand
+    interpret_alloc_index: Option<Vec<u32>>,
 }
 
 /// Abstract over the various ways one can create metadata decoders.
@@ -78,6 +81,7 @@ pub trait Metadata<'a, 'tcx>: Copy {
             last_filemap_index: 0,
             lazy_state: LazyState::NoNode,
             interpret_alloc_cache: FxHashMap::default(),
+            interpret_alloc_index: None,
         }
     }
 }
@@ -176,6 +180,17 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
         self.lazy_state = LazyState::Previous(position + min_size);
         Ok(position)
     }
+
+    fn interpret_alloc(&mut self, idx: usize) -> usize {
+        if let Some(index) = self.interpret_alloc_index.as_mut() {
+            return index[idx] as usize;
+        }
+        let index = self.cdata().root.interpret_alloc_index;
+        let index: Vec<u32> = index.decode(self.cdata()).collect();
+        let pos = index[idx];
+        self.interpret_alloc_index = Some(index);
+        pos as usize
+    }
 }
 
 impl<'a, 'tcx: 'a> TyDecoder<'a, 'tcx> for DecodeContext<'a, 'tcx> {
@@ -292,11 +307,8 @@ impl<'a, 'tcx> SpecializedDecoder<interpret::AllocId> for DecodeContext<'a, 'tcx
         if let Some(cached) = self.interpret_alloc_cache.get(&idx).cloned() {
             return Ok(cached);
         }
-        let pos = self
-            .cdata()
-            .root
-            .interpret_alloc_index[idx];
-        self.with_position(pos as usize, |this| {
+        let pos = self.interpret_alloc(idx);
+        self.with_position(pos, |this| {
             interpret::specialized_decode_alloc_id(
                 this,
                 tcx,
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index cc2d0eab233..d6c673f550d 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -265,7 +265,11 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 start - min_end
             }
             LazyState::Previous(last_min_end) => {
-                assert!(last_min_end <= position);
+                assert!(
+                    last_min_end <= position,
+                    "make sure that the calls to `lazy*` \
+                    are in the same order as the metadata fields",
+                );
                 position - last_min_end
             }
         };
@@ -439,21 +443,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             IsolatedEncoder::encode_wasm_custom_sections,
             &wasm_custom_sections);
 
-        // Encode and index the items.
-        i = self.position();
-        let items = self.encode_info_for_items();
-        let item_bytes = self.position() - i;
-
-        i = self.position();
-        let index = items.write_index(&mut self.opaque.cursor);
-        let index_bytes = self.position() - i;
-
         let tcx = self.tcx;
-        let link_meta = self.link_meta;
-        let is_proc_macro = tcx.sess.crate_types.borrow().contains(&CrateTypeProcMacro);
-        let has_default_lib_allocator =
-            attr::contains_name(tcx.hir.krate_attrs(), "default_lib_allocator");
-        let has_global_allocator = *tcx.sess.has_global_allocator.get();
 
         // Encode the allocation index
         let interpret_alloc_index = {
@@ -478,9 +468,24 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 }
                 n = new_n;
             }
-            interpret_alloc_index
+            self.lazy_seq(interpret_alloc_index)
         };
 
+        // Encode and index the items.
+        i = self.position();
+        let items = self.encode_info_for_items();
+        let item_bytes = self.position() - i;
+
+        i = self.position();
+        let index = items.write_index(&mut self.opaque.cursor);
+        let index_bytes = self.position() - i;
+
+        let link_meta = self.link_meta;
+        let is_proc_macro = tcx.sess.crate_types.borrow().contains(&CrateTypeProcMacro);
+        let has_default_lib_allocator =
+            attr::contains_name(tcx.hir.krate_attrs(), "default_lib_allocator");
+        let has_global_allocator = tcx.sess.has_global_allocator.get();
+
         let root = self.lazy(&CrateRoot {
             name: tcx.crate_name(LOCAL_CRATE),
             extra_filename: tcx.sess.opts.cg.extra_filename.clone(),
@@ -512,8 +517,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             impls,
             exported_symbols,
             wasm_custom_sections,
-            index,
             interpret_alloc_index,
+            index,
         });
 
         let total_bytes = self.position();
diff --git a/src/librustc_metadata/schema.rs b/src/librustc_metadata/schema.rs
index 4eaf08742ec..23ea5e4cc55 100644
--- a/src/librustc_metadata/schema.rs
+++ b/src/librustc_metadata/schema.rs
@@ -207,7 +207,7 @@ pub struct CrateRoot {
     pub impls: LazySeq<TraitImpls>,
     pub exported_symbols: EncodedExportedSymbols,
     pub wasm_custom_sections: LazySeq<DefIndex>,
-    pub interpret_alloc_index: Vec<u32>,
+    pub interpret_alloc_index: LazySeq<u32>,
 
     pub index: LazySeq<index::Index>,
 }