diff --git a/src/librustc/mir/interpret/allocation.rs b/src/librustc/mir/interpret/allocation.rs
index e0ada7e2f11a..bfbfffeb3b8c 100644
--- a/src/librustc/mir/interpret/allocation.rs
+++ b/src/librustc/mir/interpret/allocation.rs
@@ -13,6 +13,8 @@ use rustc_data_structures::sorted_map::SortedMap;
 use rustc_target::abi::HasDataLayout;
 use std::borrow::Cow;
 
+// NOTE: When adding new fields, make sure to adjust the Snapshot impl in
+// `src/librustc_mir/interpret/snapshot.rs`.
 #[derive(
     Clone,
     Debug,
diff --git a/src/librustc_mir/interpret/intrinsics.rs b/src/librustc_mir/interpret/intrinsics.rs
index 89c5be137a4e..a48a4abf796b 100644
--- a/src/librustc_mir/interpret/intrinsics.rs
+++ b/src/librustc_mir/interpret/intrinsics.rs
@@ -81,7 +81,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let alloc = alloc_type_name(self.tcx.tcx, substs.type_at(0));
                 let name_id = self.tcx.alloc_map.lock().create_memory_alloc(alloc);
                 let id_ptr = self.memory.tag_static_base_pointer(name_id.into());
-                let alloc_len = alloc.bytes.len() as u64;
+                let alloc_len = alloc.size.bytes();
                 let name_val = Immediate::new_slice(Scalar::Ptr(id_ptr), alloc_len, self);
                 self.write_immediate(name_val, dest)?;
             }
diff --git a/src/librustc_mir/interpret/intrinsics/type_name.rs b/src/librustc_mir/interpret/intrinsics/type_name.rs
index f207cfc6b39c..032d16a49db4 100644
--- a/src/librustc_mir/interpret/intrinsics/type_name.rs
+++ b/src/librustc_mir/interpret/intrinsics/type_name.rs
@@ -221,7 +221,7 @@ pub fn type_name<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> &'tcx ty::Const<'tcx>
         val: ConstValue::Slice {
             data: alloc,
             start: 0,
-            end: alloc.bytes.len(),
+            end: alloc.len(),
         },
         ty: tcx.mk_static_str(),
     })
diff --git a/src/librustc_mir/interpret/memory.rs b/src/librustc_mir/interpret/memory.rs
index f572651f02b5..15180265ad29 100644
--- a/src/librustc_mir/interpret/memory.rs
+++ b/src/librustc_mir/interpret/memory.rs
@@ -210,7 +210,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         let new_ptr = self.allocate(new_size, new_align, kind);
         let old_size = match old_size_and_align {
             Some((size, _align)) => size,
-            None => Size::from_bytes(self.get(ptr.alloc_id)?.bytes.len() as u64),
+            None => self.get(ptr.alloc_id)?.size,
         };
         self.copy(
             ptr,
@@ -271,20 +271,20 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             ))
         }
         if let Some((size, align)) = old_size_and_align {
-            if size.bytes() != alloc.bytes.len() as u64 || align != alloc.align {
-                let bytes = Size::from_bytes(alloc.bytes.len() as u64);
+            if size != alloc.size || align != alloc.align {
+                let bytes = alloc.size;
                 throw_unsup!(IncorrectAllocationInformation(size, bytes, align, alloc.align))
             }
         }
 
         // Let the machine take some extra action
-        let size = Size::from_bytes(alloc.bytes.len() as u64);
+        let size = alloc.size;
         AllocationExtra::memory_deallocated(&mut alloc, ptr, size)?;
 
         // Don't forget to remember size and align of this now-dead allocation
         let old = self.dead_alloc_map.insert(
             ptr.alloc_id,
-            (Size::from_bytes(alloc.bytes.len() as u64), alloc.align)
+            (alloc.size, alloc.align)
         );
         if old.is_some() {
             bug!("Nothing can be deallocated twice");
@@ -555,7 +555,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         // a) cause cycles in case `id` refers to a static
         // b) duplicate a static's allocation in miri
         if let Some((_, alloc)) = self.alloc_map.get(id) {
-            return Ok((Size::from_bytes(alloc.bytes.len() as u64), alloc.align));
+            return Ok((alloc.size, alloc.align));
         }
 
         // # Function pointers
@@ -583,7 +583,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             Some(GlobalAlloc::Memory(alloc)) =>
                 // Need to duplicate the logic here, because the global allocations have
                 // different associated types than the interpreter-local ones.
-                Ok((Size::from_bytes(alloc.bytes.len() as u64), alloc.align)),
+                Ok((alloc.size, alloc.align)),
             Some(GlobalAlloc::Function(_)) =>
                 bug!("We already checked function pointers above"),
             // The rest must be dead.
@@ -645,7 +645,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         let prefix_len = msg.len();
         let mut relocations = vec![];
 
-        for i in 0..(alloc.bytes.len() as u64) {
+        for i in 0..alloc.size.bytes() {
             let i = Size::from_bytes(i);
             if let Some(&(_, target_id)) = alloc.relocations.get(&i) {
                 if allocs_seen.insert(target_id) {
@@ -655,7 +655,12 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             }
             if alloc.undef_mask.is_range_defined(i, i + Size::from_bytes(1)).is_ok() {
                 // this `as usize` is fine, since `i` came from a `usize`
-                write!(msg, "{:02x} ", alloc.bytes[i.bytes() as usize]).unwrap();
+                let i = i.bytes() as usize;
+
+                // Checked definedness (and thus range) and relocations. This access also doesn't
+                // influence interpreter execution but is only for debugging.
+                let bytes = alloc.inspect_with_undef_and_ptr_outside_interpreter(i..i+1);
+                write!(msg, "{:02x} ", bytes[0]).unwrap();
             } else {
                 msg.push_str("__ ");
             }
@@ -664,7 +669,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         trace!(
             "{}({} bytes, alignment {}){}",
             msg,
-            alloc.bytes.len(),
+            alloc.size.bytes(),
             alloc.align.bytes(),
             extra
         );
diff --git a/src/librustc_mir/interpret/snapshot.rs b/src/librustc_mir/interpret/snapshot.rs
index 47289064f4d0..0f0c422d8d3f 100644
--- a/src/librustc_mir/interpret/snapshot.rs
+++ b/src/librustc_mir/interpret/snapshot.rs
@@ -15,7 +15,7 @@ use rustc::mir::interpret::{
 };
 
 use rustc::ty::{self, TyCtxt};
-use rustc::ty::layout::Align;
+use rustc::ty::layout::{Align, Size};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -276,6 +276,7 @@ struct AllocationSnapshot<'a> {
     relocations: Relocations<(), AllocIdSnapshot<'a>>,
     undef_mask: &'a UndefMask,
     align: &'a Align,
+    size: &'a Size,
     mutability: &'a Mutability,
 }
 
@@ -285,12 +286,27 @@ impl<'a, Ctx> Snapshot<'a, Ctx> for &'a Allocation
     type Item = AllocationSnapshot<'a>;
 
     fn snapshot(&self, ctx: &'a Ctx) -> Self::Item {
-        let Allocation { bytes, relocations, undef_mask, align, mutability, extra: () } = self;
+        let Allocation {
+            relocations,
+            size,
+            align,
+            mutability,
+            extra: (),
+            ..
+        } = self;
+
+        let all_bytes = 0..self.len();
+        // This 'inspect' is okay since following access respects undef and relocations. This does
+        // influence interpreter exeuction, but only to detect the error of cycles in evalution
+        // dependencies.
+        let bytes = self.inspect_with_undef_and_ptr_outside_interpreter(all_bytes);
+        let undef_mask = self.undef_mask();
 
         AllocationSnapshot {
             bytes,
             undef_mask,
             align,
+            size,
             mutability,
             relocations: relocations.snapshot(ctx),
         }