diff --git a/src/librustc/ich/impls_ty.rs b/src/librustc/ich/impls_ty.rs
index 0ddc9211f981..e33991bb1b6f 100644
--- a/src/librustc/ich/impls_ty.rs
+++ b/src/librustc/ich/impls_ty.rs
@@ -176,15 +176,21 @@ impl<'a> HashStable<StableHashingContext<'a>> for mir::interpret::Allocation {
         hasher: &mut StableHasher<W>,
     ) {
         let mir::interpret::Allocation {
-            bytes, relocations, undef_mask, align, mutability,
+            relocations, align, mutability, size,
             extra: _,
+            .. /* private bytes and undef_mask */
         } = self;
+
+        let bytes = self.inspect_with_undef_and_ptr_outside_interpreter(0..self.len());
+        let undef_mask = self.undef_mask();
+
         bytes.hash_stable(hcx, hasher);
         relocations.len().hash_stable(hcx, hasher);
         for reloc in relocations.iter() {
             reloc.hash_stable(hcx, hasher);
         }
         undef_mask.hash_stable(hcx, hasher);
+        size.hash_stable(hcx, hasher);
         align.hash_stable(hcx, hasher);
         mutability.hash_stable(hcx, hasher);
     }
diff --git a/src/librustc/mir/interpret/allocation.rs b/src/librustc/mir/interpret/allocation.rs
index ce04cca96e0f..61b237c0bb39 100644
--- a/src/librustc/mir/interpret/allocation.rs
+++ b/src/librustc/mir/interpret/allocation.rs
@@ -16,15 +16,17 @@ use std::borrow::Cow;
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
 pub struct Allocation<Tag=(),Extra=()> {
     /// The actual bytes of the allocation.
-    /// Note that the bytes of a pointer represent the offset of the pointer
-    pub bytes: Vec<u8>,
+    /// Note that the bytes of a pointer represent the offset of the pointer.
+    bytes: Vec<u8>,
     /// Maps from byte addresses to extra data for each pointer.
     /// Only the first byte of a pointer is inserted into the map; i.e.,
     /// every entry in this map applies to `pointer_size` consecutive bytes starting
     /// at the given offset.
     pub relocations: Relocations<Tag>,
-    /// Denotes undefined memory. Reading from undefined memory is forbidden in miri
-    pub undef_mask: UndefMask,
+    /// Denotes which part of this allocation is initialized.
+    undef_mask: UndefMask,
+    /// The size of the allocation. Currently, must always equal `bytes.len()`.
+    pub size: Size,
     /// The alignment of the allocation to detect unaligned reads.
     pub align: Align,
     /// Whether the allocation is mutable.
@@ -85,11 +87,12 @@ impl<Tag> Allocation<Tag> {
     /// Creates a read-only allocation initialized by the given bytes
     pub fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, align: Align) -> Self {
         let bytes = slice.into().into_owned();
-        let undef_mask = UndefMask::new(Size::from_bytes(bytes.len() as u64), true);
+        let size = Size::from_bytes(bytes.len() as u64);
         Self {
             bytes,
             relocations: Relocations::new(),
-            undef_mask,
+            undef_mask: UndefMask::new(size, true),
+            size,
             align,
             mutability: Mutability::Immutable,
             extra: (),
@@ -106,6 +109,7 @@ impl<Tag> Allocation<Tag> {
             bytes: vec![0; size.bytes() as usize],
             relocations: Relocations::new(),
             undef_mask: UndefMask::new(size, false),
+            size,
             align,
             mutability: Mutability::Mutable,
             extra: (),
@@ -113,6 +117,21 @@ impl<Tag> Allocation<Tag> {
     }
 }
 
+/// Raw accessors. Provide access to otherwise private bytes.
+impl<Tag, Extra> Allocation<Tag, Extra> {
+    pub fn len(&self) -> usize {
+        self.size.bytes() as usize
+    }
+
+    /// Look at a slice which may describe undefined bytes or describe a relocation. This differs
+    /// from `get_bytes_with_undef_and_ptr` in that it does no relocation checks (even on the
+    /// edges) at all. It further ignores `AllocationExtra` callbacks.
+    /// This must not be used for reads affecting the interpreter execution.
+    pub fn inspect_with_undef_and_ptr_outside_interpreter(&self, range: Range<usize>) -> &[u8] {
+        &self.bytes[range]
+    }
+}
+
 impl<'tcx> rustc_serialize::UseSpecializedDecodable for &'tcx Allocation {}
 
 /// Byte accessors
@@ -132,9 +151,9 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         );
         let end = end.bytes() as usize;
         assert!(
-            end <= self.bytes.len(),
+            end <= self.len(),
             "Out-of-bounds access at offset {}, size {} in allocation of size {}",
-            offset.bytes(), size.bytes(), self.bytes.len()
+            offset.bytes(), size.bytes(), self.len()
         );
         (offset.bytes() as usize)..end
     }
diff --git a/src/librustc/ty/print/pretty.rs b/src/librustc/ty/print/pretty.rs
index bf6741dde43b..a72ecdb5745b 100644
--- a/src/librustc/ty/print/pretty.rs
+++ b/src/librustc/ty/print/pretty.rs
@@ -944,10 +944,16 @@ pub trait PrettyPrinter<'tcx>:
                         .get_bytes(&self.tcx(), ptr, Size::from_bytes(n)).unwrap())
                 },
                 (ConstValue::Slice { data, start, end }, ty::Slice(t)) if *t == u8 => {
-                    Some(&data.bytes[start..end])
+                    // The `inspect` here is okay since we checked the bounds, and there are no
+                    // relocations (we have an active slice reference here). We don't use this
+                    // result to affect interpreter execution.
+                    Some(data.inspect_with_undef_and_ptr_outside_interpreter(start..end))
                 },
                 (ConstValue::Slice { data, start, end }, ty::Str) => {
-                    let slice = &data.bytes[start..end];
+                    // The `inspect` here is okay since we checked the bounds, and there are no
+                    // relocations (we have an active `str` reference here). We don't use this
+                    // result to affect interpreter execution.
+                    let slice = data.inspect_with_undef_and_ptr_outside_interpreter(start..end);
                     let s = ::std::str::from_utf8(slice)
                         .expect("non utf8 str from miri");
                     p!(write("{:?}", s));