diff --git a/src/librustc/mir/interpret/allocation.rs b/src/librustc/mir/interpret/allocation.rs
index 796d293e2c63c..aa8ac4902a894 100644
--- a/src/librustc/mir/interpret/allocation.rs
+++ b/src/librustc/mir/interpret/allocation.rs
@@ -353,11 +353,14 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         &mut self,
         cx: &impl HasDataLayout,
         ptr: Pointer<Tag>,
-        src: impl IntoIterator<Item=u8, IntoIter: iter::ExactSizeIterator>,
+        src: impl IntoIterator<Item=u8>,
     ) -> InterpResult<'tcx>
     {
         let mut src = src.into_iter();
-        let bytes = self.get_bytes_mut(cx, ptr, Size::from_bytes(src.len() as u64))?;
+        let (lower, upper) = src.size_hint();
+        let len = upper.expect("can only write bounded iterators");
+        assert_eq!(lower, len, "can only write iterators with a precise length");
+        let bytes = self.get_bytes_mut(cx, ptr, Size::from_bytes(len as u64))?;
         // `zip` would stop when the first iterator ends; we want to definitely
         // cover all of `bytes`.
         for dest in bytes {
diff --git a/src/librustc_mir/interpret/memory.rs b/src/librustc_mir/interpret/memory.rs
index eef1868ec65b6..d113ee33162d2 100644
--- a/src/librustc_mir/interpret/memory.rs
+++ b/src/librustc_mir/interpret/memory.rs
@@ -7,7 +7,7 @@
 //! short-circuiting the empty case!
 
 use std::collections::VecDeque;
-use std::{ptr, iter};
+use std::ptr;
 use std::borrow::Cow;
 
 use rustc::ty::{self, Instance, ParamEnv, query::TyCtxtAt};
@@ -791,11 +791,12 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
     pub fn write_bytes(
         &mut self,
         ptr: Scalar<M::PointerTag>,
-        src: impl IntoIterator<Item=u8, IntoIter: iter::ExactSizeIterator>,
+        src: impl IntoIterator<Item=u8>,
     ) -> InterpResult<'tcx>
     {
         let src = src.into_iter();
-        let size = Size::from_bytes(src.len() as u64);
+        let size = Size::from_bytes(src.size_hint().0 as u64);
+        // `write_bytes` checks that this lower bound matches the upper bound matches reality.
         let ptr = match self.check_ptr_access(ptr, size, Align::from_bytes(1).unwrap())? {
             Some(ptr) => ptr,
             None => return Ok(()), // zero-sized access