3 files changed, 26 insertions, 1 deletions
diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index f8ecd963457..51e590fe292 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -267,7 +267,12 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                 }
             };
             debug!("store to var {:?}", index);
-            self.local_qualif[index] = Some(self.qualif);
+            match &mut self.local_qualif[index] {
+                // update
+                Some(ref mut qualif) => *qualif = *qualif | self.qualif,
+                // or insert
+                qualif @ None => *qualif = Some(self.qualif),
+            }
             return;
         }
 
diff --git a/src/test/ui/consts/partial_qualif.rs b/src/test/ui/consts/partial_qualif.rs
new file mode 100644
index 00000000000..bb3fc92d4fb
--- /dev/null
+++ b/src/test/ui/consts/partial_qualif.rs
@@ -0,0 +1,11 @@
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+const FOO: &(Cell<usize>, bool) = {
+    let mut a = (Cell::new(0), false);
+    a.1 = true; // resets `qualif(a)` to `qualif(true)`
+    &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
+};
+
+fn main() {}
\ No newline at end of file
diff --git a/src/test/ui/consts/partial_qualif.stderr b/src/test/ui/consts/partial_qualif.stderr
new file mode 100644
index 00000000000..d695f64e2c3
--- /dev/null
+++ b/src/test/ui/consts/partial_qualif.stderr
@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/partial_qualif.rs:8:5
+   |
+LL |     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
+   |     ^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0492`.