Properly assign to aggregate fields

src/librustc_mir/transform/qualify_consts.rs

@@ -267,7 +267,12 @@ fn assign(&mut self, dest: &Place<'tcx>, location: Location) {
                 }
             };
             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;
         }
 

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() {}

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`.