Add regression test for overwriting qualifs by assignment

src/test/ui/consts/qualif_overwrite.rs

@@ -0,0 +1,13 @@
+// compile-pass
+
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+const FOO: &Option<Cell<usize>> = {
+    let mut a = Some(Cell::new(0));
+    a = None; // resets `qualif(a)` to `qualif(None)`
+    &{a}
+};
+
+fn main() {}

src/test/ui/consts/qualif_overwrite.stderr

@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/qualif_overwrite.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`.