Explain why we do not overwrite qualification of locals

src/librustc_mir/transform/qualify_consts.rs

@@ -277,9 +277,12 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             };
             debug!("store to var {:?}", index);
             match &mut self.local_qualif[index] {
-                // update
+                // this is overly restrictive, because even full assignments do not clear the qualif
+                // While we could special case full assignments, this would be inconsistent with
+                // aggregates where we overwrite all fields via assignments, which would not get
+                // that feature.
                 Some(ref mut qualif) => *qualif = *qualif | self.qualif,
-                // or insert
+                // insert new qualification
                 qualif @ None => *qualif = Some(self.qualif),
             }
             return;

src/test/ui/consts/qualif_overwrite.rs

@@ -1,13 +1,15 @@
-// compile-pass
-
 #![feature(const_let)]
 
 use std::cell::Cell;
 
+// this is overly conservative. The reset to `None` should clear `a` of all qualifications
+// while we could fix this, it would be inconsistent with `qualif_overwrite_2.rs`.
+// We can fix this properly in the future by allowing constants that do not depend on generics
+// to be checked by an analysis on the final value instead of looking at the body.
 const FOO: &Option<Cell<usize>> = {
     let mut a = Some(Cell::new(0));
-    a = None; // resets `qualif(a)` to `qualif(None)`
-    &{a}
+    a = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
+    &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
 };
 
 fn main() {}

src/test/ui/consts/qualif_overwrite.stderr

@@ -1,5 +1,5 @@
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
-  --> $DIR/qualif_overwrite.rs:8:5
+  --> $DIR/qualif_overwrite.rs:12:5
    |
 LL |     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
    |     ^^^^

src/test/ui/consts/qualif_overwrite_2.rs

@@ -0,0 +1,13 @@
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+// const qualification is not smart enough to know about fields and always assumes that there might
+// be other fields that caused the qualification
+const FOO: &Option<Cell<usize>> = {
+    let mut a = (Some(Cell::new(0)),);
+    a.0 = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
+    &{a.0} //~ ERROR cannot borrow a constant which may contain interior mutability
+};
+
+fn main() {}

src/test/ui/consts/qualif_overwrite_2.stderr

@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/qualif_overwrite_2.rs:10:5
+   |
+LL |     &{a.0} //~ 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`.