Reset qualifs when a storage of a local ends

to ensure that the local qualifs are affected by the state from previous
loop iterations only if the local is kept alive.

The change should be forward compatible with a stricter handling of
indirect assignments, since storage dead invalidates all existing
pointers to the local.

Author: tmiasko

compiler/rustc_const_eval/src/transform/check_consts/resolver.rs

@@ -4,7 +4,7 @@
 
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{self, BasicBlock, Local, Location};
+use rustc_middle::mir::{self, BasicBlock, Local, Location, Statement, StatementKind};
 
 use std::marker::PhantomData;
 
@@ -120,6 +120,15 @@ where
         self.super_assign(place, rvalue, location);
     }
 
+    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
+        match statement.kind {
+            StatementKind::StorageDead(local) => {
+                self.qualifs_per_local.remove(local);
+            }
+            _ => self.super_statement(statement, location),
+        }
+    }
+
     fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
         // The effect of assignment to the return place in `TerminatorKind::Call` is not applied
         // here; that occurs in `apply_call_return_effect`.

src/test/ui/consts/promoted-storage.rs

@@ -0,0 +1,20 @@
+// Check that storage statements reset local qualification.
+// check-pass
+use std::cell::Cell;
+
+const C: Option<Cell<u32>> = {
+    let mut c = None;
+    let mut i = 0;
+    while i == 0 {
+        let mut x = None;
+        c = x;
+        x = Some(Cell::new(0));
+        let _ = x;
+        i += 1;
+    }
+    c
+};
+
+fn main() {
+    let _: &'static _ = &C;
+}