Don't allow dereferencing uninitialized references inside PlaceMention

compiler/rustc_borrowck/src/lib.rs

@@ -851,8 +851,21 @@ impl<'a, 'tcx> ResultsVisitor<'tcx, Borrowck<'a, 'tcx>> for MirBorrowckCtxt<'a,
             },
             // Only relevant for mir typeck
             StatementKind::AscribeUserType(..) => {}
-            // Only relevant for liveness and unsafeck
-            StatementKind::PlaceMention(..) => {}
+            // Only relevant for liveness and unsafeck,
+            // except for dereferences which must be materialized
+            StatementKind::PlaceMention(place) => {
+                if let Some((dereferenced_place, _)) = place
+                    .iter_projections()
+                    .rfind(|(_, proj)| matches!(proj, ProjectionElem::Deref))
+                {
+                    self.check_if_full_path_is_moved(
+                        location,
+                        InitializationRequiringAction::Use,
+                        (dereferenced_place, span),
+                        state,
+                    );
+                }
+            }
             // Doesn't have any language semantics
             StatementKind::Coverage(..) => {}
             // These do not actually affect borrowck

src/tools/miri/tests/pass/placemention-box-deref.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let x = Box::new(Box::new(Box::new(0)));
+    drop(**x);
+    _ = **x;
+}

tests/ui/borrowck/placemention-deref.rs

@@ -0,0 +1,42 @@
+enum Thing {
+    Wut,
+}
+
+struct A(Box<A>);
+
+fn main() {
+    let x: &'static String;
+    let _ = *x;
+    //~^ ERROR used binding `x` isn't initialized
+
+    let x: &Thing;
+    let Thing::Wut = *x;
+    //~^ ERROR used binding `x` isn't initialized
+
+    let x = Box::new(1);
+    drop(x);
+    _ = *x;
+    //~^ ERROR use of moved value: `x`
+
+    let x: A;
+    _ = x.0.0.0.0.0;
+    //~^ ERROR used binding `x` isn't initialized
+
+    let x: A;
+    let _ = x.0.0;
+    //~^ ERROR used binding `x` isn't initialized
+
+    let x = Box::new(Box::new(Box::new(Box::new(Box::new(Box::new(1))))));
+    drop(x);
+    _ = ******x;
+    //~^ ERROR use of moved value: `x`
+
+    let x = Box::new(Box::new(Box::new(0)));
+    drop(**x);
+    _ = ***x;
+    //~^ ERROR use of moved value: `**x`
+
+    let x = Box::new(Box::new(Box::new(0)));
+    drop(**x);
+    _ = **x; // OK
+}

tests/ui/borrowck/placemention-deref.stderr

@@ -0,0 +1,96 @@
+error[E0381]: used binding `x` isn't initialized
+  --> $DIR/placemention-deref.rs:9:13
+   |
+LL |     let x: &'static String;
+   |         - binding declared here but left uninitialized
+LL |     let _ = *x;
+   |             ^^ `x` used here but it isn't initialized
+   |
+help: consider assigning a value
+   |
+LL |     let x: &'static String = &Default::default();
+   |                            +++++++++++++++++++++
+
+error[E0381]: used binding `x` isn't initialized
+  --> $DIR/placemention-deref.rs:13:22
+   |
+LL |     let x: &Thing;
+   |         - binding declared here but left uninitialized
+LL |     let Thing::Wut = *x;
+   |                      ^^ `x` used here but it isn't initialized
+   |
+help: consider assigning a value
+   |
+LL |     let x: &Thing = &/* value */;
+   |                   ++++++++++++++
+
+error[E0382]: use of moved value: `x`
+  --> $DIR/placemention-deref.rs:18:9
+   |
+LL |     let x = Box::new(1);
+   |         - move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
+LL |     drop(x);
+   |          - value moved here
+LL |     _ = *x;
+   |         ^^ value used here after move
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |     drop(x.clone());
+   |           ++++++++
+
+error[E0381]: used binding `x` isn't initialized
+  --> $DIR/placemention-deref.rs:22:9
+   |
+LL |     let x: A;
+   |         - binding declared here but left uninitialized
+LL |     _ = x.0.0.0.0.0;
+   |         ^^^^^^^^^^^ `x.0.0.0.0` used here but it isn't initialized
+   |
+help: consider assigning a value
+   |
+LL |     let x: A = /* value */;
+   |              +++++++++++++
+
+error[E0381]: used binding `x` isn't initialized
+  --> $DIR/placemention-deref.rs:26:13
+   |
+LL |     let x: A;
+   |         - binding declared here but left uninitialized
+LL |     let _ = x.0.0;
+   |             ^^^^^ `x.0` used here but it isn't initialized
+   |
+help: consider assigning a value
+   |
+LL |     let x: A = /* value */;
+   |              +++++++++++++
+
+error[E0382]: use of moved value: `x`
+  --> $DIR/placemention-deref.rs:31:9
+   |
+LL |     let x = Box::new(Box::new(Box::new(Box::new(Box::new(Box::new(1))))));
+   |         - move occurs because `x` has type `Box<Box<Box<Box<Box<Box<i32>>>>>>`, which does not implement the `Copy` trait
+LL |     drop(x);
+   |          - value moved here
+LL |     _ = ******x;
+   |         ^^^^^^^ value used here after move
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |     drop(x.clone());
+   |           ++++++++
+
+error[E0382]: use of moved value: `**x`
+  --> $DIR/placemention-deref.rs:36:9
+   |
+LL |     drop(**x);
+   |          --- value moved here
+LL |     _ = ***x;
+   |         ^^^^ value used here after move
+   |
+   = note: move occurs because `**x` has type `Box<i32>`, which does not implement the `Copy` trait
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0381, E0382.
+For more information about an error, try `rustc --explain E0381`.