[Crater experiment] Adjust PlaceMention borrow checking for MIR move elimination semantics

compiler/rustc_borrowck/src/lib.rs

@@ -851,8 +851,26 @@ 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(..) => {}
+            // We impose two requirements on `PlaceMention`s:
+            // - `Deref`s get materialized, so check that the pointer is live
+            // - Under MIR move elimination semantics, locals that are entirely
+            //   uninitialized or moved out of don't have storage, so reject that
+            StatementKind::PlaceMention(place) => {
+                let place_to_check = match place
+                    .iter_projections()
+                    .rfind(|(_, proj)| matches!(proj, ProjectionElem::Deref))
+                {
+                    Some((dereferenced_place, _)) => dereferenced_place,
+                    None => place.local.into(),
+                };
+
+                self.check_if_full_path_is_moved(
+                    location,
+                    InitializationRequiringAction::Use,
+                    (place_to_check, 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/issue-62107-match-arm-scopes.rs

@@ -1,8 +1,8 @@
 fn main() {
     let e: i32;
     match e {
-        ref u if true => {}
         //~^ ERROR E0381
+        ref u if true => {}
         ref v if true => {
             let tx = 0;
             &tx;

tests/ui/borrowck/issue-62107-match-arm-scopes.stderr

@@ -1,11 +1,10 @@
 error[E0381]: used binding `e` isn't initialized
-  --> $DIR/issue-62107-match-arm-scopes.rs:4:9
+  --> $DIR/issue-62107-match-arm-scopes.rs:3:11
    |
 LL |     let e: i32;
    |         - binding declared here but left uninitialized
 LL |     match e {
-LL |         ref u if true => {}
-   |         ^^^^^ `e` used here but it isn't initialized
+   |           ^ `e` used here but it isn't initialized
    |
 help: consider assigning a value
    |

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`.

tests/ui/borrowck/placemention-mir-move-elim.rs

@@ -0,0 +1,15 @@
+fn main() {
+    let x: String;
+    let _ = x; //~ ERROR
+    x = "foo".to_owned();
+    let _ = x; // OK
+    drop(x);
+    let _ = x; //~ ERROR
+
+    let x: (String,);
+    let _ = x; //~ ERROR
+    x = ("foo".to_owned(),);
+    let _ = x; // OK
+    drop(x.0);
+    let _ = x; // OK
+}

tests/ui/borrowck/placemention-mir-move-elim.stderr

@@ -0,0 +1,46 @@
+error[E0381]: used binding `x` isn't initialized on any path leading to this point
+  --> $DIR/placemention-mir-move-elim.rs:3:13
+   |
+LL |     let x: String;
+   |         - binding declared here but left uninitialized
+LL |     let _ = x;
+   |             ^ `x` used here but it isn't initialized on any path leading to this point
+   |
+help: consider assigning a value
+   |
+LL |     let x: String = Default::default();
+   |                   ++++++++++++++++++++
+
+error[E0382]: use of moved value: `x`
+  --> $DIR/placemention-mir-move-elim.rs:7:13
+   |
+LL |     let x: String;
+   |         - move occurs because `x` has type `String`, which does not implement the `Copy` trait
+...
+LL |     drop(x);
+   |          - value moved here
+LL |     let _ = 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 on any path leading to this point
+  --> $DIR/placemention-mir-move-elim.rs:10:13
+   |
+LL |     let x: (String,);
+   |         - binding declared here but left uninitialized
+LL |     let _ = x;
+   |             ^ `x` used here but it isn't initialized on any path leading to this point
+   |
+help: consider assigning a value
+   |
+LL |     let x: (String,) = (Default::default(),);
+   |                      +++++++++++++++++++++++
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0381, E0382.
+For more information about an error, try `rustc --explain E0381`.

tests/ui/pattern/uninit-trivial.stderr

@@ -1,10 +1,10 @@
 error[E0381]: used binding `x` isn't initialized
-  --> $DIR/uninit-trivial.rs:6:10
+  --> $DIR/uninit-trivial.rs:6:19
    |
 LL |     let x;
    |         - binding declared here but left uninitialized
 LL |     let (0 | _) = x;
-   |          ^^^^^ `x` used here but it isn't initialized
+   |                   ^ `x` used here but it isn't initialized
    |
 help: consider assigning a value
    |

tests/ui/rfcs/rfc-0000-never_patterns/check_place_is_initialized.stderr

@@ -1,10 +1,10 @@
 error[E0381]: used binding `x` isn't initialized
-  --> $DIR/check_place_is_initialized.rs:10:15
+  --> $DIR/check_place_is_initialized.rs:10:11
    |
 LL |     let x: Void;
    |         - binding declared here but left uninitialized
 LL |     match x { ! }
-   |               ^ `x` used here but it isn't initialized
+   |           ^ `x` used here but it isn't initialized
    |
 help: consider assigning a value
    |

tests/ui/rfcs/rfc-2294-if-let-guard/move-guard-if-let-chain.edition2021.stderr

@@ -1,43 +1,45 @@
 error[E0382]: use of moved value: `x`
-  --> $DIR/move-guard-if-let-chain.rs:14:23
+  --> $DIR/move-guard-if-let-chain.rs:14:27
    |
 LL |     let x: Box<_> = Box::new(1);
    |         - move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
 ...
 LL |         (1, 2) if let y = x && c => (),
    |                       - value moved here
 LL |         (1, 2) if let z = x => (),
-   |                       ^ value used here after move
+   |                           ^ value used here after move
    |
 help: borrow this binding in the pattern to avoid moving the value
    |
 LL |         (1, 2) if let ref y = x && c => (),
    |                       +++
 
 error[E0382]: use of moved value: `x`
-  --> $DIR/move-guard-if-let-chain.rs:38:23
+  --> $DIR/move-guard-if-let-chain.rs:38:27
    |
 LL |     let x: Box<_> = Box::new(1);
    |         - move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
 ...
 LL |         (1, _) if let y = x && c => (),
    |                       - value moved here
 LL |         (_, 2) if let z = x => (),
-   |                       ^ value used here after move
+   |                           ^ value used here after move
    |
 help: borrow this binding in the pattern to avoid moving the value
    |
 LL |         (1, _) if let ref y = x && c => (),
    |                       +++
 
 error[E0382]: use of moved value: `x`
-  --> $DIR/move-guard-if-let-chain.rs:61:32
+  --> $DIR/move-guard-if-let-chain.rs:61:36
    |
 LL |     let x: Box<_> = Box::new(1);
    |         - move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
 ...
 LL |         (1, _) | (_, 2) if let y = x && c => (),
-   |                                ^ value used here after move
+   |                                -   ^ value used here after move
+   |                                |
+   |                                value moved here
    |
 help: borrow this binding in the pattern to avoid moving the value
    |
@@ -61,15 +63,13 @@ LL |         (1, 2) if let ref y = x && c => false,
    |                       +++
 
 error[E0382]: use of moved value: `x`
-  --> $DIR/move-guard-if-let-chain.rs:105:41
+  --> $DIR/move-guard-if-let-chain.rs:105:45
    |
 LL |     let x: Box<_> = Box::new(1);
    |         - move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
 ...
 LL |         (1, 2) if let y = x && c && let z = x => false,
-   |                       -                 ^ value used here after move
-   |                       |
-   |                       value moved here
+   |                       - value moved here    ^ value used here after move
    |
 help: borrow this binding in the pattern to avoid moving the value
    |