Use the entire type of a dropped local to compute variance (edge direction) for Polonius alpha

compiler/rustc_borrowck/src/polonius/constraints.rs

@@ -5,6 +5,7 @@ use rustc_index::interval::SparseIntervalMatrix;
 use rustc_middle::mir::{Body, Location};
 use rustc_middle::ty::RegionVid;
 use rustc_mir_dataflow::points::PointIndex;
+use tracing::debug;
 
 use crate::BorrowSet;
 use crate::constraints::OutlivesConstraint;
@@ -255,6 +256,7 @@ fn compute_forward_successor(
 
     // 2. Otherwise, gather the edges due to explicit region liveness, when applicable.
     if !live_regions.contains(region, next_point) {
+        debug!("Region {region:?} isn't live at successor {next_point:?}; traversal stops.");
         return None;
     }
 
@@ -275,6 +277,7 @@ fn compute_forward_successor(
         ConstraintDirection::Backward => {
             // Contravariant cases: loans flow in the inverse direction, but we're only interested
             // in forward successors and there are none here.
+            debug!("Constraint direction is backwards; {region:?} has no forward successors");
             None
         }
         ConstraintDirection::Forward | ConstraintDirection::Bidirectional => {
@@ -299,6 +302,9 @@ fn compute_backward_successor(
     // Liveness flows into the regions live at the next point. So, in a backwards view, we'll link
     // the region from the current point, if it's live there, to the previous point.
     if !live_regions.contains(region, current_point) {
+        debug!(
+            "Backwards successor: {region:?} not live at current point {current_point:?}; bailing out!"
+        );
         return None;
     }
 

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -532,6 +532,7 @@ impl<'tcx> LivenessContext<'_, '_, 'tcx> {
     /// points `live_at`.
     fn add_use_live_facts_for(&mut self, value: Ty<'tcx>, live_at: &IntervalSet<PointIndex>) {
         debug!("add_use_live_facts_for(value={:?})", value);
+        Self::record_region_variance(self.typeck, value);
         Self::make_all_regions_live(self.location_map, self.typeck, value, live_at);
     }
 
@@ -573,6 +574,9 @@ impl<'tcx> LivenessContext<'_, '_, 'tcx> {
             }
         }
 
+        // Since the entire dropped local is live, record the variance of its regions.
+        Self::record_region_variance(self.typeck, dropped_ty);
+
         // All things in the `outlives` array may be touched by
         // the destructor and must be live at this point.
         for &kind in &drop_data.dropck_result.kinds {
@@ -587,6 +591,24 @@ impl<'tcx> LivenessContext<'_, '_, 'tcx> {
         }
     }
 
+    /// `live_kind` is the type of a  (use- or drop-) live local.
+    /// Record the variance of any region(s) appearing in it for Polonius. Does
+    /// nothing if Polonius is not active.
+    fn record_region_variance(
+        typeck: &mut TypeChecker<'_, 'tcx>,
+        live_kind: impl TypeVisitable<TyCtxt<'tcx>> + Relate<TyCtxt<'tcx>>,
+    ) {
+        // When using `-Zpolonius=next`, we record the variance of each live region.
+        if let Some(polonius_context) = typeck.polonius_context.as_mut() {
+            record_live_region_variance(
+                typeck.infcx.tcx,
+                &mut polonius_context.live_region_variances,
+                typeck.universal_regions,
+                live_kind,
+            );
+        }
+    }
+
     fn make_all_regions_live(
         location_map: &DenseLocationMap,
         typeck: &mut TypeChecker<'_, 'tcx>,
@@ -604,20 +626,10 @@ impl<'tcx> LivenessContext<'_, '_, 'tcx> {
             param_env: typeck.infcx.param_env,
             op: |r| {
                 let live_region_vid = typeck.universal_regions.to_region_vid(r);
-
                 typeck.constraints.liveness_constraints.add_points(live_region_vid, live_at);
             },
         });
-
-        // When using `-Zpolonius=next`, we record the variance of each live region.
-        if let Some(polonius_context) = typeck.polonius_context.as_mut() {
-            record_live_region_variance(
-                typeck.infcx.tcx,
-                &mut polonius_context.live_region_variances,
-                typeck.universal_regions,
-                value,
-            );
-        }
+        Self::record_region_variance(typeck, value);
     }
 }
 

tests/ui/borrowck/alias-liveness/drop-liveness-invariance-issue-160670.rs

@@ -0,0 +1,32 @@
+// From https://github.com/rust-lang/rust/issues/160670 This issue was
+// discovered when developing Polonius alpha. A live local (the one holding the
+// struct `D`) had its type be drop-live, but only partially. This had not
+// previously triggered any issues because it did not affect region liveness,
+// but it did affect Polonius' region variance computations, since the outer `D`
+// nesting was removed to obtain `fn(&'a T)`, which unlike the associated type
+// isn't invariant.
+//
+// The split declaration/assignment on lines 30--31 is load bearing; without
+// them the bug does not appear.
+
+struct D<T: HasArg>(T::Arg);
+
+trait HasArg {
+    type Arg;
+}
+impl<'a, T> HasArg for fn(&'a T) {
+    type Arg = &'a T;
+}
+impl<T: HasArg> Drop for D<T> {
+    fn drop(&mut self) {}
+}
+fn mk<'a, T>(r: &'a T) -> D<fn(&'a T)> {
+    D(r)
+}
+
+fn main() {
+    let b = Box::new(0u8);
+    let d;
+    d = mk(&*b);
+    drop(b); //~ ERROR cannot move out of `b` because it is borrowed
+}

tests/ui/borrowck/alias-liveness/drop-liveness-invariance-issue-160670.stderr

@@ -0,0 +1,22 @@
+error[E0505]: cannot move out of `b` because it is borrowed
+  --> $DIR/drop-liveness-invariance-issue-160670.rs:31:10
+   |
+LL |     let b = Box::new(0u8);
+   |         - binding `b` declared here
+LL |     let d;
+LL |     d = mk(&*b);
+   |            --- borrow of `*b` occurs here
+LL |     drop(b);
+   |          ^ move out of `b` occurs here
+LL | }
+   | - borrow might be used here, when `d` is dropped and runs the `Drop` code for type `D`
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL -     d = mk(&*b);
+LL +     d = mk(&b.clone());
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0505`.