Distinguish binding assignments, use Ty::needs_drop

This better captures the actual behavior, rather than using hacks around
whether the assignment has any projections.

Author: eholk

compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs

@@ -6,14 +6,14 @@ use crate::{
 use hir::{def_id::DefId, Body, HirId, HirIdMap};
 use rustc_data_structures::stable_set::FxHashSet;
 use rustc_hir as hir;
-use rustc_middle::hir::map::Map;
+use rustc_middle::ty::{ParamEnv, TyCtxt};
 
 pub(super) fn find_consumed_and_borrowed<'a, 'tcx>(
     fcx: &'a FnCtxt<'a, 'tcx>,
     def_id: DefId,
     body: &'tcx Body<'tcx>,
 ) -> ConsumedAndBorrowedPlaces {
-    let mut expr_use_visitor = ExprUseDelegate::new(fcx.tcx.hir());
+    let mut expr_use_visitor = ExprUseDelegate::new(fcx.tcx, fcx.param_env);
     expr_use_visitor.consume_body(fcx, def_id, body);
     expr_use_visitor.places
 }
@@ -36,14 +36,16 @@ pub(super) struct ConsumedAndBorrowedPlaces {
 /// Interesting values are those that are either dropped or borrowed. For dropped values, we also
 /// record the parent expression, which is the point where the drop actually takes place.
 struct ExprUseDelegate<'tcx> {
-    hir: Map<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
     places: ConsumedAndBorrowedPlaces,
 }
 
 impl<'tcx> ExprUseDelegate<'tcx> {
-    fn new(hir: Map<'tcx>) -> Self {
+    fn new(tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> Self {
         Self {
-            hir,
+            tcx,
+            param_env,
             places: ConsumedAndBorrowedPlaces {
                 consumed: <_>::default(),
                 borrowed: <_>::default(),
@@ -77,7 +79,7 @@ impl<'tcx> expr_use_visitor::Delegate<'tcx> for ExprUseDelegate<'tcx> {
         place_with_id: &expr_use_visitor::PlaceWithHirId<'tcx>,
         diag_expr_id: HirId,
     ) {
-        let parent = match self.hir.find_parent_node(place_with_id.hir_id) {
+        let parent = match self.tcx.hir().find_parent_node(place_with_id.hir_id) {
             Some(parent) => parent,
             None => place_with_id.hir_id,
         };
@@ -108,20 +110,23 @@ impl<'tcx> expr_use_visitor::Delegate<'tcx> for ExprUseDelegate<'tcx> {
         diag_expr_id: HirId,
     ) {
         debug!("mutate {assignee_place:?}; diag_expr_id={diag_expr_id:?}");
-        // Count mutations as a borrow when done through a projection.
-        //
-        // The goal here is to catch cases such as `x.y = 42`, since MIR will count this
-        // as a borrow of `x`, and we need to match that behavior.
-        //
-        // FIXME(eholk): this is probably still more conservative than we need to be. For example,
-        // we may need to count `*x = 42` as a borrow of `x`, since it overwrites all of `x`.
-        if !assignee_place.place.projections.is_empty() {
+        // If the type being assigned needs dropped, then the mutation counts as a borrow
+        // since it is essentially doing `Drop::drop(&mut x); x = new_value;`.
+        if assignee_place.place.base_ty.needs_drop(self.tcx, self.param_env) {
             self.places
                 .borrowed
                 .insert(TrackedValue::from_place_with_projections_allowed(assignee_place));
         }
     }
 
+    fn bind(
+        &mut self,
+        binding_place: &expr_use_visitor::PlaceWithHirId<'tcx>,
+        diag_expr_id: HirId,
+    ) {
+        debug!("bind {binding_place:?}; diag_expr_id={diag_expr_id:?}");
+    }
+
     fn fake_read(
         &mut self,
         _place: expr_use_visitor::Place<'tcx>,

compiler/rustc_typeck/src/expr_use_visitor.rs

@@ -51,6 +51,15 @@ pub trait Delegate<'tcx> {
     /// `diag_expr_id` is the id used for diagnostics (see `consume` for more details).
     fn mutate(&mut self, assignee_place: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId);
 
+    /// The path at `binding_place` is a binding that is being initialized.
+    ///
+    /// This covers cases such as `let x = 42;`
+    fn bind(&mut self, binding_place: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId) {
+        // Bindings can normally be treated as a regular assignment, so by default we
+        // forward this to the mutate callback.
+        self.mutate(binding_place, diag_expr_id)
+    }
+
     /// The `place` should be a fake read because of specified `cause`.
     fn fake_read(&mut self, place: Place<'tcx>, cause: FakeReadCause, diag_expr_id: hir::HirId);
 }
@@ -648,11 +657,9 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                     let pat_ty = return_if_err!(mc.node_ty(pat.hir_id));
                     debug!("walk_pat: pat_ty={:?}", pat_ty);
 
-                    // Each match binding is effectively an assignment to the
-                    // binding being produced.
                     let def = Res::Local(canonical_id);
                     if let Ok(ref binding_place) = mc.cat_res(pat.hir_id, pat.span, pat_ty, def) {
-                        delegate.mutate(binding_place, binding_place.hir_id);
+                        delegate.bind(binding_place, binding_place.hir_id);
                     }
 
                     // It is also a borrow or copy/move of the value being matched.

src/test/ui/async-await/drop-and-assign.rs

@@ -0,0 +1,19 @@
+// edition:2021
+// compile-flags: -Zdrop-tracking
+// build-pass
+
+struct A;
+impl Drop for A { fn drop(&mut self) {} }
+
+pub async fn f() {
+    let mut a = A;
+    a = A;
+    drop(a);
+    async {}.await;
+}
+
+fn assert_send<T: Send>(_: T) {}
+
+fn main() {
+    let _ = f();
+}