Consider mutations as borrows in drop tracking

This is needed to match MIR more conservative approximation of any
borrowed value being live across a suspend point (See #94067). This
change considers an expression such as `x.y = z` to be a borrow of `x`
and therefore keeps `x` live across suspend points.

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

@@ -93,19 +93,25 @@ impl<'tcx> expr_use_visitor::Delegate<'tcx> for ExprUseDelegate<'tcx> {
     fn borrow(
         &mut self,
         place_with_id: &expr_use_visitor::PlaceWithHirId<'tcx>,
-        _diag_expr_id: HirId,
+        diag_expr_id: HirId,
         _bk: rustc_middle::ty::BorrowKind,
     ) {
+        debug!("borrow {:?}; diag_expr_id={:?}", place_with_id, diag_expr_id);
         self.places
             .borrowed
             .insert(TrackedValue::from_place_with_projections_allowed(place_with_id));
     }
 
     fn mutate(
         &mut self,
-        _assignee_place: &expr_use_visitor::PlaceWithHirId<'tcx>,
-        _diag_expr_id: HirId,
+        assignee_place: &expr_use_visitor::PlaceWithHirId<'tcx>,
+        diag_expr_id: HirId,
     ) {
+        debug!("mutate {:?}; diag_expr_id={:?}", assignee_place, diag_expr_id);
+        // Count mutations as a borrow.
+        self.places
+            .borrowed
+            .insert(TrackedValue::from_place_with_projections_allowed(assignee_place));
     }
 
     fn fake_read(

src/test/ui/async-await/drop-track-field-assign-nonsend.rs

@@ -0,0 +1,45 @@
+// Derived from an ICE found in tokio-xmpp during a crater run.
+// edition:2021
+// compile-flags: -Zdrop-tracking
+
+#![allow(dead_code)]
+
+#[derive(Clone)]
+struct InfoResult {
+    node: Option<std::rc::Rc<String>>
+}
+
+struct Agent {
+    info_result: InfoResult
+}
+
+impl Agent {
+    async fn handle(&mut self) {
+        let mut info = self.info_result.clone();
+        info.node = None;
+        let element = parse_info(info);
+        let _ = send_element(element).await;
+    }
+}
+
+struct Element {
+}
+
+async fn send_element(_: Element) {}
+
+fn parse(_: &[u8]) -> Result<(), ()> {
+    Ok(())
+}
+
+fn parse_info(_: InfoResult) -> Element {
+    Element { }
+}
+
+fn assert_send<T: Send>(_: T) {}
+
+fn main() {
+    let agent = Agent { info_result: InfoResult { node: None } };
+    // FIXME: It would be nice for this to work. See #94067.
+    assert_send(agent.handle());
+    //~^ cannot be sent between threads safely
+}

src/test/ui/async-await/drop-track-field-assign-nonsend.stderr

@@ -0,0 +1,25 @@
+error: future cannot be sent between threads safely
+  --> $DIR/drop-track-field-assign-nonsend.rs:43:17
+   |
+LL |     assert_send(agent.handle());
+   |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
+   |
+   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/drop-track-field-assign-nonsend.rs:21:38
+   |
+LL |         let mut info = self.info_result.clone();
+   |             -------- has type `InfoResult` which is not `Send`
+...
+LL |         let _ = send_element(element).await;
+   |                                      ^^^^^^ await occurs here, with `mut info` maybe used later
+LL |     }
+   |     - `mut info` is later dropped here
+note: required by a bound in `assert_send`
+  --> $DIR/drop-track-field-assign-nonsend.rs:38:19
+   |
+LL | fn assert_send<T: Send>(_: T) {}
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: aborting due to previous error
+

src/test/ui/async-await/drop-track-field-assign.rs

@@ -0,0 +1,44 @@
+// Derived from an ICE found in tokio-xmpp during a crater run.
+// edition:2021
+// compile-flags: -Zdrop-tracking
+// build-pass
+
+#![allow(dead_code)]
+
+#[derive(Clone)]
+struct InfoResult {
+    node: Option<String>
+}
+
+struct Agent {
+    info_result: InfoResult
+}
+
+impl Agent {
+    async fn handle(&mut self) {
+        let mut info = self.info_result.clone();
+        info.node = Some("bar".into());
+        let element = parse_info(info);
+        let _ = send_element(element).await;
+    }
+}
+
+struct Element {
+}
+
+async fn send_element(_: Element) {}
+
+fn parse(_: &[u8]) -> Result<(), ()> {
+    Ok(())
+}
+
+fn parse_info(_: InfoResult) -> Element {
+    Element { }
+}
+
+fn main() {
+    let mut agent = Agent {
+        info_result: InfoResult { node: None }
+    };
+    let _ = agent.handle();
+}