From 5c98737715f60ce180a3b607fd1b96c709f6807d Mon Sep 17 00:00:00 2001
From: Eric Holk <ericholk@microsoft.com>
Date: Fri, 13 May 2022 16:25:22 -0700
Subject: [PATCH 1/2] Drop tracking: handle invalid assignments better

Previously this test case was crashing with an index out of bounds error
deep in the call to `needs_drop`. We avoid this by detecting clearly
invalid assignees in the `mutate` callback and ignoring these.
---
 .../drop_ranges/record_consumed_borrow.rs          |  9 +++++++++
 .../issue-73741-type-err-drop-tracking.rs          | 14 ++++++++++++++
 .../issue-73741-type-err-drop-tracking.stderr      | 11 +++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 src/test/ui/async-await/issue-73741-type-err-drop-tracking.rs
 create mode 100644 src/test/ui/async-await/issue-73741-type-err-drop-tracking.stderr

diff --git a/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs b/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
index 928daba0a7b39..b3012cc677661 100644
--- a/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
+++ b/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
@@ -180,6 +180,15 @@ impl<'tcx> expr_use_visitor::Delegate<'tcx> for ExprUseDelegate<'tcx> {
         diag_expr_id: HirId,
     ) {
         debug!("mutate {assignee_place:?}; diag_expr_id={diag_expr_id:?}");
+
+        if assignee_place.place.base == PlaceBase::Rvalue
+            && assignee_place.place.projections.len() == 0
+        {
+            // Assigning to an Rvalue is illegal unless done through a dereference. We would have
+            // already gotten a type error, so we will just return here.
+            return;
+        }
+
         // 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) {
diff --git a/src/test/ui/async-await/issue-73741-type-err-drop-tracking.rs b/src/test/ui/async-await/issue-73741-type-err-drop-tracking.rs
new file mode 100644
index 0000000000000..c3423ad629f16
--- /dev/null
+++ b/src/test/ui/async-await/issue-73741-type-err-drop-tracking.rs
@@ -0,0 +1,14 @@
+// edition:2018
+// compile-flags: -Zdrop-tracking
+// Regression test for issue #73741
+// Ensures that we don't emit spurious errors when
+// a type error ocurrs in an `async fn`
+
+async fn weird() {
+    1 = 2; //~ ERROR invalid left-hand side
+
+    let mut loop_count = 0;
+    async {}.await
+}
+
+fn main() {}
diff --git a/src/test/ui/async-await/issue-73741-type-err-drop-tracking.stderr b/src/test/ui/async-await/issue-73741-type-err-drop-tracking.stderr
new file mode 100644
index 0000000000000..d4e3b6c3bf40d
--- /dev/null
+++ b/src/test/ui/async-await/issue-73741-type-err-drop-tracking.stderr
@@ -0,0 +1,11 @@
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/issue-73741-type-err-drop-tracking.rs:8:7
+   |
+LL |     1 = 2;
+   |     - ^
+   |     |
+   |     cannot assign to this expression
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0070`.

From 6665a4328b4076285e4c233995b5a08aeff4b603 Mon Sep 17 00:00:00 2001
From: Eric Holk <ericholk@microsoft.com>
Date: Fri, 13 May 2022 19:32:53 -0700
Subject: [PATCH 2/2] Fix nit

---
 .../generator_interior/drop_ranges/record_consumed_borrow.rs    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs b/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
index b3012cc677661..e89a896199615 100644
--- a/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
+++ b/compiler/rustc_typeck/src/check/generator_interior/drop_ranges/record_consumed_borrow.rs
@@ -182,7 +182,7 @@ impl<'tcx> expr_use_visitor::Delegate<'tcx> for ExprUseDelegate<'tcx> {
         debug!("mutate {assignee_place:?}; diag_expr_id={diag_expr_id:?}");
 
         if assignee_place.place.base == PlaceBase::Rvalue
-            && assignee_place.place.projections.len() == 0
+            && assignee_place.place.projections.is_empty()
         {
             // Assigning to an Rvalue is illegal unless done through a dereference. We would have
             // already gotten a type error, so we will just return here.