Make must_not_suspend lint see through references when drop tracking is enabled

compiler/rustc_typeck/src/check/generator_interior.rs

@@ -457,7 +457,7 @@ impl<'a, 'tcx> Visitor<'tcx> for InteriorVisitor<'a, 'tcx> {
 }
 
 #[derive(Default)]
-pub struct SuspendCheckData<'a, 'tcx> {
+struct SuspendCheckData<'a, 'tcx> {
     expr: Option<&'tcx Expr<'tcx>>,
     source_span: Span,
     yield_span: Span,
@@ -472,7 +472,7 @@ pub struct SuspendCheckData<'a, 'tcx> {
 //
 // Note that this technique was chosen over things like a `Suspend` marker trait
 // as it is simpler and has precedent in the compiler
-pub fn check_must_not_suspend_ty<'tcx>(
+fn check_must_not_suspend_ty<'tcx>(
     fcx: &FnCtxt<'_, 'tcx>,
     ty: Ty<'tcx>,
     hir_id: HirId,
@@ -489,6 +489,8 @@ pub fn check_must_not_suspend_ty<'tcx>(
 
     let plural_suffix = pluralize!(data.plural_len);
 
+    debug!("Checking must_not_suspend for {}", ty);
+
     match *ty.kind() {
         ty::Adt(..) if ty.is_box() => {
             let boxed_ty = ty.boxed_ty();
@@ -580,6 +582,12 @@ pub fn check_must_not_suspend_ty<'tcx>(
                 },
             )
         }
+        // If drop tracking is enabled, we want to look through references, since the referrent
+        // may not be considered live across the await point.
+        ty::Ref(_region, ty, _mutability) if fcx.sess().opts.unstable_opts.drop_tracking => {
+            let descr_pre = &format!("{}reference{} to ", data.descr_pre, plural_suffix);
+            check_must_not_suspend_ty(fcx, ty, hir_id, SuspendCheckData { descr_pre, ..data })
+        }
         _ => false,
     }
 }

src/test/ui/lint/must_not_suspend/ref-drop-tracking.rs

@@ -0,0 +1,30 @@
+// edition:2018
+// compile-flags: -Zdrop-tracking
+#![feature(must_not_suspend)]
+#![deny(must_not_suspend)]
+
+#[must_not_suspend = "You gotta use Umm's, ya know?"]
+struct Umm {
+    i: i64
+}
+
+struct Bar {
+    u: Umm,
+}
+
+async fn other() {}
+
+impl Bar {
+    async fn uhoh(&mut self) {
+        let guard = &mut self.u; //~ ERROR `Umm` held across
+
+        other().await;
+
+        *guard = Umm {
+            i: 2
+        }
+    }
+}
+
+fn main() {
+}

src/test/ui/lint/must_not_suspend/ref-drop-tracking.stderr

@@ -0,0 +1,27 @@
+error: reference to `Umm` held across a suspend point, but should not be
+  --> $DIR/ref-drop-tracking.rs:19:13
+   |
+LL |         let guard = &mut self.u;
+   |             ^^^^^
+LL |
+LL |         other().await;
+   |                ------ the value is held across this suspend point
+   |
+note: the lint level is defined here
+  --> $DIR/ref-drop-tracking.rs:4:9
+   |
+LL | #![deny(must_not_suspend)]
+   |         ^^^^^^^^^^^^^^^^
+note: You gotta use Umm's, ya know?
+  --> $DIR/ref-drop-tracking.rs:19:13
+   |
+LL |         let guard = &mut self.u;
+   |             ^^^^^
+help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
+  --> $DIR/ref-drop-tracking.rs:19:13
+   |
+LL |         let guard = &mut self.u;
+   |             ^^^^^
+
+error: aborting due to previous error
+