Update to still be correct without drop tracking

Author: eholk

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,
@@ -582,7 +582,9 @@ pub fn check_must_not_suspend_ty<'tcx>(
                 },
             )
         }
-        ty::Ref(_region, ty, _mutability) => {
+        // 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.debugging_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 })
         }

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
+

src/test/ui/lint/must_not_suspend/ref.stderr

@@ -1,5 +1,5 @@
-error: reference to `Umm` held across a suspend point, but should not be
-  --> $DIR/ref.rs:18:13
+error: `Umm` held across a suspend point, but should not be
+  --> $DIR/ref.rs:18:26
    |
 LL |         let guard = &mut self.u;
    |                          ^^^^^^
@@ -13,15 +13,15 @@ note: the lint level is defined here
 LL | #![deny(must_not_suspend)]
    |         ^^^^^^^^^^^^^^^^
 note: You gotta use Umm's, ya know?
-  --> $DIR/ref.rs:18:13
+  --> $DIR/ref.rs:18:26
    |
 LL |         let guard = &mut self.u;
-   |             ^^^^^
+   |                          ^^^^^^
 help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
-  --> $DIR/ref.rs:18:13
+  --> $DIR/ref.rs:18:26
    |
 LL |         let guard = &mut self.u;
-   |             ^^^^^
+   |                          ^^^^^^
 
 error: aborting due to previous error