Suggest to add move keyword for generator capture

src/librustc_mir/borrow_check/conflict_errors.rs

@@ -750,6 +750,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let kind_place = kind.filter(|_| place_desc.is_some()).map(|k| (k, place_span.0));
         let explanation = self.explain_why_borrow_contains_point(location, &borrow, kind_place);
 
+        debug!(
+            "report_borrowed_value_does_not_live_long_enough(place_desc: {:?}, explanation: {:?})",
+            place_desc,
+            explanation
+        );
         let err = match (place_desc, explanation) {
             (Some(_), _) if self.is_place_thread_local(root_place) => {
                 self.report_thread_local_value_does_not_live_long_enough(drop_span, borrow_span)
@@ -790,6 +795,24 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 span,
                 &format!("`{}`", name),
             ),
+            (
+                Some(ref name),
+                BorrowExplanation::MustBeValidFor {
+                    category: category @ ConstraintCategory::OpaqueType,
+                    from_closure: false,
+                    ref region_name,
+                    span,
+                    ..
+                },
+
+            ) if borrow_spans.for_generator() => self.report_escaping_closure_capture(
+                borrow_spans.args_or_use(),
+                borrow_span,
+                region_name,
+                category,
+                span,
+                &format!("`{}`", name),
+            ),
             (
                 ref name,
                 BorrowExplanation::MustBeValidFor {
@@ -1214,6 +1237,9 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             ConstraintCategory::Return => {
                 err.span_note(constraint_span, "closure is returned here");
             }
+            ConstraintCategory::OpaqueType => {
+                err.span_note(constraint_span, "generator is returned here");
+            }
             ConstraintCategory::CallArgument => {
                 fr_name.highlight_region_name(&mut err);
                 err.span_note(

src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -17,6 +17,7 @@ use syntax_pos::Span;
 
 mod find_use;
 
+#[derive(Debug)]
 pub(in crate::borrow_check) enum BorrowExplanation {
     UsedLater(LaterUseKind, Span),
     UsedLaterInLoop(LaterUseKind, Span),
@@ -35,7 +36,7 @@ pub(in crate::borrow_check) enum BorrowExplanation {
     Unexplained,
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 pub(in crate::borrow_check) enum LaterUseKind {
     TraitCapture,
     ClosureCapture,

src/test/ui/async-await/async-borrowck-escaping-block-error.fixed

@@ -0,0 +1,12 @@
+// edition:2018
+// run-rustfix
+
+fn foo() -> Box<impl std::future::Future<Output = u32>> {
+    let x = 0u32;
+    Box::new(async move { x } )
+    //~^ ERROR E0373
+}
+
+fn main() {
+    let _foo = foo();
+}

src/test/ui/async-await/async-borrowck-escaping-block-error.rs

@@ -0,0 +1,12 @@
+// edition:2018
+// run-rustfix
+
+fn foo() -> Box<impl std::future::Future<Output = u32>> {
+    let x = 0u32;
+    Box::new(async { x } )
+    //~^ ERROR E0373
+}
+
+fn main() {
+    let _foo = foo();
+}

src/test/ui/async-await/async-borrowck-escaping-block-error.stderr

@@ -0,0 +1,22 @@
+error[E0373]: closure may outlive the current function, but it borrows `x`, which is owned by the current function
+  --> $DIR/async-borrowck-escaping-block-error.rs:6:20
+   |
+LL |     Box::new(async { x } )
+   |                    ^^-^^
+   |                    | |
+   |                    | `x` is borrowed here
+   |                    may outlive borrowed value `x`
+   |
+note: generator is returned here
+  --> $DIR/async-borrowck-escaping-block-error.rs:4:13
+   |
+LL | fn foo() -> Box<impl std::future::Future<Output = u32>> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: to force the closure to take ownership of `x` (and any other referenced variables), use the `move` keyword
+   |
+LL |     Box::new(async move { x } )
+   |                    ^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0373`.