1 files changed, 56 insertions, 0 deletions
diff --git a/tests/ui/borrowck/borrow-immutable-upvar-mutation.rs b/tests/ui/borrowck/borrow-immutable-upvar-mutation.rs
new file mode 100644
index 000000000..a3350024e
--- /dev/null
+++ b/tests/ui/borrowck/borrow-immutable-upvar-mutation.rs
@@ -0,0 +1,56 @@
+#![feature(unboxed_closures, tuple_trait)]
+
+// Tests that we can't assign to or mutably borrow upvars from `Fn`
+// closures (issue #17780)
+
+fn set(x: &mut usize) {
+    *x = 5;
+}
+
+fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+    f
+}
+fn to_fn_mut<A: std::marker::Tuple, F: FnMut<A>>(f: F) -> F {
+    f
+}
+
+fn main() {
+    // By-ref captures
+    {
+        let mut x = 0;
+        let _f = to_fn(|| x = 42); //~ ERROR cannot assign
+
+        let mut y = 0;
+        let _g = to_fn(|| set(&mut y)); //~ ERROR cannot borrow
+
+        let mut z = 0;
+        let _h = to_fn_mut(|| {
+            set(&mut z);
+            to_fn(|| z = 42); //~ ERROR cannot assign
+        });
+    }
+
+    // By-value captures
+    {
+        let mut x = 0;
+        let _f = to_fn(move || x = 42); //~ ERROR cannot assign
+
+        let mut y = 0;
+        let _g = to_fn(move || set(&mut y)); //~ ERROR cannot borrow
+
+        let mut z = 0;
+        let _h = to_fn_mut(move || {
+            set(&mut z);
+            to_fn(move || z = 42);
+            //~^ ERROR cannot assign
+        });
+    }
+}
+
+fn foo() -> Box<dyn Fn() -> usize> {
+    let mut x = 0;
+    Box::new(move || {
+        x += 1; //~ ERROR cannot assign
+        x
+    })
+}