1 files changed, 43 insertions, 0 deletions
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-recursive-fn-using-fn-mut.rs b/src/test/ui/unboxed-closures/unboxed-closures-recursive-fn-using-fn-mut.rs
new file mode 100644
index 000000000..5e354cb6f
--- /dev/null
+++ b/src/test/ui/unboxed-closures/unboxed-closures-recursive-fn-using-fn-mut.rs
@@ -0,0 +1,43 @@
+#![feature(fn_traits, unboxed_closures)]
+
+use std::marker::PhantomData;
+
+// An erroneous variant of `run-pass/unboxed_closures-infer-recursive-fn.rs`
+// where we attempt to perform mutation in the recursive function. This fails to compile
+// because it winds up requiring `FnMut` which enforces linearity.
+
+struct YCombinator<F,A,R> {
+    func: F,
+    marker: PhantomData<(A,R)>,
+}
+
+impl<F,A,R> YCombinator<F,A,R> {
+    fn new(f: F) -> YCombinator<F,A,R> {
+        YCombinator { func: f, marker: PhantomData }
+    }
+}
+
+impl<A,R,F : FnMut(&mut dyn FnMut(A) -> R, A) -> R> FnMut<(A,)> for YCombinator<F,A,R> {
+    extern "rust-call" fn call_mut(&mut self, (arg,): (A,)) -> R {
+        (self.func)(self, arg)
+            //~^ ERROR cannot borrow `*self` as mutable more than once at a time
+    }
+}
+
+impl<A,R,F : FnMut(&mut dyn FnMut(A) -> R, A) -> R> FnOnce<(A,)> for YCombinator<F,A,R> {
+    type Output = R;
+    extern "rust-call" fn call_once(mut self, args: (A,)) -> R {
+        self.call_mut(args)
+    }
+}
+
+fn main() {
+    let mut counter = 0;
+    let factorial = |recur: &mut dyn FnMut(u32) -> u32, arg: u32| -> u32 {
+        counter += 1;
+        if arg == 0 {1} else {arg * recur(arg-1)}
+    };
+    let mut factorial: YCombinator<_,u32,u32> = YCombinator::new(factorial);
+    let mut r = factorial(10);
+    assert_eq!(3628800, r);
+}