1 files changed, 32 insertions, 0 deletions
diff --git a/tests/ui/fmt/debug-single-call.rs b/tests/ui/fmt/debug-single-call.rs
new file mode 100644
index 00000000000..b59a766c71a
--- /dev/null
+++ b/tests/ui/fmt/debug-single-call.rs
@@ -0,0 +1,32 @@
+//! Test that Debug::fmt is called exactly once during formatting.
+//!
+//! This is a regression test for PR https://github.com/rust-lang/rust/pull/10715
+
+//@ run-pass
+//@ needs-threads
+
+use std::cell::Cell;
+use std::{fmt, thread};
+
+struct Foo(Cell<isize>);
+
+impl fmt::Debug for Foo {
+    fn fmt(&self, _fmt: &mut fmt::Formatter) -> fmt::Result {
+        let Foo(ref f) = *self;
+        assert_eq!(f.get(), 0);
+        f.set(1);
+        Ok(())
+    }
+}
+
+pub fn main() {
+    thread::spawn(move || {
+        let mut f = Foo(Cell::new(0));
+        println!("{:?}", f);
+        let Foo(ref mut f) = f;
+        assert_eq!(f.get(), 1);
+    })
+    .join()
+    .ok()
+    .unwrap();
+}