Add test for issue #52240

Closes #52240

2 files changed, 25 insertions, 0 deletions

diff --git a/src/test/ui/issues/issue-52240.rs b/src/test/ui/issues/issue-52240.rs
new file mode 100644
index 00000000000..9ac7e9905da
--- /dev/null
+++ b/src/test/ui/issues/issue-52240.rs
@@ -0,0 +1,16 @@
+// issue-52240: Can turn immutable into mut with `ref mut`
+
+enum Foo {
+    Bar(i32),
+}
+
+fn main() {
+    let arr = vec!(Foo::Bar(0));
+    if let (Some(Foo::Bar(ref mut val)), _) = (&arr.get(0), 0) {
+        //~^ ERROR cannot borrow field of immutable binding as mutable
+        *val = 9001;
+    }
+    match arr[0] {
+        Foo::Bar(ref s) => println!("{}", s)
+    }
+}
diff --git a/src/test/ui/issues/issue-52240.stderr b/src/test/ui/issues/issue-52240.stderr
new file mode 100644
index 00000000000..c2c2524816d
--- /dev/null
+++ b/src/test/ui/issues/issue-52240.stderr
@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow field of immutable binding as mutable
+  --> $DIR/issue-52240.rs:9:27
+   |
+LL |     if let (Some(Foo::Bar(ref mut val)), _) = (&arr.get(0), 0) {
+   |                           ^^^^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.