Add wrong_covariance_unsize_coercion invalid program test

tests-extra/invalid/wrong_covariance_unsize_coercion.rs

@@ -0,0 +1,32 @@
+use std::cell::RefCell;
+use std::fmt;
+
+use self_cell::self_cell;
+
+self_cell! {
+    struct WrongVarianceExample {
+        owner: (),
+
+        #[covariant]
+        dependent: Dependent,
+    }
+}
+
+// this type is not covariant
+type Dependent<'a> = RefCell<Box<dyn fmt::Display + 'a>>;
+
+fn main() {
+    let cell = WrongVarianceExample::new((), |_| RefCell::new(Box::new("")));
+    let s = String::from("Hello World");
+
+    // borrow_dependent unsound due to incorrectly checked variance
+    *cell.borrow_dependent().borrow_mut() = Box::new(s.as_str());
+
+    // s still exists
+    cell.with_dependent(|_, d| println!("{}", d.borrow()));
+
+    drop(s);
+
+    // s is gone
+    cell.with_dependent(|_, d| println!("{}", d.borrow()));
+}

tests-extra/invalid/wrong_covariance_unsize_coercion.stderr

@@ -0,0 +1,16 @@
+error[E0623]: lifetime mismatch
+  --> $DIR/wrong_covariance_unsize_coercion.rs:6:1
+   |
+6  | / self_cell! {
+7  | |     struct WrongVarianceExample {
+8  | |         owner: (),
+9  | |
+...  |
+12 | |     }
+13 | | }
+   | | ^
+   | | |
+   | |_these two types are declared with different lifetimes...
+   |   ...but data from `x` flows into `x` here
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)