message: Document AnyLifetime is unsound
  Summary: Using dtolnay example from [workplace](https://fb.workplace.com/groups/buck2eng/posts/2988664691431011).
diff: @@ -327,4 +327,15 @@ mod tests {
  #[derive(ProvidesStaticType)]
  struct FooBar<'x, P: My<'x>>(&'x P);
  }
  +
  + #[test]
  + fn test_any_lifetime_unsound() {
  + fn _use_after_free(s: &mut &str) {
  + let ss = (s as &mut dyn AnyLifetime).downcast_mut().unwrap();
  + // We allocate a string here, but do not store it anywhere, so it is dropped.
  + // But the pointer to it is available in `s` at call site.
  + // This is use after free.
  + *ss = &".".repeat(40);
  + }
  + }
  }
  