pub unsafe fn increment_strong_count(ptr: *const T) {
        // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
        let rc = unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) };
        // Now increase refcount, but don't drop new refcount either
        let _rc_clone: mem::ManuallyDrop<_> = rc.clone();
    }
Returns a mutable reference into the given `Rc`,
without any check.

See also [`get_mut`], which is
benchmarks/haystacks/code/rust-library.rs