> Rust has lifetime tracking but I would be interested to know how it performs against Nimrod's GC when the ownership gets really tricky.

You can use GC or reference counting in Rust—the same that Nimrod uses. (You could use DRC like Nimrod in Rust if you wanted, but it's not my preferred form of memory management due to the cost of making it thread safe.)

I think one issue is that people have different opinions on whether "ownership gets really tricky" is a common case or an edge case. Many people with unique_ptr experience seem to assume that Rust's owned pointer is more of the same and not really applicable when ownership is even slightly complex, but this is not the case. Rust's owned pointer is vastly more powerful than unique_ptr and in my experience (and in Rust compiler and Servo browser's experience) and "ownership gets really tricky" is an edge case.