I have added that now.

I originally didn't do this, because I didn't feel the need to invoke UB without necessity. Also I wanted to demonstrate, that the error can (and is) caught in C, without actually committing the double-free (so that the program behaviour is still well-defined, but the abstraction is already violated).

I'm not familiar with Splint, but my understanding is that the error is because of the @only@ marker indicating that the string is not shared. That's an incorrect translation of the Rust program, where the reference to the string is explicitly a shared reference rather than an owned reference. The issue isn't the sharing; it's that rustc mistakenly allows the code to assign a reference that is valid until the end of the current scope to a field that expects a reference valid forever (like a string constant in the program binary). The Rust program with equivalent semantics to your C program (passing an owned String instead of a shared reference) is correctly rejected by the compiler.

Also, the Q_free function should not free q->t. It may access it (e.g. printing it), but it does not own that reference and is not responsible for deallocating its memory.

Sorry, I should have looked closer the first time! My excuse is that it was too early in the morning to be thinking about such things.