This can use C-u prefix, like it is implemented in https://github.com/ayrat555/cargo-mode .
Where M-x cargo-process-test run predefined command,
but if you run C-u M-x cargo-process-test then it is allowed you to edit predefined command and add wherever you want, like -- --show-output or -- --no-capture.

Thanks! How about modifying rust-test to accept universal argument instead and allow it configure arbitrary argument instead ?

This would allow us to accept any arbitrary options to be passed instead of this specific usecase. You can look into rustic mode for inspiration: https://github.com/emacs-rustic/rustic?tab=readme-ov-file#test

Thank you, will definitely look into this suggestion when I get chance.

Updated with the suggested change. Feel free to merge unless you want more changes, in which case let me know and I'll take a look.

Hold on, I seem to have misread the suggestion, I'm going to convert to a draft while I figure this out.

Sorry for the wait on this, should get to it soon :]

I wanted to add an empty list of arguments that would be added to whenever you passed a C-u prefix arg (and persisted across sessions), but I couldn't get it to work- so I just had to settle with this. You can pass prefix with C-u, and if you want access to previously inputted args you'll have to use the built-in minibuffer history (should be accessible via the up and down keys).

I'm going to squash the commits into one then it should be ready to merge.

Sorry about that "Merge branch 'rust-lang:master' into master" commit, I didn't think it would do that (never mind, managed to fix it!). Anyway, this should be ready to merge now.

@psibi Sorry for the ping, just wanted to ask if this is good to merge? Or if there's anything else you want added, let me know and I'll take a look.