fn uncompressStream: clean up state machine mess

[folkertdev]

c2rust really made a mess of things here, so it's probably best to match this versus the original C code.

My theory is that c2rust doesn't use the information about functions/branches diverging in the relooper algorithm, though in theory it does know that functions diverge (it gives then the ! return type). Idk though, it could be something else.

Incidentally, this function would be a great candidate for rust-lang/rfcs#3720, the logic can be expressed as nicely as a state machine. I had to initialize the bzf in my cleanup, but that would not be required using loop match with const continue _

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[bjorn3]

Would it be possible to structure the code something like:

'outer: {
    loop {
        // ...
        if should_goto_trycat {
            state = TryCat;
            break 'outer;
        }
        if should_goto_errhandler {
            state = ErrHandler;
            break 'outer;
        }
    }
}

'state: loop {
    match state {
        // ...
    }
}

given that the Standard state is never re-entered. Or does that make the code less clear?

[folkertdev]

for now I went with a correct transformation. There is a bunch of stuff in there that still needs further cleanup (e.g. casts) and I suspect we can play around with this control flow a bit too.