Functions should be able to initialize their arguments

[eholk]

This came up in the course of writing comms as a library. I want to be able to write this:

fn recv_into(v : &T) {
    rustrt::port_recv(unsafe::reinterpret_cast(ptr::addr_of(v)),
                      **raw_port);
}

fn recv() -> T {
    let x : T;
    self.recv_into(x);
    ret x;
}

recv_into has the form it does because it calls into the runtime library, which needs a pointer to the place where the data should go. Ideally, we won't always want to initialize this, and in fact in some cases we may not be able to (for example, a tag type is exported from a module, but not its constructor).

What would be nice is some time of typestate annotation that says a parameter may not be initialized (or perhaps even stronger, is not initialized), and then declare as a postcondition that its argument is initialized.

I'm not sure what ramifications this might have as far as safety and typestate analysis, but this pattern is relatively common in a lot of systems code I've seen.

[graydon]

See also #1487. This will take some work but it seems useful in a few places.

[eholk]

Now that typestate is gone, is this issue still relevant?

It seems like it might be related to moving out of closures.

[catamorphism]

It's still relevant -- liveness analysis does the same analysis that typestate did before.

[catamorphism]

No activity for a long time; it seems like people aren't crying out for this feature. Rust has changed enough since this was filed that I suspect there's a way to address it now, though I don't know offhand.

Closing.