allow for native mod functions to initialize records

[erickt]

Some C libraries want you to create a structure on the stack and pass it off to be initialized by some function. Right now the typestate requires all structures to be fully defined before being passed to a function, so I'm finding myself having to write:

type foo_t = { p: *ctypes::void, i: ctypes::c_int };

native mod foo {
    fn f(foo: foo_t);
}

fn bar() {
    let x = { p: ptr::null(), i: 0 as ctypes::c_int };
    foo::f(x);
    ...
}

Since I'm passing raw pointers to c, it's naturally unsafe, so it'd be nice if we could allow foo_t to be opaque to the typestate system. What if we allow native module functions to have a flag saying they can initialize an uninitialized value? It'd help simplify binding to third party libraries.

[graydon]

I don't think this is invasive enough to warrant an RFC, un-tagging.

It's not a bad idea, and I don't object; we'd just need support for function postconditions (#586) and a way to denote the init predicate. Certainly an unsafe operation.

[catamorphism]

@graydon : now that we don't have typestate, can liveness do this or is it too hard now?

[yichoi]

Rust does not support record anymore

[catamorphism]

It's still relevant with structs, but I think we have a good enough solution for this (passing in a reference to a zeroed-out struct; for example, as with libc::stat). Closing.