casting an array to slice copies constant data instead of using the pointer

[andrewrk]

test "foo" {
    const array = "hi";
    const slice: []const u8 = array;
    return bar(array, slice);
}

fn bar(args: ...) {
    const s1 = ([]const u8)(args[0]);
    const s2 = args[1];
    assert(s1.ptr == s2.ptr);
}

This behavior is problematic because you expect to be able to pass a string literal to a var args function, implicitly cast the argument to a slice, and have the pointer have the static lifetime. Instead you end up with a pointer to the stack.

I'm not quite sure how to fix this, but one idea is to automatically cast array literals to const slices for var args functions. It's kind of an arbitrary restriction though.

Maybe this is a different bug. It looks like the array is getting passed by value, and we're intending to make it an error to pass things by value with some exceptions. Maybe for var args it can implicitly cast to &const T if T is passed and T is not copyable.

[andrewrk]

The part about passing args to var args is solved, but there's another issue, which has to do with constant values. Here's a simple test:

test "aoeu" {
    const array = "hi";
    const slice: []const u8 = array;
    assert(&array[0] == slice.ptr);
}

I believe that if this fixed the other test will now pass.