struct passed by value OK if function is not called

[PavelVozenilek]

Copied on request from #548 (comment).

Docs states:

In Zig, structs, unions, and enums with payloads cannot be passed by value to a function.

However this compiles:

const foo = struct 
{
  x : i32,
  y : i32,
};

fn bar(f : foo) {}

pub fn main() -> %void {

  var f : foo = undefined;
  //bar(f);  // <<== it stops to compile when I uncomment this
}

This feels like a bug, the offending function has to be called for error to show.

[andrewrk]

Zig has lazy analysis of top level declarations.

It's so you can do something like this:

if (is_windows) { // assume `is_windows` is compile-time known
    functionThatOnlyWorksOnWindows();
}

functionThatOnlyWorksOnWindows is only analyzed if it is called.

This also helps us compile faster; we don't waste effort on dead code.