`with` clauses

language design
8

This desugaring doesn't feel right:

  • What are get_arg() and Arg here?
  • How did the &self parameter become &mut self? This is unsound in presence of code that calls visit while expecting it to take an &self.
// `Foo`'s definition is valid today, so it has to compile
trait Foo {
    fn foo(&self);
    fn bar(&self, f: impl Fn());
    fn baz(&self) {
        // This is possible only because both `bar` and `foo` take `&self`.
        self.bar(|| self.foo())
    }
}

impl Foo for MyNode
with(ob: &mut Option<Box<i32>>)
{
    fn foo(&self) {
        *ob = None; // Invalidates any reference to the inside of `ob`
    }
    fn bar(&self, f: impl Fn()) {
        // Let's assume that the box is valid now
        let box_ref: &Box<i32> = ob.as_ref().unwrap();
        f(); // This calls `self.foo()` which invalidates `box_ref`
        println!("{}", box_ref); // Oops, now we're printing invalid data
    }
}

fn main() {
    with(cx: &mut Some(Box::new(0)) {
        MyNode.baz();
    }
}