Unsound: Borrowck missing that closure needs to be "move"

[diwic]

Here's as far as I've come in minifying the problem. It gives bad result on both Stable, Beta and Nightly according to playground.

pub trait MethodType: Sized + Default {
    type GetProp: ?Sized;
}

#[derive(Default, Debug, Copy, Clone)]
pub struct MTFn;

impl<'a> MethodType for MTFn {
    type GetProp = Fn(&mut String) -> Result<(), ()> + 'a;
}

pub struct Property<M: MethodType> {
    get_cb: Option<Box<M::GetProp>>,
}

impl<'a> Property<MTFn> {
    pub fn on_get<H>(mut self, handler: H) -> Property<MTFn>
        where H: Fn(&mut String) -> Result<(), ()> + 'a {
        self.get_cb = Some(Box::new(handler) as Box<_>);
        self
    }
}

fn main() {
    let z: Option<Property<MTFn>>;
    {
        let mut v = format!("ABC DEF GHI JKL");
        let p = Property { get_cb: None };

        // This line should give an error indicating we need a "move" closure
        z = Some(p.on_get(|s| { *s = v.clone(); Ok(()) }));

        // Destroy v to show the error
        v = format!("MNO PQR");
    }
    let mut q = format!("STU");
    let _ = (z.unwrap().get_cb.unwrap())(&mut q);
    println!("{}", q); // Prints "STU PQR" !
}

[eefriedman]

The problem has to do with type GetProp = Fn(&mut String) -> Result<(), ()> + 'a;. It probably should be triggering E0207 ("the lifetime parameter 'a is not constrained by the impl trait, self type, or predicates").

[diwic]

@eefriedman, hmm, but this would be a bad solution in my real code. I want to indicate that the GetProp function can borrow from its environment (as opposed to + 'static, which is the default and does not allow borrows at all. I believe?).

[eefriedman]

You can change MTFn to pub struct MTFn<'a>(PhantomData<&'a u8>);, then write impl<'a> MethodType for MTFn<'a> {, and everything should work.