Existential type can have inconsistent concrete type

[dtolnay]

#![feature(existential_type)]

use std::fmt::Debug;

// Parser workaround. https://github.com/rust-lang/rust/issues/52631
macro_rules! this_is_an_item {
    ($i:item) => { $i };
}

fn main() {
    this_is_an_item! {
        existential type Existential: Debug;
    }

    fn _unused() -> Existential { String::new() }

    let null = || -> Existential { 0 };
    println!("{:?}", null());
}

Segmentation fault (core dumped)

Mentioning the existential types tracking issue #34511
Mentioning @oli-obk and @cramertj

[oli-obk]

O_o I don't think I considered closures at all.

Should closures be defining uses? Or just uses (and thus only be able to return that type by calling a function that defines the existential)?

[dtolnay]

As a motivating use case, existential types defined by a closure are required if we want to extend lazy_static! to support referring to local variables.

#![feature(existential_type, untagged_unions)]

#[macro_use]
extern crate lazy_static;

use std::ops::Deref;

#[derive(Debug)]
struct ExpensiveResult;

// TODO: move this inside of `lazy_local!`.
existential type Init: FnOnce() -> ExpensiveResult;

macro_rules! lazy_local {
    (ref $name:ident : $ty:ty = $init:expr;) => {
        #[allow(unions_with_drop_fields)]
        union BrieflyUninit {
            uninit: (),
            // If $name is never deref'd, this initializer is never dropped.
            value: Init,
        }

        static mut INIT: BrieflyUninit = BrieflyUninit { uninit: () };
        let init = move || -> Init { move || $init };
        unsafe {
            std::ptr::write(&mut INIT.value, init());
        }

        lazy_static! {
            static ref $name: $ty = unsafe { std::ptr::read(&INIT.value)() };
        }
    };
}

fn lazy_deref_with_args(arg: i32) -> &'static impl Deref<Target = ExpensiveResult> {
    lazy_local! {
        ref STATE: ExpensiveResult = { println!("arg={}", arg); ExpensiveResult };
    }

    &STATE
}

fn main() {
    let state = lazy_deref_with_args(1);
    println!("{:?}", **state);
}

[oli-obk]

I chose a simpler logic: any in scope use must be a defining use.

[oli-obk]

We've moved from segmentation faults to an ICE:

internal compiler error: broken MIR in DefId(0/1:9 ~ playground[d6cf]::main[0]::{{closure}}[0]) (bb0[0]): equate_inputs_and_outputs: `Existential==i32` failed with `NoSolution`