ICE with unsized associated type

[petertodd]

use std::mem;

pub trait Arena {
    type Dyn : ?Sized;
}

pub struct DynRef {
    _dummy: [()],
}

pub struct Ref<T, A: Arena> {
    _value: T,
    _dyn_arena: A::Dyn,
}

pub struct Obstack;

impl Arena for Obstack {
    type Dyn = DynRef;
}

fn main() {
    mem::size_of::<&Ref<u8, Obstack>>();
}

produces:

$ rustc ice-dyn.rs 
error: internal compiler error: src/librustc_codegen_llvm/context.rs:867: failed to get layout for `&Ref<u8, Obstack>`: the type `<Obstack as Arena>::Dyn` has an unknown layout

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:620:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: aborting due to previous error


note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.34.1 (fc50f328b 2019-04-24) running on x86_64-unknown-linux-gnu

Some context as to what that's actually for: the idea was to use the len metadata of the slice within the DynRef to smuggle a pointer to the arena in question. At first glance you'd think you could also do:

pub struct Ref<T, A: Arena> {
    marker: PhantomData<A>,
    value: T,
    arena: [()],
}

But the problem is that is mutable borrows: if Ref implements DerefMut there's no way to both access the value and the arena at the same time. You need them separate, and deliberately public:

pub struct Ref<T, A: Arena> {
    pub this: T,
    pub arena: A::Dyn,
}

allowing implementations like:

impl<A: Arena> Foo {
    fn frob(self: &mut Ref<Foo, A>) {
        self.arena.get(self.this.bar);
    }
}

...and as for why the object can't just own a reference to the arena it's in, tl;dr: memmap;

[pnkfelix]

triage: P-high due to ICE. Removing nomination since there's no much to discuss beyond severity.

[pnkfelix]

Discussed with eddyb on zulip; they identified the cause and a fix pretty readily. Working on putting in the fix now.