Rust: New compiler error in beta: this constant cannot be used, attempted to read undefined bytes

[sdroege]

Since latest beta (and nightly), some of my crates have errors like the following

error: this constant cannot be used
  --> src/udpsrc.rs:47:1
   |
47 | const DEFAULT_CAPS: Option<gst::Caps> = None;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes

A standalone testcase replicating all the involved types can be found below (depends only on the libc crate).

I don't know if the error is valid, but in any case I don't understand what it means so at least the error message could be improved a bit :)

use std::marker;
use std::ptr;

extern crate libc;
use libc::{c_int, c_uint, c_void, size_t};

#[repr(C)]
pub struct CapsRef(ffi::GstCaps);

pub type Caps = GstRc<CapsRef>;

pub trait MiniObject {}
impl MiniObject for CapsRef {}

pub struct GstRc<T: MiniObject> {
    obj: ptr::NonNull<T>,
    borrowed: bool,
    phantom: marker::PhantomData<T>,
}

mod ffi {
    use super::*;

    #[repr(C)]
    #[derive(Copy, Clone)]
    pub struct GstCaps {
        pub mini_object: GstMiniObject,
    }

    #[repr(C)]
    #[derive(Copy, Clone)]
    pub struct GstMiniObject {
        pub type_: GType,
        pub refcount: c_int,
        pub lockstate: c_int,
        pub flags: c_uint,
        pub copy: GstMiniObjectCopyFunction,
        pub dispose: GstMiniObjectDisposeFunction,
        pub free: GstMiniObjectFreeFunction,
        pub n_qdata: c_uint,
        pub qdata: gpointer,
    }

    pub type GstMiniObjectCopyFunction =
        Option<unsafe extern "C" fn(*const GstMiniObject) -> *mut GstMiniObject>;
    pub type GstMiniObjectDisposeFunction =
        Option<unsafe extern "C" fn(*mut GstMiniObject) -> gboolean>;
    pub type GstMiniObjectFreeFunction = Option<unsafe extern "C" fn(*mut GstMiniObject)>;

    pub type gboolean = c_int;

    pub type GType = size_t;

    pub type gpointer = *mut c_void;

}

const FOO: Option<Caps> = None;

fn main() {
    let _meh = FOO;
}

[csmoe]

the testcase compiles with stable-1.29.0.

[hanna-kruppe]

Minimized:

pub struct GstRc {
    _obj: *const (),
    _borrowed: bool,
}

const FOO: Option<GstRc> = None;

fn main() {
    let _meh = FOO;
}

[pnkfelix]

Visited during T-compiler meeting. Tentatively assigning to @eddyb for investigation.

[eddyb]

cc @RalfJung @oli-obk I can't find anything in the validation code that's reading the wrong thing - it should be able to tell this is a None, which has 0 fields.

[AstralSorcerer]

I've been looking into this. It seems that the issue stems from here. Specifically, because the niche-filling enum representation is used for Option<GstRc> and similar types, and the niche that gets filled is the second field (the bool), it is actually the second part of the ScalarPair that is defined, not the first.

[eddyb]

Ahh, you're right! I don't know why only the second component was special-cased.

[RalfJung]

Seems like @oli-obk's quest to keep Undef contained within the CTFE engine failed at last. :/

However, in this case nothing should even be trying to convert to a ConstValue. So I am not sure if that's really the problem.

[RalfJung]

Actually it looks like this is not a ScalarPair, but a Scalar being undef. At least that's what the line numbers are saying. It does not make much sense though.

EDIT: It is definitely a ScalarPair, just the line numbers seem to be wrong.

[RalfJung]

I am working on a fix.

EDIT: Submitted as https://github.com/rust-lang/rust/pull/54693

[eddyb]

Should this be closed before we get a beta backport?

[pietroalbini]

Backported to beta. Closing this.