Debuginfo changes causing ICE

[sophiajt]

A recent change to debug info (possibly 0db7abe) seems to be causing ICEs with code like the following:

#![feature(never_type)]
#![feature(nll)]

#[derive(Copy, Clone)]
pub struct Entity {
    private: std::num::NonZeroU32,
}

#[derive(Copy, Clone, PartialEq, Eq)]
pub struct Declaration;

impl TypeFamily for Declaration {
    type Base = Base;
    type Placeholder = !;

    fn intern_base_data(_: BaseKind<Self>) {}
}

#[derive(Copy, Clone)]
pub struct Base;

pub trait TypeFamily: Copy + 'static {
    type Base: Copy;
    type Placeholder: Copy;

    fn intern_base_data(_: BaseKind<Self>);
}

#[derive(Copy, Clone)]
pub enum BaseKind<F: TypeFamily> {
    Named(Entity),
    Placeholder(F::Placeholder),
    Error,
}

Compiled with:

rustc src/lib.rs -C debuginfo=2  --crate-type lib 

Edit: Platform info:

jonathan@pop-os:~/Source/lark/components/lark-ty$ rustc --version
rustc 1.31.0-nightly (de9666f12 2018-10-31)

[nikomatsakis]

cc @tromey -- we believe this was caused by #54004 (bisection)

[nikomatsakis]

Also, note that the exact rustc line is required (in particular, --crate-type lib).

The assertion failures I see are:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `18446744073709551615`,
 right: `340282366920938463463374607431768211455`', librustc_codegen_llvm/debuginfo/metadata.rs:1367:29
note: Run with `RUST_BACKTRACE=1` for a backtrace.

with this backtrace found here.

[tromey]

You can make it fail without --crate-type lib by adding a main that uses BaseKind:

pub fn main() {
    let x = BaseKind::Error::<Declaration>;
    let y = 7;
}

[tromey]

This assert:

assert_eq!(niche as u64 as u128, niche);

is wrong if niche underflows to -1u128.

Just removing the assert is probably fine. However I wonder if the values should actually be truncated to the size of the tag slot.