ICE when combining const (fn) and unions

[jonas-schievink]

This code posted by @sfleischman105 in #24111 currently compiles:

#![feature(const_fn)]

use std::time::Instant;

pub struct GlobalTimer {
    time: Instant,
}

union DummyUnion {
    time: Instant,
    junk: u32
}

impl GlobalTimer {
    pub const fn init() -> GlobalTimer {
        const DUMMY: DummyUnion = DummyUnion {
            junk: 0
        };
        
        const UNINIT: Instant = unsafe {
            DUMMY.time
        };
        
        // Ta-da!
        GlobalTimer {
            time: UNINIT,
        }
    }
}

fn main() {
    //GlobalTimer::init();
}

With this suspicious (and wrong?) warning:

warning: constant evaluation error: nonexistent struct field
  --> src/main.rs:20:33
   |
20 |           const UNINIT: Instant = unsafe {
   |  _________________________________^
21 | |             DUMMY.time
22 | |         };
   | |_________^
   |
   = note: #[warn(const_err)] on by default

However, trying to use the const fn by uncommenting the line in main causes an ICE:

error: internal compiler error: librustc_trans/type_of.rs:386: TyLayout::llvm_field_index(TyLayout { ty: DummyUnion, details: LayoutDetails { variants: Single { index: 0 }, fields: Union(2), abi: Aggregate { sized: true }, align: Align { abi: 3, pref: 3 }, size: Size { raw: 16 } } }): not applicable

[etaoins]

The ICE can be reduced to:

union DummyUnion {
    field1: u32,
    field2: u32,
}

pub fn init() -> u32 {
    const UNION: DummyUnion = DummyUnion { field1: 0 };
    const FIELD: u32 = unsafe { UNION.field1 };
    FIELD
}

fn main() {
    init();
}

Note that this does not require const_fn. This also looks like a regression from stable to nightly.

[etaoins]

The non-existent field warning can be reduced to:

union DummyUnion {
    field1: i32,
    field2: i32,
}

const UNION: DummyUnion = DummyUnion { field1: 1 };

fn main() {
    assert_eq!(unsafe { UNION.field2 }, 1);
}

It looks like referencing any field except the first will trigger the warning.