ICE around validation of bool and char

[RalfJung]

@niluxv found an ICE around validation of uninitialized char. Here's a self-contained example; this embeds linked-hash-map. It would be nice to minimize this further.

The actual ICE occurs here and it is unclear why, because uninitialized char usually work fine.

[niluxv]

I used this example (playground link). It only includes the code from linked-hash-map that is required for the example to compile.

[RalfJung]

Actually, even this ICEs:

fn main() {
    let _b: bool = unsafe { std::mem::MaybeUninit::uninit().assume_init() };
}

Something is very odd...

[niluxv]

Actually droping a LinkedHashMap is enough: playground link.
@RalfJung Oh, sorry, didn't see your comment.

[RalfJung]

After fixing the ICE, things are worse: there is no error at all...

I'll need to do a full rustc bootstrap to get more debugging information, right now nothing makes any sense.^^ This will take a while.

[RalfJung]

rust-lang/rust#74059 should fix this.