Miri comparison sanity check is wrong

[RalfJung]

The following ICEs (example by @eddyb):

#![feature(const_fn)]

const fn nested(x: (for<'a> fn(&'a ()), String)) -> (fn(&'static ()), String) {
    x
}

pub const TEST: (fn(&'static ()), String) = nested((|_x| (), String::new()));

fn main() {}

shows

thread 'rustc' panicked at 'type mismatch when copying!
src: (for<'r> fn(&'r ()), std::string::String),
dest: (fn(&()), std::string::String)', /rustc/853c4774e26ea97b45fe74de9a6f68e526784323/src/libstd/macros.rs:16:9

This means that the check here is wrong. It might be possible to also trigger this without nightly features through const_prop.

[RalfJung]

Baring a way to do the proper structural subtyping check, the only solution I see is to go back to comparing layouts in all cases (as a fallback after types mismatch) -- but @eddyb does not like this, as far as I recall. Any other proposals?