[security] library user `MUST` able to forbid bypassing `miri`

[loynoir]

Recently, I encounter UB, which makes me take UB very very seriously.

Under release mode, same X, both X == 0 and X != 0 are triggered

Which makes me migrate away from all UB dependencies except hard dependency.

I do see bypassing miri, which still makes me very very concerned.

    #[cfg(all(XXX, not(miri)))]
    ...
    #[cfg(any(not(XXX), miri))]
    ...

I think, the security design should be

• library developer MAY able to bypass miri

• library user MUST able to forbid bypassing miri

cargo miri test --forbid-reachable-bypassing-miri --forbid-unreachable-bypassing-miri

When branch XXX is reachable, --forbid-reachable-bypassing-miri

• If not(miri) and miri have different behavior, should be fatal.

When branch XXX is unreachable, --forbid-unreachable-bypassing-miri

• If not(miri) and miri have different behavior, should be fatal.

[loynoir]

Close in favor of rust-lang/miri#4522