We allow infinitely expanding types behind pointer indirection

[lcnr]

This compiles

struct Foo<T> {
    y: T,
    x: Option<*const Foo<(T,)>>,
}

fn main() {
    let x = Foo {
        y: 1u32,
        x: None,
    };
}

It feels quite brittle however, e.g. the following does not:

struct Foo<T> {
    y: T,
    // We recur due to drop checking
    x: Option<Box<Foo<(T,)>>>,
}

fn main() {
    let x = Foo {
        y: 1u32,
        x: None,
    };
}

[lcnr]

Also see #156977 (comment) where the opsem inhabitedness check explicitly avoids erroring on such types to avoid errors only happening when running with MIRI