Bear in mind that the optimization on `Option<Infallible>` isn't specific to that type; it applies to anything that results in one possible value. For example, `Foo` in this example is also zero-sized:

    pub enum Foo {
        Foo,
        Bar(A),
        Baz(B),
        Qux(C),
    }

    pub enum A {}
    pub enum B {}
    pub enum C {}

The optimization applied to Vec also applies to any zero-sized type.