/// In a more ideal world, we could check that requirement by just checking that
/// the given type implements both the `StructuralPartialEq` trait *and*
/// the `Eq` trait. However, you can have ADTs that *do* `derive(PartialEq, Eq)`,
/// and be a case that we want the compiler to accept, and yet the constant's
/// type fails to implement `Eq`.
///
/// Namely, a case like this:
///
/// ```rust
/// #[derive(PartialEq, Eq)]
/// struct Wrap<X>(X);
///
/// fn higher_order(_: &()) { }
///
/// const CFN: Wrap<fn(&())> = Wrap(higher_order);
///
/// fn main() {
///     match CFN {
///         CFN => {}
///         _ => {}
///     }
/// }
/// ```
///
/// (The problem in the above code is that `Wrap<fn(&())>` does not implement
/// `PartialEq`, nor `Eq`, because `for<'a> fn(&'a _)` does not implement those
/// traits.)
///
/// Therefore, we cannot rely on naive check for `StructuralPartialEq` and
/// mere `Eq`.