Fix transparency for top-level shared attribute in Display

[tyranron]

Synopsis

The transparency is not considered when the top-level shared #[display(...)] attribute on enum uses a field directly.

#[derive(Display)]
#[display("{_0}")]
enum Foo<A> {
    A(A),
}

assert_eq!(format!("{:03}", Foo::<i8>::A(7)), "007");

results in:

assertion `left == right` failed
  left: "7"
 right: "007"

Solution

Consider field's transparency for the top-level shared #[display(...)] attribute.

Checklist

• Documentation is updated (not required)
• Tests are added/updated
• CHANGELOG entry is added