Lint: Removal of a field in a enum struct variant

[obi1kenobi]

For example:

pub enum Foo {
    Bar {
        a: usize,
        b: u64,
    }
}

Removing e.g. the b field here is a breaking change.

The fields of struct variants always have "default" visibility, just like variants themselves: they inherit the visibility of the enum itself.

This lint is similar to lints checking for removal of pub fields in structs and for checks over enum variants.

It may require a small amount of new schema and adapter code: currently the schema for StructVariant doesn't have a field edge like Struct does, and probably should have one. I believe the type of struct variant fields should still be StructField.