What is the proper way to create a Span when inserting a new field with a Rust compiler plugin?

Question

I want a compiler plugin to annotate a structure with some information. For example, the original struct has only one field:

struct X { x: i32 }

And I want to add another field:

struct X { x: i32, y: MARKTYPE }

As I looked into Rust compiler plugins, I decided to use the MultiModifier(SyntaxExtension) to do my work. enum ItemKind defines Struct(VariantData, Generics) and the VariantData stores the data fields:

#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
pub enum VariantData {
    /// Struct variant.
    ///
    /// E.g. `Bar { .. }` as in `enum Foo { Bar { .. } }`
    Struct(Vec<StructField>, NodeId),
    /// Tuple variant.
    ///
    /// E.g. `Bar(..)` as in `enum Foo { Bar(..) }`
    Tuple(Vec<StructField>, NodeId),
    /// Unit variant.
    ///
    /// E.g. `Bar = ..` as in `enum Foo { Bar = .. }`
    Unit(NodeId),
}

StructField is defined as:

#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
pub struct StructField {
    pub span: Span,
    pub ident: Option<Ident>,
    pub vis: Visibility,
    pub id: NodeId,
    pub ty: P<Ty>,
    pub attrs: Vec<Attribute>,
}

I planned to insert a StructField, but I don't know how to make a Span for the field. Each Span contains a lo and a hi BytePos. The StructFields information looks like:

Fields 0: StructField { 
    span: Span {
        lo: BytePos(432),
        hi: BytePos(437),
        expn_id: ExpnId(4294967295)
    }, 
    ident: Some(x#0),
    vis: Inherited, 
    id: NodeId(4294967295), 
    ty: type(i32),
    attrs: []
}

What is the proper way to insert the new field?

---

I know it would be easier to use a macro to do this job, but I want to know if it is plausible to insert the field by modifying the VariantData.