Function fmt_field_projection 

fn fmt_field_projection<C: AstFormatter>(
    ctx: &C,
    f: &mut Formatter<'_>,
    sub: impl Display,
    sub_ty: &Ty,
    variant_id: Option<VariantId>,
    field_id: FieldId,
) -> Result

Formats the projection of field field_id of sub, which has type sub_ty.