Macro sql_struct 

macro_rules! sql_struct {
    (
        impl $ty:ty {
            $(
                $field:ident : { db: $db:literal, $(orig: $orig:literal,)? tags: [ $($tag:literal),* $(,)? ], omitempty: [ $($omit:literal),* $(,)? ], quote: $quote:literal, as: $as:expr }
            ),* $(,)?
        }
    ) => { ... };
}

声明一个可用于 Struct<T> 的业务 struct 元数据与取值逻辑。

用法示例：

#[derive(Default)]
struct User { id: i64, name: String }

sql_builder::sql_struct! {
  impl User {
    id:  { db: "id", tags: ["pk"], omitempty: [], quote: false, as: None },
    name:{ db: "name", tags: [],     omitempty: [""], quote: true,  as: None },
  }
}