我尝试将Postgres中的数组字段加载到Rust结构体中，如下所示

use sqlx::{Pool, PgConnection, PgPool, FromRow};
use sqlx::postgres::PgQueryAs;

#[derive(Copy, Clone, sqlx::Type)]
#[sqlx(rename = "VARCHAR")]
#[sqlx(rename_all = "snake_case")]
enum Coupon {
    ChristmasSaleCoupon,
    NewYearSaleCoupon,
}

#[derive(FromRow, Clone)]
struct UserCouponMap {
    pub id: i64,
    pub user_id: i64,
    pub coupons: Vec<Coupon>,
}

impl UserCouponMap {
    pub async fn get_for_userid(db_pool: Pool<PgConnection>, user_id: i64) -> Vec<UserCouponMap> {
        let user_coupon_map: Vec<UserCouponMap> = sqlx::query_as("SELECT * FROM user_coupon_map WHERE user_id = $1")
            .bind(user_id)
            .fetch_all(db_pool)
            .await
            .expect("failed to fetch user coupon map");
        user_coupon_map
    }
}

#[tokio::main]
async fn fetch_coupons_for_user_id(user_id: i64) {
    let pool = PgPool::new("postgresql://asnim@dbhost:5732").await.expect("expected unwrap to succeed");
    let user_coupon_map = UserCouponMap::get_for_userid(pool, user_id).await;
}

fn main() {
    fetch_coupons_for_user_id(20);
}

根据数组documentation，我已经为Coupon实现了sqlx::Type。
尽管如此，编译器还是会说，某些特性并不令人满意。

Compiling playground v0.1.0 (/Users/asnimansari/CLionProjects/playground)
error[E0277]: the trait bound `Vec<Coupon>: Type<Postgres>` is not satisfied
  --> src/main.rs:23:14
   |
23 |             .fetch_all(db_pool)
   |              ^^^^^^^^^ the trait `Type<Postgres>` is not implemented for `Vec<Coupon>`
   |
   = help: the following implementations were found:
             <Vec<&[u8]> as Type<Postgres>>
             <Vec<&str> as Type<Postgres>>
             <Vec<(T1, T2)> as Type<Postgres>>
             <Vec<(T1, T2, T3)> as Type<Postgres>>
           and 29 others
   = note: required because of the requirements on the impl of `for<'c> FromRow<'c, PgRow<'c>>` for `UserCouponMap`

error[E0277]: the trait bound `[Coupon]: Type<Postgres>` is not satisfied
  --> src/main.rs:23:14
   |
23 |             .fetch_all(db_pool)
   |              ^^^^^^^^^ the trait `Type<Postgres>` is not implemented for `[Coupon]`
   |
   = help: the following implementations were found:
             <[&[u8]] as Type<Postgres>>
             <[&str] as Type<Postgres>>
             <[(T1, T2)] as Type<Postgres>>
             <[(T1, T2, T3)] as Type<Postgres>>
           and 29 others
   = note: required because of the requirements on the impl of `sqlx::decode::Decode<'_, Postgres>` for `Vec<Coupon>`
   = note: required because of the requirements on the impl of `for<'c> FromRow<'c, PgRow<'c>>` for `UserCouponMap`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

我错过了什么？
我的Cargo文件具有以下依赖项

sqlx = { version = "0.3.5", default-features = false, features = ["runtime-tokio", "macros", "postgres", "all-type"] }
tokio = { version = "0.2.21", features = ["full"] }