EthereumCheckpointStorageBuilder的build_with_provider泛型实现问题

问题背景 #

为泛型结构体EthereumCheckpointStorageBuilder<M>实现build_with_provider方法时，结构体与方法的泛型约束均为M: Middleware + 'static，希望仅声明一次泛型参数M并复用，却触发错误：method build_with_provider has 0 type parameters but its trait declaration has 1 type parameter。尝试为方法添加独立泛型参数P后，不仅冗余还引发From相关类型转换问题。

用户提供的核心代码如下：

// 结构体定义
pub struct EthereumCheckpointStorageBuilder<M> {
    _phantom: std::marker::PhantomData<M>,
}

// Trait实现（报错版本）
#[async_trait]
impl<M> BuildableWithProvider for EthereumCheckpointStorageBuilder<M>
where
    M: Middleware + 'static,
{
    type Output = EthereumCheckpointStorage<M>;
    const NEEDS_SIGNER: bool = true;

    async fn build_with_provider(
        &self,
        provider: M,
        conn: &ConnectionConf,
        locator: &ContractLocator,
    ) -> Self::Output {
        EthereumCheckpointStorage::new(Arc::new(provider), conn, locator)
    }
}

// Trait定义
#[async_trait]
pub trait BuildableWithProvider {
    async fn build_with_provider<M>(
        &self,
        provider: M,
        conn: &ConnectionConf,
        locator: &ContractLocator,
    ) -> Self::Output
    where
        M: Middleware + 'static;
}

错误根源 #

trait定义中的build_with_provider方法自带独立泛型参数M，要求所有实现该trait的类型必须支持任意符合约束的M类型。但你的结构体EthereumCheckpointStorageBuilder<M>绑定了固定的泛型M，实现方法时试图用结构体的M替代方法的泛型M，违反了trait的定义规则，导致类型不匹配报错。

解决方案 #

方案1：将泛型参数移至trait层面（推荐） #

修改trait定义，将泛型M绑定到trait本身，让结构体的泛型与trait泛型对齐，彻底避免方法层的冗余泛型：

// 修改后的Trait定义
#[async_trait]
pub trait BuildableWithProvider<M>
where
    M: Middleware + 'static,
{
    type Output;
    const NEEDS_SIGNER: bool;

    async fn build_with_provider(
        &self,
        provider: M,
        conn: &ConnectionConf,
        locator: &ContractLocator,
    ) -> Self::Output;
}

// 对应的结构体实现
#[async_trait]
impl<M> BuildableWithProvider<M> for EthereumCheckpointStorageBuilder<M>
where
    M: Middleware + 'static,
{
    type Output = EthereumCheckpointStorage<M>;
    const NEEDS_SIGNER: bool = true;

    async fn build_with_provider(
        &self,
        provider: M,
        conn: &ConnectionConf,
        locator: &ContractLocator,
    ) -> Self::Output {
        EthereumCheckpointStorage::new(Arc::new(provider), conn, locator)
    }
}

此方案将泛型约束统一到trait层级，结构体实现时直接复用同一个M，完全符合复用泛型的需求，无冗余代码，彻底解决错误。

方案2：适配原trait定义，调整实现逻辑 #

若无法修改trait定义，需保留方法的泛型参数，同时通过From trait确保参数类型可转换为结构体的M类型：

#[async_trait]
impl<M> BuildableWithProvider for EthereumCheckpointStorageBuilder<M>
where
    M: Middleware + 'static,
{
    type Output = EthereumCheckpointStorage<M>;
    const NEEDS_SIGNER: bool = true;

    async fn build_with_provider<P>(
        &self,
        provider: P,
        conn: &ConnectionConf,
        locator: &ContractLocator,
    ) -> Self::Output
    where
        P: Middleware + 'static,
        M: From<P>, // 确保P可转换为结构体的M类型
    {
        EthereumCheckpointStorage::new(Arc::new(M::from(provider)), conn, locator)
    }
}

此方案虽保留了方法泛型，但通过类型转换约束解决了之前的From相关问题，适用于无法修改trait的场景。

内容的提问来源于stack exchange，提问作者Paul Yu