Generic bound when deriving AsExpression for new array type

[Zstorm999]

Hello!

I have an array wrapper type that is expected to be generic (it’s a simple wrapper around Vec). I can leverage the implementation of FromSql and ToSql for Vec, but I can’t get it to work with AsExpression.

So for example I have the following definition:

#[derive(FromSqlRow, AsExpression)]
#[diesel(sql_type=sql_types::Array<T>)]
pub struct UniqueVec<T: Hash + Eq> {
    inner: Vec<T>,
}

I know that the bound Array<T> is not correct, because T is not an sql_type. I do not want to use any pre-defined sql type, because I don’t know what will be used in place of T (that’s the whole point of the generic).

Right now this code:

struct A {
    v: UniqueVec<String>,
}

fails with the following error:

rustc: the trait bound `UniqueVec<std::string::String>: diesel::Expression` is not satisfied

Is there any way I can solve this ?

[weiznich]

The short answer is: You cannot solve this while using the derive and only using stable diesel API's. You can likely write the relevant impls on your own by relaying on the internals generated by the macro, but we don't guarantee that these API's exist in the next diesel version anymore. Tools like cargo expand or rust-analyzer let you expand macros. Such output then can be adjusted to your needs.

That written: On a conceptual level that would be something we might want to support at some point, but I don't have any good ideas how to describe the relevant information (mostly the "unbound" generic for the SQL type) for the derive.

[Zstorm999]

Thanks for your answer ! I will rely on the internals for now then