Not possible to insert arrays of custom types.

[vicky5124]

If a custom PgSql type is made, such as an enum, and a table column needs an array of that custom type, there's currently no way to be able to insert to that column.

Reproduceable example:

CREATE TYPE CATEGORIES AS ENUM (
    'API',
    'Editor'
);

CREATE TABLE test (categories CATEGORIES[]);

#[derive(Debug, sqlx::Type)]
#[sqlx(rename = "categories")]
pub enum Categories {
    #[sqlx(rename = "API")]
    Api,
    Editor,
}

let categories: Vec<Categories> = vec![Categories::Api, Categories::Editor];

sqlx::query!("INSERT INTO mods (categories) VALUES ($1)", &categories)
    .execute(&pool)
    .await?;

This fails to compile with:

error[E0277]: the trait bound `Vec<Categories>: Type<Postgres>` is not satisfied
...
    = 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 30 others
    = note: required because of the requirements on the impl of `Encode<'_, Postgres>` for `Vec<Categories>`
    = note: required because of the requirements on the impl of `Encode<'_, Postgres>` for `&Vec<Categories>`
    = note: required by `sqlx::Encode::size_hint`

And currently trying to manually implement this type is impossible due to having required fields are private.

#[derive(Debug, Clone)]
pub struct CategoriesList(Vec<Categories>);

impl Type<Postgres> for CategoriesList {
    fn type_info() -> PgTypeInfo {
        PgTypeInfo(
            PgType::Custom(
                Arc::new(PgCustomType {
                    // fields are private
                })
            )
        )
    }

    fn compatible(ty: &PgTypeInfo) -> bool {
        ...
    }
}

impl Encode<'_, Postgres> for CategoriesList {
    fn encode_by_ref(&self, buf: &mut PgArgumentBuffer) -> IsNull {
        <CategoriesList as Encode<Postgres>>::encode(self.clone(), buf)
    }
}

[abonander]

Duplicate of #298