Returning a type in Rust

Question

I'm trying to create a trait(StringToTable) that return a struct with an implementation of another trait(Table).

When I have read this answer How to infer the return type of a function?, I tried the "boxed type" approach, but without success.

I don't know the struct type on call my StringToTable, so I can't use approaches like 'to_table< T >' or 'StringToTable< T >' ..

Playground with the error: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=cac035a6e6357c204156fab13449e865

My code:

1° approach:

fn to_table(table: String) -> Option<Box<dyn Table<DeserializeOwned + Serialize + SearchIndex>>>;

I got:

error[E0225]: only auto traits can be used as additional traits in a trait object
  --> src/main.rs:18:75
   |
18 |     fn to_table(table: String) -> Option<Box<dyn Table<DeserializeOwned + Serialize + SearchIndex>>>;
   |                                                        ----------------   ^^^^^^^^^
   |                                                        |                  |
   |                                                        |                  additional non-auto trait
   |                                                        |                  trait alias used in trait object type (additional use)
   |                                                        first non-auto trait
   |                                                        trait alias used in trait object type (first use)

2° approach:

fn to_table2(table: String) -> Option<Box<dyn Table<T>>> where T: DeserializeOwned + Serialize + SearchIndex;

I got:

error[E0412]: cannot find type `T` in this scope
  --> src/main.rs:20:68
   |
20 |     fn to_table2(table: String) -> Option<Box<dyn Table<T>>> where T: DeserializeOwned + Serialize + SearchIndex;
   |                                                                    ^ not found in this scope

** My code has been simplified

What I'm trying to do: receive a random name on a tcp server and got the equivalent struct for it.

Peter Zeller Peter Zeller · Accepted Answer · 2020-02-27T10:17:50

In the second approach, you can fix the compilation error by declaring the type parameter T. You can do this on the function or on the trait.

On the function:

fn to_table2<T>(table: String) -> Option<Box<dyn Table<T>>> where T: DeserializeOwned + Serialize + SearchIndex;

On the trait:

pub trait StringToTable<T> where T: DeserializeOwned + Serialize + SearchIndex {

    fn to_table2(table: String) -> Option<Box<dyn Table<T>>>;
}

Another option is to use an associated type:

pub trait StringToTable {
    type T : DeserializeOwned + Serialize + SearchIndex;

    fn to_table2(table: String) -> Option<Box<dyn Table<Self::T>>>;
}

Returning a type in Rust

1 Answers