Automatic string conversion for type variables

[ryzhyk]

At the moment, the compiler refuses to convert type variables to strings.

This can be fixed by:

1. Infering that a function expects a type variable to be printable
2. In the generated Rust function, add the Display trait for this function
3. Whenever the function is called, check that its concrete arguments satisfy the trait
4. Implement Display trait for types that have a user-defined 2String method

[blp]

Here's a function that I wrote that will compile to Rust, then won't compile at that point. I think it's exactly this issue:

 function stringize_and_join(vec: Vec<'A>, sep: string): string {
    var strings: Vec<string> = vec_with_capacity(vec_len(vec));
    for (x in vec) {
        vec_push(strings, __builtin_2string(x))
    };
    string_join(strings, sep)
}

[ryzhyk]

Yes, exactly. There is no easy fix at the moment. We don't have the notion of traits or interfaces or anything like that, so there is no way to define a function that only accepts printable arguments.