How do I know what types I have to put into the generic type parameters?

Question

I'm learning Rust by looking at and manipulating examples from others. I tried to encapsulate the following code into a struct:

let mut encoder: gfx::Encoder<_, _> = factory.create_command_buffer().into();

I want to create a struct like this:

pub struct Window {
    encoder: gfx::Encoder<?, ?>,
    // ...
}

How do I know what types I have to put into the question marks?

score 3 · Accepted Answer · edited May 23 '17 at 10:28

Change the type of your variable (encoder) to cause a type mismatch. The easiest type to use for this is ():

let mut encoder: gfx::Encoder<(), ()> = factory.create_command_buffer().into();

This will generate an error with the concrete types, which you can then clean up and use directly.

See also How do I print the type of a variable in Rust?.

In many cases, you'd use something a bit simpler:

let mut encoder: () = factory.create_command_buffer().into();

But that's unlikely to work in this specific case because into has a polymorphic return type. It needs to have some concrete type specified in order to know which implementation should be called.

How do I know what types I have to put into the generic type parameters?

1 Answers1