[SOLVED] Type annotations needed for none variant

Type annotations needed for none variant - ergonomics

I have a struct with a field 'payload' of Option<T> where T can be one of several primitive integer types. Like so:

pub struct Request<T: MyIntTrait> {
    pub cmd: Cmd,
    pub payload: Option<T>,
    a bunch of other fields...
}

When instantiating this struct with None as its payload obviously type annotations are needed:

let req: Request<u8> = Request { cmd: Cmd::Get, payload: None, ...}

or like this:

let req = Request { cmd: Cmd::Get, payload: None::<u8>, ...}

This makes for a very unergonomic api that is counterintuitive to use. Has anybody got an idea how to get around this or will I just have to live with it?

Solution

Unless the T can be inferred somewhere else, you have to specify it somehow. You could specify a default for the type argument, which may be sufficient in your case:

pub struct Request<T: MyIntTrait = u8>