How can a trait select an implementation at compile time?

[tones111]

I have a serialization crate that provides writers that are able to serialize streams of big or little endian data. In another crate I have functions that use these writers to serialize a number of structs. I would like to define a trait (Message) that specifies the endianness for a message to consolidate the serialization functions into one generic implementation but the compiler is unable to determine that the trait's associated type has a writer implementation.

I've reproduced the problem in a single file: playground link

Is there a way to tweak the pattern to help the compiler?

If I try to express endianness as a trait instead of separate structs the compiler will no longer be able to select the appropriate implementation at compile-time. The end goal is for calling code to be able to pass an instance of a type implementing Message to one function instead of having to implement them separately.

Thanks for any help. I'm still learning how to express compile-time patterns.

[quinedot]

You can add a where clause expressing the required functionality:

fn pack<M: Message>(_mesg: &M)
where
    Writer<M::Endian>: Write,
{
    let w = Writer::<M::Endian>::new();
    w.write();
}

(A where clause is just like the M: Message bound, but can syntactically express more complicated bounds.)

[tones111]

Wow, I didn't know where clauses could be so expressive. This really helped resolve my confusion with it working in the other cases. Thank you!