Select trait exposes private types

[brson]

The Select trait - a public interface - uses the following pattern to hide private implementation details:

priv trait SelectInner { ... implementation details ... }

pub trait Select: SelectInner { ... public API ... }

This is the only place where we use this pattern and I think it's kind of dubious - Select's signature refers to a type that, from the user's perspective, doesn't exist. It's a leaky abstraction.

There may be similar patterns that are more palatable but uses a private enum variant - the enum type is public but opaque.

[bblum]

I both agree and disagree. On the one hand, I'm not particularly proud of it; it was just the only way I could think of to hide the internal types in SelectInner's functions. Suggestions for other ways to do it welcome.

On the other hand, I see it less as "select uses a type that doesn't exist", and more as "Select is an opaque trait, that is somehow/magically implemented by X, Y, and Z types of ports, which are the only things select() can be used on."

[glaebhoerl]

FWIW Haskell does this a lot so you're in good company. I agree it's not super nice. I don't know what a cleaner way could be. (Exposing the trait and hiding the methods maybe, but that didn't generate too much enthusiasm last time I mentioned it.)

[alexcrichton]

No longer a problem after #10830