On: Improving Rust compile times to enable adoption of...

[nicoburns]

It would allow you to call a function to check for trait B and downcast to "implements trait A and B" in the case that it does implement the trait.

[tialaramex → nicoburns]

It seems like a way to ask "Can this thing implement X and if so how?" from say the Any trait would be what you want here, I have no idea how hard that would be to deliver but I also don't see how the previous trait thing is relevant, like, why do we need to say up front that maybe we will care whether trait B is implemented?

[merely-unlikely → nicoburns]

I’m still learning the language but couldn’t you use an enum containing two types to accomplish the same thing?

[nicoburns → merely-unlikely]

You can if you know all of the possible types in advance. But if you want to expose this as an interface from a library that allows users to provide their own custom implementation then you need to use traits.