Given that Send is autoimplemented for things that should be Send, this is a strong indication towards your code attempting to send actually not-thread-safe things across threads (things containing borrowed references or Rc<T>)

The error message does drill down and tell you the actual type causing the lack of Send impl.

The only time you have to manually impl Send is for custom container types that are built from raw primitives.

Please file bugs if the errors are confusing!

Eh, they’re probably fine for regular users. They are just nonsensical when you’re starting out.

We care just as much for people just starting out as we do for regular users; possibly even more!

Well, there's plenty to say about C++ compiler errors too!

Clang has done a really amazing job of cleaning those up. I find them super helpful these days.

If you want compiler ambiguity in C++ look no further than Templates... C++ is rarely ergonomic to use.

I like templates, because to date there is not a popular language that can do compile time stuff it can do. And no joke, some of the stuff you can do is quite amazing.