[ninkendo]

> Types don't carry behavioral information about what the method does internally.

I don’t remember specifics, but I very distinctly remember changing a method in some way and Rust determining that my type is now not Send, and getting errors very far away in the codebase because of it.[0]

If I have time in a bit I’ll try and reproduce it, but I think Send conformance may be an exception to your statement, particularly around async code. (It also may have been a compiler bug.)

[0] It had something to do with carrying something across an await boundary, and if I got rid of the async call it went back to being Send again. I didn’t change the signature, it was an async method in both cases.

[kd5bjo]

`Send`, `Sync`, and `Unpin` are special because they're so-called 'auto traits': The compiler automatically implements them for all compound types whose fields also implement those traits. That turns out to be a double-edged sword: The automatic implementation makes them pervasive in a way that `Clone` or `Debug` could never be, but it also means that changes which might be otherwise private can have unintended far-reaching effects.

In your case, what happens is that async code conceptually generates an `enum` with one variant per await point which contains the locals held across that point, and it's this enum that actually gets returned from the async method/block and implements the `Future` trait.