Cow<T> isn't Send, even if T and <T as ToOwned>::Owned are

[renato-zannon]

The following doesn't compile, but I believe it should:

use std::borrow::{ToOwned, Cow};

fn main() {
  assert_send(Cow::Borrowed("foo"));
}

fn assert_send<T: Send>(_: T) {}

Error:

cow.rs:4:3: 4:14 error: the trait `core::marker::Send` is not implemented for the type `<str as collections::borrow::ToOwned>::Owned` [E0277]
cow.rs:4   assert_send(Cow::Borrowed("foo"));
           ^~~~~~~~~~~
cow.rs:4:3: 4:14 note: `<str as collections::borrow::ToOwned>::Owned` cannot be sent between threads safely
cow.rs:4   assert_send(Cow::Borrowed("foo"));
           ^~~~~~~~~~~
error: aborting due to previous error

Even though <str as ToOwned>::Owned == String, and String: Send. In fact, this compiles:

assert_send("foo");
assert_send("foo".to_owned());

rustc 1.0.0-nightly (522d09dfe 2015-02-19) (built 2015-02-20)

[japaric]

Cow<'static, str> should be Send, because the old definition that didn't use assoc types was Send. Probably the code that detects builtin bounds wasn't updated to handle associated types. cc @nikomatsakis @flaper87

[seanmonstar]

This breaks us in hyper. For now, I'll have to manually impl Send for Request, knowing that it's safe.,,