fix spawn_local

[lcnr]

spawn_local caused uncaught exception: undefined on firefox when supplied with an error variant. This should now be fixed.

Examples

spawn_local(future::err::<(), ()>(()));

I don't know if and how I should write a test for this.

[richard-uk1]

Oops my bad.

If you were to add a test (good to make sure this doesn't happen again), the place to do it is after this test, or possibly inside it.

[lcnr]

I might require some help with testing this. While I do get the following warning when testing

(node:14013) UnhandledPromiseRejectionWarning: undefined
(node:14013) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:14013) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js processwith a non-zero exit code.

the tests are successful and I don't know how to add an eventlistener for unhandledrejection in Node.js, as the window object does not exist.

[alexcrichton]

@lcnr hm I'm not entirely sure what those warnings are, but want to commit the test here and we can help through CI logs?

[lcnr]

@alexcrichton I did commit the test without reverting the fix, so it runs successfully without this warning.

[alexcrichton]

Thanks!

I think it's ok for us to manually inspect output for now to make sure this doesn't regress