Specify that runtime should have task scheduler

[olegnn]

Motivation

If runtime doesn't have task scheduler,tokio::spawn from context of this runtime will produce panic must be called from the context of Tokio runtime, which is unclear, because it's actually done from the context of runtime. New panic message specifies that runtime should have task scheduler in order to execute spawned tasks.

Solution

Changed runtime panic message. Closes #1837.

[carllerche]

What about:

Suggested change

	panic!("must be called from the context of Tokio runtime with task scheduler");
	panic!("must be called from the context of Tokio runtime configured with either `basic_scheduler` or `threaded_scheduler`");

?

[carllerche]

I think that committing ^^ as is will make rustfmt unhappy though.

[carllerche]

Thanks for the panic message PR 👍 I greatly appreciate these kinds of contributions. Fixing pain points is great.

[olegnn]

Also there's still a thing that can confuse people - calling num_threads() on Builder without call of threaded_scheduler in fact does nothing.
Maybe it's a good idea to make debug assertion for this case or warning at least?
Because when I write code like

let rt = tokio::runtime::Builder::new().num_threads(4).build().unwrap();

I don't expect that num_threads is useless unless threaded_scheduler method is called.

[carllerche]

Thanks 👍