Add helpers to implement main() about error-chain

[Yamakaky]

Something like https://play.rust-lang.org/?gist=f98139dcdd16cceb898731f0fe649a34&version=stable&backtrace=0?

[joshtriplett]

Wrapping the whole thing in a macro (which expects a closure as an argument) seems a little too magic to me. I'd prefer a helper like the toplevel_unwrap() I suggested above, or at most, a macro that takes a function name (chain_main!(real_main);).

Then again, I suppose you could pass a function name instead of a closure, so sure. But I'd still like the helper function version as well. And both the helper function and the macro should print the causes and possible backtrace, as shown in that blog entry.

[Yamakaky]

Yeah, it's just that I was on play.rust-lang.org, which doesn't allow the use of other crates.
How would you implement it? A trait on Result?

[joshtriplett]

Right. error-chain already provides a trait on Result, so this would just add one method to it.

[Yamakaky]

In fact, not exactly: ResultExt is defined on Result<T, E> where E: Error, and not E: ChainedError.

[joshtriplett]

Good point; in the absence of some kind of specialization, that trait wouldn't work. But another trait could.