Make an "I just want to use `?`" type for use as a `main` return type

[scottmcm]

Inspired by rust-lang/rust#49162 (comment)

With ?-in-main about to stabilize, it would be nice to have an "it just works" return type for main so that one can start a bin with fn main() -> InsertSomethingEasyHere { until you care about doing something else.

Edit: Would also be nice for tests, since they never care about the error type, just whether there was one. (cc rust-lang/rust#48854)

Tagging lang in case this needs tweaks to ? to work well with heterogeneous error types.
Tagging libs in case this should be an alias for something like Result<(), Box<Error>> or involve failure in some way.

[JustAPerson]

Does impl Termination not suffice?

[scottmcm]

@JustAPerson It's currently incompatible with ?, see rust-lang/rust#43301 (comment)

[clarfonthey]

Wouldn't Result<(), impl Debug> suffice?

[scottmcm]

@clarcharr That also doesn't work today, for the same reason as impl Termination.

[sgrif]

Something like fn main() -> ? does seem like it'd be nice to have (not as nice as fn main() { just "working" when ? is used, but that sounds significantly more complicated and would have greater implications)

[Centril]

@sgrif Personally I'd like to reserve ? in such situations for typed toles (as used in Agda, etc.)

[scottmcm]

I put some thoughts about how changing the desugaring of ? could help support this in the try_trait tracking issue: rust-lang/rust#42327 (comment)