Can we split `E0308` into different errors for better `--explain`?

[scottmcm]

(Inspired by the question in https://users.rust-lang.org/t/returning-different-types-in-match-arms/73508?u=scottmcm)

When you get an error like https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b1f5ac9c4d61db01d47d3a3927cfb544

error[E0308]: match arms have incompatible types

It would be nice to have that be a separate error code from the "you passed the wrong type to a function" errors.

Because the existing explain for E0308 talks more about the "there's only one valid type" case.

Ideally we could have a separate error code for the match/if arms with different types case, so that the explain for that would could talk about the "make an enum" or "use Box-of-dyn" solutions.

[zirconium-n]

Just my 2 cents. Most of time I'm hitting this error, it's a {integer}/{float} disagreement, or a forgotten Ok/Some, or a additional semicolon caused one branch becoming ().

I agree a separate error code is good, but the proposed solution may not.

The use case in the link almost always come from a dynamic typed language background (e.g. python), which I doubt use a dyn like Box<dyn Debug> would solve the problem. Yes, it would make this particular code snippet compiles and run, but it's quite likely there would be much more "generic" code follows, which may even use traits that's not object safe (IntoIterator) or just not from a trait.

The enum solution is more applicable, but I imagine the resulting code would be a mess since you need match everywhere.

I don't really have a nice solve-all solution, but we should try not fall into the XY problem.