Rust: Put back "try with rustc --explain E0XXX"

[GuillaumeGomez]

This sentence has been removed here in this PR. However, I think it's quite useful considering the amount of extra information it provides.

cc @rust-lang/compiler

[estebank]

I would like to wait for a bit until we have a good story and coverage around -Zteach (at least on nightly). I think that's going to be more useful than always suggesting --explain E0XXX.

[GuillaumeGomez]

The old "--explain" suggestion system was a bit heavy. By rewording it, it might be better than what it was. However, -Zteach seems to be a potential great replacement in the future. I think that at first, we should put back --explain while waiting for -Zteach to be complete. What do you think about this?

[estebank]

I think it is reasonable to go that way. I don't know if there was any conversation before removal. I would be very much in favor of having a single note suggesting using --explain generically though. I wouldn't want to add 2 more lines per error with a code, it is redundant and would get old very quickly. It'd be even great if there were a way to silence it (which makes me go back to the increasing need of having a rustc.toml config file somewhere...).

[GuillaumeGomez]

I was thinking about just adding a line at the end of errors. Something like: "You got error(s), if you want more explanations about this/them, try with rustc --explain E0XXX" (XXX being one of the errors reported).

[nikomatsakis]

I like the idea of putting it as part of the final "here's how many errors you got" message.

[GuillaumeGomez]

I'll write a PR and we'll iterate about the formulation then.