Language Parser: poor error recovery on faulty DU declaration / improve error reporting

[smoothdeveloper]

// lots of code

type MyBrokenDU =
| A B // error FS0010: Unexpected identifier in member definition

// lots of code

I miss to put "of" between A and B, the whole file is failing to provide tooling support.

I'd expect if another declaration (a type, a binding) is encountered, it could determine the broken DU is closed.

I'd expect the error message to guide me in either:

• you are missing of before case member definition
• you are missing | before next case definition

related: #1103

[kerams]

Could #10872 help here?

[smoothdeveloper]

@kerams, I'd say @auduchinok could help for sure 🙂

also

changes were done by @dsyme during our parser recovery work today

what would help is surfacing postmortem notes of those occurrences of collaboration from main maintainers and a detailed TOC of compiler workshop videos: (https://www.youtube.com/watch?v=3Zr0HNVcooU), both those could be .md files under fsharp.org repository.

All would help for this type of issues to get taken up by the community.

We can split the issue in two: parser recovery (hard to fix for a person not familiar with parser infrastructure) and error message (easy to tackle by community at large).

[smoothdeveloper]

the thriller starts there: https://youtu.be/3Zr0HNVcooU?t=4977

[auduchinok]

Could #10872 help here?

In general it'd need a separate recovery for each place in the grammar (which sometimes would only work for some common kind of similar mistakes), so recovery in match expressions clauses won't help much with union case declarations. Though, a similar technique could be applied here, so some errors would be recovered.