Towards better error messages in Scalac

[soronpo]

@jvican

What makes a good error?
A good error message has IMO the following properties:

A unique identifier.
The source code is the main part in a message (*).
The content is short (the less unnecessary cruft, the better).
Descriptions are clear, verbs are in the present or imperative tense.
There are actionable steps to address the issue.

In addition, I would love a clickable link to search for the message online, as I mentioned before on Compiler warnings/errors with http document pointers to help resolve them

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@jvican

Unique identifiers must be compiler specific, that is an id for the same error in, say, Dotty and Scalac have to be different to avoid interferences between each other. This way, users can quickly google an error id to know more about an unclear or difficult to address error.

I’m not entirely sure that’s good. Dotty and Scala have common ground. In the future they are supposed to be merged into one. Why should the same error messages that exist in both have a different ID?

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Possibly relevant issues:

• Local warning suppression. It would be great if we can properly reference a warning ID and suppress it for a specific scope/line.
• Somehow provide better implicitNotFound error cascading messages: Allow cascading custom error messages with `@implicitNotFound`