This is an imple proposal for @ckipp01's Roadmap for actionable diagnostics, which will enable Scala compiler to suggest (semi-automatic) code edits together with compilation errors and warnings, likely useful for things like deprecation and migration warnings.

What's been added to Zinc thus far

1. Problem#actions - https://github.com/sbt/sbt/blob/v1.9.0-RC3/internal/util-interface/src/main/java/xsbti/Problem.java#L81-L83 (Action contains WorkspaceEdit, which contains TextEdit)
2. Scala 2.x compiler bridge is now capable of sending Problem#actions since actions follow-up sbt/zinc#1186. As a toy example, it scans for "procedure syntax is deprecated:" in the error message to generate an action.

What I'd like to add to Scala 2.13

1. Methods in Reporter (https://github.com/scala/scala/blob/v2.13.11-M1/src/reflect/scala/reflect/internal/Reporting.scala#L102) so compiler implementation can report structural code suggestion:

    protected def doReport(pos: Position, msg: String, severity: Severity, force: Boolean, actions: List[Action]): Unit
    final def echo(pos: Position, msg: String, actions: List[Action]): Unit
    final def warning(pos: Position, msg: String, actions: List[Action]): Unit
    final def error(pos: Position, msg: String, actions: List[Action]): Unit

2. Data structure for Action, WorkspaceEdit, and TextEdit (trait in api + data type in internal).
3. Add a few actions to deprecations. For example:
   • Deprecate auto-application of (non-Java-defined) methods with a single empty parameter list scala#8833
   • procedure syntax - scala/scala@1d4d901
   • val in for comprehension Remove val in for comprehension under -Xsource:2.14 scala#6348
   • Warn about change to parenless lambda parens under -Xsource:3 scala#10320

Impact to existing users

Hopefully none. The errors and warning message should remain as-is including code examples. Actions will be additional information that compatible editors like IntelliJ and Metals can consume.