Checking -Vphases does not work in Run, but that is not urgent. Other settings that represent phase names could be added to settings to check.

Showing plugins with highlighting:
~/projects/dotty/bin/scalacQ -Xplugin:research.jar -Xplugin:div-zero.jar -Vphases:research,divideZero -Ystop-after:refchecks

The wild error:

Error:  -- [E177] Reference Error: /__w/scala3/scala3/compiler/src/dotty/tools/dotc/config/CliCommand.scala:122:19 
Error:  122 |    val compiler = Compiler()
Error:      |                   ^^^^^^^^
Error:      |Reference to constructor proxy for class Compiler in package dotty.tools.dotc
Error:      |shadows outer reference to object Compiler in package java.lang
Error:      |
Error:      |The instance needs to be created with an explicit `new`.
Error:      |
Error:      | longer explanation available when compiling with `-explain`
Error:  one error found
Error:  (scala3-compiler / Compile / compileIncremental) Compilation failed

https://github.com/scala/scala3/actions/runs/11796179302/job/32857488523?pr=17563

Edit: it was deprecated since 9 and removed in 21, stale interface for JIT.

This is where a mechanism to exclude API from compilation would really shine.

Scala 2 partest supports testing plugins by taking -Xplugin:. to mean "current output directory". Not sure Scala 3 even supports loading from a dir; or how hard it would be to get vulpix to massage options; or whether Scala 3 supports multiple plugin classes in the plugin.properties.

Currently there is no automated test for -Vprint. That's also useful for stability of phase order.

Drafting to solve scalac -Vprint file.scala where the option consumes the arg.

In Scala 2, a PhasesSetting distinguishes tryToSet (which does not consume args) from tryToSetColon (which uses the supplied args if present).

In Scala 3, settings are just strings. Possibly, it is not too late to require PhasesSettings to have a special tag PhaseName . Not sure if anyone in Dottyland ever writes -Vprint typer or -Ystop-after refchecks. Settings with "choices" require a choice after the colon in -Xfoo:bar, and a phase setting is effectively a choice of phase names, albeit tweaked (-Vprint:typer+, and matching phase names by prefix).

Edit: resolved by requiring a "colon arg" if a multivalued (list) option has no default value.

That currently applies only to -Vprint and -Vphases, surprisingly. -Wunused imports, for example. -Wunused with no selected choice is rejected.

@ckipp01 I think of you every time I return to this PR, which is two years now. So many colors!