Add a default set of rules for running `scalafix`

[Gedochao]

Is your feature request related to a problem? Please describe.
We currently require a .scalafix.conf file to be present in the project.
We should use a predefined default set of rules, if none are provided.

Describe the solution you'd like

• when scalafix is run in a directory with no .scalafix.conf, a set of default rules should be run (the temp config file can be kept in the .scala-build workspace directory)
• there should be a command line option for persisting the default set of rules in a .scalafix.conf file in the project root directory

[Gedochao]

@bjaglin @dos65 @Vigorge @tgodzik
Any suggestions on what rules should be enabled by default?

[tgodzik]

That is an interesting question, since it might be highly subjective. 🤔

I think OrganizeImports is a pretty sure one.

Let's maybe create a list:

• ExplicitResultTypes <- this still needs some work for Scala 3

• NoAutoTupling Inserts explicit tuples for adapted argument lists for compatibility with -Yno-adapted-args <- not super neccessary

• OrganizeImports <- 100% and should be safe

• RemoveUnused <- this one is also safe and only removes anything reported by the compiler under -Wunused

• DisableSyntax Reports an error for disabled features such as var or XML literals. <- I don't think we need this one

• LeakingImplicitClassVal - Adds 'private' to val parameters of implicit value classes <- also looks pretty safe

• NoValInForComprehension Removes deprecated val inside for-comprehension binders <- should also be safe

• ProcedureSyntax Replaces deprecated Scala 2.x procedure syntax with explicit ': Unit =' <- feels sensible

• RedundantSyntax Removes redundant syntax such as final modifiers on an object <- also ok

[dos65]

I like Tomek's list above.

I think OrganizeImports + RemoveUnused fall into MUST category.
Also it would be nice to automatically or by default add -Wunused flag without explicit notification to user that it's required.