Create the syntax rewrite in Scalafix for Scala 3

Context

Scalafix

Scalafix is the refactoring and linting tool for Scala. Scalafix is a tool and a collection of rule that one can apply in its codebase to refactor some patterns or to get some linting errors. Examples of rules are:

Remove unused: remove the unused import or unused variables
Explicit result type: add the return type of public members automatically
Disable syntax: disable some of the Scala syntax (throw, var...)

Scalafix is also a library that one can use to define its own rule.

Scala 3 new syntax

In Scala 3, we have a new control structure syntax and the new indentation syntax.

Control structure syntax

https://docs.scala-lang.org/scala3/reference/other-new-features/control-syntax.html

Traditional Scala 2 syntax:

if (cond) {
  branch1
} else  {
  branch2
}

while (cond) {
  loop
}

New Scala 3 syntax:

if cond then
  branch1
else
  branch2

while cond do
  loop

Indentation syntax

https://docs.scala-lang.org/scala3/reference/other-new-features/indentation.html#

Traditional curly-brace syntax:

class A {
  def foo = {
    body
  }
}

New Scala 3 significant indentation syntax:

class A :
  def foo =
    body

Description of the project

The goal of the project is to create Scalafix rules to refactor a codebase to the new control structure syntax and to the significant indentation syntax.

Those rules are already implemented in the Scala 3 compiler itself, however they are not very flexible:

we cannot have configuration on them
we can hardly configure which files to rewrite and which not to rewrite

With Scalafix we can have some configuration:

to keep or remove braces of the control structures
to activate or deactivate the fewer-braces rewrite
to add back-ticks around operator or keep braces

We can try to merge both rules in a single one.

We can integrate the rule in Metals to run the rule in a single file.

Expected outcome

We expect the student to implement the rewrite rules and to define some meaningful configuration on them.

An important requirement are the tests: each rule and configuration should be well-tested using the Scalafix test infrastructure.

Supervisor

Adrien Piquerez(adrien.piquerez@epfl.ch): Tooling Engineer at the Scala Center

scalacenter / student-projects