Scalafix rule to remove instance imports when upgrading to 2.2.0 (#3566)

* Bump scalafix

* Move the v1.0.0 tests to a subdirectory

* WIP scalafix rule to remove instance imports

* Catch the low-hanging fruit

* Support for removing "import cats.instances.all._"

The import is only removed if there is no possibility it is being used
to import Future instances.

* Support for removing "import cats.implicits._"

* Update the scalafix readme

* Update test command in .travis.yml

* Put sbt-scalafix back how it was

Upgrading it broke one of the v1.0.0 rules and I can't be bothered
investigating

* No need to list all the positives, just check the negatives

* Add -Ypartial-unification

The v1.0.0 expected output files won't compile without this.

* Remove special handling of Future instances

* Rewrite "import cats.implicits._" to "import cats.syntax.all._"

* Bump cats-core dependency in scalafix build.sbt

This is so we get Future instances in implicit scope

* Bump cats dependency to 2.2.0-RC4

* Add a test for rewriting of global imports

i.e. imports at the top of the file

* Work around a known Scalafix issue

* Improve the check for use of extension methods

* Handle implicit conversions

Do not remove an import of cats.implicits._ if it is being used to
import an implicit conversion

* Reorganise readme

Author: cb372

.travis.yml

@@ -63,7 +63,7 @@ jobs:
     - stage: test
       name: Scalafix tests
       env: TEST="scalafix"
-      script: cd scalafix && sbt tests/test
+      script: cd scalafix && sbt test
 
     - &bincompat
       stage: test

scalafix/README.md

@@ -1,22 +1,37 @@
 # Scalafix rules for cats
 
-## Try this!
+## How to use
 
-[Install the Scalafix sbt plugin](https://scalacenter.github.io/scalafix/docs/users/installation)
+1. [Install the Scalafix sbt plugin](https://scalacenter.github.io/scalafix/docs/users/installation)
 
-To run all rules that apply to version `1.0.0-RC1` run
+2. Run the rules appropriate to your Cats version (see below)
+
+## Migration to Cats v2.2.0
+
+First configure the SemanticDB compiler plugin to enable synthetics:
+
+```
+ThisBuild / scalacOptions += "-P:semanticdb:synthetics:on"
+```
+
+Then run Scalafix:
 
 ```sh
-sbt scalafix github:typelevel/cats/v1.0.0?sha=v1.0.0-RC1
+sbt scalafix github:typelevel/cats/Cats_v2_2_0
 ```
 
-to run all rules that apply to the current `1.0.0-SNAPSHOT` run
+### Available rules
+
+- Type class instances are now available in implicit scope, so there is a rule to
+  remove imports that are no longer needed
+
+## Migration to Cats v1.0.0
 
 ```sh
-sbt scalafix github:typelevel/cats/v1.0.0
+sbt scalafix github:typelevel/cats/Cats_v1_0_0
 ```
 
-## Available rules
+### Available rules
 
 - [x] All Unapply enabled methods, e.g. sequenceU, traverseU, etc. are removed. Unapply enabled syntax ops are also removed. Please use the partial unification SI-2712 fix instead. The easiest way might be this sbt-plugin.
 
@@ -40,7 +55,7 @@ sbt scalafix github:typelevel/cats/v1.0.0
 
 - [x] Split is removed, and the method split is moved to Arrow. Note that only under CommutativeArrow does it guarantee the non-interference between the effects. see #1567
 
-# WIP
+### WIP
 
 - [ ] cats no longer publishes the all-inclusive bundle package "org.typelevel" % "cats", use cats-core, cats-free, or cats-law accordingly instead. If you need cats.free, use "org.typelevel" % "cats-free", if you need cats-laws use "org.typelevel" % "cats-laws", if neither, use "org.typelevel" % "cats-core".
 
@@ -53,10 +68,9 @@ sbt scalafix github:typelevel/cats/v1.0.0
 - [ ] iteratorFoldM was removed from Foldable due to #1716
 
 
-## To test scala fix
+## To test the Scalafix rules
 
 ```bash
-sbt coreJVM/publishLocal freeJVM/publishLocal
 cd scalafix
 sbt test
 ```

scalafix/build.sbt

@@ -11,32 +11,68 @@ lazy val rules = project.settings(
   libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % V.scalafixVersion
 )
 
-lazy val input = project.settings(
-  libraryDependencies ++= Seq(
-    "org.typelevel" %% "cats" % "0.9.0"
-  ),
-  scalacOptions += "-language:higherKinds"
-)
+lazy val v1_0_0_input = project.in(file("v1_0_0/input"))
+  .settings(
+    libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats" % "0.9.0"
+    ),
+    scalacOptions += "-language:higherKinds"
+  )
 
-lazy val output = project.settings(
-  libraryDependencies ++= Seq(
-    "org.typelevel" %% "cats-core" % "1.0.0",
-    "org.typelevel" %% "cats-free" % "1.0.0"
-  ),
-  scalacOptions += "-language:higherKinds"
-)
+lazy val v1_0_0_output = project.in(file("v1_0_0/output"))
+  .settings(
+    libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats-core" % "1.0.0",
+      "org.typelevel" %% "cats-free" % "1.0.0"
+    ),
+    scalacOptions ++= Seq(
+      "-language:higherKinds",
+      "-Ypartial-unification"
+    )
+  )
+
+lazy val v1_0_0_tests = project.in(file("v1_0_0/tests"))
+  .settings(
+    libraryDependencies += "ch.epfl.scala" % "scalafix-testkit" % V.scalafixVersion % Test cross CrossVersion.full,
+    compile.in(Compile) :=
+      compile.in(Compile).dependsOn(compile.in(v1_0_0_input, Compile)).value,
+    scalafixTestkitOutputSourceDirectories :=
+      sourceDirectories.in(v1_0_0_output, Compile).value,
+    scalafixTestkitInputSourceDirectories :=
+      sourceDirectories.in(v1_0_0_input, Compile).value,
+    scalafixTestkitInputClasspath :=
+      fullClasspath.in(v1_0_0_input, Compile).value
+  )
+  .dependsOn(v1_0_0_input, rules)
+  .enablePlugins(ScalafixTestkitPlugin)
+
+lazy val v2_2_0_input = project.in(file("v2_2_0/input"))
+  .settings(
+    libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats-core" % "2.1.0"
+    ),
+    scalacOptions ++= Seq("-language:higherKinds", "-P:semanticdb:synthetics:on")
+  )
+
+lazy val v2_2_0_output = project.in(file("v2_2_0/output"))
+  .settings(
+    libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats-core" % "2.2.0-RC4"
+    ),
+    scalacOptions += "-language:higherKinds"
+  )
 
-lazy val tests = project
+lazy val v2_2_0_tests = project.in(file("v2_2_0/tests"))
   .settings(
     libraryDependencies += "ch.epfl.scala" % "scalafix-testkit" % V.scalafixVersion % Test cross CrossVersion.full,
     compile.in(Compile) :=
-      compile.in(Compile).dependsOn(compile.in(input, Compile)).value,
+      compile.in(Compile).dependsOn(compile.in(v2_2_0_input, Compile)).value,
     scalafixTestkitOutputSourceDirectories :=
-      sourceDirectories.in(output, Compile).value,
+      sourceDirectories.in(v2_2_0_output, Compile).value,
     scalafixTestkitInputSourceDirectories :=
-      sourceDirectories.in(input, Compile).value,
+      sourceDirectories.in(v2_2_0_input, Compile).value,
     scalafixTestkitInputClasspath :=
-      fullClasspath.in(input, Compile).value
+      fullClasspath.in(v2_2_0_input, Compile).value
   )
-  .dependsOn(input, rules)
+  .dependsOn(v2_2_0_input, rules)
   .enablePlugins(ScalafixTestkitPlugin)

scalafix/rules/src/main/scala/fix/Cats_v2_2_0.scala

@@ -0,0 +1,89 @@
+package fix
+package v2_2_0
+
+import scalafix.v0._
+import scalafix.syntax._
+import scala.meta._
+import scala.meta.contrib._
+import scala.meta.Term.Apply
+
+// ref: https://github.com/typelevel/cats/issues/3563
+case class RemoveInstanceImports(index: SemanticdbIndex)
+  extends SemanticRule(index, "RemoveInstanceImports") {
+
+  override def fix(ctx: RuleCtx): Patch = ctx.tree.collect {
+    // e.g. "import cats.instances.int._" or "import cats.instances.all._"
+    case i @ Import(Importer(Select(Select(Name("cats"), Name("instances")), x), _) :: _) =>
+      removeImportLine(ctx)(i)
+
+    // "import cats.implicits._"
+    case i @ Import(Importer(Select(Name("cats"), Name("implicits")), _) :: _) =>
+      val boundary = findLexicalBoundary(i)
+
+      // Find all synthetics between the import statement and the end of the lexical boundary
+      val lexicalStart = i.pos.end
+      val lexicalEnd = boundary.pos.end
+      try {
+        val relevantSynthetics =
+          ctx.index.synthetics.filter(x => x.position.start >= lexicalStart && x.position.end <= lexicalEnd)
+
+        val usesImplicitConversion = relevantSynthetics.exists(containsImplicitConversion)
+        val usesSyntax = relevantSynthetics.exists(containsCatsSyntax)
+
+        if (usesImplicitConversion) {
+          // the import is used to enable an implicit conversion,
+          // so we have to keep it
+          Patch.empty
+        } else if (usesSyntax) {
+          // the import is used to enable an extension method,
+          // so replace it with "import cats.syntax.all._"
+          ctx.replaceTree(i, "import cats.syntax.all._")
+        } else {
+          // the import is only used to import instances,
+          // so it's safe to remove
+          removeImportLine(ctx)(i)
+        }
+      } catch {
+        case e: scalafix.v1.MissingSymbolException =>
+          // see https://github.com/typelevel/cats/pull/3566#issuecomment-684007028
+          // and https://github.com/scalacenter/scalafix/issues/1123
+          println(s"Skipping rewrite of 'import cats.implicits._' in file ${ctx.input.label} because we ran into a Scalafix bug. $e")
+          e.printStackTrace()
+          Patch.empty
+      }
+  }.asPatch
+
+  private def removeImportLine(ctx: RuleCtx)(i: Import): Patch =
+    ctx.removeTokens(i.tokens) + removeWhitespaceAndNewlineBefore(ctx)(i.tokens.start)
+
+  private def containsImplicitConversion(synthetic: Synthetic) =
+    synthetic.names.exists(x => isCatsKernelConversion(x.symbol))
+
+  private def isCatsKernelConversion(symbol: Symbol) =
+    symbol.syntax.contains("cats/kernel") && symbol.syntax.contains("Conversion")
+
+  private def containsCatsSyntax(synthetic: Synthetic) =
+    synthetic.names.exists(x => isCatsSyntax(x.symbol))
+
+  private def isCatsSyntax(symbol: Symbol) =
+    symbol.syntax.contains("cats") && (symbol.syntax.contains("syntax") || symbol.syntax.contains("Ops"))
+
+  private def findLexicalBoundary(t: Tree): Tree = {
+    t.parent match {
+      case Some(b: Term.Block) => b
+      case Some(t: Template) => t
+      case Some(parent) => findLexicalBoundary(parent)
+      case None => t
+    }
+  }
+
+  private def removeWhitespaceAndNewlineBefore(ctx: RuleCtx)(index: Int): Patch = {
+    val whitespaceAndNewlines = ctx.tokens.take(index).takeRightWhile(t =>
+      t.is[Token.Space] ||
+      t.is[Token.Tab] ||
+      t.is[Token.LF]
+    )
+    ctx.removeTokens(whitespaceAndNewlines)
+  }
+
+}