add Scalafix rule to add cats-tagless instances for use with asyncMapK

running the Scalafix rule after generating Finagle code with Scrooge
will result in implicit instances being added to the companion objects
of the generated Finagle service traits.

README.md

@@ -36,6 +36,8 @@ object FooService {
 }
 ```
 
+These implicit instances can be automatically added to the code generated by Scrooge by running the `AddCatsTaglessInstances` scalafix rule! See below for more detail.
+
 Now you can safely convert your `Future`-based implementation into one in `F[_] : Async`:
 
 ```scala
@@ -64,10 +66,8 @@ trait FooScroogeService[F[_]] {
 }
 
 object FooScroogeService {
-  // these implicits won't be generated by scrooge—but we're working on that!
-  // for now you'll have to add them yourself, either here or somewhere in the 
-  // implicit scope available to the calls shown below under "Finagle Clients" 
-  // and "Finagle Servers"
+  // Let Scalafix generate these instances for you! 
+  // Follow the instructions in the `Scalafix Rule` section below.
   implicit def FooScroogeServiceReaderT[F[_]]: FooScroogeService[ReaderT[F, FooScroogeService[F], *]] =
     Derive.readerT[FooScroogeService, F]
   implicit val FooScroogeServiceFunctorK: FunctorK[FooScroogeService] = Derive.functorK[FooScroogeService]
@@ -100,6 +100,43 @@ val fooImpl: FooScroogeService[IO] = new FooScroogeService[IO] {
 val thriftServer: IO[Nothing] = ThriftServer("address", fooImpl)
 ```
 
+## Scalafix Rule
+
+Add Scalafix to your project's build by [following the instructions](https://scalacenter.github.io/scalafix/docs/users/installation.html#sbt):
+
+1. Add the Scalafix plugin to the project by adding this to `project/plugins.sbt`:
+    ```scala
+    addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.31")
+    ```
+
+2. Enable SemanticDB by adding this to `build.sbt`:
+    ```scala
+    ThisBuild / semanticdbEnabled := true
+    ThisBuild / semanticdbVersion := scalafixSemanticdb.revision
+    ThisBuild / scalafixScalaBinaryVersion := CrossVersion.binaryScalaVersion(scalaVersion.value)
+    ThisBuild / scalafixDependencies += "com.dwolla" %% "finagle-tagless-scalafix" % "0.0.4"
+    ```
+
+3. Run the Scalafix rule automatically after generating the Thrift sources by adding this to `build.sbt`:
+    ```scala
+    Compile / scalafix / unmanagedSources := (Compile / sources).value
+    Compile / compile := Def.taskDyn {
+      val compileOutput = (Compile / compile).value
+
+      Def.task {
+        (Compile / scalafix).toTask(" AddCatsTaglessInstances").value
+        compileOutput
+      }
+    }.value
+    libraryDependencies ++= {
+      val catsTaglessV = "0.14.0"
+      Seq(
+        "org.typelevel" %% "cats-tagless-core" % catsTaglessV,
+        "org.typelevel" %% "cats-tagless-macros" % catsTaglessV,
+      )
+    }
+    ```
+
 ## Artifacts
 
 The Group ID for each artifact is `"com.dwolla"`. All artifacts are published to Maven Central.
@@ -153,6 +190,14 @@ The Group ID for each artifact is `"com.dwolla"`. All artifacts are published to
 <td align="center">Cats Effect 3</td>
 </tr>
 <tr>
+<td><code>"finagle-tagless-scalafix"</code></td>
+<td>Automatically adds implicit instances needed by `asyncMapK` to the companion objects of Finagle services generated by Scrooge</td>
+<td align="center">All</td>
+<td align="center"><g-emoji class="g-emoji" alias="white_check_mark" fallback-src="https://github.githubassets.com/images/icons/emoji/unicode/2705.png">✅</g-emoji></td>
+<td align="center"><g-emoji class="g-emoji" alias="white_check_mark" fallback-src="https://github.githubassets.com/images/icons/emoji/unicode/2705.png">✅</g-emoji></td>
+<td align="center">N/A</td>
+</tr>
+<tr>
 <td><code>"async-utils-twitter-19-4-ce2"</code></td>
 <td rowspan="2">Implementation for Twitter <code>Future</code><br>(with <code>"com.twitter" %% "util-core" % "19.4.0"</code>)</td>
 <td align="center">Cats Effect 2</td>

build.sbt

@@ -21,6 +21,7 @@ lazy val CatsEffect3V = "3.2.9"
 lazy val TwitterUtilsLatestV = "21.8.0"
 lazy val TwitterUtils19_4V = "19.4.0"
 lazy val CatsTaglessV = "0.14.0"
+lazy val libthriftV = "0.10.0"
 
 lazy val SCALA_2_13 = "2.13.6"
 lazy val SCALA_2_12 = "2.12.15"
@@ -252,13 +253,72 @@ lazy val examples = (projectMatrix in file("examples"))
   .jvmPlatform(scalaVersions = Scala2Versions)
   .dependsOn(`async-utils`)
 
+lazy val `scalafix-rules` = (projectMatrix in file("scalafix/rules"))
+  .jvmPlatform(scalaVersions = Scala2Versions)
+  .settings(
+    moduleName := "finagle-tagless-scalafix",
+    libraryDependencies ++= Seq(
+      "ch.epfl.scala" %% "scalafix-core" % _root_.scalafix.sbt.BuildInfo.scalafixVersion,
+
+    ),
+    scalacOptions ~= { _.filterNot(_ == "-Xfatal-warnings") },
+  )
+
+lazy val `scalafix-input` = (project in file("scalafix/input"))
+  .settings(
+    publish / skip := true,
+    scalaVersion := Scala2Versions.head,
+    libraryDependencies ++= Seq(
+      "com.twitter" %% "scrooge-core" % TwitterUtilsLatestV,
+      "com.twitter" %% "finagle-thrift" % TwitterUtilsLatestV,
+      "org.apache.thrift" % "libthrift" % libthriftV,
+    ),
+    scalacOptions += "-nowarn",
+    scalacOptions ~= { _.filterNot(_ == "-Xfatal-warnings") },
+    semanticdbEnabled := true,
+    semanticdbVersion := scalafixSemanticdb.revision,
+  )
+  .disablePlugins(ScalafixPlugin)
+
+lazy val `scalafix-output` = (project in file("scalafix/output"))
+  .settings(
+    publish / skip := true,
+    scalaVersion := Scala2Versions.head,
+    libraryDependencies ++= Seq(
+      "com.twitter" %% "scrooge-core" % TwitterUtilsLatestV,
+      "com.twitter" %% "finagle-thrift" % TwitterUtilsLatestV,
+      "org.apache.thrift" % "libthrift" % libthriftV,
+      "org.typelevel" %% "cats-tagless-core" % CatsTaglessV,
+      "org.typelevel" %% "cats-tagless-macros" % CatsTaglessV,
+    ),
+    scalacOptions += "-nowarn",
+    scalacOptions ~= { _.filterNot(_ == "-Xfatal-warnings") },
+  )
+  .disablePlugins(ScalafixPlugin)
+
+lazy val `scalafix-tests` = (projectMatrix in file("scalafix/tests"))
+  .jvmPlatform(scalaVersions = Scala2Versions)
+  .settings(
+    publish / skip := true,
+    libraryDependencies += "ch.epfl.scala" % "scalafix-testkit" % _root_.scalafix.sbt.BuildInfo.scalafixVersion % Test cross CrossVersion.full,
+    scalafixTestkitOutputSourceDirectories := (`scalafix-output` / Compile / unmanagedSourceDirectories).value,
+    scalafixTestkitInputSourceDirectories := (`scalafix-input` / Compile / unmanagedSourceDirectories).value,
+    scalafixTestkitInputClasspath := (`scalafix-input` / Compile / fullClasspath).value,
+    scalafixTestkitInputScalacOptions := (`scalafix-input` / Compile / scalacOptions).value,
+    scalafixTestkitInputScalaVersion := (`scalafix-input` / Compile / scalaVersion).value,
+  )
+  .dependsOn(`scalafix-rules`)
+  .enablePlugins(ScalafixTestkitPlugin)
+
 lazy val `async-utils-root` = (project in file("."))
   .aggregate(
     Seq(
       `async-utils-core`,
       `async-utils`,
       `async-utils-twitter`,
       `async-utils-finagle`,
+      `scalafix-rules`,
+      `scalafix-tests`,
       examples,
     ).flatMap(_.projectRefs): _*
   )

project/plugins.sbt

@@ -2,3 +2,4 @@ addSbtPlugin("io.github.davidgregory084" % "sbt-tpolecat" % "0.1.20")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.5.9")
 addSbtPlugin("com.eed3si9n" % "sbt-projectmatrix" % "0.8.0")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.7.0")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.31")

scalafix/README.md

@@ -0,0 +1,33 @@
+# Scalafix Notes
+
+## Scrooge Updates
+
+Scrooge updates roughly monthly, and occasionally the updates introduce 
+breaking changes in the generated code. (For example, between the `21.5.0` 
+and `21.8.0` versions, [a new abstract method was added to the `ValidatingThriftStructCodec3`](https://github.com/twitter/scrooge/commit/fdb8f8f2d9cc30d6ca06b117fe7a82003f330ba8#diff-1fb7ff7b4c7a64ec907dfa620403b5b190f9829f3d5e6e0d57d07685dd53411f)
+trait, which implementations generated by the older version will be missing.)
+
+As a result, the files in `scalafix/input/src/main/scala/example/thrift` and
+`scalafix/output/src/main/scala/example/thrift` may need to be regenerated when
+the Scrooge version is updated by running the `regenerate-inputs.sh` script with
+the new Scrooge version as the first parameter.
+
+```bash
+./regenerate-inputs.sh 21.8.0
+```
+
+Then edit `scalafix/output/src/main/scala/example/thrift/SimpleService.scala` and replace the
+blank line at the end of the `SimpleService` object with these lines:
+
+```scala
+
+
+  implicit def SimpleServiceInReaderT[F[_]]: SimpleService[({type Λ[β0] = _root_.cats.data.ReaderT[F, SimpleService[F], β0]})#Λ] =
+    _root_.cats.tagless.Derive.readerT[SimpleService, F]
+
+  implicit val SimpleServiceFunctorK: _root_.cats.tagless.FunctorK[SimpleService] = _root_.cats.tagless.Derive.functorK[SimpleService]
+
+
+```
+
+(Newlines and whitespace matter to the tests!)