Add a TypelevelMTLSubmarine rule

.github/workflows/ci.yml

@@ -74,11 +74,11 @@ jobs:
 
       - name: Make target directories
         if: github.event_name != 'pull_request' && (startsWith(github.ref, 'refs/tags/v') || github.ref == 'refs/heads/main')
-        run: mkdir -p modules/fs2/rules/target target/rules-aggregate/target modules/cats-effect/rules/target modules/http4s/rules/target modules/cats/rules/target project/target
+        run: mkdir -p modules/fs2/rules/target target/rules-aggregate/target modules/cats-effect/rules/target modules/http4s/rules/target modules/mtl/rules/target modules/cats/rules/target project/target
 
       - name: Compress target directories
         if: github.event_name != 'pull_request' && (startsWith(github.ref, 'refs/tags/v') || github.ref == 'refs/heads/main')
-        run: tar cf targets.tar modules/fs2/rules/target target/rules-aggregate/target modules/cats-effect/rules/target modules/http4s/rules/target modules/cats/rules/target project/target
+        run: tar cf targets.tar modules/fs2/rules/target target/rules-aggregate/target modules/cats-effect/rules/target modules/http4s/rules/target modules/mtl/rules/target modules/cats/rules/target project/target
 
       - name: Upload target directories
         if: github.event_name != 'pull_request' && (startsWith(github.ref, 'refs/tags/v') || github.ref == 'refs/heads/main')
@@ -195,7 +195,7 @@ jobs:
       - name: Submit Dependencies
         uses: scalacenter/sbt-dependency-submission@v2
         with:
-          modules-ignore: cats-output_2.13 cats-output_2.12 http4s_2.13 http4s_2.12 fs2_2.13 fs2_2.12 cats-effect_2.13 cats-effect_2.12 cats-tests_2.13 cats-tests_2.12 fs2-tests_2.13 fs2-tests_2.12 cats-effect-tests_2.13 cats-effect-tests_2.12 typelevel-scalafix_2.13 typelevel-scalafix_2.12 fs2-output_2.13 fs2-output_2.12 fs2-input_2.13 fs2-input_2.12 cats-effect-input_2.13 cats-effect-input_2.12 http4s-tests_2.13 http4s-tests_2.12 http4s-input_2.13 http4s-input_2.12 cats-effect-output_2.13 cats-effect-output_2.12 http4s-output_2.13 http4s-output_2.12 cats_2.13 cats_2.12 cats-input_2.13 cats-input_2.12
+          modules-ignore: mtl-output_2.13 mtl-output_2.12 cats-output_2.13 cats-output_2.12 http4s_2.13 http4s_2.12 fs2_2.13 fs2_2.12 mtl-tests_2.13 mtl-tests_2.12 cats-effect_2.13 cats-effect_2.12 cats-tests_2.13 cats-tests_2.12 fs2-tests_2.13 fs2-tests_2.12 cats-effect-tests_2.13 cats-effect-tests_2.12 typelevel-scalafix_2.13 typelevel-scalafix_2.12 fs2-output_2.13 fs2-output_2.12 fs2-input_2.13 fs2-input_2.12 cats-effect-input_2.13 cats-effect-input_2.12 http4s-tests_2.13 http4s-tests_2.12 mtl-input_2.13 mtl-input_3 http4s-input_2.13 http4s-input_2.12 mtl_2.13 mtl_2.12 cats-effect-output_2.13 cats-effect-output_2.12 http4s-output_2.13 http4s-output_2.12 cats_2.13 cats_2.12 cats-input_2.13 cats-input_2.12
           configs-ignore: test scala-tool scala-doc-tool test-internal
 
   validate-steward:

.scalafmt.conf

@@ -11,3 +11,9 @@ indent {
   defnSite = 2
   extendSite = 2
 }
+
+fileOverride {
+  "glob:**/scala-3/**" {
+    runner.dialect = scala3
+  }
+}

README.md

@@ -21,6 +21,8 @@ ThisBuild / scalafixDependencies += "org.typelevel" %% "typelevel-scalafix" % "0
 ThisBuild / scalafixDependencies += "org.typelevel" %% "typelevel-scalafix-cats" % "0.2.0"
 // To add only cats-effect Scalafix rules
 ThisBuild / scalafixDependencies += "org.typelevel" %% "typelevel-scalafix-cats-effect" % "0.2.0"
+// To add only cats-mtl Scalafix rules
+ThisBuild / scalafixDependencies += "org.typelevel" %% "typelevel-scalafix-cats-mtl" % "0.2.0"
 // To add only fs2 Scalafix rules
 ThisBuild / scalafixDependencies += "org.typelevel" %% "typelevel-scalafix-fs2" % "0.2.0"
 // To add only http4s Scalafix rules
@@ -41,6 +43,7 @@ rules = [
   TypelevelFs2SyncCompiler
   TypelevelHttp4sLiteralsSyntax
   TypelevelIORandomUUID
+  TypelevelMTLSubmarine
 ]
 ```
 
@@ -62,6 +65,7 @@ Not all rules function with Scala 3 yet.
 | TypelevelUnusedShowInterpolator | :white_check_mark: | :x:                |
 | TypelevelFs2SyncCompiler        | :white_check_mark: | :x:                |
 | TypelevelHttp4sLiteralsSyntax   | :white_check_mark: | :white_check_mark: |
+| TypelevelMTLSubmarine           | :white_check_mark: | :white_check_mark: |
 
 ## Rules for cats
 
@@ -170,6 +174,36 @@ val test = IO.randomUUID
 
 This rule works on variable declarations, usaged within methods as well as for comprehensions.
 
+## Rules for cats-mtl
+
+### TypelevelMTLSubmarine
+
+See https://typelevel.org/blog/2025/09/02/custom-error-types.html.
+
+This rule forbids calling error-handling methods (`handleError`, `recover`, `onError`, etc.) 
+on expressions that require `cats.mtl.Raise[F, E]`.
+`Raise` provided by `Handle.allow` uses a traceless exception type called `cats.mtl.Handle#Submarine`, 
+so handling it through `ApplicativeError`, `MonadError`, or `IO` error-handling methods is not always desirable.
+
+For example:
+```scala
+import cats.effect.IO
+import cats.mtl.{Raise, Handle}
+
+def raiseError: Raise[IO, String] ?=> IO[Unit] = r =>
+  r.raise("boom")
+
+def standardError: IO[Unit] =
+  IO.raiseError(new RuntimeException("boom"))
+
+Handle.allow[String] {
+  for {
+    _ <- raiseError.onError(e => IO.println("Error: " + e)) // forbidden
+    _ <- standardError.onError(e => IO.println("Error: " + e)) // allowed
+  } yield 
+}
+```
+
 ## Rules for fs2
 
 ### TypelevelFs2SyncCompiler

build.sbt

@@ -7,6 +7,7 @@ lazy val CatsVersion       = "2.12.0"
 lazy val CatsEffectVersion = "3.6.3"
 lazy val Fs2Version        = "3.12.2"
 lazy val Http4sVersion     = "0.23.32"
+lazy val MtlVersion        = "1.6.0"
 
 ThisBuild / startYear := Some(2022)
 ThisBuild / developers ++= List(
@@ -19,12 +20,12 @@ ThisBuild / scalafixScalaBinaryVersion := CrossVersion.binaryScalaVersion(scalaV
 
 lazy val `typelevel-scalafix` = project
   .in(file("."))
-  .aggregate(`typelevel-scalafix-rules`, cats.all, catsEffect.all, fs2.all, http4s.all)
+  .aggregate(`typelevel-scalafix-rules`, cats.all, catsEffect.all, fs2.all, http4s.all, mtl.all)
   .enablePlugins(NoPublishPlugin)
 
 lazy val `typelevel-scalafix-rules` = project
   .in(file("target/rules-aggregate"))
-  .dependsOn(cats.rules, catsEffect.rules, fs2.rules, http4s.rules)
+  .dependsOn(cats.rules, catsEffect.rules, fs2.rules, http4s.rules, mtl.rules)
   .settings(
     moduleName := "typelevel-scalafix",
     tlVersionIntroduced ++= List("2.12", "2.13").map(_ -> "0.1.2").toMap,
@@ -83,3 +84,17 @@ lazy val http4s = scalafixProject("http4s")
       "org.http4s" %% "http4s-core" % Http4sVersion
     )
   )
+
+// typelevel/mtl Scalafix rules
+lazy val mtl = scalafixProject("mtl")
+  .rulesSettings(
+    tlVersionIntroduced ++= List("2.12", "2.13").map(_ -> "0.6.0").toMap
+  )
+  .inputSettings(
+    scalaVersion       := "3.7.2",
+    crossScalaVersions := Seq(V.scala213, "3.7.2"),
+    libraryDependencies ++= Seq(
+      "org.typelevel" %% "cats-effect" % CatsEffectVersion,
+      "org.typelevel" %% "cats-mtl"    % MtlVersion
+    )
+  )