Issue 1747 docs cross compilation

docs/users/cross-building.md

@@ -0,0 +1,137 @@
+---
+id: cross-building
+title: Cross-building setups
+---
+
+Cross-building the same codebase with Scala 2.13 and Scala 3.3+ is common, but
+it collides with how Scalafix resolves rules and compiler options. A rule or
+build-tool flag that only works on Scala 2 can crash Scalafix when the Scala 3
+compilation unit is processed, and vice versa. Split your configuration per
+Scala version and let the build tool wire the right file.
+
+## Why separate configs are required today
+
+* Scala 2 only scalac flags such as `-Ywarn-unused-import` or SemanticDB options
+  using `-P:semanticdb` belong in the build tool. Those settings differ per
+  target and cannot be inferred by Scalafix.
+* Some built-in or community rules rely on compiler symbols present only in a
+  single Scala major version.
+* The CLI currently ingests a single `.scalafix.conf`; there is no per-target
+  override like sbt’s `CrossVersion`.
+
+## Limitations
+
+Scalafix cannot conditionally toggle rules or scalac options based on the input
+Scala version. Provide a separate config file per Scala version and have sbt,
+mill, or your CLI invocation choose the right file along with the correct
+compiler flags. See [issue #1747](https://github.com/scalacenter/scalafix/issues/1747)
+for additional background.
+
+## File layout suggestion
+
+Keep shared defaults in one file and let version-specific files `include` it via
+the HOCON `include` syntax. One convenient layout is:
+
+```
+.
+└── scalafix/
+    ├── common.conf
+    ├── scala2.conf
+    └── scala3.conf
+```
+
+`common.conf`
+```scala
+rules = [
+  DisableSyntax,
+  OrganizeImports
+]
+
+DisableSyntax {
+  noFinalize = true
+}
+```
+
+`scala2.conf`
+```scala
+include "common.conf"
+
+rules += RemoveUnused
+
+// Scala 2 only rule settings
+RemoveUnused {
+  imports = true
+}
+```
+
+`scala3.conf`
+```scala
+include "common.conf"
+
+rules += LeakingImplicitClassVal
+
+// Scala 3 specific tweaks go here
+OrganizeImports {
+  groupedImports = Keep
+}
+```
+
+### Multiple include files
+
+You may split out even more granular snippets (for example `linting.conf`,
+`rewrites.conf`) and include them from both `scala2.conf` and `scala3.conf`. The
+HOCON syntax supports nested includes, so feel free to create the hierarchy that
+matches your team conventions.
+
+## Selecting the right config in sbt
+
+Point `scalafixConfig` at the version-specific file, typically inside a helper
+setting applied to all cross-built projects:
+
+```scala
+import scalafix.sbt.ScalafixPlugin.autoImport._
+
+lazy val commonSettings = Seq(
+  scalafixConfig := {
+    val base = (ThisBuild / baseDirectory).value / "scalafix"
+    val file =
+      CrossVersion.partialVersion(scalaVersion.value) match {
+        case Some((3, _)) => base / "scala3.conf"
+        case _            => base / "scala2.conf"
+      }
+    Some(file)
+  }
+)
+
+lazy val core = project
+  .settings(commonSettings)
+  .settings(
+    scalaVersion := "3.3.3",
+    crossScalaVersions := Seq("2.13.14", "3.3.3")
+  )
+```
+
+For builds that already differentiate per configuration (`Compile`, `Test`,
+`IntegrationTest`), you can set `Compile / scalafixConfig` and
+`Test / scalafixConfig` separately if the inputs diverge.
+
+## Command-line usage
+
+When invoking the CLI directly, pass the desired config with `--config`:
+
+```
+scalafix --config scalafix/scala2.conf
+scalafix --config scalafix/scala3.conf
+```
+
+Your CI job can loop over each target Scala version, selecting the matching
+config before running `scalafix --check`.
+
+## Recommendations
+
+* Keep rules that truly work on both versions inside `common.conf`.
+* Manage scalac and SemanticDB flags inside the build tool per target; Scalafix
+  configs focus solely on rules and rule-specific settings.
+* Document in `README.md` or `CONTRIBUTING.md` which rules run on which Scala
+  version to reduce confusion for new contributors.
+

website/i18n/en.json

@@ -82,6 +82,9 @@
       "users/configuration": {
         "title": "Configuration"
       },
+      "users/cross-building": {
+        "title": "Cross-building setups"
+      },
       "users/installation": {
         "title": "Installation"
       },

website/sidebars.json

@@ -1,6 +1,11 @@
 {
   "users": {
-    "Usage": ["users/installation", "users/configuration", "users/suppression"],
+    "Usage": [
+      "users/installation",
+      "users/configuration",
+      "users/cross-building",
+      "users/suppression"
+    ],
     "Rules": [
       "rules/overview",
       "rules/DisableSyntax",