Allow publishing bridge for a single version (#2822)

Releasing a new version of the bridge just for adding a Scala version is
wasteful.
We can publish the bridge just for the new version (in our case
`2.13.12`) without tagging a `0.0.2`.
This PR adds a `bridge_versions` input to the `workflow_dispatch` which
can be `all` for the current behavior or a comma separated list of Scala
versions. This way we can release the current bridge for Scala `2.13.12`
and then add `2.13.12` to the list of supported bridges in a later PR.

Pull Request: #2822

.github/workflows/publish-bridges.yml

@@ -5,6 +5,12 @@ name: Publish Bridges
 # publishing workflow that runs every Mill version
 on:
   workflow_dispatch:
+    inputs:
+      bridge_versions:
+        description: 'comma-separated list of Scala versions to publish or `all` for all supported versions'
+        required: true
+        type: string
+
 jobs:
   publish-bridges:
     runs-on: ubuntu-latest
@@ -19,7 +25,7 @@ jobs:
       LANG: "en_US.UTF-8"
       LC_MESSAGES: "en_US.UTF-8"
       LC_ALL: "en_US.UTF-8"
-      MILL_BUILD_COMPILER_BRIDGES: "true"
+      MILL_COMPILER_BRIDGE_VERSIONS: ${{ inputs.bridge_versions }}
 
     steps:
       - uses: actions/checkout@v4

build.sc

@@ -185,13 +185,6 @@ def millBinPlatform: T[String] = T {
 
 def baseDir = build.millSourcePath
 
-// We limit the number of compiler bridges to compile and publish for local
-// development and testing, because otherwise it takes forever to compile all
-// of them. Compiler bridges not in this set will get downloaded and compiled
-// on the fly anyway. For publishing, we publish everything.
-val buildAllCompilerBridges = interp.watchValue(sys.env.contains("MILL_BUILD_COMPILER_BRIDGES"))
-val bridgeVersion = "0.0.1"
-
 val bridgeScalaVersions = Seq(
   // Our version of Zinc doesn't work with Scala 2.12.0 and 2.12.4 compiler
   // bridges. We skip 2.12.1 because it's so old not to matter, and we need a
@@ -223,7 +216,17 @@ val bridgeScalaVersions = Seq(
   "2.13.11"
 )
 
-val buildBridgeScalaVersions = if (!buildAllCompilerBridges) Seq() else bridgeScalaVersions
+// We limit the number of compiler bridges to compile and publish for local
+// development and testing, because otherwise it takes forever to compile all
+// of them. Compiler bridges not in this set will get downloaded and compiled
+// on the fly anyway. For publishing, we publish everything or a specific version
+// if given.
+val compilerBridgeScalaVersions = interp.watchValue(sys.env.get("MILL_COMPILER_BRIDGE_VERSIONS")) match {
+  case None => Seq.empty[String]
+  case Some("all") => bridgeScalaVersions
+  case Some(versions) => versions.split(',').map(_.trim).toSeq
+}
+val bridgeVersion = "0.0.1"
 
 trait MillJavaModule extends JavaModule {
 
@@ -395,7 +398,7 @@ trait MillStableScalaModule extends MillPublishScalaModule with Mima {
   def skipPreviousVersions: T[Seq[String]] = T(Seq.empty[String])
 }
 
-object bridge extends Cross[BridgeModule](buildBridgeScalaVersions)
+object bridge extends Cross[BridgeModule](compilerBridgeScalaVersions)
 trait BridgeModule extends MillPublishJavaModule with CrossScalaModule {
   def scalaVersion = crossScalaVersion
   def publishVersion = bridgeVersion