Mask colons in segment paths

main/eval/src/mill/eval/EvaluatorPaths.scala

@@ -3,6 +3,8 @@ package mill.eval
 import mill.api.internal
 import mill.define.{NamedTask, Segment, Segments}
 
+import java.util.regex.Matcher
+
 case class EvaluatorPaths private (dest: os.Path, meta: os.Path, log: os.Path) {
   // scalafix:off; we want to hide the generic copy method
   private def copy(dest: os.Path = dest, meta: os.Path = meta, log: os.Path = log): EvaluatorPaths =
@@ -47,10 +49,25 @@ object EvaluatorPaths {
   // case-insensitive match on reserved names
   private val ReservedWinNames =
     raw"^([cC][oO][nN]|[pP][rR][nN]|[aA][uU][xX]|[nN][uU][lL]|[cC][oO][mM][0-9¹²³]|[lL][pP][tT][0-9¹²³])($$|[.].*$$)".r
-  def sanitizePathSegment(segment: String): os.PathChunk = {
-    segment match {
+  // Colons are not supported on Windows
+  private val Colon = "[:]".r
+  // Dollar sign `$` is our masking-character
+  private val Dollar = "[$]".r
+
+  private val steps: Seq[String => String] = Seq(
+    // Step 1: mask all existing dollar signs, so we can use the dollar as masking character
+    s => Dollar.replaceAllIn(s, Matcher.quoteReplacement("$$")),
+    // Step 2: mask reserved Windows names, like CON1 or LPT1
+    _ match {
       case ReservedWinNames(keyword, rest) => s"${keyword}~${rest}"
       case s => s
-    }
+    },
+    // Step 3: Replace colon (:) with $colon
+    s => Colon.replaceAllIn(s, Matcher.quoteReplacement("$colon"))
+  )
+
+  def sanitizePathSegment(segment: String): os.PathChunk = {
+    // sanitize and implicitly convert to PathChunk
+    steps.foldLeft(segment) { (segment, f) => f(segment) }
   }
 }

main/eval/test/src/mill/eval/EvaluatorPathsTests.scala

@@ -8,8 +8,15 @@ object EvaluatorPathsTests extends TestSuite {
     "sanitizedPathSegment" - {
       "WindowsReservedNames" - {
         val replace = Seq(
+          // reserved file names under Windows
           "com1.json" -> "com1~.json",
-          "LPT¹" -> "LPT¹~"
+          "LPT¹" -> "LPT¹~",
+          // a unsupported character under Windows
+          "a:b" -> "a$colonb",
+          // do not collide with the applied `$`-masking character
+          "a$colonb" -> "a$$colonb",
+          // replace not just the first $
+          "a$$b" -> "a$$$$b"
         )
         val noReplace = Seq(
           "con10.json"