Add MappedRoots.requireMappedPaths API

Author: lefou

core/api/src/mill/api/MappedRoots.scala

@@ -78,4 +78,17 @@ trait MappedRootsImpl {
     }
   }
 
+  /**
+   * Use this to assert at runtime, that a root path with the given `key` is defined.
+   * @throws NoSuchElementException when no path was mapped under the given `key`.
+   */
+  def requireMappedPaths(key: String*): Unit = {
+    val map = toMap
+    for {
+      singleKey <- key
+    } {
+      if (!map.contains(singleKey)) throw new NoSuchElementException(s"No root path mapping defined for '${key}'")
+    }
+  }
+
 }

runner/daemon/src/mill/daemon/MillBuildBootstrap.scala

@@ -86,6 +86,14 @@ class MillBuildBootstrap(
   }
 
   def evaluateRec(depth: Int): RunnerState = {
+
+    // We need relocatable PathRef for meta-builds for a stable classpathSig
+    MappedRoots.requireMappedPaths(
+      mill.constants.PathVars.WORKSPACE,
+      mill.constants.PathVars.HOME,
+      mill.constants.PathVars.MILL_OUT
+    )
+
     logger.withChromeProfile(s"meta-level $depth") {
       // println(s"+evaluateRec($depth) " + recRoot(projectRoot, depth))
       val currentRoot = recRoot(projectRoot, depth)