Cleanup mill.api package (#5478)

- Rename `RootModule0` -> `RootModule`
- Rename `BaseModule` -> `RootModule0`
- Privatize `mill.api.Lazy`
- Rename `TaskModule` to `DefaultTaskModule`
- Privatize `Mirrors`

Author: lihaoyi

ci/mill-bootstrap.patch

@@ -1,5 +1,5 @@
 diff --git a/build.mill b/build.mill
-index bb60d3b7bee..78ec19509ea 100644
+index c87279ddc31..ab3f79762c0 100644
 --- a/build.mill
 +++ b/build.mill
 @@ -10,12 +10,12 @@ import millbuild.*
@@ -18,7 +18,7 @@ index bb60d3b7bee..78ec19509ea 100644
  import mill.api.SelectMode
  import mill.T
 diff --git a/contrib/package.mill b/contrib/package.mill
-index 38f05bccde9..1cbbd237d33 100644
+index 82bcca15874..7b019a583a6 100644
 --- a/contrib/package.mill
 +++ b/contrib/package.mill
 @@ -5,7 +5,7 @@ import coursier.maven.MavenRepository
@@ -193,7 +193,7 @@ index ef25e2a891f..11e78faf6da 100644
 +//  def mimaExcludeAnnotations = Seq("mill.api.internal.internal", "mill.api.experimental")
  }
 diff --git a/website/package.mill b/website/package.mill
-index dd0965f8778..ee1476b87fa 100644
+index 9b215a2dfc2..3184bc4112d 100644
 --- a/website/package.mill
 +++ b/website/package.mill
 @@ -105,7 +105,7 @@ object `package` extends mill.Module {
@@ -223,6 +223,15 @@ index dd0965f8778..ee1476b87fa 100644
      )
      PathRef(Task.dest)
    }
+@@ -304,7 +304,7 @@ object `package` extends mill.Module {
+       )
+ 
+   def oldDocSources = Task {
+-    Task.traverse(oldDocs.items.map(_.module))(_.value.oldDocSource)()
++    Task.traverse(oldDocs.items.map(_.module))(_().oldDocSource)()
+   }
+ 
+   object oldDocs extends Cross[OldDocModule](versionLabels)
 @@ -352,7 +352,7 @@ object `package` extends mill.Module {
 
        expandDiagramsInDirectoryAdocFile(

contrib/playlib/readme.adoc

@@ -190,7 +190,7 @@ be find for a production deployment.
 
 == Using `RootModule`
 
-The `RootModule0` abstract class allows you to have the build descriptor at the same level as the
+The `RootModule` abstract class allows you to have the build descriptor at the same level as the
 source code on the filesystem. You can move from there to a multi-module build either by refactoring
 your directory layout into multiple subdirectories or by using mill's nested modules feature.
 
@@ -237,7 +237,7 @@ The directory layout was:
         └── controllers
 ----
 
-by extending `RootModule0` in your build:
+by extending `RootModule` in your build:
 
 .`build.mill`
 [source,scala]

core/api/src/mill/api/BaseModule.scala

@@ -158,12 +158,17 @@ object Cross {
  */
 trait Cross[M <: Cross.Module[?]](factories: Cross.Factory[M]*) extends mill.api.Module {
 
+  private class Lazy[T](t: () => T) extends Function0[T] {
+    lazy val value: T = t()
+    def apply(): T = value
+  }
+
   val ctx: ModuleCtx = moduleCtx
 
   trait Item {
     def crossValues: List[Any]
     def crossSegments: List[String]
-    def module: Lazy[M]
+    def module: Function0[M]
     def cls: Class[?]
   }
 
@@ -209,7 +214,7 @@ trait Cross[M <: Cross.Module[?]](factories: Cross.Factory[M]*) extends mill.api
    * A list of the cross modules, in
    * the order the original cross values were given in
    */
-  lazy val crossModules: Seq[M] = items.map(_.module.value)
+  lazy val crossModules: Seq[M] = items.map(_.module())
 
   /**
    * A mapping of the raw cross values to the cross modules, in
@@ -219,7 +224,7 @@ trait Cross[M <: Cross.Module[?]](factories: Cross.Factory[M]*) extends mill.api
     .map { i => (i.crossValues, i.module) }
     .to(collection.mutable.LinkedHashMap)
     .view
-    .mapValues(_.value)
+    .mapValues(_())
 
   /**
    * A mapping of the string-ified string segments to the cross modules, in
@@ -229,7 +234,7 @@ trait Cross[M <: Cross.Module[?]](factories: Cross.Factory[M]*) extends mill.api
     .map { i => (i.crossSegments, i.module) }
     .to(collection.mutable.LinkedHashMap)
     .view
-    .mapValues(_.value)
+    .mapValues(_())
 
   /**
    * The default cross segments to use, when no cross value is specified.

core/api/src/mill/api/Cross.scala

@@ -4,7 +4,7 @@ package mill.api
  * A [[Module]] that has a [[defaultTask]] that will be automatically
  * executed if the module name is provide at the Mill command line
  */
-trait TaskModule extends Module {
+trait DefaultTaskModule extends Module {
 
   /**
    * The name of the default command, which will be automatically executed if

core/api/src/mill/api/TaskModule.scala renamed to core/api/src/mill/api/DefaultTaskModule.scala

@@ -5,6 +5,8 @@ import mill.api.*
 import mill.api.daemon.Watchable
 import mill.api.BuildCtx
 import mill.api.daemon.internal.{EvaluatorApi, TaskApi}
+import mill.api.internal.RootModule0
+
 import scala.util.DynamicVariable
 import scala.collection.mutable
 import scala.jdk.CollectionConverters.*
@@ -25,7 +27,7 @@ trait Evaluator extends AutoCloseable with EvaluatorApi {
   private[mill] def outPath: os.Path
   private[mill] def outPathJava = outPath.toNIO
   private[mill] def codeSignatures: Map[String, Int]
-  private[mill] def rootModule: BaseModule
+  private[mill] def rootModule: RootModule0
   private[mill] def workerCache: mutable.Map[String, (Int, Val)]
   private[mill] def env: Map[String, String]
   private[mill] def effectiveThreadCount: Int

core/api/src/mill/api/Evaluator.scala

@@ -1,5 +1,7 @@
 package mill.api
 
+import mill.api.internal.RootModule0
+
 /**
  * A module defined outside of the `build.mill` file, and is instead
  * provided builtin by some Mill library or plugin
@@ -13,7 +15,7 @@ abstract class ExternalModule(implicit
     millModuleEnclosing0: sourcecode.Enclosing,
     millModuleLine0: sourcecode.Line,
     millFile0: sourcecode.File
-) extends BaseModule(BuildCtx.workspaceRoot, external0 = true)(
+) extends RootModule0(BuildCtx.workspaceRoot, external0 = true)(
       millModuleEnclosing0,
       millModuleLine0,
       millFile0

core/api/src/mill/api/ExternalModule.scala

@@ -1,6 +1,6 @@
 package mill.api
 
-import mill.api.internal.OverrideMapping
+import mill.api.internal.{RootModule0, OverrideMapping}
 
 import scala.annotation.{compileTimeOnly, implicitNotFound}
 
@@ -80,7 +80,7 @@ object ModuleCtx extends LowPriCtx {
     private[mill] def millSourcePath: os.Path
 
     /**
-     * The full path of this task or module, from the [[BaseModule]]
+     * The full path of this task or module, from the [[RootModule0]]
      */
     private[mill] def segments: Segments
 
@@ -90,7 +90,7 @@ object ModuleCtx extends LowPriCtx {
     private[mill] def external: Boolean
 
     /**
-     * The [[Discover]] instance associate with this [[BaseModule]] hierarchy
+     * The [[Discover]] instance associate with this [[RootModule0]] hierarchy
      */
     private[mill] def discover: Discover
 

core/api/src/mill/api/Lazy.scala

@@ -2,7 +2,7 @@ package mill.api.internal
 
 import mill.api.BuildCtx
 
-class BuildFileCls(rootModule0: => mill.api.internal.RootModule0)
+class BuildFileCls(rootModule0: => mill.api.internal.RootModule)
     extends mill.api.daemon.internal.BuildFileApi {
   def value = this
   def checker = mill.api.internal.ResolveChecker(BuildCtx.workspaceRoot)