Consolidate build-global APIs in mill.define.BuildCtx (#5142)

This PR standardizes the location of APIs that are global to the build
on `mill.define.BuildCtx`, meant to be analogous to `EvalCtx` and
`ModuleCtx`. Previously they were scattered on
`mill.define.WorkspaceApi`,`build.interp`, and some direct calls to
`os.checker.withValue`. Now, they all are consolidated under `BuildCtx`
where they can be discovered and documented.

Since the `watch` lists aren't conveniently bundled together with
`RootModule` anymore, we cannot rely on `RootModuleApi` when passing
them around `MillBuildBootstrap`, so I wrapped them in a `BuildFileApi`
object to pass them around conveniently. This subsumes the previous
`wrapper_object_getter` that we code-gened

Author: lihaoyi

ci/mill-bootstrap.patch

@@ -0,0 +1,35 @@
+diff --git a/build.mill b/build.mill
+index 4465e163a55..6b8da38aae7 100644
+--- a/build.mill
++++ b/build.mill
+@@ -124,7 +124,7 @@ val bridgeScalaVersions = Seq(
+ // 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 {
++  mill.define.BuildCtx.watchValue(sys.env.get("MILL_COMPILER_BRIDGE_VERSIONS")) match {
+     case None | Some("") | Some("none") => Seq.empty[String]
+     case Some("all") => (essentialBridgeScalaVersions ++ bridgeScalaVersions).distinct
+     case Some("essential") => essentialBridgeScalaVersions
+@@ -181,7 +181,7 @@ def formatDep(dep: Dep) = {
+   s"${d.module.organization.value}:${d.module.name.value}:${d.versionConstraint.asString}"
+ }
+ 
+-def listIn(path: os.Path) = interp.watchValue(os.list(path).map(_.last))
++def listIn(path: os.Path) = mill.define.BuildCtx.watchValue(os.list(path).map(_.last))
+ 
+ val dummyDeps: Seq[Dep] = Seq(
+   Deps.DocDeps.millScip,
+diff --git a/runner/codesig/package.mill b/runner/codesig/package.mill
+index 3950422474a..1ed233d24f7 100644
+--- a/runner/codesig/package.mill
++++ b/runner/codesig/package.mill
+@@ -18,7 +18,7 @@ object `package` extends MillPublishScalaModule {
+ 
+   override lazy val test: CodeSigTests = new CodeSigTests {}
+   trait CodeSigTests extends MillScalaTests {
+-    val caseKeys = build.interp.watchValue(
++    val caseKeys = mill.define.BuildCtx.watchValue(
+       os.walk(moduleDir / "cases", maxDepth = 3)
+         .map(_.subRelativeTo(moduleDir / "cases").segments)
+         .collect { case Seq(a, b, c) => s"$a-$b-$c" }

core/api/src/mill/api/internal/BuildFileApi.scala

@@ -0,0 +1,15 @@
+package mill.api.internal
+
+import mill.api.Watchable
+
+trait BuildFileApi {
+  def rootModule: RootModuleApi
+  def moduleWatchedValues: Seq[Watchable]
+  def evalWatchedValues: collection.mutable.Buffer[Watchable]
+}
+object BuildFileApi {
+  class Bootstrap(val rootModule: RootModuleApi) extends BuildFileApi {
+    def moduleWatchedValues = Nil
+    def evalWatchedValues = collection.mutable.Buffer[Watchable]()
+  }
+}

core/api/src/mill/api/internal/RootModuleApi.scala

@@ -1,9 +1,3 @@
 package mill.api.internal
 
-import mill.api.Watchable
-
-import scala.collection.mutable
-trait RootModuleApi {
-  protected[mill] def watchedValues: mutable.Buffer[Watchable]
-  protected[mill] def evalWatchedValues: mutable.Buffer[Watchable]
-}
+trait RootModuleApi {}

core/define/src/mill/define/BuildCtx.scala

@@ -0,0 +1,36 @@
+package mill.define
+import collection.mutable
+import mill.api.Watchable
+import mill.constants.EnvVars
+object BuildCtx {
+  val workspaceRoot: os.Path =
+    sys.env.get(EnvVars.MILL_WORKSPACE_ROOT).fold(os.pwd)(os.Path(_, os.pwd))
+
+  def withFilesystemCheckerDisabled[T](block: => T): T =
+    os.checker.withValue(os.Checker.Nop) { block }
+
+  protected[mill] val watchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty[Watchable]
+  protected[mill] val evalWatchedValues: mutable.Buffer[Watchable] = mutable.Buffer.empty[Watchable]
+  def watchValue[T](v0: => T)(implicit fn: sourcecode.FileName, ln: sourcecode.Line): T = {
+    withFilesystemCheckerDisabled {
+      val v = v0
+      val watchable = Watchable.Value(
+        () => v0.hashCode,
+        v.hashCode(),
+        fn.value + ":" + ln.value
+      )
+      watchedValues.append(watchable)
+      v
+    }
+  }
+
+  def watch(p: os.Path): os.Path = {
+    val watchable = Watchable.Path(p.toNIO, false, PathRef(p).sig)
+    watchedValues.append(watchable)
+    p
+  }
+
+  def watch0(w: Watchable): Unit = watchedValues.append(w)
+
+  def evalWatch0(w: Watchable): Unit = evalWatchedValues.append(w)
+}

core/define/src/mill/define/Evaluator.scala

@@ -91,7 +91,7 @@ trait Evaluator extends AutoCloseable with EvaluatorApi {
       serialCommandExec: Boolean = false,
       selectiveExecution: Boolean = false
   ): EvaluatorApi.Result[T] = {
-    os.checker.withValue(os.Checker.Nop) {
+    mill.define.BuildCtx.withFilesystemCheckerDisabled {
       execute(
         targets.map(_.asInstanceOf[Task[T]]),
         reporter,

core/define/src/mill/define/ExternalModule.scala

@@ -13,7 +13,7 @@ abstract class ExternalModule(implicit
     millModuleEnclosing0: sourcecode.Enclosing,
     millModuleLine0: sourcecode.Line,
     millFile0: sourcecode.File
-) extends BaseModule(WorkspaceRoot.workspaceRoot, external0 = true)(
+) extends BaseModule(BuildCtx.workspaceRoot, external0 = true)(
       millModuleEnclosing0,
       millModuleLine0,
       millFile0

core/define/src/mill/define/JsonFormatters.scala

@@ -79,6 +79,6 @@ object JsonFormatters extends JsonFormatters {
   private object PathTokensReader0 extends mainargs.TokensReader.Simple[os.Path] {
     def shortName = "path"
     def read(strs: Seq[String]): Either[String, Path] =
-      Right(os.Path(strs.last, WorkspaceRoot.workspaceRoot))
+      Right(os.Path(strs.last, BuildCtx.workspaceRoot))
   }
 }

core/define/src/mill/define/WorkspaceRoot.scala

@@ -0,0 +1,10 @@
+package mill.define.internal
+
+class BuildFileCls(rootModule0: => mill.define.RootModule0) extends mill.api.internal.BuildFileApi {
+  def value = this
+  def checker = mill.define.internal.ResolveChecker(mill.define.BuildCtx.workspaceRoot)
+  val rootModule = os.checker.withValue(checker) { rootModule0 }
+
+  def moduleWatchedValues = mill.define.BuildCtx.watchedValues.toSeq
+  def evalWatchedValues = mill.define.BuildCtx.evalWatchedValues
+}

core/define/src/mill/define/internal/BuildFileCls.scala

@@ -24,7 +24,7 @@ private[mill] class FileLogger(
     var folderCreated = false
     // Lazily create the folder and file that we're logging to, so as to avoid spamming the out/
     // folder with empty folders/files for the vast majority of tasks that do not have any logs
-    lazy val inner = os.checker.withValue(os.Checker.Nop) {
+    lazy val inner = mill.define.BuildCtx.withFilesystemCheckerDisabled {
       if (!os.exists(file / os.up)) os.makeDir.all(file / os.up)
       folderCreated = true
       Files.newOutputStream(file.toNIO, options*)