[DRAFT] Allow customisation of metaprogramming via scalac flags

Author: japgolly

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -44,6 +44,7 @@ trait CommonScalaSettings { self: Settings.SettingGroup =>
   /** Other settings */
   val encoding: Setting[String] = StringSetting("-encoding", "encoding", "Specify character encoding used by source files.", Properties.sourceEncoding, aliases = List("--encoding"))
   val usejavacp: Setting[Boolean] = BooleanSetting("-usejavacp", "Utilize the java.class.path in classpath resolution.", aliases = List("--use-java-class-path"))
+  val compileTimeEnv: Setting[List[String]] = MultiStringSetting("-E", "key[=value]", "Options to pass to the metaprogramming environment. e.g. -Ecom.example.app.mode=RELEASE")
 
   /** Plugin-related setting */
   val plugin: Setting[List[String]]             = MultiStringSetting  ("-Xplugin", "paths", "Load a plugin from each classpath.")

compiler/src/dotty/tools/dotc/config/Settings.scala

@@ -166,6 +166,14 @@ object Settings {
 
       def matches(argName: String) = (name :: aliases).exists(_ == argName)
 
+      // TODO Do properly
+      if name == "-E" && prefix.isEmpty && arg.startsWith(name) then
+        val s = arg.drop(name.length)
+        if s.isEmpty || s.startsWith("=") then
+          return state // Upstream will report as a bad option
+        else
+          return update(s :: Nil, args)
+
       if (prefix != "" && arg.startsWith(prefix))
         doSet(arg drop prefix.length)
       else if (prefix == "" && matches(arg.takeWhile(_ != ':')))

compiler/src/dotty/tools/dotc/core/CompileTimeEnvMap.scala

@@ -0,0 +1,32 @@
+package dotty.tools.dotc.core
+
+import dotty.tools.dotc.config.ScalaSettings
+import dotty.tools.dotc.config.Settings.Setting.value
+import Contexts._
+
+// TODO doc
+final case class CompileTimeEnvMap(env: Map[String, String]) {
+  def apply(key: String): Option[String] =
+    env.get(key)
+}
+
+object CompileTimeEnvMap {
+
+  def fromSettings(using Context): CompileTimeEnvMap = {
+    var m = Map.empty[String, String]
+
+    for (s <- ctx.settings.compileTimeEnv.value)
+      val i = s.indexOf('=')
+      if i > 0 then
+        // -Ekey=value
+        val key = s.take(i)
+        val value = s.drop(i + 1)
+        m = m.updated(key, value)
+      else if i < 0 then
+        // -Ekey
+        val key = s
+        if !m.contains(key) then m = m.updated(key, "")
+
+    new CompileTimeEnvMap(m)
+  }
+}

compiler/src/dotty/tools/dotc/core/Contexts.scala

@@ -491,6 +491,9 @@ object Contexts {
     /** Is the explicit nulls option set? */
     def explicitNulls: Boolean = base.settings.YexplicitNulls.value
 
+    lazy val compileTimeEnvMap: CompileTimeEnvMap =
+      CompileTimeEnvMap.fromSettings
+
     /** Initialize all context fields, except typerState, which has to be set separately
      *  @param  outer   The outer context
      *  @param  origin  The context from which fields are copied

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -226,14 +226,15 @@ class Definitions {
   @tu lazy val ScalaXmlPackageClass: Symbol = getPackageClassIfDefined("scala.xml")
 
   @tu lazy val CompiletimePackageClass: Symbol = requiredPackage("scala.compiletime").moduleClass
-    @tu lazy val Compiletime_codeOf: Symbol = CompiletimePackageClass.requiredMethod("codeOf")
+    @tu lazy val Compiletime_codeOf       : Symbol = CompiletimePackageClass.requiredMethod("codeOf")
     @tu lazy val Compiletime_erasedValue  : Symbol = CompiletimePackageClass.requiredMethod("erasedValue")
     @tu lazy val Compiletime_uninitialized: Symbol = CompiletimePackageClass.requiredMethod("uninitialized")
     @tu lazy val Compiletime_error        : Symbol = CompiletimePackageClass.requiredMethod(nme.error)
     @tu lazy val Compiletime_requireConst : Symbol = CompiletimePackageClass.requiredMethod("requireConst")
     @tu lazy val Compiletime_constValue   : Symbol = CompiletimePackageClass.requiredMethod("constValue")
     @tu lazy val Compiletime_constValueOpt: Symbol = CompiletimePackageClass.requiredMethod("constValueOpt")
     @tu lazy val Compiletime_summonFrom   : Symbol = CompiletimePackageClass.requiredMethod("summonFrom")
+    @tu lazy val Compiletime_envGetOrNull : Symbol = CompiletimePackageClass.requiredMethod("envGetOrNull")
   @tu lazy val CompiletimeTestingPackage: Symbol = requiredPackage("scala.compiletime.testing")
     @tu lazy val CompiletimeTesting_typeChecks: Symbol = CompiletimeTestingPackage.requiredMethod("typeChecks")
     @tu lazy val CompiletimeTesting_typeCheckErrors: Symbol = CompiletimeTestingPackage.requiredMethod("typeCheckErrors")

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -665,19 +665,37 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
       case _ => EmptyTree
     }
 
+  private def extractConstantValue(t: Tree): Option[Any] =
+    t match
+      case ConstantValue(v)                            => Some(v)
+      case Inlined(_, Nil, Typed(ConstantValue(v), _)) => Some(v)
+      case _                                           => None
+
+  private def reportErrorConstantValueRequired(arg: Tree): Unit =
+    report.error(em"expected a constant value but found: $arg", arg.srcPos)
+
   /** The Inlined node representing the inlined call */
   def inlined(sourcePos: SrcPos): Tree = {
 
-    // Special handling of `requireConst` and `codeOf`
+    // Special handling of `requireConst`, `codeOf`, `compileTimeEnv`
     callValueArgss match
       case (arg :: Nil) :: Nil =>
         if inlinedMethod == defn.Compiletime_requireConst then
-          arg match
-            case ConstantValue(_) | Inlined(_, Nil, Typed(ConstantValue(_), _)) => // ok
-            case _ => report.error(em"expected a constant value but found: $arg", arg.srcPos)
+          if extractConstantValue(arg).isEmpty then
+            reportErrorConstantValueRequired(arg)
           return Literal(Constant(())).withSpan(sourcePos.span)
         else if inlinedMethod == defn.Compiletime_codeOf then
           return Intrinsics.codeOf(arg, call.srcPos)
+        else if inlinedMethod == defn.Compiletime_envGetOrNull then
+          extractConstantValue(arg) match
+            case Some(key: String) =>
+              ctx.compileTimeEnvMap(key) match {
+                case Some(v) => return Literal(Constant(v))
+                case None    => return Literal(Constant(null))
+              }
+            case _ =>
+              reportErrorConstantValueRequired(arg)
+              return Literal(Constant(null))
       case _ =>
 
   	// Special handling of `constValue[T]` and `constValueOpt[T]`

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -190,6 +190,7 @@ class CompilationTests {
       compileFile("tests/run-custom-args/fors.scala", defaultOptions.and("-source", "future")),
       compileFile("tests/run-custom-args/no-useless-forwarders.scala", defaultOptions and "-Xmixin-force-forwarders:false"),
       compileFile("tests/run-custom-args/defaults-serizaliable-no-forwarders.scala", defaultOptions and "-Xmixin-force-forwarders:false"),
+      compileFilesInDir("tests/run-custom-args/compileTimeEnv", defaultOptions.and("-Ea", "-Eb=1", "-Ec.b.a=x.y.z=1", "-EmyLogger.level=INFO", "-Xfatal-warnings")),
       compileFilesInDir("tests/run-custom-args/erased", defaultOptions.and("-language:experimental.erasedDefinitions")),
       compileFilesInDir("tests/run-deep-subtype", allowDeepSubtypes),
       compileFilesInDir("tests/run", defaultOptions.and("-Ysafe-init"))

library/src/scala/compiletime/package.scala

@@ -156,6 +156,25 @@ inline def summonAll[T <: Tuple]: T =
   res.asInstanceOf[T]
 end summonAll
 
+@compileTimeOnly("Illegal reference to `scala.compiletime.envGetOrNull`")
+transparent inline def envGetOrNull(inline key: String): String | Null =
+  // implemented in dotty.tools.dotc.typer.Inliner
+  error("Compiler bug: `envGetOrNull` was not evaluated by the compiler")
+
+@compileTimeOnly("Illegal reference to `scala.compiletime.envGet`")
+transparent inline def envGet(inline key: String): Option[String] =
+  inline envGetOrNull(key) match {
+    case null => None
+    case v    => Some[v.type](v)
+  }
+
+@compileTimeOnly("Illegal reference to `scala.compiletime.envGetOrElse`")
+transparent inline def envGetOrElse(inline key: String, inline default: String): String =
+  inline envGetOrNull(key) match {
+    case null => default
+    case v    => v
+  }
+
 /** Assertion that an argument is by-name. Used for nullability checking. */
 def byName[T](x: => T): T = x
 
@@ -168,4 +187,3 @@ def byName[T](x: => T): T = x
   */
 extension [T](x: T)
   transparent inline def asMatchable: x.type & Matchable = x.asInstanceOf[x.type & Matchable]
-

tests/run-custom-args/compileTimeEnv/basic.check

@@ -0,0 +1,3 @@
+a = []
+b = [1]
+c.b.a = [x.y.z=1]

tests/run-custom-args/compileTimeEnv/basic.scala

@@ -0,0 +1,15 @@
+import scala.compiletime.*
+
+object Test {
+
+  inline def logEnv(inline k: String): Unit =
+    inline envGet(k) match
+      case Some(v) => println(s"$k = [$v]")
+      case None    => println(k + " is not defined")
+
+  def main(args: Array[String]): Unit = {
+    logEnv("a")
+    logEnv("b")
+    logEnv("c.b.a")
+  }
+}

tests/run-custom-args/compileTimeEnv/logging.check

@@ -0,0 +1,2 @@
+I'm a info msg
+I'm a warn msg

tests/run-custom-args/compileTimeEnv/logging.scala

@@ -0,0 +1,38 @@
+import scala.compiletime.*
+
+object Logging {
+
+  // Just use your imagination for now :)
+  private inline val Trace = 0
+  private inline val Debug = 1
+  private inline val Info  = 2
+  private inline val Warn  = 3
+
+  private transparent inline def chosenThreshold: Int =
+    inline envGet("myLogger.level") match
+      case Some("TRACE") => Trace
+      case Some("DEBUG") => Debug
+      case Some("INFO")  => Info
+      case Some("WARN")  => Warn
+      case Some(x)       => error("Unsupported logging level: " + x)
+      case None          => Trace
+
+  private inline def log(inline lvl: Int, inline msg: String): Unit =
+    inline if lvl >= chosenThreshold then println(msg) else ()
+
+  inline def trace(inline msg: String) = log(Trace, msg)
+  inline def debug(inline msg: String) = log(Debug, msg)
+  inline def info (inline msg: String) = log(Info , msg)
+  inline def warn (inline msg: String) = log(Warn , msg)
+}
+
+object Test {
+  import Logging.*
+
+  def main(args: Array[String]): Unit = {
+    trace("I'm a trace msg")
+    debug("I'm a debug msg")
+    info("I'm a info msg")
+    warn("I'm a warn msg")
+  }
+}