-Ylog:X now only log phase X, use -Ylog:X+ to also log phase X+1

src/dotty/tools/dotc/config/CompilerCommand.scala

@@ -19,11 +19,13 @@ object CompilerCommand extends DotClass {
     |Where multiple values are accepted, they should be comma-separated.
     |  example: -Xplugin:plugin1,plugin2
     |<phases> means one or a comma-separated list of:
-    |  (partial) phase names, phase ids, phase id ranges, or the string "all".
+    |  - (partial) phase names with an optional "+" suffix to include the next phase
+    |  - the string "all"
     |  example: -Xprint:all prints all phases.
-    |  example: -Xprint:expl,24-26 prints phases explicitouter, closelim, dce, jvm.
-    |  example: -Xprint:-4 prints only the phases up to typer.
-    |
+    |  example: -Xprint:front,mixin prints the frontend and mixin phases.
+    |  example: -Ylog:erasure+ logs the erasure phase and the phase after the erasure phase.
+    |           This is useful because during the tree transform of phase X, we often
+    |           already are in phase X+1.
   """.stripMargin.trim + "\n"
 
   def shortUsage = s"Usage: $cmdName <options> <source files>"

src/dotty/tools/dotc/core/Decorators.scala

@@ -128,15 +128,22 @@ object Decorators {
     def show(implicit ctx: Context) = text.mkString(ctx.settings.pageWidth.value)
   }
 
-  /** Implements a test whether a list of strings representing phases contains
-   *  a given phase. The test returns true if the given phase starts with
-   *  one of the names in the list of strings, or if the list of strings
-   *  contains "all".
+  /** Test whether a list of strings representing phases contains
+   *  a given phase. See [[config.CompilerCommand#explainAdvanced]] for the
+   *  exact meaning of "contains" here.
    */
   implicit class PhaseListDecorator(val names: List[String]) extends AnyVal {
-    def containsPhase(phase: Phase): Boolean = phase  match {
+    def containsPhase(phase: Phase): Boolean = phase match {
       case phase: TreeTransformer => phase.transformations.exists(trans => containsPhase(trans.phase))
-      case _ => names exists (n => n == "all" || phase.phaseName.startsWith(n))
+      case _ =>
+        names exists { name =>
+          name == "all" || {
+            val strippedName = name.stripSuffix("+")
+            val logNextPhase = name ne strippedName
+            phase.phaseName.startsWith(strippedName) ||
+              (logNextPhase && phase.prev.phaseName.startsWith(strippedName))
+          }
+        }
     }
   }
 

src/dotty/tools/dotc/reporting/Reporter.scala

@@ -101,16 +101,12 @@ trait Reporting { this: Context =>
   def incompleteInputError(msg: String, pos: SourcePosition = NoSourcePosition)(implicit ctx: Context): Unit =
     reporter.incomplete(new Error(msg, pos))(ctx)
 
-  /** Log msg if current phase or its precedessor is mentioned in
-   *  settings.log.
-   *  The reason we also pick the predecessor is that during the
-   *  tree transform of phase X, we often are already in phase X+1.
-   *  It's convenient to have logging work independently of whether
-   *  we have advanced the phase or not.
+  /** Log msg if settings.log contains the current phase.
+   *  See [[config.CompilerCommand#explainAdvanced]] for the exact meaning of
+   *  "contains" here.
    */
   def log(msg: => String): Unit =
-    if (this.settings.log.value.containsPhase(phase) ||
-        this.settings.log.value.containsPhase(phase.prev))
+    if (this.settings.log.value.containsPhase(phase))
       echo(s"[log ${ctx.phasesStack.reverse.mkString(" -> ")}] $msg")
 
   def debuglog(msg: => String): Unit =