Tweak -Xprint output

Most importantly, move "unchanged since $phase" to be on the same line
as the lead-in.  And then I copied over the lead-in from Scala 2,
because I like it.  It doesn't look as nice because MegaPhases don't
have their own trim name, but it starts nicely:

    [[syntax trees at end of                     typer]] // a.scala
    package <empty> {
      class C() extends Object() {
        def foo: Int = 1
      }
    }

    [[syntax trees at end of          inlinedPositions]] // a.scala: unchanged since typer
    [[syntax trees at end of                 posttyper]] // a.scala
    package <empty> {
      @scala.annotation.internal.SourceFile("a.scala") class C() extends Object() {
        def foo: Int = 1
      }
    }

    [[syntax trees at end of                   pickler]] // a.scala: unchanged since posttyper
    [[syntax trees at end of                  inlining]] // a.scala: unchanged since posttyper
    [[syntax trees at end of              postInlining]] // a.scala: unchanged since posttyper
    [[syntax trees at end of                   staging]] // a.scala: unchanged since posttyper
    [[syntax trees at end of              pickleQuotes]] // a.scala: unchanged since posttyper
    [[syntax trees at end of MegaPhase{firstTransform, checkReentrant, elimPackagePrefixes, cookComments, checkStatic, betaReduce, inlineVals, expandSAMs}]] // a.scala: unchanged since posttyper
    [[syntax trees at end of MegaPhase{elimRepeated, protectedAccessors, extmethods, uncacheGivenAliases, byNameClosures, hoistSuperArgs, specializeApplyMethods, refchecks, tryCatchPatterns, patternMatcher}]] // a.scala: unchanged since posttyper
    [[syntax trees at end of MegaPhase{elimOpaque, explicitOuter, explicitSelf, elimByName, stringInterpolatorOpt}]] // a.scala: unchanged since posttyper
    [[syntax trees at end of MegaPhase{pruneErasedDefs, uninitializedDefs, inlinePatterns, vcInlineMethods, seqLiterals, intercepted, getters, specializeFunctions, liftTry, collectNullableFields, elimOuterSelect, resolveSuper, functionXXLForwarders, paramForwarding, genericTuples, letOverApply, arrayConstructors}]] // a.scala: unchanged since posttyper

Author: dwijnand

compiler/src/dotty/tools/dotc/Run.scala

@@ -268,28 +268,23 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     val unit = ctx.compilationUnit
     val prevPhase = ctx.phase.prev // can be a mini-phase
     val fusedPhase = ctx.base.fusedContaining(prevPhase)
-    val tree =
-      if (ctx.isAfterTyper) unit.tpdTree
-      else unit.untpdTree
+    val echoHeader = f"[[syntax trees at end of $fusedPhase%25s]] // ${unit.source}"
+    val tree = if ctx.isAfterTyper then unit.tpdTree else unit.untpdTree
     val treeString = tree.show(using ctx.withProperty(XprintMode, Some(())))
 
-    report.echo(s"result of $unit after $fusedPhase:")
-
     last match {
-      case SomePrintedTree(phase, lastTreeSting) if lastTreeSting != treeString =>
-        val msg =
-          if (!ctx.settings.XprintDiff.value && !ctx.settings.XprintDiffDel.value) treeString
-          else DiffUtil.mkColoredCodeDiff(treeString, lastTreeSting, ctx.settings.XprintDiffDel.value)
-        report.echo(msg)
-        SomePrintedTree(fusedPhase.toString, treeString)
-
-      case SomePrintedTree(phase, lastTreeSting) =>
-        report.echo("  Unchanged since " + phase)
+      case SomePrintedTree(phase, lastTreeString) if lastTreeString == treeString =>
+        report.echo(s"$echoHeader: unchanged since $phase")
         last
 
-      case NoPrintedTree =>
-        report.echo(treeString)
-        SomePrintedTree(fusedPhase.toString, treeString)
+      case SomePrintedTree(phase, lastTreeString) if ctx.settings.XprintDiff.value || ctx.settings.XprintDiffDel.value =>
+        val diff = DiffUtil.mkColoredCodeDiff(treeString, lastTreeString, ctx.settings.XprintDiffDel.value)
+        report.echo(s"$echoHeader\n$diff\n")
+        SomePrintedTree(fusedPhase.phaseName, treeString)
+
+      case _ =>
+        report.echo(s"$echoHeader\n$treeString\n")
+        SomePrintedTree(fusedPhase.phaseName, treeString)
     }
   }
 

tests/printing/i620.check

@@ -1,4 +1,4 @@
-result of tests/printing/i620.scala after typer:
+[[syntax trees at end of                     typer]] // tests/printing/i620.scala
 package O {
   package O.A {
     class D() extends Object() {
@@ -30,3 +30,4 @@ package O {
     }
   }
 }
+