Partial revert of previous commit.

Instead of avoiding fully qualified names,
use a different separator in zincMangledName.

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -79,6 +79,9 @@ class ExtractAPI extends Phase {
       val pw = new PrintWriter(File(sourceFile.file.jpath).changeExtension("inc").toFile
         .bufferedWriter(append = true), true)
       try {
+        pw.println()
+        pw.println("API:")
+        pw.println("====")
         classes.foreach(source => pw.println(DefaultShowAPI(source)))
       } finally pw.close()
     }

compiler/src/dotty/tools/dotc/sbt/package.scala

@@ -4,17 +4,24 @@ import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.Symbols.Symbol
 import dotty.tools.dotc.core.NameOps.stripModuleClassSuffix
 import dotty.tools.dotc.core.Names.Name
+import dotty.tools.dotc.core.Names.termName
+import dotty.tools.dotc.core.NameKinds.QualifiedNameKind
 
 inline val TermNameHash = 1987 // 300th prime
 inline val TypeNameHash = 1993 // 301st prime
 inline val InlineParamHash = 1997 // 302nd prime
 
 extension (sym: Symbol)
 
-  def constructorName(using Context) =
-    sym.owner.name ++ ";init;"
-
-  /** Mangle a JVM symbol name in a format better suited for internal uses by sbt. */
-  def zincMangledName(using Context): Name =
-    if (sym.isConstructor) constructorName
-    else sym.name.stripModuleClassSuffix
+  /** Mangle a JVM symbol name in a format better suited for internal uses by sbt.
+   *  WARNING: output must not be written to TASTy, as it is not a valid TASTy name.
+   */
+  private[sbt] def zincMangledName(using Context): Name =
+    if sym.isConstructor then
+      // TODO: ideally we should avoid unnecessarily caching these Zinc specific
+      // names in the global chars array. But we would need to restructure
+      // ExtractDependencies caches to avoid expensive `toString` on
+      // each member reference.
+      termName(sym.owner.fullName.mangledString.replace(".", ";").nn ++ ";init;")
+    else
+      sym.name.stripModuleClassSuffix

sbt-bridge/test/xsbt/ExtractUsedNamesSpecification.scala

@@ -306,7 +306,7 @@ class ExtractUsedNamesSpecification {
     // All classes extend Object
     "Object",
     // All classes have a default constructor called <init>
-    "Object;init;",
+    "java;lang;Object;init;",
     // the return type of the default constructor is Unit
     "Unit"
   )