Fix #3515: Do not save tasty in .class with -YemitTasty

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -223,20 +223,24 @@ class GenBCodePipeline(val entryPoints: List[Symbol], val int: DottyBackendInter
 
         if (claszSymbol.isClass) // @DarkDimius is this test needed here?
           for (binary <- ctx.compilationUnit.pickled.get(claszSymbol.asClass)) {
-            val dataAttr = new CustomAttr(nme.TASTYATTR.mangledString, binary)
             val store = if (mirrorC ne null) mirrorC else plainC
+            val tasty =
+              if (ctx.settings.emitTasty.value) {
+                val outTastyFile = getFileForClassfile(outF, store.name, ".tasty")
+                val outstream = new DataOutputStream(outTastyFile.bufferedOutput)
+                try outstream.write(binary)
+                finally outstream.close()
+                // TASTY attribute is created but 0 bytes are stored in it.
+                // A TASTY attribute has length 0 if and only if the .tasty file exists.
+                Array.empty[Byte]
+              } else {
+                // Create an empty file to signal that a tasty section exist in the corresponding .class
+                // This is much cheaper and simpler to check than doing classfile parsing
+                getFileForClassfile(outF, store.name, ".hasTasty")
+                binary
+              }
+            val dataAttr = new CustomAttr(nme.TASTYATTR.mangledString, tasty)
             store.visitAttribute(dataAttr)
-            if (ctx.settings.emitTasty.value) {
-              val outTastyFile = getFileForClassfile(outF, store.name, ".tasty")
-              val outstream = new DataOutputStream(outTastyFile.bufferedOutput)
-
-              try outstream.write(binary)
-              finally outstream.close()
-            } else {
-              // Create an empty file to signal that a tasty section exist in the corresponding .class
-              // This is much cheaper and simpler to check than doing classfile parsing
-              getFileForClassfile(outF, store.name, ".hasTasty")
-            }
           }
 
         // -------------- bean info class, if needed --------------

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -13,7 +13,7 @@ import scala.collection.{ mutable, immutable }
 import scala.collection.mutable.{ ListBuffer, ArrayBuffer }
 import scala.annotation.switch
 import typer.Checking.checkNonCyclic
-import io.AbstractFile
+import io.{AbstractFile, PlainFile}
 import scala.util.control.NonFatal
 
 object ClassfileParser {
@@ -781,7 +781,13 @@ class ClassfileParser(
 
       if (scan(tpnme.TASTYATTR)) {
         val attrLen = in.nextInt
-        return unpickleTASTY(in.nextBytes(attrLen))
+        if (attrLen == 0) {
+          // A tasty attribute implies the existence of the .tasty file
+          val file = new PlainFile(io.File(classfile.jpath).changeExtension("tasty"))
+          if (file.exists) return unpickleTASTY(new AbstractFileReader(file).nextBytes(file.sizeOption.get))
+          else ctx.error("Could not find " + file)
+        }
+        else return unpickleTASTY(in.nextBytes(attrLen))
       }
 
       if (scan(tpnme.ScalaATTR) && !scalaUnpickleWhitelist.contains(classRoot.name)) {

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

@@ -96,6 +96,7 @@ class CompilationTests extends ParallelTesting {
     compileFilesInDir("../tests/pos-scala2", scala2Mode) +
     compileFilesInDir("../tests/pos", defaultOptions) +
     compileFilesInDir("../tests/pos-deep-subtype", allowDeepSubtypes) +
+    compileDir("../tests/pos/i1137-1", defaultOptions and "-YemitTasty") +
     compileFile(
       // succeeds despite -Xfatal-warnings because of -nowarn
       "../tests/neg/customArgs/xfatalWarnings.scala",