Avoid reassigning this = this in the tailrec transform.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -349,10 +349,13 @@ class TailRec extends MiniPhase {
             yield
               (getVarForRewrittenParam(param), arg)
 
-            val assignThisAndParamPairs =
-              if (prefix eq EmptyTree) assignParamPairs
-              else
-                // TODO Opt: also avoid assigning `this` if the prefix is `this.`
+            val assignThisAndParamPairs = prefix match
+              case EmptyTree =>
+                assignParamPairs
+              case prefix: This if prefix.symbol == enclosingClass =>
+                // Avoid assigning `this = this`
+                assignParamPairs
+              case _ =>
                 (getVarForRewrittenThis(), noTailTransform(prefix)) :: assignParamPairs
 
             val assignments = assignThisAndParamPairs match {

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -976,28 +976,24 @@ class TestBCode extends DottyBytecodeTest {
         Op(ICONST_0),
         Jump(IF_ICMPNE, Label(7)),
         VarOp(ILOAD, 2),
-        Jump(GOTO, Label(26)),
+        Jump(GOTO, Label(22)),
         Label(7),
-        VarOp(ALOAD, 0),
-        VarOp(ASTORE, 3),
         VarOp(ILOAD, 1),
         Op(ICONST_1),
         Op(ISUB),
-        VarOp(ISTORE, 4),
+        VarOp(ISTORE, 3),
         VarOp(ILOAD, 2),
         VarOp(ILOAD, 1),
         Op(IMUL),
-        VarOp(ISTORE, 5),
-        VarOp(ALOAD, 3),
-        VarOp(ASTORE, 0),
-        VarOp(ILOAD, 4),
+        VarOp(ISTORE, 4),
+        VarOp(ILOAD, 3),
         VarOp(ISTORE, 1),
-        VarOp(ILOAD, 5),
+        VarOp(ILOAD, 4),
         VarOp(ISTORE, 2),
-        Jump(GOTO, Label(29)),
-        Label(26),
+        Jump(GOTO, Label(25)),
+        Label(22),
         Op(IRETURN),
-        Label(29),
+        Label(25),
         Jump(GOTO, Label(0)),
         Op(NOP),
         Op(ATHROW),