Ensure **kwargs initialized by positionalOnlyCall is mutable

The fix in cf94101 made an empty **kwargs immutable when initialized via
positionalOnlyCall. Unfortunately, that was an incompatible change (although
unlikely to affect anything in practice given how positionalOnlyCall is
currently used): the Starlark language spec does not specify whether **kwargs
is mutable, but **kwargs is mutable in Python 3 and was always mutable
previously in the Starlark interpreter.

See bazelbuild/starlark#295 - perhaps it ought to
be immutable in Starlark, but that requires a language spec change and an
incompatible flag.

PiperOrigin-RevId: 712920880
Change-Id: I151bb29e9646b82def8944143198f3dffb398583

src/main/java/net/starlark/java/eval/StarlarkFunction.java

@@ -205,7 +205,7 @@ public Object positionalOnlyCall(StarlarkThread thread, Object... positional)
     checkRecursive(thread);
     ArgumentProcessor argumentProcessor = new ArgumentProcessor(this);
     // Feed only positional arguments into the argument processor.
-    argumentProcessor.processPositionalOnly(positional);
+    argumentProcessor.processPositionalOnly(positional, thread.mutability());
     return callWithArguments(thread, argumentProcessor);
   }
 
@@ -392,13 +392,14 @@ void processPositionalAndNamed(Object[] positional, Object[] named, Mutability m
       bindNamedArgsToLocals(named, mu);
     }
 
-    void processPositionalOnly(Object[] positional) throws EvalException {
+    void processPositionalOnly(Object[] positional, Mutability mu) throws EvalException {
       numNonSurplusPositionalArgs = getNumNonSurplusPositionalArgs(positional);
       bindPositionalArgsToLocals(positional);
       bindSurplusPositionalArgsToVarArgs(positional);
-      // Bind an empty dict to **kwargs if present.
+      // Bind an empty dict to **kwargs if present. (The dict, unfortunately, needs to be mutable;
+      // see https://github.com/bazelbuild/starlark/issues/295)
       if (owner.rfn.hasKwargs()) {
-        locals[owner.rfn.getParameters().size() - 1] = Dict.empty();
+        locals[owner.rfn.getParameters().size() - 1] = Dict.of(mu);
       }
     }
 

src/test/java/net/starlark/java/eval/FunctionTest.java

@@ -668,13 +668,14 @@ public void positionalOnlyCall_setsKeywordArgsVarargsAndKwargs() throws Exceptio
     ev.exec(
         """
         def f(a, b, *args, k = 42, **kwargs):
+            kwargs["mutable"] = True  # verify that **kwargs is a mutable dict
             return "k=%s args=%s kwargs=%s" % (repr(k), repr(args), repr(kwargs))
         """);
     StarlarkFunction f = (StarlarkFunction) ev.lookup("f");
     try (Mutability mu = Mutability.create("test")) {
       StarlarkThread thread = StarlarkThread.createTransient(mu, StarlarkSemantics.DEFAULT);
       assertThat((String) Starlark.positionalOnlyCall(thread, f, "a", "b", "c", "d"))
-          .isEqualTo("k=42 args=(\"c\", \"d\") kwargs={}");
+          .isEqualTo("k=42 args=(\"c\", \"d\") kwargs={\"mutable\": True}");
     }
   }
 }