fix(decode): terminate the schema walk on a self-embedded pointer struct

Assisted-by: GLM 5.3

decode_test.go

@@ -538,6 +538,34 @@ func TestUnmarshalEmbeddedPointerStruct(t *testing.T) {
 	}
 }
 
+// A struct embedding a pointer to itself is legal Go; decoding into it must
+// terminate. The schema walk used to recurse through the embedded type
+// forever.
+func TestUnmarshalSelfEmbeddedPointerStructTerminates(t *testing.T) {
+	type SelfLink struct {
+		*SelfLink
+		X int    `toml:"x"`
+		Y string `toml:"y"`
+	}
+	var n SelfLink
+	if err := Unmarshal([]byte("x = 1\ny = \"s\"\n"), &n); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if n.X != 1 || n.Y != "s" {
+		t.Fatalf("decoded: %+v", n)
+	}
+
+	// A nil self pointer on the encode side stays skippable, as any nil
+	// embedded pointer is.
+	out, err := Marshal(SelfLink{X: 2})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if want := "x = 2\ny = \"\"\n"; string(out) != want {
+		t.Errorf("output mismatch:\ngot:  %q\nwant: %q", out, want)
+	}
+}
+
 type RoundTripExtra map[string]int
 
 type RoundTripMapCfg struct {