feat: carry the byte offset and column in SyntaxError

Assisted-by: GLM 5.3 Flash

decode_test.go

@@ -1220,3 +1220,56 @@ func TestNumberMethods(t *testing.T) {
 		}
 	}
 }
+
+func TestSyntaxErrorPosition(t *testing.T) {
+	src := []byte("alpha = 1\nbeta x = 2\n")
+	_, err := ParseMap(src)
+	se, ok := errors.AsType[*SyntaxError](err)
+	if !ok {
+		t.Fatalf("err = %v, want a SyntaxError", err)
+	}
+	if se.Line != 2 {
+		t.Errorf("Line = %d, want 2", se.Line)
+	}
+	if want := strings.Index(string(src), "x"); se.Offset != want {
+		t.Errorf("Offset = %d, want %d", se.Offset, want)
+	}
+	if se.Column != 6 {
+		t.Errorf("Column = %d, want 6", se.Column)
+	}
+	want := "beta x = 2\n     ^"
+	if got := se.SourceLine(src); got != want {
+		t.Errorf("SourceLine =\n%s\nwant:\n%s", got, want)
+	}
+}
+
+func TestSyntaxErrorUTF8Offset(t *testing.T) {
+	src := []byte("a = \"ok\"\nb = \"\xff\xfe\"\n")
+	_, err := ParseMap(src)
+	se, ok := errors.AsType[*SyntaxError](err)
+	if !ok {
+		t.Fatalf("err = %v, want a SyntaxError", err)
+	}
+	if !strings.Contains(se.Msg, "byte offset 14") {
+		t.Errorf("Msg = %q, want it to name byte offset 14", se.Msg)
+	}
+	if se.Offset != 14 {
+		t.Errorf("Offset = %d, want 14", se.Offset)
+	}
+	want := "b = \"\xff\xfe\"\n     ^"
+	if got := se.SourceLine(src); got != want {
+		t.Errorf("SourceLine =\n%q\nwant:\n%q", got, want)
+	}
+}
+
+func TestSourceLineEdgePositions(t *testing.T) {
+	src := []byte("a = 1\n")
+	e := &SyntaxError{Line: 1, Msg: "no position"}
+	if got, want := e.SourceLine(src), "a = 1\n^"; got != want {
+		t.Errorf("SourceLine(zero offset) =\n%q\nwant:\n%q", got, want)
+	}
+	e = &SyntaxError{Line: 2, Offset: 100, Msg: "past the end"}
+	if got, want := e.SourceLine(src), "\n^"; got != want {
+		t.Errorf("SourceLine(offset past end) =\n%q\nwant:\n%q", got, want)
+	}
+}