Resolve rule of fill violation in JSX and add assertion. (#16700)

* add stricter assertion into fill()

* resolve rule of fill issue. some comments and naming can be problematic.

* cleanup

* update comments

* rename printWhitespace -> printStringOrLine

* refine isValidSeparator() a bit

* remove FIXME comment for fixed issue

* fix idempotency problem in MDX

* add changelog

* rename assertDocFill() -> assertDocFillParts()

* add test for assertDocFillParts()

* replace cleanDoc(doc) === "" by isEmptyDoc(doc)

* Update src/document/utils/assert-doc.js

Co-authored-by: fisker Cheung <lionkay@gmail.com>

---------

Co-authored-by: fisker Cheung <lionkay@gmail.com>

tests/unit/doc-builders.js

@@ -11,6 +11,7 @@ import {
   indentIfBreak,
   join,
   label,
+  line,
   lineSuffix,
   markAsRoot,
 } from "../../src/document/builders.js";
@@ -49,6 +50,10 @@ describe("doc builders", () => {
     [() => fill(), TypeError],
     [() => fill(notArray), TypeError],
     () => fill([invalidDoc]),
+    [() => fill(["abc", "abc"]), Error],
+    [() => fill(["abc", line, "def", "ghi"]), Error],
+    [() => fill(["abc", [line, "def"], "ghi"]), Error],
+    [() => fill(["abc", "", "def"]), Error],
 
     () => ifBreak(),
     () => ifBreak(invalidDoc),

tests/unit/is-empty-doc.js

@@ -0,0 +1,30 @@
+import {
+  breakParent,
+  fill,
+  group,
+  line,
+  lineSuffix,
+} from "../../src/document/builders.js";
+import { cleanDoc, isEmptyDoc } from "../../src/document/utils.js";
+
+it.each([
+  { name: "empty string", doc: "" },
+  { name: "non-empty string", doc: "a" },
+  { name: "empty array", doc: [] },
+  { name: "nested empty array", doc: [[[]]] },
+  { name: "array with empty string", doc: [""] },
+  { name: "non-empty array", doc: ["", "a", ""] },
+  { name: "empty group", doc: group([]) },
+  { name: "nested empty group", doc: group([group([])]) },
+  { name: "group with empty string", doc: group([""]) },
+  { name: "non-empty group", doc: group(["a"]) },
+  { name: "empty fill", doc: fill([]) },
+  { name: "fill with empty content", doc: fill([["", ""]]) },
+  { name: "non-empty fill", doc: fill(["", line]) },
+  { name: "empty line", doc: line },
+  { name: "empty lineSuffix", doc: lineSuffix([]) },
+  { name: "breakParent", doc: breakParent },
+])("isEmptyDoc: $name", ({ doc }) => {
+  const expected = cleanDoc(doc) === "";
+  expect(isEmptyDoc(doc)).toBe(expected);
+});