Fix lineSuffixBoundary (#10122)

tests_integration/__tests__/line-suffix-boundary.js

@@ -0,0 +1,41 @@
+"use strict";
+
+/** @type {import('prettier')} */
+const prettier = require("prettier-local");
+
+const {
+  group,
+  indent,
+  line,
+  lineSuffix,
+  lineSuffixBoundary,
+  softline,
+} = prettier.doc.builders;
+
+const printDoc = require("../printDoc");
+
+describe("lineSuffixBoundary", () => {
+  test("should be correctly treated as a potential line break in `fits`", () => {
+    const doc = group([
+      "let foo = [",
+      indent([
+        softline,
+        [lineSuffixBoundary, "item1,"],
+        line,
+        [lineSuffixBoundary, "item2,", lineSuffix(" // comment")],
+        line,
+        [lineSuffixBoundary, "item3"],
+      ]),
+      softline,
+      "];",
+    ]);
+
+    const expected = `let foo = [
+  item1,
+  item2, // comment
+  item3
+];`;
+
+    expect(printDoc(doc)).toBe(expected);
+  });
+});

tests_integration/printDoc.js

@@ -0,0 +1,30 @@
+"use strict";
+
+/** @type {import('prettier')} */
+const prettier = require("prettier-local");
+
+function printDoc(doc) {
+  // This dummy plugin ignores the input and simply returns the given doc.
+  // This is to make sure that the doc will go through all the stages a real doc
+  // returned by a real plugin would go (e.g. `propagateBreaks`).
+  const dummyPlugin = {
+    parsers: {
+      dummy: {
+        parse: () => ({}),
+        astFormat: "dummy",
+      },
+    },
+    printers: {
+      dummy: {
+        print: () => doc,
+      },
+    },
+  };
+
+  return prettier.format("_", {
+    plugins: [dummyPlugin],
+    parser: "dummy",
+  });
+}
+
+module.exports = printDoc;