Stop mutate doc.parts when printing fill (#13315)

Co-authored-by: Alexander Kachkaev <alexander@kachkaev.ru>

changelog_unreleased/api/13315.md

@@ -0,0 +1,3 @@
+#### Stop doc mutation in `prettier.doc.printDocToString` (#13315 by @fisker)
+
+For performance reason, `prettier.doc.printDocToString` used to mutate `.parts` of the [`fill`](https://github.com/prettier/prettier/blob/main/commands.md#fill) command during print. It was converted to a [pure function](https://en.wikipedia.org/wiki/Pure_function) to ensure output correctness.

changelog_unreleased/javascript/13315-2.md

@@ -0,0 +1,19 @@
+#### Fix template literal print with array (#13315 by @fisker, @syi0808)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const string = `${[[1, 2], [3, 4]]}`
+
+// Prettier stable
+const string = `${[
+  [1, 2],
+  [3, 4],
+]}`;
+
+// Prettier main
+const string = `${[
+  [1, 2],
+  [3, 4],
+]}`;
+```

src/document/printer.js

@@ -1,6 +1,6 @@
 import { convertEndOfLineToChars } from "../common/end-of-line.js";
 import getStringWidth from "../utils/get-string-width.js";
-import { fill, hardlineWithoutBreakParent, indent } from "./builders.js";
+import { hardlineWithoutBreakParent, indent } from "./builders.js";
 import {
   DOC_TYPE_ALIGN,
   DOC_TYPE_ARRAY,
@@ -32,6 +32,8 @@ const MODE_FLAT = Symbol("MODE_FLAT");
 
 const CURSOR_PLACEHOLDER = Symbol("cursor");
 
+const IS_MUTABLE_FILL = Symbol("IS_MUTABLE_FILL");
+
 function rootIndent() {
   return { value: "", length: 0, queue: [] };
 }
@@ -503,16 +505,26 @@ function printDocToString(doc, options) {
           break;
         }
 
-        // At this point we've handled the first pair (context, separator)
+        const secondContent = parts[2];
-        // and will create a new fill doc for the rest of the content.
-        // Ideally we wouldn't mutate the array here but copying all the
-        // elements to a new array would make this algorithm quadratic,
-        // which is unusable for large arrays (e.g. large texts in JSX).
-        parts.splice(0, 2);
-        /** @type {Command} */
-        const remainingCmd = { ind, mode, doc: fill(parts) };
 
-        const secondContent = parts[0];
+        // At this point we've handled the first pair (context, separator)
+        // and will create a new *mutable* fill doc for the rest of the content.
+        // Copying all the elements to a new array would make this algorithm quadratic,
+        // which is unusable for large arrays (e.g. large texts in JSX).
+        // https://github.com/prettier/prettier/issues/3263#issuecomment-344275152
+        let remainingDoc = doc;
+        if (doc[IS_MUTABLE_FILL]) {
+          parts.splice(0, 2);
+        } else {
+          remainingDoc = {
+            ...doc,
+            parts: parts.slice(2),
+            [IS_MUTABLE_FILL]: true,
+          };
+        }
+
+        /** @type {Command} */
+        const remainingCmd = { ind, mode, doc: remainingDoc };
 
         /** @type {Command} */
         const firstAndSecondContentFlatCmd = {

tests/format/js/template-literals/__snapshots__/format.test.js.snap

@@ -164,6 +164,8 @@ descirbe('something', () => {
 
 throw new Error(\`pretty-format: Option "theme" has a key "\${key}" whose value "\${value}" is undefined in ansi-styles.\`,)
 
+a = \`\${[[1, 2, 3], [4, 5, 6]]}\`
+
 =====================================output=====================================
 const long1 = \`long \${
   a.b //comment
@@ -224,6 +226,11 @@ throw new Error(
   \`pretty-format: Option "theme" has a key "\${key}" whose value "\${value}" is undefined in ansi-styles.\`,
 );
 
+a = \`\${[
+  [1, 2, 3],
+  [4, 5, 6],
+]}\`;
+
 ================================================================================
 `;
 

tests/format/js/template-literals/expressions.js

@@ -44,3 +44,5 @@ descirbe('something', () => {
 })
 
 throw new Error(`pretty-format: Option "theme" has a key "${key}" whose value "${value}" is undefined in ansi-styles.`,)
+
+a = `${[[1, 2, 3], [4, 5, 6]]}`

tests/unit/doc-printer.js

@@ -0,0 +1,45 @@
+// TODO: Find a better way to test the performance
+import { fill, join, line } from "../../src/document/builders.js";
+import { printDocToString } from "../../src/document/printer.js";
+
+test("`printDocToString` should not manipulate docs", () => {
+  const printOptions = { printWidth: 40, tabWidth: 2 };
+  const doc = fill(
+    join(
+      line,
+      Array.from({ length: 255 }, (_, index) => String(index + 1)),
+    ),
+  );
+
+  expect(doc.parts.length).toBe(255 + 254);
+
+  const { formatted: firstPrint } = printDocToString(doc, printOptions);
+
+  expect(doc.parts.length).toBe(255 + 254);
+
+  const { formatted: secondPrint } = printDocToString(doc, printOptions);
+
+  expect(firstPrint).toBe(secondPrint);
+
+  {
+    // About 1000 lines, #3263
+    const WORD = "word";
+    const hugeParts = join(
+      line,
+      Array.from(
+        { length: 1000 * Math.ceil(printOptions.printWidth / WORD.length) },
+        () => WORD,
+      ),
+    );
+    const orignalLength = hugeParts.length;
+
+    const startTime = performance.now();
+    const { formatted } = printDocToString(fill(hugeParts), printOptions);
+    const endTime = performance.now();
+    expect(hugeParts.length).toBe(orignalLength);
+
+    const lines = formatted.split("\n");
+    expect(lines.length).toBeGreaterThan(1000);
+    expect(endTime - startTime).toBeLessThan(1000);
+  }
+});