fix(formatter): keep trailing double spaces on JSDoc lines (#26861)

Prettier keeps a trailing double space on the middle lines of a `/**`
comment (prettier/prettier#18594, in the pinned 3.9.6): it is a Markdown
hard line break that doc tooling renders (prettier/prettier#6793). We
printed every aligned line through `trim()`, so the spaces were dropped:

```tsx
/**
 * JSDoc with new line using double space  
 * It's on another line
 */
```

Match Prettier's `printIndentableBlockComment`: in a `/**` comment (not
`/***`), a middle line whose source ends with two or more spaces keeps
exactly two after its trimmed content. A bare `*` line, a single
trailing space, the opening line, and non-JSDoc block comments are
trimmed as before. The check is textual rather than `comment.is_jsdoc()`
because the parser also tags `/***` as JSDoc and Prettier's test is
textual. The printer never trims text at a line end, so writing the two
spaces before the next hard line break is enough.

Fixture `js/comments/jsdoc-trailing-double-space.js` mirrors Prettier's
`tests/format/js/comments/trailing-spaces` cases plus a plain `/*`
block; its output is byte-identical to Prettier 3.9.8. One line added to
`js/crlf/block-comment.js` pins the CRLF output. `cargo test -p
oxc_formatter`, clippy and fmt are clean; I did not run the
`tasks/coverage` idempotency suites locally.

Written with help from an AI assistant (Claude); reviewed and tested by
me.

Closes #26854

---------

Co-authored-by: leaysgur <6259812+leaysgur@users.noreply.github.com>

crates/oxc_formatter/src/formatter/trivia.rs

@@ -564,9 +564,17 @@ impl<'a> Format<'a, JsFormatContext<'a>> for FormatCommentText<'_> {
                     let first_line = lines.next().unwrap();
                     write!(f, [text(first_line.trim_end())]);
 
+                    // Not `comment.is_jsdoc()`, which also accepts `/***`
+                    let is_jsdoc = content.starts_with("/**") && !content.starts_with("/***");
+
                     // Indent the remaining lines by one space so that all `*` are aligned.
                     for line in lines {
-                        write!(f, [hard_line_break(), " ", text(line.trim())]);
+                        let trimmed = line.trim();
+                        write!(f, [hard_line_break(), " ", text(trimmed)]);
+                        // Keep a Markdown hard line break in JSDoc, as 2 trailing spaces
+                        if is_jsdoc && trimmed != "*" && line.ends_with("  ") {
+                            write!(f, ["  "]);
+                        }
                     }
                 } else {
                     // Normalize line endings `\r\n` to `\n`

crates/oxc_formatter/tests/fixtures/js/comments/jsdoc-trailing-double-space.js

@@ -0,0 +1,46 @@
+// Prettier keeps a trailing double space on JSDoc lines (prettier/prettier#18594).
+/**
+ * Two spaces stay.  
+ * It's on another line
+ */
+function two() {}
+
+/**
+ * Three spaces collapse to two.   
+ * next
+ */
+function three() {}
+
+/**
+ * One space goes. 
+ * next
+ */
+function one() {}
+
+/**
+ *  
+ * A bare star line drops its spaces.
+ */
+function bare() {}
+
+/**  
+ * Spaces on the opening line go, spaces here stay.  
+ */
+function opening() {}
+
+function nested() {
+        /**
+         * Misaligned and indented, spaces stay.  
+        */
+  return 1;
+}
+
+/***
+ * Not JSDoc, spaces go.  
+ */
+function notJsdoc() {}
+
+/*
+ * Plain block comment, spaces go.  
+ */
+function plain() {}

crates/oxc_formatter/tests/fixtures/js/comments/jsdoc-trailing-double-space.js.snap

@@ -0,0 +1,153 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// Prettier keeps a trailing double space on JSDoc lines (prettier/prettier#18594).
+/**
+ * Two spaces stay.  
+ * It's on another line
+ */
+function two() {}
+
+/**
+ * Three spaces collapse to two.   
+ * next
+ */
+function three() {}
+
+/**
+ * One space goes. 
+ * next
+ */
+function one() {}
+
+/**
+ *  
+ * A bare star line drops its spaces.
+ */
+function bare() {}
+
+/**  
+ * Spaces on the opening line go, spaces here stay.  
+ */
+function opening() {}
+
+function nested() {
+        /**
+         * Misaligned and indented, spaces stay.  
+        */
+  return 1;
+}
+
+/***
+ * Not JSDoc, spaces go.  
+ */
+function notJsdoc() {}
+
+/*
+ * Plain block comment, spaces go.  
+ */
+function plain() {}
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// Prettier keeps a trailing double space on JSDoc lines (prettier/prettier#18594).
+/**
+ * Two spaces stay.  
+ * It's on another line
+ */
+function two() {}
+
+/**
+ * Three spaces collapse to two.  
+ * next
+ */
+function three() {}
+
+/**
+ * One space goes.
+ * next
+ */
+function one() {}
+
+/**
+ *
+ * A bare star line drops its spaces.
+ */
+function bare() {}
+
+/**
+ * Spaces on the opening line go, spaces here stay.  
+ */
+function opening() {}
+
+function nested() {
+  /**
+   * Misaligned and indented, spaces stay.  
+   */
+  return 1;
+}
+
+/***
+ * Not JSDoc, spaces go.
+ */
+function notJsdoc() {}
+
+/*
+ * Plain block comment, spaces go.
+ */
+function plain() {}
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// Prettier keeps a trailing double space on JSDoc lines (prettier/prettier#18594).
+/**
+ * Two spaces stay.  
+ * It's on another line
+ */
+function two() {}
+
+/**
+ * Three spaces collapse to two.  
+ * next
+ */
+function three() {}
+
+/**
+ * One space goes.
+ * next
+ */
+function one() {}
+
+/**
+ *
+ * A bare star line drops its spaces.
+ */
+function bare() {}
+
+/**
+ * Spaces on the opening line go, spaces here stay.  
+ */
+function opening() {}
+
+function nested() {
+  /**
+   * Misaligned and indented, spaces stay.  
+   */
+  return 1;
+}
+
+/***
+ * Not JSDoc, spaces go.
+ */
+function notJsdoc() {}
+
+/*
+ * Plain block comment, spaces go.
+ */
+function plain() {}
+
+===================== End =====================

crates/oxc_formatter/tests/fixtures/js/crlf/block-comment.js

@@ -6,6 +6,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */

crates/oxc_formatter/tests/fixtures/js/crlf/block-comment.js.snap

@@ -10,6 +10,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */
@@ -28,6 +29,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */
@@ -45,6 +47,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */
@@ -62,6 +65,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */
@@ -79,6 +83,7 @@ block
 
 /**
  * js doc
+ * hard break kept  
  * @type {string}
  * @description This is a description.
  */