fix(formatter): keep trailing spaces on normal block comments (#27037)

Closes #17786

---

I've decided to keep trailing spaces in normal block comments as belonging to the user.

If the aim is to clean up unnecessary whitespace, we should presumably remove it from the other lines as well, but the current behavior only trims the first line, it is inconsistent.

Furthermore, this doesn't match the behavior of CSS block comments.

crates/oxc_formatter/src/formatter/trivia.rs

@@ -78,9 +78,8 @@
 //!    — see [`DanglingIndentMode`] for which variant an empty container takes
 //! 3. Preserves comment relationships and spacing
 //! 4. Advances cursor for processed comments
-use oxc_allocator::ArenaStringBuilder;
 use oxc_ast::{Comment, CommentContent, CommentKind};
-use oxc_formatter_core::SourceText;
+use oxc_formatter_core::{LINE_TERMINATORS, SourceText, arena_cow_str, normalize_newlines};
 use oxc_span::Span;
 use oxc_syntax::line_terminator::LineTerminatorSplitter;
 
@@ -560,8 +559,8 @@ impl<'a> Format<'a, JsFormatContext<'a>> for FormatCommentText<'_> {
         } else {
             let content = f.source_text().text_for(&comment.span);
             if comment.is_multiline_block() {
-                let mut lines = LineTerminatorSplitter::new(content);
                 if is_alignable_comment(content) {
+                    let mut lines = LineTerminatorSplitter::new(content);
                     // `unwrap` is safe because `content` contains at least one line.
                     let first_line = lines.next().unwrap();
                     write!(f, [text(first_line.trim_end())]);
@@ -578,17 +577,8 @@ impl<'a> Format<'a, JsFormatContext<'a>> for FormatCommentText<'_> {
                         }
                     }
                 } else {
-                    // Normalize line endings `\r\n` to `\n`
-                    let mut string =
-                        ArenaStringBuilder::with_capacity_in(content.len(), f.allocator());
-                    // `unwrap` is safe because `content` contains at least one line.
-                    string.push_str(lines.next().unwrap().trim_end());
-
-                    for str in lines {
-                        string.push('\n');
-                        string.push_str(str);
-                    }
-                    write!(f, [text(string.into_str())]);
+                    let normalized = normalize_newlines(content, LINE_TERMINATORS);
+                    write!(f, [text(arena_cow_str(&normalized, f))]);
                 }
             } else {
                 write!(f, [text(content.trim_end())]);

crates/oxc_formatter/tests/fixtures/js/comments/block-comment-trailing-space.js

@@ -0,0 +1,5 @@
+/* See -> 
+   this  ^
+   trailing spaces stay 
+*/
+function plain() {}

crates/oxc_formatter/tests/fixtures/js/comments/block-comment-trailing-space.js.snap

@@ -0,0 +1,30 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+/* See -> 
+   this  ^
+   trailing spaces stay 
+*/
+function plain() {}
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+/* See -> 
+   this  ^
+   trailing spaces stay 
+*/
+function plain() {}
+
+-------------------
+{ printWidth: 100 }
+-------------------
+/* See -> 
+   this  ^
+   trailing spaces stay 
+*/
+function plain() {}
+
+===================== End =====================

tasks/track_memory_allocations/allocs_formatter.yaml

@@ -1,8 +1,8 @@
 checker.ts:
   file size: 2922154 # 2.92 MB
-  sys allocs: 19383
-  sys reallocs: 1166
-  sys deallocs: 19382
+  sys allocs: 19384
+  sys reallocs: 1169
+  sys deallocs: 19383
   sys alloc bytes: 6767781 # 6.77 MB
   sys peak growth: 2973674 # 2.97 MB
   arena allocs: 1070124
@@ -16,7 +16,7 @@ App.tsx:
   sys deallocs: 5191
   sys alloc bytes: 1564670 # 1.56 MB
   sys peak growth: 435358 # 435.36 kB
-  arena allocs: 210975
+  arena allocs: 210972
   arena reallocs: 364
   arena size: 16925816 # 16.93 MB
 
@@ -49,9 +49,9 @@ antd.js:
   sys deallocs: 90042
   sys alloc bytes: 76472316 # 76.47 MB
   sys peak growth: 50737424 # 50.74 MB
-  arena allocs: 2558365
+  arena allocs: 2555858
   arena reallocs: 1889
-  arena size: 246661408 # 246.66 MB
+  arena size: 246149128 # 246.15 MB
 
 binder.ts:
   file size: 193077 # 193.08 kB