fix(parser): should regard comments where after = as leading comments of next token (#6355)

A part of fixing https://github.com/rolldown/rolldown/pull/2375/files#r1789011257

### Compare to `Babel`
This is also among its behavior, see

<img width="1470" alt="image" src="https://github.com/user-attachments/assets/1b1f9c78-3e44-495d-b2f7-38e6c3bc8bf1">

### Compare to `esbuild`

I haven't looked through related code from esbuild, from its playground, the behavior looks the same

https://esbuild.github.io/try/#dAAwLjI0LjAAAGNvbnN0IEEgPSAvLyBzZGRkZGQKKCkgPT4ge30

crates/oxc_parser/src/lexer/mod.rs

@@ -218,7 +218,7 @@ impl<'a> Lexer<'a> {
         self.token.end = self.offset();
         debug_assert!(self.token.start <= self.token.end);
         let token = self.token;
-        self.trivia_builder.handle_token(token.start);
+        self.trivia_builder.handle_token(token);
         self.token = Token::default();
         token
     }

crates/oxc_parser/src/lexer/trivia_builder.rs

@@ -1,6 +1,8 @@
 use oxc_ast::{Comment, CommentKind, CommentPosition, Trivias};
 use oxc_span::Span;
 
+use super::{Kind, Token};
+
 #[derive(Debug)]
 pub struct TriviaBuilder {
     // This is a set of unique comments. Duplicated
@@ -16,11 +18,20 @@ pub struct TriviaBuilder {
 
     /// Saw a newline before this position
     saw_newline: bool,
+
+    /// Previous token kind, used to indicates comments are trailing from what kind
+    previous_kind: Kind,
 }
 
 impl Default for TriviaBuilder {
     fn default() -> Self {
-        Self { comments: vec![], irregular_whitespaces: vec![], processed: 0, saw_newline: true }
+        Self {
+            comments: vec![],
+            irregular_whitespaces: vec![],
+            processed: 0,
+            saw_newline: true,
+            previous_kind: Kind::Undetermined,
+        }
     }
 }
 
@@ -57,13 +68,14 @@ impl TriviaBuilder {
         self.saw_newline = true;
     }
 
-    pub fn handle_token(&mut self, token_start: u32) {
+    pub fn handle_token(&mut self, token: Token) {
         let len = self.comments.len();
+        self.previous_kind = token.kind;
         if self.processed < len {
             // All unprocessed preceding comments are leading comments attached to this token start.
             for comment in &mut self.comments[self.processed..] {
                 comment.position = CommentPosition::Leading;
-                comment.attached_to = token_start;
+                comment.attached_to = token.start;
             }
             self.processed = len;
         }
@@ -85,8 +97,8 @@ impl TriviaBuilder {
         if comment.is_line() {
             // A line comment is always followed by a newline. This is never set in `handle_newline`.
             comment.followed_by_newline = true;
-            // A line comment is trailing when it is no preceded by a newline.
-            if !self.saw_newline {
+            // A line comment is trailing when it is no preceded by a newline and it is not after `=`
+            if !self.saw_newline && self.previous_kind != Kind::Eq {
                 self.processed = self.comments.len() + 1; // +1 to include this comment.
             }
             self.saw_newline = true;
@@ -236,4 +248,35 @@ token /* Trailing 1 */
         ];
         assert_eq!(comments, expected);
     }
+
+    #[test]
+    fn leading_comments_after_eq() {
+        let source_text = "
+            const v1 = // Leading comment 1
+            foo();
+            function foo(param =// Leading comment 2
+            new Foo()
+            ) {}
+        ";
+        let comments = get_comments(source_text);
+        let expected = vec![
+            Comment {
+                span: Span::new(26, 44),
+                kind: CommentKind::Line,
+                position: CommentPosition::Leading,
+                attached_to: 57,
+                preceded_by_newline: false,
+                followed_by_newline: true,
+            },
+            Comment {
+                span: Span::new(98, 116),
+                kind: CommentKind::Line,
+                position: CommentPosition::Leading,
+                attached_to: 129,
+                preceded_by_newline: false,
+                followed_by_newline: true,
+            },
+        ];
+        assert_eq!(comments, expected);
+    }
 }