fix: no source map when starts with a newline

crates/swc/tests/fixture/sourcemap/issue-9567/output/index.map

@@ -1,5 +1,5 @@
 {
-  "mappings": "AAAA,SAASA,EAAIC,CAAI,CAAEC,CAAK,EACtBD,EAAKE,SAAS,CAAC,CACXC,QAAS,CAAC,EAAEF,EAAME,OAAO,CAAC,CAAC,EACvBF,EAAMG,IAAI,CACJ,CAAC;AAAE;AAAS;AAC5B;AAAS;AACT;AAAI,GAAG,EAAEH,EAAMG,IAAI,CAAC,CAAC,CACL,QACT,CAAC,AACN,EACF,CACAL",
+  "mappings": "AAAA,SAASA,EAAIC,CAAI,CAAEC,CAAK,EACtBD,EAAKE,SAAS,CAAC,CACXC,QAAS,CAAC,EAAEF,EAAME,OAAO,CAAC,CAAC,EACvBF,EAAMG,IAAI,CACJ;AAAG;AAAS;AAC5B;AAAS;AACT;AAAI,GAAG,EAAEH,EAAMG,IAAI,CAAC,CAAC,CACL,QACT,CAAC,AACN,EACF,CACAL",
   "names": [
     "foo",
     "span",

crates/swc_ecma_codegen/src/lib.rs

@@ -139,6 +139,8 @@ fn replace_close_inline_script(raw: &str) -> Cow<str> {
     Cow::Owned(result)
 }
 
+static NEW_LINE_TPL_REGEX: Lazy<regex::Regex> = Lazy::new(|| regex::Regex::new(r"\\n|\n").unwrap());
+
 impl<W, S: SourceMapper> Emitter<'_, W, S>
 where
     W: WriteJs,
@@ -1986,22 +1988,27 @@ where
             let v = get_template_element_from_raw(&raw, self.cfg.ascii_only);
             let span = node.span();
 
-            // Search for newline chars. We search only for `\n`, since both `\r` and
-            // `\r\n` are normalized to `\n` during parse. We exclude `\u2028` and
-            // `\u2029` for performance reasons, they're so uncommon that it's probably
-            // ok. It's also unclear how other sourcemap utilities handle them...
-            let mut last = 0;
-            for (i, _) in v.match_indices('\n') {
-                if i + 1 < v.len() {
-                    // We mark the start of each line, which happens directly after this newline
-                    // char unless this is the last char.
-                    self.wr.add_srcmap(span.lo + BytePos((i + 1) as u32))?;
+            let mut last_offset_gen = 0;
+            let mut last_offset_origin = 0;
+            for ((offset_gen, _), mat) in v
+                .match_indices('\n')
+                .zip(NEW_LINE_TPL_REGEX.find_iter(&raw))
+            {
+                // If the string starts with a newline char, then adding a mark is redundant.
+                // This catches both "no newlines" and "newline after several chars".
+                if offset_gen != 0 {
+                    self.wr
+                        .add_srcmap(span.lo + BytePos(last_offset_origin as u32))?;
                 }
-                self.wr.write_str_lit(DUMMY_SP, &v[last..=i])?;
-                last = i;
+
+                self.wr
+                    .write_str_lit(DUMMY_SP, &v[last_offset_gen..=offset_gen])?;
+                last_offset_gen = offset_gen + 1;
+                last_offset_origin = mat.end();
             }
-            self.wr.add_srcmap(span.lo + BytePos(last as u32))?;
-            self.wr.write_str_lit(DUMMY_SP, &v[last..])?;
+            self.wr
+                .add_srcmap(span.lo + BytePos(last_offset_origin as u32))?;
+            self.wr.write_str_lit(DUMMY_SP, &v[last_offset_gen..])?;
             self.wr.add_srcmap(span.hi)?;
         } else {
             self.wr.write_str_lit(node.span(), &raw)?;