fix(hstr): Skip only \u for unicode (#11216)

**Description:**

Attempt to fix #11214.

The reason for unespaced `\\` was special handling for unicode `\u`
codepoint, that escapes any string literal starts with `\u` - for
windows, path separator with u (`\\u`) matches with this case.

PR attempts to solve by lookahead, confirming all 4 following char is
hex digit to represent unicode hex, otherwise consider it as plain
string. To be honest I'm not sure if this is acceptable approach or not,
feel free to close if there's better way to fix.

**Related issue:**

 - Closes #11214

Author: kwonoj

.changeset/rotten-frogs-drive.md

@@ -0,0 +1,6 @@
+---
+swc_core: patch
+hstr: patch
+---
+
+fix(atom): skip only unicode \u

crates/hstr/src/wtf8_atom.rs

@@ -100,9 +100,43 @@ impl serde::ser::Serialize for Wtf8Atom {
                     // By escaping literal '\u' to '\\u', we ensure:
                     // - Unpaired surrogates serialize as '\uXXXX'
                     // - Literal '\u' text serializes as '\\uXXXX'
+                    //
+                    // However, we should only escape '\u' if it's followed by exactly 4 hex digits,
+                    // which would indicate a Unicode escape sequence. Otherwise, '\u' followed by
+                    // non-hex characters (like '\util') should not be escaped.
                     if c == '\\' && iter.peek().map(|cp| cp.to_u32()) == Some('u' as u32) {
-                        iter.next(); // skip 'u'
-                        result.push_str("\\\\u");
+                        // Look ahead to see if this is followed by exactly 4 hex digits
+                        let mut lookahead = iter.clone();
+                        lookahead.next(); // skip 'u'
+
+                        let mut hex_count = 0;
+                        let mut all_hex = true;
+                        for _ in 0..4 {
+                            if let Some(next_cp) = lookahead.next() {
+                                if let Some(next_c) = next_cp.to_char() {
+                                    if next_c.is_ascii_hexdigit() {
+                                        hex_count += 1;
+                                    } else {
+                                        all_hex = false;
+                                        break;
+                                    }
+                                } else {
+                                    all_hex = false;
+                                    break;
+                                }
+                            } else {
+                                all_hex = false;
+                                break;
+                            }
+                        }
+
+                        // Only escape if we have exactly 4 hex digits after '\u'
+                        if hex_count == 4 && all_hex {
+                            iter.next(); // skip 'u'
+                            result.push_str("\\\\u");
+                        } else {
+                            result.push(c);
+                        }
                     } else {
                         result.push(c)
                     }
@@ -553,4 +587,32 @@ mod tests {
         let err_atom = result.unwrap_err();
         assert_eq!(err_atom.to_string_lossy(), "\u{FFFD}");
     }
+
+    #[test]
+    fn test_backslash_util_issue_11214() {
+        let atom =
+            Wtf8Atom::from("C:\\github\\swc-plugin-coverage-instrument\\spec\\util\\verifier.ts");
+        let serialized = serde_json::to_string(&atom).unwrap();
+
+        assert!(
+            !serialized.contains("spec\\\\\\\\util"),
+            "Found quadruple backslashes in spec segment! Serialized: {serialized}"
+        );
+
+        assert!(
+            serialized.contains("spec\\\\util"),
+            "Expected double backslashes in spec segment not found! Serialized: {serialized}",
+        );
+
+        // The expected serialized value should have consistent escaping
+        let expected = r#""C:\\github\\swc-plugin-coverage-instrument\\spec\\util\\verifier.ts""#;
+        assert_eq!(
+            serialized, expected,
+            "Serialized value should have consistent backslash escaping"
+        );
+
+        // Test round-trip
+        let deserialized: Wtf8Atom = serde_json::from_str(&serialized).unwrap();
+        assert_eq!(atom, deserialized);
+    }
 }

crates/swc_ecma_parser/tests/js/issue-11214/input.js

@@ -0,0 +1,3 @@
+var coverageData = {
+    path: "C:\\github\\swc-plugin-coverage-instrument\\spec\\util\\verifier.ts",   
+}

crates/swc_ecma_parser/tests/js/issue-11214/input.js.json

@@ -0,0 +1,70 @@
+{
+  "type": "Script",
+  "span": {
+    "start": 1,
+    "end": 107
+  },
+  "body": [
+    {
+      "type": "VariableDeclaration",
+      "span": {
+        "start": 1,
+        "end": 107
+      },
+      "ctxt": 0,
+      "kind": "var",
+      "declare": false,
+      "declarations": [
+        {
+          "type": "VariableDeclarator",
+          "span": {
+            "start": 5,
+            "end": 107
+          },
+          "id": {
+            "type": "Identifier",
+            "span": {
+              "start": 5,
+              "end": 17
+            },
+            "ctxt": 0,
+            "value": "coverageData",
+            "optional": false,
+            "typeAnnotation": null
+          },
+          "init": {
+            "type": "ObjectExpression",
+            "span": {
+              "start": 20,
+              "end": 107
+            },
+            "properties": [
+              {
+                "type": "KeyValueProperty",
+                "key": {
+                  "type": "Identifier",
+                  "span": {
+                    "start": 26,
+                    "end": 30
+                  },
+                  "value": "path"
+                },
+                "value": {
+                  "type": "StringLiteral",
+                  "span": {
+                    "start": 32,
+                    "end": 101
+                  },
+                  "value": "C:\\github\\swc-plugin-coverage-instrument\\spec\\util\\verifier.ts",
+                  "raw": "\"C:\\\\github\\\\swc-plugin-coverage-instrument\\\\spec\\\\util\\\\verifier.ts\""
+                }
+              }
+            ]
+          },
+          "definite": false
+        }
+      ]
+    }
+  ],
+  "interpreter": null
+}