fix(lint/useTemplate): correctly escape characters

CHANGELOG.md

@@ -79,6 +79,12 @@ our [guidelines for writing a good changelog entry](https://github.com/biomejs/b
 
   Contributed by @Conaclos
 
+- Fix [useTemplate](https://biomejs.dev/linter/rules/use-template/) that wrongly escaped strings in some edge cases ([#2580](https://github.com/biomejs/biome/issues/2580)).
+
+  Previously, the rule didn't correctly escaped characters preceded by an escaped character.
+
+  Contributed by @Conaclos
+
 ### Parser
 
 ## 1.7.1 (2024-04-22)

crates/biome_js_analyze/tests/specs/style/useTemplate/invalidIssue2580.js

@@ -0,0 +1,2 @@
+// Issue https://github.com/biomejs/biome/issues/2580
+'```ts\n' + x + '\n```';

crates/biome_js_analyze/tests/specs/style/useTemplate/invalidIssue2580.js.snap

@@ -0,0 +1,28 @@
+---
+source: crates/biome_js_analyze/tests/spec_tests.rs
+expression: invalidIssue2580.js
+---
+# Input
+```jsx
+// Issue https://github.com/biomejs/biome/issues/2580
+'```ts\n' + x + '\n```';
+```
+
+# Diagnostics
+```
+invalidIssue2580.js:2:1 lint/style/useTemplate  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  ! Template literals are preferred over string concatenation.
+
+    1 │ // Issue https://github.com/biomejs/biome/issues/2580
+  > 2 │ '```ts\n' + x + '\n```';
+      │ ^^^^^^^^^^^^^^^^^^^^^^^
+
+  i Unsafe fix: Use a template literal.
+
+    1 1 │   // Issue https://github.com/biomejs/biome/issues/2580
+    2   │ - '```ts\n'·+·x·+·'\n```';
+      2 │ + `\`\`\`ts\n${x}\n\`\`\``;
+
+
+```

crates/biome_js_factory/src/utils.rs

@@ -17,41 +17,36 @@ pub fn escape<'a>(
     needs_escaping: &[&str],
     escaping_char: char,
 ) -> std::borrow::Cow<'a, str> {
+    debug_assert!(!needs_escaping.is_empty());
     let mut escaped = String::new();
     let mut iter = unescaped_string.char_indices();
-    let mut is_escaped = false;
-    'unescaped: while let Some((idx, chr)) = iter.next() {
+    let mut last_copied_idx = 0;
+    while let Some((idx, chr)) = iter.next() {
         if chr == escaping_char {
-            is_escaped = !is_escaped;
+            // The next character is esacaped
+            iter.next();
         } else {
             for candidate in needs_escaping {
                 if unescaped_string[idx..].starts_with(candidate) {
-                    if !is_escaped {
-                        if escaped.is_empty() {
-                            escaped = String::with_capacity(unescaped_string.len() * 2);
-                            escaped.push_str(&unescaped_string[0..idx]);
-                        }
-                        escaped.push(escaping_char);
-                        escaped.push_str(candidate);
-                        for _ in candidate.chars().skip(1) {
-                            iter.next();
-                        }
-                        continue 'unescaped;
-                    } else {
-                        is_escaped = false;
+                    if escaped.is_empty() {
+                        escaped = String::with_capacity(unescaped_string.len() * 2 - idx);
                     }
+                    escaped.push_str(&unescaped_string[last_copied_idx..idx]);
+                    escaped.push(escaping_char);
+                    escaped.push_str(candidate);
+                    for _ in candidate.chars().skip(1) {
+                        iter.next();
+                    }
+                    last_copied_idx = idx + candidate.len();
+                    break;
                 }
             }
         }
-
-        if !escaped.is_empty() {
-            escaped.push(chr);
-        }
     }
-
     if escaped.is_empty() {
         std::borrow::Cow::Borrowed(unescaped_string)
     } else {
+        escaped.push_str(&unescaped_string[last_copied_idx..unescaped_string.len()]);
         std::borrow::Cow::Owned(escaped)
     }
 }
@@ -87,5 +82,7 @@ mod tests {
         );
         assert_eq!(escape(r"abc \${bca}", &["${", "`"], '\\'), r"abc \${bca}");
         assert_eq!(escape(r"abc \`bca", &["${", "`"], '\\'), r"abc \`bca");
+
+        assert_eq!(escape(r"\n`", &["`"], '\\'), r"\n\`");
     }
 }