From ac313ee0bd96a06e895d738b2aac8ba0a234c35f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bartek=20Iwa=C5=84czuk?= <bartek@deno.com>
Date: Sun, 7 Jun 2026 18:30:52 +0200
Subject: [PATCH] fix: don't double backslash escapes in non-CSS embedded
 templates

---
 src/generation/generate.rs              | 19 +++++++++++++++++--
 tests/specs/external_formatter/html.txt |  7 +++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/generation/generate.rs b/src/generation/generate.rs
index 7fb3a332..c50196d5 100644
--- a/src/generation/generate.rs
+++ b/src/generation/generate.rs
@@ -3064,8 +3064,23 @@ fn maybe_gen_tagged_tpl_with_external_formatter<'a>(node: &TaggedTpl<'a>, contex
   .unwrap();
 
   // Then formats the text with the external formatter.
-  let formatted_tpl = match external_formatter(embedded_lang, text.replace(r"\\", "\\"), context.config) {
-    Ok(formatted_tpl) => formatted_tpl?.replace("\\", r"\\"),
+  //
+  // Only CSS needs the `\\` -> `\` round-trip, so the CSS parser sees escapes
+  // like Tailwind's `\\[\\#86efac\\]` as `\[\#86efac\]`. For other embedded
+  // languages the raw text must be passed through verbatim; unescaping there
+  // doubles lone escape sequences such as `\n` or `` \` `` (see
+  // denoland/deno#30103 and #30948).
+  let needs_unescape = embedded_lang == "css";
+  let input = if needs_unescape { text.replace(r"\\", "\\") } else { text };
+  let formatted_tpl = match external_formatter(embedded_lang, input, context.config) {
+    Ok(formatted_tpl) => {
+      let formatted = formatted_tpl?;
+      if needs_unescape {
+        formatted.replace("\\", r"\\")
+      } else {
+        formatted
+      }
+    }
     Err(err) => {
       context.diagnostics.push(context::GenerateDiagnostic {
         message: format!("Error formatting tagged template literal at line {}: {}", node.start_line() + 1, err),
diff --git a/tests/specs/external_formatter/html.txt b/tests/specs/external_formatter/html.txt
index ef31cd05..a105816a 100644
--- a/tests/specs/external_formatter/html.txt
+++ b/tests/specs/external_formatter/html.txt
@@ -97,3 +97,10 @@ const a = html`
             <circle r="5" />
         </svg></a>
 `;
+
+== should preserve a lone newline escape, not double it (deno#30103) ==
+html`\n`;
+[expect]
+html`
+    \n
+`;