Fix pattern matching of UTF-8 strings containing escape chars

CHANGELOG.md

@@ -264,6 +264,11 @@
   the package interface file if some modules were cached.
   ([Surya Rose](https://github.com/GearsDatapacks))
 
+- Fixed a bug where pattern matching using a UTF-8 string constant would not
+  work correctly on the JavaScript target when the string contained escape
+  characters.
+  ([Richard Viney](https://github.com/richard-viney))
+
 ## v1.8.1 - 2025-02-11
 
 ### Bug fixes

compiler-core/src/javascript/pattern.rs

@@ -739,7 +739,7 @@ impl<'module_ctx, 'expression_gen, 'a> Generator<'module_ctx, 'expression_gen, '
 
                             [Opt::Utf8 { .. }] => match segment.value.as_ref() {
                                 Pattern::String { value, .. } => {
-                                    for byte in value.as_bytes() {
+                                    for byte in convert_string_escape_chars(value).as_bytes() {
                                         if offset.bits % 8 == 0 {
                                             self.push_byte_at(offset.bits / 8);
                                         } else {

compiler-core/src/javascript/tests/bit_arrays.rs

@@ -270,6 +270,17 @@ fn go(x) {
     );
 }
 
+#[test]
+fn match_utf8_with_escape_chars() {
+    assert_js!(
+        r#"
+fn go(x) {
+  let assert <<"\"\\\r\n\t\f\u{1f600}">> = x
+}
+"#,
+    );
+}
+
 #[test]
 fn match_utf8() {
     assert_js!(

compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__bit_arrays__match_utf8_with_escape_chars.snap

@@ -0,0 +1,39 @@
+---
+source: compiler-core/src/javascript/tests/bit_arrays.rs
+expression: "\nfn go(x) {\n  let assert <<\"\\\"\\\\\\r\\n\\t\\f\\u{1f600}\">> = x\n}\n"
+---
+----- SOURCE CODE
+
+fn go(x) {
+  let assert <<"\"\\\r\n\t\f\u{1f600}">> = x
+}
+
+
+----- COMPILED JAVASCRIPT
+import { makeError } from "../gleam.mjs";
+
+function go(x) {
+  if (
+    x.byteAt(0) !== 34 ||
+    x.byteAt(1) !== 92 ||
+    x.byteAt(2) !== 13 ||
+    x.byteAt(3) !== 10 ||
+    x.byteAt(4) !== 9 ||
+    x.byteAt(5) !== 12 ||
+    x.byteAt(6) !== 240 ||
+    x.byteAt(7) !== 159 ||
+    x.byteAt(8) !== 152 ||
+    x.byteAt(9) !== 128 ||
+    !(x.bitSize == 80)
+  ) {
+    throw makeError(
+      "let_assert",
+      "my/mod",
+      3,
+      "go",
+      "Pattern match failed, no pattern matched the value.",
+      { value: x }
+    )
+  }
+  return x;
+}