fix(wasm): preserve leading BOM in custom section names (#2102)

scripts/verify-wasm-artifact.mjs

@@ -248,6 +248,10 @@ try {
           '"fake"\nSection Details:\n   Type start=0x0000000a end=0x0000000b (size=0x00000001) count: 0\n"end"',
           '"unicode-é\\\t"',
           '"visible"',
+          '"\uFEFFsection"',
+          '"\uFEFFname"',
+          '"\uFEFF"',
+          '""',
         ],
       );
     if (debugNames) {

src/wasm/WabtOutput.test.ts

@@ -166,7 +166,7 @@ it("bounds section header bytes while accepting a five-byte size LEB", () => {
       "Sections:",
       "",
       "     Type start=0x0000000a end=0x0000000b (size=0x00000001) count: 0",
-      `   Custom start=0x${start.toString(16).padStart(8, "0")} end=0x${(start + 1).toString(16).padStart(8, "0")} (size=0x00000001) "\"`,
+      `   Custom start=0x${start.toString(16).padStart(8, "0")} end=0x${(start + 1).toString(16).padStart(8, "0")} (size=0x00000001) ""`,
       "",
       "Section Details:",
       "",
@@ -211,6 +211,19 @@ it("frames custom section names at their NUL-terminated objdump display", async
     /custom-section header/u,
   );
 });
+it("preserves leading U+FEFF as custom-name data, not a debug-name marker", () => {
+  const parsed = parseWabtWat(
+    '(module (@custom "\\ef\\bb\\bfsection" "payload") (@custom "\\ef\\bb\\bfname" "payload") (@custom "\\ef\\bb\\bf" "payload") (@custom "" "payload"))',
+  );
+  expect(parsed.customSectionNames).toEqual([
+    "\uFEFFsection",
+    "\uFEFFname",
+    "\uFEFF",
+    "",
+  ]);
+  expect(parsed.debugModuleNames).toEqual([]);
+  expect(() => parseWabtWat('(module (@custom "\\ff" "payload"))')).toThrow();
+});
 it("extracts escaped WAT import/export forms while retaining multiline producer text", async () => {
   const text = await readFile(
     new URL("../../tests/fixtures/wasm/decoded.wat", import.meta.url),

src/wasm/WabtWat.ts

@@ -147,9 +147,11 @@ export const parseWabtWat = (text: string) => {
           if (prefix === null)
             throw new Error("Unsupported WABT custom-section name.");
           const nameString = watString(form, prefix[0].length);
-          const name = new TextDecoder("utf-8", { fatal: true }).decode(
-            nameString.bytes,
-          );
+          // A WASM name is length-delimited UTF-8; leading U+FEFF is name data.
+          const name = new TextDecoder("utf-8", {
+            fatal: true,
+            ignoreBOM: true,
+          }).decode(nameString.bytes);
           customSectionNames.push(name);
           if (name === "name") {
             const payloadPrefix = /^\s+"/u.exec(form.slice(nameString.end));

tests/fixtures/wasm/README.md

@@ -12,3 +12,5 @@ WABT and compares exact producer output against the public CLI/MCP surfaces.
 `custom-names.wat` covers newlines, quotes, Unicode, backslashes, embedded
 section markers and NUL in custom names; objdump displays those names without
 escaping and ends each at its first NUL.
+It also covers leading U+FEFF, including a name distinct from the special `name`
+section, and distinguishes a U+FEFF-only name from an empty name.

tests/fixtures/wasm/custom-names.wat

@@ -2,4 +2,8 @@
   (@custom "test\0aline" "payload")
   (@custom "fake\22\0aSection Details:\0a   Type start=0x0000000a end=0x0000000b (size=0x00000001) count: 0\0a\22end" "payload")
   (@custom "unicode-\c3\a9\\\t" "payload")
-  (@custom "visible\00hidden" "payload"))
+  (@custom "visible\00hidden" "payload")
+  (@custom "\ef\bb\bfsection" "payload")
+  (@custom "\ef\bb\bfname" "payload")
+  (@custom "\ef\bb\bf" "payload")
+  (@custom "" "payload"))