module: validate that strip-types does not insert any code

lib/internal/modules/helpers.js

@@ -307,10 +307,49 @@ function lazyLoadTSParser() {
   return parseTS;
 }
 
+function doesTSStripTypesResultMatchSource(code, source) {
+  const sourceBuf = Buffer.from(source);
+  const codeBuf = Buffer.from(code);
+  if (sourceBuf.length !== codeBuf.length) return false;
+  for (let i = 0; i < codeBuf.length; i++) {
+    // should match either the source buffer or spaces (sometimes multi-byte) or semicolon
+    const val = codeBuf[i];
+    if (val === sourceBuf[i]) continue; // source match
+    // https://github.com/nodejs/amaro/blob/e533394f576f946add41dd8816816435e8100c3b/deps/swc/crates/swc_fast_ts_strip/src/lib.rs#L400-L414
+    if (val === 0x3b) continue; // Semicolon 3b
+    // https://github.com/nodejs/amaro/blob/e533394f576f946add41dd8816816435e8100c3b/deps/swc/crates/swc_fast_ts_strip/src/lib.rs#L200-L226
+    if (val === 0x20) continue; // Space 20
+    if (val === 0xc2) {
+      // No-Break Space 00A0
+      if (i + 1 >= codeBuf.length) return false;
+      if (codeBuf[++i] !== 0xa0) return false;
+      continue;
+    }
+    if (val === 0xc2) {
+      // En Space 2002
+      if (i + 2 >= codeBuf.length) return false;
+      if (codeBuf[++i] !== 0x80) return false;
+      if (codeBuf[++i] !== 0x82) return false;
+      continue;
+    }
+    if (val === 0xef) {
+      // ZWNBSP FEFF
+      if (i < 1 || codeBuf[i - 1] !== 0x20) return false; // only can get insterted after a space
+      if (i + 2 >= codeBuf.length) return false;
+      if (codeBuf[++i] !== 0xbb) return false;
+      if (codeBuf[++i] !== 0xbf) return false;
+      continue;
+    }
+    return false;
+  }
+  return true;
+}
+
 function tsParse(source) {
   if (!source || typeof source !== 'string') { return; }
   const transformSync = lazyLoadTSParser();
   const { code } = transformSync(source, { mode: 'strip-only' });
+  assert(doesTSStripTypesResultMatchSource(code, source), 'Unexpected strip-types result');
   return code;
 }