diff --git a/pkgs/ffigen/CHANGELOG.md b/pkgs/ffigen/CHANGELOG.md
index 83243bff0..661143c13 100644
--- a/pkgs/ffigen/CHANGELOG.md
+++ b/pkgs/ffigen/CHANGELOG.md
@@ -6,6 +6,9 @@
   `Categories`, `Interfaces`, and `Protocols`.
 - __Breaking change__: Remove deprecated `wrapperName` field from
   `NativeExternalBindings`.
+- __Breaking change__: Certain synthetic USRs have been modified to ensure they
+  cannot collide with real USRs. It's very unlikely that any user facing USRs
+  are affected.
 
 ## 20.1.1
 
diff --git a/pkgs/ffigen/lib/src/code_generator/objc_block.dart b/pkgs/ffigen/lib/src/code_generator/objc_block.dart
index 7e2bbec70..53697c301 100644
--- a/pkgs/ffigen/lib/src/code_generator/objc_block.dart
+++ b/pkgs/ffigen/lib/src/code_generator/objc_block.dart
@@ -4,6 +4,7 @@
 
 import '../code_generator.dart';
 import '../context.dart';
+import '../strings.dart' as strings;
 import '../visitor/ast.dart';
 
 import 'binding_string.dart';
@@ -108,14 +109,12 @@ class ObjCBlock extends BindingType with HasLocalScope {
   ) {
     // Create a fake USR code for the block. This code is used to dedupe blocks
     // with the same signature. Not intended to be human readable.
-    final usr = StringBuffer();
-    usr.write(
-      'objcBlock: ${returnType.cacheKey()} ${returnsRetained ? 'R' : ''}',
-    );
-    for (final param in params) {
-      usr.write(' ${param.type.cacheKey()} ${param.objCConsumed ? 'C' : ''}');
-    }
-    return usr.toString();
+    return [
+      '${strings.synthUsrChar} objcBlock:',
+      '${returnType.cacheKey()} ${returnsRetained ? 'R' : ''}',
+      for (final param in params)
+        '${param.type.cacheKey()} ${param.objCConsumed ? 'C' : ''}',
+    ].join(' ');
   }
 
   bool get hasListener => returnType == voidType;
diff --git a/pkgs/ffigen/lib/src/header_parser/sub_parsers/functiondecl_parser.dart b/pkgs/ffigen/lib/src/header_parser/sub_parsers/functiondecl_parser.dart
index 71f0a23b3..895eb1f44 100644
--- a/pkgs/ffigen/lib/src/header_parser/sub_parsers/functiondecl_parser.dart
+++ b/pkgs/ffigen/lib/src/header_parser/sub_parsers/functiondecl_parser.dart
@@ -6,6 +6,7 @@ import '../../code_generator.dart';
 import '../../config_provider/config.dart';
 import '../../config_provider/config_types.dart';
 import '../../context.dart';
+import '../../strings.dart';
 import '../clang_bindings/clang_bindings.dart' as clang_types;
 import '../utils.dart';
 import 'api_availability.dart';
@@ -119,7 +120,7 @@ List<Func> parseFunctionDeclaration(
     );
 
     // Initialized with a single value with no prefix and empty var args.
-    var varArgFunctions = [VarArgFunction('', [])];
+    var varArgFunctions = <VarArgFunction?>[null];
     if (config.functions.varArgs.containsKey(funcName)) {
       if (clang.clang_isFunctionTypeVariadic(cursor.type()) == 1) {
         varArgFunctions = config.functions.varArgs[funcName]!;
@@ -131,6 +132,8 @@ List<Func> parseFunctionDeclaration(
       }
     }
     for (final vaFunc in varArgFunctions) {
+      var usr = funcUsr;
+      if (vaFunc != null) usr += '$synthUsrChar vaFunc: ${vaFunc.postfix}';
       funcs.add(
         Func(
           dartDoc: getCursorDocComment(
@@ -139,13 +142,13 @@ List<Func> parseFunctionDeclaration(
             indent: nesting.length + commentPrefix.length,
             availability: apiAvailability.dartDoc,
           ),
-          usr: funcUsr + vaFunc.postfix,
-          name: config.functions.rename(decl) + vaFunc.postfix,
+          usr: usr,
+          name: config.functions.rename(decl) + (vaFunc?.postfix ?? ''),
           originalName: funcName,
           returnType: returnType,
           parameters: parameters,
           varArgParameters: [
-            for (final ta in vaFunc.types)
+            for (final ta in vaFunc?.types ?? const <Type>[])
               Parameter(type: ta, name: 'va', objCConsumed: false),
           ],
           exposeSymbolAddress: config.functions.includeSymbolAddress(decl),
diff --git a/pkgs/ffigen/lib/src/header_parser/utils.dart b/pkgs/ffigen/lib/src/header_parser/utils.dart
index cfbb20130..95a022bde 100644
--- a/pkgs/ffigen/lib/src/header_parser/utils.dart
+++ b/pkgs/ffigen/lib/src/header_parser/utils.dart
@@ -10,6 +10,7 @@ import 'package:logging/logging.dart';
 import '../code_generator.dart';
 import '../config_provider/config_types.dart';
 import '../context.dart';
+import '../strings.dart';
 import 'clang_bindings/clang_bindings.dart' as clang_types;
 import 'type_extractor/extractor.dart';
 
@@ -91,8 +92,9 @@ extension CXCursorExt on clang_types.CXCursor {
 
   String usr() {
     var res = clang.clang_getCursorUSR(this).toStringAndDispose();
+    assert(!res.contains(synthUsrChar));
     if (isAnonymousRecordDecl()) {
-      res += '@offset:${sourceFileOffset()}';
+      res += '$synthUsrChar anonRec: offset:${sourceFileOffset()}';
     }
     return res;
   }
diff --git a/pkgs/ffigen/lib/src/strings.dart b/pkgs/ffigen/lib/src/strings.dart
index b6a5fdef2..4e5c18cf1 100644
--- a/pkgs/ffigen/lib/src/strings.dart
+++ b/pkgs/ffigen/lib/src/strings.dart
@@ -274,6 +274,9 @@ const doubleNaN = 'double.nan';
 /// USR for struct `_Dart_Handle`.
 const dartHandleUsr = 'c:@S@_Dart_Handle';
 
+// A character that will never appear in real USRs, for making synthetic USRs.
+const synthUsrChar = '~';
+
 const ffiNative = 'ffi-native';
 const ffiNativeAsset = 'asset-id';
 
diff --git a/pkgs/ffigen/test/regen.dart b/pkgs/ffigen/test/regen.dart
index 3ca5c0fe3..db23d0fc3 100644
--- a/pkgs/ffigen/test/regen.dart
+++ b/pkgs/ffigen/test/regen.dart
@@ -22,7 +22,7 @@ $ dart run test/setup.dart && dart run test/regen.dart && dart test
 void _regenConfig(Logger logger, String yamlConfigPath) {
   final path = p.join(packagePathForTests, yamlConfigPath);
   Directory.current = File(path).parent;
-  testConfigFromPath(path).generate(logger: logger);
+  testConfigFromPath(path, logger: logger).generate(logger: logger);
 }
 
 Future<void> main(List<String> args) async {
diff --git a/pkgs/ffigen/test/test_utils.dart b/pkgs/ffigen/test/test_utils.dart
index 762fd3341..d5665b1c8 100644
--- a/pkgs/ffigen/test/test_utils.dart
+++ b/pkgs/ffigen/test/test_utils.dart
@@ -235,10 +235,10 @@ FfiGenerator testConfig(String yamlBody, {String? filename, Logger? logger}) {
   ).configAdapter();
 }
 
-FfiGenerator testConfigFromPath(String path) {
+FfiGenerator testConfigFromPath(String path, {Logger? logger}) {
   final file = File(path);
   final yamlBody = file.readAsStringSync();
-  return testConfig(yamlBody, filename: path);
+  return testConfig(yamlBody, filename: path, logger: logger);
 }
 
 bool isFlutterTester = Platform.resolvedExecutable.contains('flutter_tester');