diff --git a/test/quick-lint-js/diagnostic-assertion.cpp b/test/quick-lint-js/diagnostic-assertion.cpp
index 0290157f91..42526002a3 100644
--- a/test/quick-lint-js/diagnostic-assertion.cpp
+++ b/test/quick-lint-js/diagnostic-assertion.cpp
@@ -151,6 +151,7 @@ Diagnostic_Assertion::parse(const Char8* specification) {
   Diagnostic_Assertion_Lexer lexer(specification);
 
   Diagnostic_Assertion out_assertion;
+  out_assertion.needs_adjustment = true;
 
   // Names of diagnostic member variables.
   //
@@ -388,6 +389,10 @@ Diagnostic_Assertion Diagnostic_Assertion::parse_or_exit(
 
 Diagnostic_Assertion Diagnostic_Assertion::adjusted_for_escaped_characters(
     String8_View code) const {
+  if (!this->needs_adjustment) {
+    return *this;
+  }
+
   static auto has_utf8_continuation = [](Char8 c) -> bool {
     return (static_cast<std::uint8_t>(c) & 0x80) != 0;
   };
@@ -426,6 +431,7 @@ Diagnostic_Assertion Diagnostic_Assertion::adjusted_for_escaped_characters(
   };
 
   Diagnostic_Assertion result = *this;
+  result.needs_adjustment = false;
   for (Member& member : result.members) {
     if (member.type != Diagnostic_Arg_Type::source_code_span) {
       continue;
@@ -443,6 +449,16 @@ Diagnostic_Assertion Diagnostic_Assertion::adjusted_for_escaped_characters(
   return result;
 }
 
+Diagnostic_Assertion Diagnostic_Assertion::make_raw(
+    Diag_Type type, std::initializer_list<Member> members) {
+  Diagnostic_Assertion assertion;
+  assertion.type = type;
+  for (const Member& m : members) {
+    assertion.members.push_back(m);
+  }
+  return assertion;
+}
+
 Diagnostic_Assertion operator""_diag(
     const Char8* specification,
     [[maybe_unused]] std::size_t specification_length) {
diff --git a/test/quick-lint-js/diagnostic-assertion.h b/test/quick-lint-js/diagnostic-assertion.h
index 062fda56f7..d66b19536e 100644
--- a/test/quick-lint-js/diagnostic-assertion.h
+++ b/test/quick-lint-js/diagnostic-assertion.h
@@ -13,6 +13,8 @@
 #include <quick-lint-js/diag/diagnostic-types.h>
 #include <quick-lint-js/port/char8.h>
 #include <quick-lint-js/port/span.h>
+#include <quick-lint-js/util/cast.h>
+#include <quick-lint-js/util/cpp.h>
 #include <string>
 #include <vector>
 
@@ -105,8 +107,14 @@ struct Diagnostic_Assertion {
   Diag_Type type = Diag_Type();
   Fixed_Vector<Member, 3> members;
 
+  // Whether adjusted_for_escaped_characters should be called to compensate for
+  // control characters.
+  bool needs_adjustment = false;
+
   // If the specification is malformed, return a list of messages to report to
   // the user.
+  //
+  // Postcondition: return.needs_adjustment
   static Result<Diagnostic_Assertion, std::vector<std::string>> parse(
       const Char8* specification);
 
@@ -129,10 +137,44 @@ struct Diagnostic_Assertion {
   // from the offsets in the _diag string so that assert_diagnostics will work
   // correctly.
   //
+  // If this->needs_adjustment is false, returns an unmodified copy of *this.
+  //
+  // Postcondition: !return.needs_adjustment
+  //
   // TODO(strager): Support Unicode escape sequences (\u2063 for example).
   Diagnostic_Assertion adjusted_for_escaped_characters(String8_View code) const;
+
+  // Manually create a Diagnostic_Assertion without the short-hand syntax.
+  //
+  // See DIAGNOSTIC_ASSERTION_SPAN.
+  //
+  // Postcondition: !return.needs_adjustment
+  static Diagnostic_Assertion make_raw(Diag_Type,
+                                       std::initializer_list<Member>);
 };
 
+// Create a Diagnostic_Assertion which matches 'type_'. It asserts that
+// 'type_::member_' is a Source_Code_Span beginning at 'begin_offset_' and
+// ending at 'begin_offset_ + span_string_.size()'.
+#define DIAGNOSTIC_ASSERTION_SPAN(type_, member_, begin_offset_, span_string_) \
+  (::quick_lint_js::Diagnostic_Assertion::make_raw(                            \
+      Diag_Type::type_,                                                        \
+      {                                                                        \
+          ::quick_lint_js::Diagnostic_Assertion::Member{                       \
+              .name = QLJS_CPP_QUOTE_U8_SV(member_),                           \
+              .offset = offsetof(type_, member_),                              \
+              .type = Diagnostic_Arg_Type::source_code_span,                   \
+              .span_begin_offset =                                             \
+                  ::quick_lint_js::narrow_cast<Padded_String_Size>(            \
+                      (begin_offset_)),                                        \
+              .span_end_offset =                                               \
+                  ::quick_lint_js::narrow_cast<Padded_String_Size>(            \
+                      (begin_offset_)) +                                       \
+                  ::quick_lint_js::narrow_cast<Padded_String_Size>(            \
+                      (span_string_).size()),                                  \
+          },                                                                   \
+      }))
+
 // See [_diag-syntax].
 //
 // Exits the program at run-time if the specification is malformed.
diff --git a/test/test-lex.cpp b/test/test-lex.cpp
index a3c880189e..8dc52a0355 100644
--- a/test/test-lex.cpp
+++ b/test/test-lex.cpp
@@ -14,6 +14,7 @@
 #include <quick-lint-js/container/padded-string.h>
 #include <quick-lint-js/diag-collector.h>
 #include <quick-lint-js/diag-matcher.h>
+#include <quick-lint-js/diagnostic-assertion.h>
 #include <quick-lint-js/fe/lex.h>
 #include <quick-lint-js/fe/source-code-span.h>
 #include <quick-lint-js/fe/token.h>
@@ -2145,17 +2146,17 @@ TEST_F(Test_Lex, ascii_control_characters_are_disallowed) {
   for (String8_View control_character : control_characters_except_whitespace) {
     Padded_String input(String8(control_character) + u8"hello");
     SCOPED_TRACE(input);
-    Diag_Collector v;
+    Diag_List_Diag_Reporter diags(&this->memory_);
 
-    Lexer l(&input, &v);
+    Lexer l(&input, &diags);
     EXPECT_EQ(l.peek().type, Token_Type::identifier)
         << "control character should be skipped";
-    EXPECT_THAT(
-        v.errors,
-        ElementsAreArray({
-            DIAG_TYPE_OFFSETS(&input, Diag_Unexpected_Control_Character,  //
-                              character, 0, control_character),
-        }));
+    assert_diagnostics(
+        &input, diags.diags(),
+        {
+            DIAGNOSTIC_ASSERTION_SPAN(Diag_Unexpected_Control_Character,  //
+                                      character, 0, control_character),
+        });
   }
 }