[nullability] Define `__assert_nullability` centrally.

We now always prepend the definition of `__assert_nullability` in
`checkDiagnostics()`, so it will be available in all tests.

PiperOrigin-RevId: 522324205

nullability_verification/test/check_diagnostics.cc

diff --git a/nullability_verification/test/check_diagnostics.cc b/nullability_verification/test/check_diagnostics.cc
index d830671..328e7ec 100644
--- a/nullability_verification/test/check_diagnostics.cc
+++ b/nullability_verification/test/check_diagnostics.cc
@@ -15,6 +15,17 @@
 namespace tidy {
 namespace nullability {
 
+constexpr char kPreamble[] = R"cc(
+  enum NullabilityKind {
+    NK_nonnull,
+    NK_nullable,
+    NK_unspecified,
+  };
+
+  template <NullabilityKind... NK, typename T>
+  void __assert_nullability(const T&);
+)cc";
+
 bool checkDiagnostics(llvm::StringRef SourceCode) {
   std::vector<CFGElement> Diagnostics;
   PointerNullabilityDiagnoser Diagnoser;
@@ -35,8 +46,10 @@
                   Diagnostics.push_back(EltDiagnostics.value());
                 }
               })
+              .withASTBuildVirtualMappedFiles({{"preamble.h", kPreamble}})
               .withASTBuildArgs({"-fsyntax-only", "-std=c++17",
-                                 "-Wno-unused-value", "-Wno-nonnull"}),
+                                 "-Wno-unused-value", "-Wno-nonnull",
+                                 "-include", "preamble.h"}),
           [&Diagnostics, &Failed](
               const llvm::DenseMap<unsigned, std::string> &Annotations,
               const dataflow::test::AnalysisOutputs &AnalysisData) {

nullability_verification/test/pointer_nullability_verification_test.cc

diff --git a/nullability_verification/test/pointer_nullability_verification_test.cc b/nullability_verification/test/pointer_nullability_verification_test.cc
index 91a026c..23989f3 100644
--- a/nullability_verification/test/pointer_nullability_verification_test.cc
+++ b/nullability_verification/test/pointer_nullability_verification_test.cc
@@ -1607,22 +1607,9 @@
   )cc"));
 }
 
-// TODO: Move the definitions of `NullabilityKind` and `__assert_nullability()`
-// into a preamble that `checkDiagnostics()` prepends to every test.
 TEST(PointerNullabilityTest, AssertNullability) {
-  const std::string Declarations = R"cc(
-    enum NullabilityKind {
-      NK_nonnull,
-      NK_nullable,
-      NK_unspecified,
-    };
-
-    template <NullabilityKind... NK, typename T>
-    void __assert_nullability(const T&);
-  )cc";
-
   // Concrete struct.
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     struct StructNonnullNullable {
       int* _Nonnull nonnull;
       int* _Nullable nullable;
@@ -1641,7 +1628,7 @@
   )cc"));
 
   // Struct with two template type parameters.
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     template <typename T0, typename T1>
     struct Struct2Arg {};
 
@@ -1663,7 +1650,7 @@
   )cc"));
 
   // Struct with one type and non-type template parameters.
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     template <int I0, typename T1, typename T2>
     struct Struct3ArgWithInt {};
 
@@ -1685,7 +1672,7 @@
   )cc"));
 
   // Nested template arguments.
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     template <typename T0, typename T1>
     struct Struct2Arg {};
 
@@ -1708,7 +1695,7 @@
   )cc"));
 
   // Struct with two template parameters substituted with concrete structs.
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     struct StructUnknownNullable {
       int* unknown;
       int* _Nullable nullable;
@@ -1739,7 +1726,7 @@
     }
   )cc"));
 
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     template <typename T0, typename T1>
     struct Struct2Arg {
       T0 arg0;
@@ -1793,7 +1780,7 @@
     }
   )cc"));
 
-  EXPECT_TRUE(checkDiagnostics(Declarations + R"cc(
+  EXPECT_TRUE(checkDiagnostics(R"cc(
     void target(int* _Nullable p, int* _Nonnull q, int* r) {
       __assert_nullability<NK_nonnull, NK_nullable>(&p);
       __assert_nullability<NK_nonnull, NK_nonnull>(&q);
@@ -1974,15 +1961,6 @@
 
   // Call expression with template parameter substituted with a concrete struct.
   EXPECT_TRUE(checkDiagnostics(R"cc(
-    enum NullabilityKind {
-      NK_nonnull,
-      NK_nullable,
-      NK_unspecified,
-    };
-
-    template <NullabilityKind... NK, typename T>
-    void __assert_nullability(const T &);
-
     struct StructUnknownNullable {
       int *var0;
       int *_Nullable var1;