Combine the test discovery implementations for WASI and static Mach-O. (#761)

This PR reduces code duplication between the two supported platforms
that do not use dynamic linking during test discovery.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Diff for: Package.swift

@@ -162,6 +162,13 @@ extension Array where Element == PackageDescription.CXXSetting {
   static var packageSettings: Self {
     var result = Self()
 
+    result += [
+      .define("SWT_NO_EXIT_TESTS", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),
+      .define("SWT_NO_SNAPSHOT_TYPES", .when(platforms: [.linux, .windows, .wasi])),
+      .define("SWT_NO_DYNAMIC_LINKING", .when(platforms: [.wasi])),
+      .define("SWT_NO_PIPES", .when(platforms: [.wasi])),
+    ]
+
     // Capture the testing library's version as a C++ string constant.
     if let git = Context.gitInformation {
       let testingLibraryVersion = if let tag = git.currentTag {

Diff for: Sources/_TestingInternals/Discovery.cpp

@@ -239,8 +239,8 @@ struct SWTTypeMetadataRecord {
   }
 };
 
-#if defined(__APPLE__)
 #if !defined(SWT_NO_DYNAMIC_LINKING)
+#if defined(__APPLE__)
 #pragma mark - Apple implementation
 
 /// Get a copy of the currently-loaded type metadata sections list.
@@ -318,28 +318,6 @@ static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
   }
 }
 
-#else
-#pragma mark - Apple implementation (statically linked)
-
-// This environment does not have a dynamic linker/loader. Therefore, there is
-// only one image (this one) with Swift code in it.
-// SEE: https://github.com/swiftlang/swift/tree/main/stdlib/public/runtime/ImageInspectionStatic.cpp
-
-extern "C" const char sectionBegin __asm("section$start$__TEXT$__swift5_types");
-extern "C" const char sectionEnd __asm("section$end$__TEXT$__swift5_types");
-
-template <typename SectionEnumerator>
-static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
-  SWTSectionBounds<SWTTypeMetadataRecord> sb = {
-    nullptr,
-    &sectionBegin,
-    static_cast<size_t>(std::distance(&sectionBegin, &sectionEnd))
-  };
-  bool stop = false;
-  body(sb, &stop);
-}
-#endif
-
 #elif defined(_WIN32)
 #pragma mark - Windows implementation
 
@@ -434,29 +412,6 @@ static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
   }
 }
 
-#elif defined(__wasi__)
-#pragma mark - WASI implementation (statically linked)
-
-extern "C" const char __start_swift5_type_metadata;
-extern "C" const char __stop_swift5_type_metadata;
-
-template <typename SectionEnumerator>
-static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
-  // WASI only has a single image (so far) and it is statically linked, so all
-  // Swift metadata ends up in the same section bounded by the named symbols
-  // above. So we can just yield the section betwixt them.
-  const auto& sectionBegin = __start_swift5_type_metadata;
-  const auto& sectionEnd = __stop_swift5_type_metadata;
-
-  SWTSectionBounds<SWTTypeMetadataRecord> sb = {
-    nullptr,
-    &sectionBegin,
-    static_cast<size_t>(std::distance(&sectionBegin, &sectionEnd))
-  };
-  bool stop = false;
-  body(sb, &stop);
-}
-
 #elif defined(__linux__) || defined(__FreeBSD__) || defined(__ANDROID__)
 #pragma mark - ELF implementation
 
@@ -517,11 +472,38 @@ static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
   }, const_cast<SectionEnumerator *>(&body));
 }
 #else
-#warning Platform-specific implementation missing: Runtime test discovery unavailable
+#warning Platform-specific implementation missing: Runtime test discovery unavailable (dynamic)
 template <typename SectionEnumerator>
 static void enumerateTypeMetadataSections(const SectionEnumerator& body) {}
 #endif
 
+#else
+#pragma mark - Statically-linked implementation
+
+#if defined(__APPLE__)
+extern "C" const char sectionBegin __asm__("section$start$__TEXT$__swift5_types");
+extern "C" const char sectionEnd __asm__("section$end$__TEXT$__swift5_types");
+#elif defined(__wasi__)
+extern "C" const char sectionBegin __asm__("__start_swift5_type_metadata");
+extern "C" const char sectionEnd __asm__("__stop_swift5_type_metadata");
+#else
+#warning Platform-specific implementation missing: Runtime test discovery unavailable (static)
+static const char sectionBegin = 0;
+static const char& sectionEnd = sectionBegin;
+#endif
+
+template <typename SectionEnumerator>
+static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
+  SWTSectionBounds<SWTTypeMetadataRecord> sb = {
+    nullptr,
+    &sectionBegin,
+    static_cast<size_t>(std::distance(&sectionBegin, &sectionEnd))
+  };
+  bool stop = false;
+  body(sb, &stop);
+}
+#endif
+
 #pragma mark -
 
 void swt_enumerateTypesWithNamesContaining(const char *nameSubstring, void *context, SWTTypeEnumerator body) {