Remove C++-based section discovery logic. (#902)

This PR removes the C++ _section_ discovery logic that's used to look up
Swift's type metadata sections. Instead, we reuse the new logic used for
test content sections (written in Swift.)

I did not attempt to translate to Swift the logic to extract Swift types
from Swift's metadata records. This logic relies pretty heavily on
precise C++ data structure layouts and uses pointer authentication as
well as relative pointers. Swift does not guarantee the same structure
layout as C++, nor does it provide API for pointer authentication, nor
do value types in Swift have stable addresses that can be used to
compute addresses from relative offsets.

Resolves #764.

### 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

Documentation/Porting.md

@@ -66,7 +66,7 @@ platform-specific attention.
 > These errors are produced when the configuration you're trying to build has
 > conflicting requirements (for example, attempting to enable support for pipes
 > without also enabling support for file I/O.) You should be able to resolve
-> these issues by updating Package.swift and/or CompilerSettings.cmake.
+> these issues by updating `Package.swift` and/or `CompilerSettings.cmake`.
 
 Most platform dependencies can be resolved through the use of platform-specific
 API. For example, Swift Testing uses the C11 standard [`timespec`](https://en.cppreference.com/w/c/chrono/timespec)
@@ -123,69 +123,110 @@ Once the header is included, we can call `GetDateTime()` from `Clock.swift`:
 ## Runtime test discovery
 
 When porting to a new platform, you may need to provide a new implementation for
-`enumerateTypeMetadataSections()` in `Discovery.cpp`. Test discovery is
-dependent on Swift metadata discovery which is an inherently platform-specific
-operation.
-
-_Most_ platforms will be able to reuse the implementation used by Linux and
-Windows that calls an internal Swift runtime function to enumerate available
-metadata. If you are porting Swift Testing to Classic, this function won't be
-available, so you'll need to write a custom implementation instead. Assuming
-that the Swift compiler emits section information into the resource fork on
-Classic, you could use the [Resource Manager](https://developer.apple.com/library/archive/documentation/mac/pdf/MoreMacintoshToolbox.pdf)
+`_sectionBounds(_:)` in `Discovery+Platform.swift`. Test discovery is dependent
+on Swift metadata discovery which is an inherently platform-specific operation.
+
+_Most_ platforms in use today use the ELF image format and will be able to reuse
+the implementation used by Linux.
+
+Classic does not use the ELF image format, so you'll need to write a custom
+implementation of `_sectionBounds(_:)` instead. Assuming that the Swift compiler
+emits section information into the resource fork on Classic, you would use the
+[Resource Manager](https://developer.apple.com/library/archive/documentation/mac/pdf/MoreMacintoshToolbox.pdf)
 to load that information:
 
 ```diff
---- a/Sources/_TestingInternals/Discovery.cpp
-+++ b/Sources/_TestingInternals/Discovery.cpp
+--- a/Sources/Testing/Discovery+Platform.swift
++++ b/Sources/Testing/Discovery+Platform.swift
 
  // ...
-+#elif defined(macintosh)
-+template <typename SectionEnumerator>
-+static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
-+  ResFileRefNum refNum;
-+  if (noErr == GetTopResourceFile(&refNum)) {
-+    ResFileRefNum oldRefNum = refNum;
-+    do {
-+      UseResFile(refNum);
-+      Handle handle = Get1NamedResource('swft', "\p__swift5_types");
-+      if (handle && *handle) {
-+        auto imageAddress = reinterpret_cast<const void *>(static_cast<uintptr_t>(refNum));
-+        SWTSectionBounds sb = { imageAddress, *handle, GetHandleSize(handle) };
-+        bool stop = false;
-+        body(sb, &stop);
-+        if (stop) {
-+          break;
-+        }
-+      }
-+    } while (noErr == GetNextResourceFile(refNum, &refNum));
-+    UseResFile(oldRefNum);
++#elseif os(Classic)
++private func _sectionBounds(_ kind: SectionBounds.Kind) -> [SectionBounds] {
++  let resourceName: Str255 = switch kind {
++  case .testContent:
++    "__swift5_tests"
++  case .typeMetadata:
++    "__swift5_types"
++  }
++
++  let oldRefNum = CurResFile()
++  defer {
++    UseResFile(oldRefNum)
++  }
++
++  var refNum = ResFileRefNum(0)
++  guard noErr == GetTopResourceFile(&refNum) else {
++    return []
 +  }
++
++  var result = [SectionBounds]()
++  repeat {
++    UseResFile(refNum)
++    guard let handle = Get1NamedResource(ResType("swft"), resourceName) else {
++      continue
++    }
++    let sb = SectionBounds(
++      imageAddress: UnsafeRawPointer(bitPattern: UInt(refNum)),
++      start: handle.pointee!,
++      size: GetHandleSize(handle)
++    )
++    result.append(sb)
++  } while noErr == GetNextResourceFile(refNum, &refNum))
++  return result
 +}
  #else
- #warning Platform-specific implementation missing: Runtime test discovery unavailable (dynamic)
- template <typename SectionEnumerator>
- static void enumerateTypeMetadataSections(const SectionEnumerator& body) {}
+ private func _sectionBounds(_ kind: SectionBounds.Kind) -> [SectionBounds] {
+   #warning("Platform-specific implementation missing: Runtime test discovery unavailable (dynamic)")
+   return []
+ }
  #endif
 ```
 
+You will also need to update the `makeTestContentRecordDecl()` function in the
+`TestingMacros` target to emit the correct `@_section` attribute for your
+platform. If your platform uses the ELF image format and supports the
+`dl_iterate_phdr()` function, add it to the existing `#elseif os(Linux) || ...`
+case. Otherwise, add a new case for your platform:
+
+```diff
+--- a/Sources/TestingMacros/Support/TestContentGeneration.swift
++++ b/Sources/TestingMacros/Support/TestContentGeneration.swift
+   // ...
++  #elseif os(Classic)
++  @_section(".rsrc,swft,__swift5_tests")
+   #else
+   @__testing(warning: "Platform-specific implementation missing: test content section name unavailable")
+   #endif
+```
+
+Keep in mind that this code is emitted by the `@Test` and `@Suite` macros
+directly into test authors' test targets, so you will not be able to use
+compiler conditionals defined in the Swift Testing package (including those that
+start with `"SWT_"`).
+
 ## Runtime test discovery with static linkage
 
 If your platform does not support dynamic linking and loading, you will need to
 use static linkage instead. Define the `"SWT_NO_DYNAMIC_LINKING"` compiler
-conditional for your platform in both Package.swift and CompilerSettings.cmake,
-then define the `sectionBegin` and `sectionEnd` symbols in Discovery.cpp:
+conditional for your platform in both `Package.swift` and
+`CompilerSettings.cmake`, then define the symbols `testContentSectionBegin`,
+`testContentSectionEnd`, `typeMetadataSectionBegin`, and
+`typeMetadataSectionEnd` in `Discovery.cpp`.
 
 ```diff
 diff --git a/Sources/_TestingInternals/Discovery.cpp b/Sources/_TestingInternals/Discovery.cpp
  // ...
 +#elif defined(macintosh)
-+extern "C" const char sectionBegin __asm__("...");
-+extern "C" const char sectionEnd __asm__("...");
++extern "C" const char testContentSectionBegin __asm__("...");
++extern "C" const char testContentSectionEnd __asm__("...");
++extern "C" const char typeMetadataSectionBegin __asm__("...");
++extern "C" const char typeMetadataSectionEnd __asm__("...");
  #else
  #warning Platform-specific implementation missing: Runtime test discovery unavailable (static)
- static const char sectionBegin = 0;
- static const char& sectionEnd = sectionBegin;
+ static const char testContentSectionBegin = 0;
+ static const char& testContentSectionEnd = testContentSectionBegin;
+ static const char typeMetadataSectionBegin = 0;
+ static const char& typeMetadataSectionEnd = testContentSectionBegin;
  #endif
 ```
 
@@ -195,27 +236,6 @@ respectively. Their linker-level names will be platform-dependent: refer to the
 linker documentation for your platform to determine what names to place in the
 `__asm__` attribute applied to each.
 
-If you can't use `__asm__` on your platform, you can declare these symbols as
-C++ references to linker-defined symbols:
-
-```diff
-diff --git a/Sources/_TestingInternals/Discovery.cpp b/Sources/_TestingInternals/Discovery.cpp
- // ...
-+#elif defined(macintosh)
-+extern "C" const char __linker_defined_begin_symbol;
-+extern "C" const char __linker_defined_end_symbol;
-+static const auto& sectionBegin = __linker_defined_begin_symbol;
-+static const auto& sectionEnd = __linker_defined_end_symbol;
- #else
- #warning Platform-specific implementation missing: Runtime test discovery unavailable (static)
- static const char sectionBegin = 0;
- static const char& sectionEnd = sectionBegin;
- #endif
-```
-
-The names of `__linker_defined_begin_symbol` and `__linker_defined_end_symbol`
-in this example are, as with the shorter implementation, platform-dependent.
-
 ## C++ stub implementations
 
 Some symbols defined in C and C++ headers, especially "complex" macros, cannot