Add a distinct hook for swift_willThrowTypedImpl(). (#71651)

Add a distinct hook for `swift_willThrowTypedImpl()`.

This PR adds a hook for `swift_willThrowTypedImpl()` that is distinct from
`swift_willThrow()`. The current implementation of `swift_willThrowTypedImpl()` creates a
temporary existential box for the thrown error and calls the `_swift_willThrow` hook that
is set by XCTest and swift-testing. Unfortunately, this temporary existential box isn't
very useful because its address is not expected to be stable.

Instead, expose a separate hook, `_swift_willThrowTypedImpl`, whose signature matches
that of the new ABI function. This hook can then be used by XCTest and swift-testing to
distinguish between typed throws and untyped throws and avoids creating a temporary
existential box when it won't be useful.

As implemented, if `_swift_willThrowTypedImpl` is not set but `_swift_willThrow` is, the
`swift_willThrowTypedImpl()` will fall back to calling `_swift_willThrow` with a temporary
existential box. We might decide this is unnecessary, but I figured it would allow us more
time to stage adoption of the new hook in the testing libraries.

Resolves rdar://122824443.

Author: grynspan

stdlib/public/runtime/ErrorObjectCommon.cpp

@@ -43,26 +43,37 @@ swift::swift_willThrow(SWIFT_CONTEXT void *unused,
   }
 }
 
+std::atomic<void (*)(
+  OpaqueValue *value,
+  const Metadata *type,
+  const WitnessTable *errorConformance
+)> swift::_swift_willThrowTypedImpl;
+
 /// Breakpoint hook for debuggers that is called for typed throws, and calls
-/// _swift_willThrow if set. This implicitly boxes the typed error in an
-/// any Error for the call.
+/// _swift_willThrowTypedImpl if set. If not set and _swift_willThrow is set, this calls
+/// that hook instead and implicitly boxes the typed error in an any Error for that call.
 SWIFT_CC(swift) void
 swift::swift_willThrowTypedImpl(OpaqueValue *value,
                                 const Metadata *type,
                                 const WitnessTable *errorConformance) {
   // Cheap check to bail out early, since we expect there to be no callbacks
   // the vast majority of the time.
-  auto handler = _swift_willThrow.load(std::memory_order_acquire);
+  auto handler = _swift_willThrowTypedImpl.load(std::memory_order_acquire);
   if (SWIFT_UNLIKELY(handler)) {
-    // Form an error box containing the error.
-    BoxPair boxedError = swift_allocError(
+    (* handler)(value, type, errorConformance);
+  } else {
+    auto fallbackHandler = _swift_willThrow.load(std::memory_order_acquire);
+    if (SWIFT_UNLIKELY(fallbackHandler)) {
+      // Form an error box containing the error.
+      BoxPair boxedError = swift_allocError(
         type, errorConformance, value, /*isTake=*/false);
 
-    // Hand the boxed error off to the handler.
-    auto errorBox = reinterpret_cast<SwiftError *>(boxedError.object);
-    (* handler)(errorBox);
+      // Hand the boxed error off to the handler.
+      auto errorBox = reinterpret_cast<SwiftError *>(boxedError.object);
+      (* fallbackHandler)(errorBox);
 
-    // Release the error box.
-    swift_errorRelease(errorBox);
+      // Release the error box.
+      swift_errorRelease(errorBox);
+    }
   }
 }

stdlib/public/runtime/ErrorObjectTestSupport.h

@@ -23,6 +23,11 @@ namespace swift {
 
 #if defined(__cplusplus)
 SWIFT_RUNTIME_EXPORT std::atomic<void (*)(SwiftError *error)> _swift_willThrow;
+SWIFT_RUNTIME_EXPORT std::atomic<void (*)(
+  OpaqueValue *value,
+  const Metadata *type,
+  const WitnessTable *errorConformance
+)> _swift_willThrowTypedImpl;
 #endif
 
 /// Set the value of @c _swift_willThrow atomically.

test/abi/macOS/arm64/stdlib.swift

@@ -256,4 +256,5 @@ Added: __swift_pod_indirect_initializeBufferWithCopyOfBuffer
 Added: __swift_validatePrespecializedMetadata
 Added: __swift_exceptionPersonality
 Added: _swift_willThrowTypedImpl
+Added: __swift_willThrowTypedImpl
 Added: __swift_enableSwizzlingOfAllocationAndRefCountingFunctions_forInstrumentsOnly

test/abi/macOS/x86_64/stdlib.swift

@@ -256,4 +256,5 @@ Added: __swift_pod_indirect_initializeBufferWithCopyOfBuffer
 Added: __swift_validatePrespecializedMetadata
 Added: __swift_exceptionPersonality
 Added: _swift_willThrowTypedImpl
+Added: __swift_willThrowTypedImpl
 Added: __swift_enableSwizzlingOfAllocationAndRefCountingFunctions_forInstrumentsOnly