Optimizations to Backtrace.current(). (#647)

This PR optimizes `Backtrace.current()`:

- There are now fewer library symbols are captured in a backtrace. This
particularly affects debug builds where nested scoped accesses aren't
optimized away.
- I've eliminated almost all copies of the backtrace's underlying data.
On 64-bit targets, the only copy required is to final `Array` storage
and is handled by the standard library.
- I've eliminated runtime bounds checks on backtrace counts that need to
be cast from one integer type to another.
- I've renamed the internal symbol `_startCachingForThrownErrors` to
`__SWIFT_TESTING_IS_CAPTURING_A_BACKTRACE_FOR_A_THROWN_ERROR__`. This
symbol unavoidably shows up in backtraces captured when
`swift_willThrow` is called, so I gave it a name that (hopefully)
clearly explains why it's there.

### 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

Sources/Testing/SourceAttribution/Backtrace.swift

@@ -37,11 +37,7 @@ public struct Backtrace: Sendable {
   /// The pointers in `addresses` are converted to instances of ``Address``. Any
   /// `nil` addresses are represented as `0`.
   public init(addresses: some Sequence<UnsafeRawPointer?>) {
-    self.init(
-      addresses: addresses.lazy
-        .map(UInt.init(bitPattern:))
-        .map(Address.init)
-    )
+    self.addresses = addresses.map { Address(UInt(bitPattern: $0)) }
   }
 
   /// Get the current backtrace.
@@ -61,33 +57,42 @@ public struct Backtrace: Sendable {
   public static func current(maximumAddressCount addressCount: Int = 128) -> Self {
     // NOTE: the exact argument/return types for backtrace() vary across
     // platforms, hence the use of .init() when calling it below.
-    let addresses = [UnsafeRawPointer?](unsafeUninitializedCapacity: addressCount) { addresses, initializedCount in
-      addresses.withMemoryRebound(to: UnsafeMutableRawPointer?.self) { addresses in
+    withUnsafeTemporaryAllocation(of: UnsafeMutableRawPointer?.self, capacity: addressCount) { addresses in
+      var initializedCount = 0
 #if SWT_TARGET_OS_APPLE
-        if #available(_backtraceAsyncAPI, *) {
-          initializedCount = backtrace_async(addresses.baseAddress!, addresses.count, nil)
-        } else {
-          initializedCount = .init(backtrace(addresses.baseAddress!, .init(addresses.count)))
-        }
-#elseif os(Linux)
-        initializedCount = .init(backtrace(addresses.baseAddress!, .init(addresses.count)))
+      if #available(_backtraceAsyncAPI, *) {
+        initializedCount = backtrace_async(addresses.baseAddress!, addresses.count, nil)
+      } else {
+        initializedCount = .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
+      }
 #elseif os(Android)
-        addresses.withMemoryRebound(to: UnsafeMutableRawPointer.self) { addresses in
-          initializedCount = .init(backtrace(addresses.baseAddress!, .init(addresses.count)))
-        }
+      initializedCount = addresses.withMemoryRebound(to: UnsafeMutableRawPointer.self) { addresses in
+        .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
+      }
+#elseif os(Linux)
+      initializedCount = .init(clamping: backtrace(addresses.baseAddress!, .init(clamping: addresses.count)))
 #elseif os(Windows)
-        initializedCount = Int(RtlCaptureStackBackTrace(0, ULONG(addresses.count), addresses.baseAddress!, nil))
+      initializedCount = Int(clamping: RtlCaptureStackBackTrace(0, ULONG(clamping: addresses.count), addresses.baseAddress!, nil))
 #elseif os(WASI)
-        // SEE: https://github.com/WebAssembly/WASI/issues/159
-        // SEE: https://github.com/swiftlang/swift/pull/31693
-        initializedCount = 0
+      // SEE: https://github.com/WebAssembly/WASI/issues/159
+      // SEE: https://github.com/swiftlang/swift/pull/31693
 #else
 #warning("Platform-specific implementation missing: backtraces unavailable")
-        initializedCount = 0
 #endif
+
+      let endIndex = addresses.index(addresses.startIndex, offsetBy: initializedCount)
+#if _pointerBitWidth(_64)
+      // The width of a pointer equals the width of an `Address`, so we can just
+      // bitcast the memory rather than mapping through UInt first.
+      return addresses[..<endIndex].withMemoryRebound(to: Address.self) { addresses in
+        Self(addresses: addresses)
       }
+#else
+      return addresses[..<endIndex].withMemoryRebound(to: UnsafeRawPointer?.self) { addresses in
+        Self(addresses: addresses)
+      }
+#endif
     }
-    return Self(addresses: addresses)
   }
 }
 
@@ -164,12 +169,12 @@ extension Backtrace {
   ///   - errorAddress: The error that is about to be thrown. This pointer
   ///     refers to an instance of `SwiftError` or (on platforms with
   ///     Objective-C interop) an instance of `NSError`.
-  @Sendable private static func _willThrow(_ errorAddress: UnsafeMutableRawPointer) {
+  ///   - backtrace: The backtrace from where the error was thrown.
+  private static func _willThrow(_ errorAddress: UnsafeMutableRawPointer, from backtrace: Backtrace) {
     _oldWillThrowHandler.rawValue?(errorAddress)
 
     let errorObject = unsafeBitCast(errorAddress, to: (any AnyObject & Sendable).self)
     let errorID = ObjectIdentifier(errorObject)
-    let backtrace = Backtrace.current()
     let newEntry = _ErrorMappingCacheEntry(errorObject: errorObject, backtrace: backtrace)
 
     _errorMappingCache.withLock { cache in
@@ -183,9 +188,15 @@ extension Backtrace {
 
   /// The implementation of ``Backtrace/startCachingForThrownErrors()``, run
   /// only once.
-  private static let _startCachingForThrownErrors: Void = {
+  ///
+  /// This value is named oddly so that it shows up clearly in symbolicated
+  /// backtraces.
+  private static let __SWIFT_TESTING_IS_CAPTURING_A_BACKTRACE_FOR_A_THROWN_ERROR__: Void = {
     _oldWillThrowHandler.withLock { oldWillThrowHandler in
-      oldWillThrowHandler = swt_setWillThrowHandler { _willThrow($0) }
+      oldWillThrowHandler = swt_setWillThrowHandler { errorAddress in
+        let backtrace = Backtrace.current()
+        _willThrow(errorAddress, from: backtrace)
+      }
     }
   }()
 
@@ -196,7 +207,7 @@ extension Backtrace {
   /// developer-supplied code to ensure that thrown errors' backtraces are
   /// always captured.
   static func startCachingForThrownErrors() {
-    _startCachingForThrownErrors
+    __SWIFT_TESTING_IS_CAPTURING_A_BACKTRACE_FOR_A_THROWN_ERROR__
   }
 
   /// Flush stale entries from the error-mapping cache.