[stdlib] Add build option to enable _debugPrecondition in Release mode

It is sometimes desirable to always perform (relatively cheap) runtime checks in the stdlib, even in configurations where we’d otherwise elide them.

Add a build-time option to the standard library (`SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE`) to make `_debugPrecondition` work like `_precondition`, i.e., to enable it even in ReleaseAssert configurations.

This option will keep additional checks in the following places, even in release mode:

- Range checking in the subscript operations of all unsafe buffer pointer types.
- Nil checking in `Optional.unsafelyUnwrapped`
- Additional argument validation in `Unsafe[Mutable][Raw]Pointer`’s initialization/assignment/move/deinitialization/binding methods
- Protection against initializing `Unsafe[Mutable][Raw]Pointer` with invalid data. (Negative count, nil pointer for non-empty buffer, etc)
- Checks against index overflow in `Unsafe[Mutable]BufferPointer`’s index manipulation methods
- Checks against backward ranges in `Range(uncheckedBounds:)`, `ClosedRange(uncheckedBounds:)`
- Dynamic isa check in `unsafeDowncast(_:to:)`
- Additional [cheap] checks to catch invalid Sequence/Collection implementations in `Array.init<S:Sequence>(_:)` and elsewhere
- Checks against `Character` containing multiple grapheme clusters
- More index validation in `EmptyCollection`

(Additional cases will get added as the stdlib evolves.)

The option is disabled by default — so `_debugPrecondition`s continue to be disabled in optimized builds, even after this change lands, unless someone specifically builds a stdlib that enables them.

rdar://89118585

Author: lorentey

CMakeLists.txt

@@ -434,6 +434,10 @@ option(SWIFT_ENABLE_STDLIBCORE_EXCLUSIVITY_CHECKING
   "Build stdlibCore with exclusivity checking enabled"
   FALSE)
 
+option(SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE
+  "Enable _debugPrecondition checks in the stdlib in Release configurations"
+  FALSE)
+
 option(SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING
   "Enable experimental Swift differentiable programming features"
   FALSE)

stdlib/public/core/Assert.swift

@@ -261,24 +261,32 @@ internal func _debugPrecondition(
   _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
   file: StaticString = #file, line: UInt = #line
 ) {
+#if SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE
+  _precondition(condition(), message, file: file, line: line)
+#else
   // Only check in debug mode.
   if _slowPath(_isDebugAssertConfiguration()) {
     if !_fastPath(condition()) {
       _fatalErrorMessage("Fatal error", message, file: file, line: line,
         flags: _fatalErrorFlags())
     }
   }
+#endif
 }
 
 @usableFromInline @_transparent
 internal func _debugPreconditionFailure(
   _ message: StaticString = StaticString(),
   file: StaticString = #file, line: UInt = #line
 ) -> Never {
+#if SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE
+  _preconditionFailure(message, file: file, line: line)
+#else
   if _slowPath(_isDebugAssertConfiguration()) {
     _precondition(false, message, file: file, line: line)
   }
   _conditionallyUnreachable()
+#endif
 }
 
 /// Internal checks.

stdlib/public/core/AssertCommon.swift

@@ -58,6 +58,17 @@ func _isStdlibInternalChecksEnabled() -> Bool {
 #endif
 }
 
+@_transparent
+@_alwaysEmitIntoClient // Introduced in 5.7
+public // @testable
+func _isStdlibDebugChecksEnabled() -> Bool {
+#if SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE
+  return !_isFastAssertConfiguration()
+#else
+  return _isDebugAssertConfiguration()
+#endif
+}
+
 @usableFromInline @_transparent
 internal func _fatalErrorFlags() -> UInt32 {
   // The current flags are:

stdlib/public/core/CMakeLists.txt

@@ -272,6 +272,9 @@ endif()
 if(SWIFT_STDLIB_ENABLE_STDLIBCORE_EXCLUSIVITY_CHECKING)
   list(APPEND swift_stdlib_compile_flags "-enforce-exclusivity=checked")
 endif()
+if(SWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE)
+  list(APPEND swift_stdlib_compile_flags "-DSWIFT_STDLIB_ENABLE_DEBUG_PRECONDITIONS_IN_RELEASE")
+endif()
 set(compile_flags_for_final_build)
 if(SWIFT_ENABLE_ARRAY_COW_CHECKS)
   list(APPEND compile_flags_for_final_build "-DCOW_CHECKS_ENABLED")