[stdlib] Re-enable assertions on malloc size

Round up our malloc requests and assert that breadcrumbs are aligned
and where we think they should be. This is necessary because ASAN
doesn't give us pointer-rounded sizes.

Author: milseman

stdlib/public/core/StringStorage.swift

@@ -181,12 +181,14 @@ fileprivate func _allocate<T: AnyObject>(
 ) -> (T, realNumTailBytes: Int) {
   _internalInvariant(getSwiftClassInstanceExtents(T.self).1 == numHeaderBytes)
 
+  func roundUp(_ x: Int) -> Int { (x + 15) & ~15 }
+
   let numBytes = numHeaderBytes + numTailBytes
 
   let linearBucketThreshold = 128
   if _fastPath(numBytes < linearBucketThreshold) {
     // Allocate up to the nearest bucket of 16
-    let realNumBytes = (numBytes+15) & ~15
+    let realNumBytes = roundUp(numBytes)
     let realNumTailBytes = realNumBytes - numHeaderBytes
     _internalInvariant(realNumTailBytes >= numTailBytes)
     let object = tailAllocator(realNumTailBytes)
@@ -200,9 +202,14 @@ fileprivate func _allocate<T: AnyObject>(
     growTailBytes = numTailBytes
   }
 
-  let object = tailAllocator(growTailBytes)
+  let total = roundUp(numHeaderBytes + growTailBytes)
+  let totalTailBytes = total - numHeaderBytes
+
+  let object = tailAllocator(totalTailBytes)
   let mallocSize = _swift_stdlib_malloc_size(
     UnsafeRawPointer(Builtin.bridgeToRawPointer(object)))
+  _internalInvariant(mallocSize % MemoryLayout<Int>.stride == 0)
+
   let realNumTailBytes = mallocSize - numHeaderBytes
   _internalInvariant(realNumTailBytes >= numTailBytes)
   return (object, realNumTailBytes)
@@ -447,6 +454,8 @@ extension __StringStorage {
   // required nul-terminator.
   //
   // NOTE: Callers who wish to mutate this storage should enfore nul-termination
+  //
+  // TODO: Refactoring or removing. Excluding the last byte is awkward.
   @inline(__always)
   private var unusedStorage: UnsafeMutableBufferPointer<UInt8> {
     UnsafeMutableBufferPointer(
@@ -485,8 +494,8 @@ extension __StringStorage {
 
     // Check that capacity end matches our notion of unused storage, and also
     // checks that breadcrumbs were dutifully aligned.
-    // _internalInvariant(UnsafeMutablePointer<UInt8>(_realCapacityEnd)
-    //   == unusedStorage.baseAddress! + (unusedStorage.count + 1))
+    _internalInvariant(UnsafeMutablePointer<UInt8>(_realCapacityEnd)
+      == unusedStorage.baseAddress! + (unusedStorage.count + 1))
   }
   #endif // INTERNAL_CHECKS_ENABLED
 }