Less experimental version

Author: Catfish-Man

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -274,15 +274,15 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
          realMinimumCapacity._builtinWordValue, Element.self)
 
       let storageAddr = UnsafeMutableRawPointer(Builtin.bridgeToRawPointer(_storage))
-//      if let allocSize = _mallocSize(ofAllocation: storageAddr) {
-//        let endAddr = storageAddr + allocSize
-//        let realCapacity = endAddr.assumingMemoryBound(to: Element.self) - firstElementAddress
-//        _initStorageHeader(
-//          count: uninitializedCount, capacity: realCapacity)
-//      } else {
+      if let allocSize = _mallocSize(ofAllocation: storageAddr) {
+        let endAddr = storageAddr + allocSize
+        let realCapacity = endAddr.assumingMemoryBound(to: Element.self) - firstElementAddress
+        _initStorageHeader(
+          count: uninitializedCount, capacity: realCapacity)
+      } else {
         _initStorageHeader(
           count: uninitializedCount, capacity: realMinimumCapacity)
-//      }
+      }
     }
   }
 

stdlib/public/core/Shims.swift

@@ -37,7 +37,9 @@ internal let _fastEnumerationStorageMutationsPtr =
   UnsafeMutablePointer<CUnsignedLong>(Builtin.addressof(&_fastEnumerationStorageMutationsTarget))
 #endif
 
-@usableFromInline @_alwaysEmitIntoClient
+@usableFromInline @_alwaysEmitIntoClient @_effects(readonly)
 internal func _mallocSize(ofAllocation ptr: UnsafeRawPointer) -> Int? {
-  return _swift_stdlib_has_malloc_size() ? _swift_stdlib_malloc_size(ptr) : nil
+  return !Builtin.isOnStack(ptr) && _swift_stdlib_has_malloc_size() ?
+    _swift_stdlib_malloc_size(ptr) :
+    nil
 }

stdlib/public/core/StringStorage.swift

@@ -206,17 +206,17 @@ fileprivate func _allocate<T: AnyObject>(
   let totalTailBytes = total - numHeaderBytes
 
   let object = tailAllocator(totalTailBytes)
-//  if let allocSize = _mallocSize(ofAllocation:
-//    UnsafeRawPointer(Builtin.bridgeToRawPointer(object))) {
-//    _internalInvariant(allocSize % MemoryLayout<Int>.stride == 0)
-//
-//    let realNumTailBytes = allocSize - numHeaderBytes
-//    _internalInvariant(realNumTailBytes >= numTailBytes)
-//
-//    return (object, realNumTailBytes)
-//  } else {
+  if let allocSize = _mallocSize(ofAllocation:
+    UnsafeRawPointer(Builtin.bridgeToRawPointer(object))) {
+    _internalInvariant(allocSize % MemoryLayout<Int>.stride == 0)
+
+    let realNumTailBytes = allocSize - numHeaderBytes
+    _internalInvariant(realNumTailBytes >= numTailBytes)
+
+    return (object, realNumTailBytes)
+  } else {
     return (object, totalTailBytes)
-//  }
+  }
 }
 
 fileprivate func _allocateStringStorage(