Perform collection force-casts by force-casting the elements

instead of forcing conditional casts of the elements.

This should produce better and more compact code, allow more
efficient runtime behavior, and generate much better runtime
diagnostics if the cast fails.

Author: rjmccall

stdlib/public/core/ArrayCast.swift

@@ -47,7 +47,7 @@ public func _arrayForceCast<SourceElement, TargetElement>(
     return Array(_immutableCocoaArray: source._buffer._asCocoaArray())
   }
 #endif
-  return _arrayConditionalCast(source)!
+  return source.map { $0 as! TargetElement }
 }
 
 internal struct _UnwrappingFailed : Error {}

stdlib/public/core/HashedCollections.swift.gyb

@@ -1278,15 +1278,6 @@ internal func _stdlib_NSSet_allObjects(_ nss: _NSSet) ->
 
 //===--- Compiler conversion/casting entry points for Set<Element> --------===//
 
-func _impossible<T>(_:T.Type) -> T {
-  Builtin.unreachable()
-}
-
-func _unsafeUpcast<T, U>(_ x: T, to: U.Type) -> U {
-  _sanityCheck(x is U)
-  return x as? U ?? _impossible(U.self)
-}
-
 /// Perform a non-bridged upcast that always succeeds.
 ///
 /// - Precondition: `BaseValue` is a base class or base `@objc`
@@ -1295,7 +1286,7 @@ public func _setUpCast<DerivedValue, BaseValue>(_ source: Set<DerivedValue>)
   -> Set<BaseValue> {
   var builder = _SetBuilder<BaseValue>(count: source.count)
   for x in source {
-    builder.add(member: _unsafeUpcast(x, to: BaseValue.self))
+    builder.add(member: x as! BaseValue)
   }
   return builder.take()
 }
@@ -2220,8 +2211,7 @@ public func _dictionaryUpCast<DerivedKey, DerivedValue, BaseKey, BaseValue>(
   var result = Dictionary<BaseKey, BaseValue>(minimumCapacity: source.count)
 
   for (k, v) in source {
-    result[_unsafeUpcast(k, to: BaseKey.self)]
-      = _unsafeUpcast(v, to: BaseValue.self)
+    result[k as! BaseKey] = v as! BaseValue
   }
   return result
 }