Remove overly-permissive UnsafePointer init

Author: airspeedswift

stdlib/public/core/Pointer.swift

@@ -93,34 +93,6 @@ extension _Pointer {
     guard let unwrapped = other else { return nil }
     self.init(unwrapped._rawValue)
   }
-
-  // all pointers are creatable from mutable pointers
-  
-  /// Creates a new pointer from the given mutable pointer.
-  ///
-  /// Use this initializer to explicitly convert `other` to an `UnsafeRawPointer`
-  /// instance. This initializer creates a new pointer to the same address as
-  /// `other` and performs no allocation or copying.
-  ///
-  /// - Parameter other: The typed pointer to convert.
-  @_transparent
-  public init<T>(_ other: UnsafeMutablePointer<T>) {
-    self.init(other._rawValue)
-  }
-
-  /// Creates a new raw pointer from the given typed pointer.
-  ///
-  /// Use this initializer to explicitly convert `other` to an `UnsafeRawPointer`
-  /// instance. This initializer creates a new pointer to the same address as
-  /// `other` and performs no allocation or copying.
-  ///
-  /// - Parameter other: The typed pointer to convert. If `other` is `nil`, the
-  ///   result is `nil`.
-  @_transparent
-  public init?<T>(_ other: UnsafeMutablePointer<T>?) {
-    guard let unwrapped = other else { return nil }
-    self.init(unwrapped)
-  }
 }
 
 // well, this is pretty annoying

stdlib/public/core/StringBreadcrumbs.swift

@@ -126,7 +126,8 @@ extension _StringGuts {
     }
 
     _internalInvariant(mutPtr.pointee != nil)
-    return UnsafePointer(mutPtr)
+    // assuming optional class reference and class reference can alias
+    return UnsafeRawPointer(mutPtr).assumingMemoryBound(to: _StringBreadcrumbs.self)
   }
 
   @inline(never) // slow-path
@@ -137,6 +138,6 @@ extension _StringGuts {
     // Thread-safe compare-and-swap
     let crumbs = _StringBreadcrumbs(String(self))
     _stdlib_atomicInitializeARCRef(
-      object: UnsafeMutablePointer(mutPtr), desired: crumbs)
+      object: UnsafeMutablePointer(mutating: UnsafeRawPointer(mutPtr).assumingMemoryBound(to: Optional<AnyObject>.self)), desired: crumbs)
   }
 }

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -85,3 +85,5 @@ Subscript String.UnicodeScalarView.subscript(_:) has been removed
 Subscript Substring.subscript(_:) has been removed
 
 Func Collection.makeIterator() has self access kind changing from NonMutating to __Consuming
+
+Constructor _Pointer.init(_:) has been removed

validation-test/compiler_crashers_2_fixed/0060-sr2702.swift

@@ -46,7 +46,7 @@ public func XUAllSubclassesOfClass<T: AnyObject>(_ aClass: T.Type) -> [T.Type] {
 			free(memory)
 		}
 
-		let classesPtr = memory.assumingMemoryBound(to: Optional<AnyClass>.self)
+		let classesPtr = memory.assumingMemoryBound(to: AnyClass.self)
 		let classes = AutoreleasingUnsafeMutablePointer<AnyClass>(classesPtr)
 		numClasses = objc_getClassList(classes, numClasses)