[abi] preserve original implementations of withMemoryRebound

Author: glessard

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -699,6 +699,12 @@ extension Unsafe${Mutable}BufferPointer {
   /// - Returns: The return value, if any, of the `body` closure parameter.
   @inlinable // unsafe-performance
   @_alwaysEmitIntoClient
+  // This custom silgen name is chosen to not interfere with the old ABI
+% if Mutable:
+  @_silgen_name("$_swift_se0333_UnsafeMutableBufferPointer_withMemoryRebound")
+% else:
+  @_silgen_name("$_swift_se0333_UnsafeBufferPointer_withMemoryRebound")
+% end
   public func withMemoryRebound<T, Result>(
     to type: T.Type,
     _ body: (_ buffer: ${Self}<T>) throws -> Result
@@ -727,6 +733,33 @@ extension Unsafe${Mutable}BufferPointer {
     return try body(.init(start: .init(base), count: newCount))
   }
 
+  // This unavailable implementation uses the expected mangled name
+  // of `withMemoryRebound`, and provides an entry point for any
+  // binary compiled against the stlib binary for Swift 5.6 and older.
+  @available(*, unavailable)
+% if Mutable:
+  @_silgen_name("$sSr17withMemoryRebound2to_qd_0_qd__m_qd_0_Sryqd__GKXEtKr0_lF")
+% else:
+  @_silgen_name("$sSR17withMemoryRebound2to_qd_0_qd__m_qd_0_SRyqd__GKXEtKr0_lF")
+% end
+  public func _legacy_se0333_withMemoryRebound<T, Result>(
+    to type: T.Type, _ body: (${Self}<T>) throws -> Result
+  ) rethrows -> Result {
+    if let base = _position {
+      _debugPrecondition(MemoryLayout<Element>.stride == MemoryLayout<T>.stride)
+      Builtin.bindMemory(base._rawValue, count._builtinWordValue, T.self)
+      defer {
+        Builtin.bindMemory(base._rawValue, count._builtinWordValue, Element.self)
+      }
+
+      return try body(${Self}<T>(
+        start: Unsafe${Mutable}Pointer<T>(base._rawValue), count: count))
+    }
+    else {
+      return try body(${Self}<T>(start: nil, count: 0))
+    }
+  }
+
   /// A pointer to the first element of the buffer.
   ///
   /// If the `baseAddress` of this buffer is `nil`, the count is zero. However,

stdlib/public/core/UnsafePointer.swift

@@ -303,6 +303,8 @@ public struct UnsafePointer<Pointee>: _Pointer {
   /// - Returns: The return value, if any, of the `body` closure parameter.
   @inlinable
   @_alwaysEmitIntoClient
+  // This custom silgen name is chosen to not interfere with the old ABI
+  @_silgen_name("_swift_se0333_UnsafePointer_withMemoryRebound")
   public func withMemoryRebound<T, Result>(
     to type: T.Type, capacity count: Int,
     _ body: (_ pointer: UnsafePointer<T>) throws -> Result
@@ -318,6 +320,21 @@ public struct UnsafePointer<Pointee>: _Pointer {
     return try body(.init(_rawValue))
   }
 
+  // This unavailable implementation uses the expected mangled name
+  // of `withMemoryRebound`, and provides an entry point for any
+  // binary compiled against the stlib binary for Swift 5.6 and older.
+  @available(*, unavailable)
+  @_silgen_name("$sSP17withMemoryRebound2to8capacity_qd_0_qd__m_Siqd_0_SPyqd__GKXEtKr0_lF")
+  public func _legacy_se0333_withMemoryRebound<T, Result>(to type: T.Type, capacity count: Int,
+    _ body: (UnsafePointer<T>) throws -> Result
+  ) rethrows -> Result {
+    Builtin.bindMemory(_rawValue, count._builtinWordValue, T.self)
+    defer {
+      Builtin.bindMemory(_rawValue, count._builtinWordValue, Pointee.self)
+    }
+    return try body(.init(_rawValue))
+  }
+
   /// Accesses the pointee at the specified offset from this pointer.
   ///
   ///
@@ -970,6 +987,8 @@ public struct UnsafeMutablePointer<Pointee>: _Pointer {
   /// - Returns: The return value, if any, of the `body` closure parameter.
   @inlinable
   @_alwaysEmitIntoClient
+  // This custom silgen name is chosen to not interfere with the old ABI
+  @_silgen_name("$_swift_se0333_UnsafeMutablePointer_withMemoryRebound")
   public func withMemoryRebound<T, Result>(
     to type: T.Type, capacity count: Int,
     _ body: (_ pointer: UnsafeMutablePointer<T>) throws -> Result
@@ -985,6 +1004,21 @@ public struct UnsafeMutablePointer<Pointee>: _Pointer {
     return try body(.init(_rawValue))
   }
 
+  // This unavailable implementation uses the expected mangled name
+  // of `withMemoryRebound`, and provides an entry point for any
+  // binary compiled against the stlib binary for Swift 5.6 and older.
+  @available(*, unavailable)
+  @_silgen_name("$sSp17withMemoryRebound2to8capacity_qd_0_qd__m_Siqd_0_Spyqd__GKXEtKr0_lF")
+  public func _legacy_se0333_withMemoryRebound<T, Result>(to type: T.Type, capacity count: Int,
+    _ body: (UnsafeMutablePointer<T>) throws -> Result
+  ) rethrows -> Result {
+    Builtin.bindMemory(_rawValue, count._builtinWordValue, T.self)
+    defer {
+      Builtin.bindMemory(_rawValue, count._builtinWordValue, Pointee.self)
+    }
+    return try body(.init(_rawValue))
+  }
+
   /// Accesses the pointee at the specified offset from this pointer.
   ///
   /// For a pointer `p`, the memory at `p + i` must be initialized when reading