internal/pragma: package to statically enforce certain properties

Package pragma provides certain constructs that can be used to statically enforce
certain language properties.

Change-Id: I58a87bf7429f3be1b5aae418595fadda32f9d039
Reviewed-on: https://go-review.googlesource.com/127822
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>

internal/pragma/pragma.go

@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+// Package pragma provides types that can be embedded into a struct to
+// statically enforce or prevent certain language properties.
+package pragma
+
+import "sync"
+
+// NoUnkeyedLiterals can be embedded in a struct to prevent unkeyed literals.
+type NoUnkeyedLiterals struct{}
+
+// DoNotImplement can be embedded in an interface to prevent trivial
+// implementations of the interface.
+//
+// This is useful to prevent unauthorized implementations of an interface
+// so that it can be extended in the future for any protobuf language changes.
+type DoNotImplement interface{ ProtoInternal(DoNotImplement) }
+
+// DoNotCompare can be embedded in a struct to prevent comparability.
+type DoNotCompare [0]func()
+
+// DoNotCopy can be embedded in a struct to help prevent shallow copies.
+// This does not rely on a Go language feature, but rather a special case
+// within the vet checker.
+//
+// See https://golang.org/issues/8005.
+type DoNotCopy [0]sync.Mutex