internal/weakdeps: put APIv1 weak dependency behind a build constraint
By using a build constraint that is never satisfied, we can add a weak
dependency on a module. If the module is part of a build, our go.mod
enforces a minimum version on it, but we never add it to the build
dependencies ourselves.
This is the same trick used for tool dependencies:
https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
Dropped the TODO to remove the APIv1 dependency, since I think this
removes any need to do so.
Change-Id: I45b1a3f45535bcdc9abf34fb562d2869f1712bb6
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219499
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/depv1/depv1.go b/internal/depv1/depv1.go
deleted file mode 100644
index 01d81f0..0000000
--- a/internal/depv1/depv1.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// Copyright 2020 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 file.
-
-// Package depv1 exists to depend on github.com/golang/protobuf.
-//
-// We include this dependency to ensure that any program using
-// APIv2 also uses a sufficiently new version of APIv1. At some
-// point in the future when old versions of APIv1 are no longer
-// of concern, we may drop this dependency.
-package depv1
-
-// TODO: Delete this dependency when it no longer serves a purpose.
-import _ "github.com/golang/protobuf/proto"
diff --git a/internal/weakdeps/doc.go b/internal/weakdeps/doc.go
new file mode 100644
index 0000000..bc7d7c5
--- /dev/null
+++ b/internal/weakdeps/doc.go
@@ -0,0 +1,12 @@
+// Copyright 2020 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 file.
+
+// Package weakdeps exists to add weak module dependencies.
+//
+// We want to ensure that this module is used with a minimum
+// version of certain other modules, without actually importing
+// those modules in normal builds. We do that by adding an
+// import of a package in the module under a build constraint
+// that is never satisfied in normal usage.
+package weakdeps
diff --git a/internal/weakdeps/weakdeps.go b/internal/weakdeps/weakdeps.go
new file mode 100644
index 0000000..456fbfd
--- /dev/null
+++ b/internal/weakdeps/weakdeps.go
@@ -0,0 +1,11 @@
+// Copyright 2020 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 file.
+
+// +build weak_dependency
+
+package weakdeps
+
+// Ensure that any program using "github.com/golang/protobuf"
+// uses a version that wraps this module.
+import _ "github.com/golang/protobuf/proto"
