internal/jsonrpc2_v2: add Func convenience wrappers for the Binder and Preempter interfaces

Change-Id: Ib21dabb908c13d9bbf50f053a342a8644d3ee68b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/388596
Run-TryBot: Bryan Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/jsonrpc2_v2/conn.go

@@ -28,6 +28,15 @@ type Binder interface {
 	Bind(context.Context, *Connection) (ConnectionOptions, error)
 }
 
+// A BinderFunc implements the Binder interface for a standalone Bind function.
+type BinderFunc func(context.Context, *Connection) (ConnectionOptions, error)
+
+func (f BinderFunc) Bind(ctx context.Context, c *Connection) (ConnectionOptions, error) {
+	return f(ctx, c)
+}
+
+var _ Binder = BinderFunc(nil)
+
 // ConnectionOptions holds the options for new connections.
 type ConnectionOptions struct {
 	// Framer allows control over the message framing and encoding.

internal/jsonrpc2_v2/jsonrpc2.go

@@ -47,6 +47,15 @@ type Preempter interface {
 	Preempt(ctx context.Context, req *Request) (result interface{}, err error)
 }
 
+// A PreempterFunc implements the Preempter interface for a standalone Preempt function.
+type PreempterFunc func(ctx context.Context, req *Request) (interface{}, error)
+
+func (f PreempterFunc) Preempt(ctx context.Context, req *Request) (interface{}, error) {
+	return f(ctx, req)
+}
+
+var _ Preempter = PreempterFunc(nil)
+
 // Handler handles messages on a connection.
 type Handler interface {
 	// Handle is invoked sequentially for each incoming request that has not
@@ -75,12 +84,15 @@ func (defaultHandler) Handle(context.Context, *Request) (interface{}, error) {
 	return nil, ErrNotHandled
 }
 
+// A HandlerFunc implements the Handler interface for a standalone Handle function.
 type HandlerFunc func(ctx context.Context, req *Request) (interface{}, error)
 
 func (f HandlerFunc) Handle(ctx context.Context, req *Request) (interface{}, error) {
 	return f(ctx, req)
 }
 
+var _ Handler = HandlerFunc(nil)
+
 // async is a small helper for operations with an asynchronous result that you
 // can wait for.
 type async struct {