Add a generic BindFor that can usually replace BindTo

When this infers the type of interfaces at compile-time it’s usually
more ergonomic.

Author: jwreschnig-utiq

options.go

@@ -248,6 +248,13 @@ func BindTo(impl, iface any) Option {
 	})
 }
 
+// BindFor allows binding of implementations to the compile-time type. It
+// is equivalent to BindTo(v, (*T)(nil)), but can usually infer the
+// correct type T.
+func BindFor[T any](v T) Option {
+	return BindTo(v, (*T)(nil))
+}
+
 // BindToProvider binds an injected value to a provider function.
 //
 // The provider function must have one of the following signatures:

options_test.go

@@ -1,6 +1,7 @@
 package kong
 
 import (
+	"context"
 	"reflect"
 	"strings"
 	"testing"
@@ -43,6 +44,23 @@ func TestBindTo(t *testing.T) {
 	assert.Equal(t, "foo", saw)
 }
 
+func TestBindFor(t *testing.T) {
+	type key struct{}
+	var saw any
+	method := func(ctx context.Context) error {
+		saw = ctx.Value(key{})
+		return nil
+	}
+
+	var cli struct{}
+
+	p, err := New(&cli, BindFor(context.WithValue(context.Background(), key{}, "foo")))
+	assert.NoError(t, err)
+	err = callFunction(reflect.ValueOf(method), p.bindings)
+	assert.NoError(t, err)
+	assert.Equal(t, "foo", saw)
+}
+
 func TestInvalidCallback(t *testing.T) {
 	type iface interface {
 		Method()