feat(core): deprecate OptionalUpgrade

This is dead-code, we don't use it in our codebase. I am suggesting to remove it because it creates an unnecessary API surface.

Related: #3747.
Related: #3271.

Pull-Request: #3806.

core/CHANGELOG.md

@@ -7,8 +7,12 @@
 - Deprecate `{In,Out}boundUpgradeExt`, as they are not used in rust-libp2p.
   See [PR 3807].
 
+- Deprecate `OptionalUpgrade` without replacement.
+  See [PR 3806].
+
 [PR 3747]: https://github.com/libp2p/rust-libp2p/pull/3747
 [PR 3807]: https://github.com/libp2p/rust-libp2p/pull/3807
+[PR 3806]: https://github.com/libp2p/rust-libp2p/pull/3806
 
 ## 0.39.1
 

core/src/upgrade.rs

@@ -77,17 +77,17 @@ pub use self::{
     apply::{apply, apply_inbound, apply_outbound, InboundUpgradeApply, OutboundUpgradeApply},
     denied::DeniedUpgrade,
     error::UpgradeError,
-    optional::OptionalUpgrade,
     pending::PendingUpgrade,
     ready::ReadyUpgrade,
     select::SelectUpgrade,
     transfer::{read_length_prefixed, read_varint, write_length_prefixed, write_varint},
 };
 pub use crate::Negotiated;
-pub use multistream_select::{NegotiatedComplete, NegotiationError, ProtocolError, Version};
-
 #[allow(deprecated)]
 pub use map::{MapInboundUpgrade, MapInboundUpgradeErr, MapOutboundUpgrade, MapOutboundUpgradeErr};
+pub use multistream_select::{NegotiatedComplete, NegotiationError, ProtocolError, Version};
+#[allow(deprecated)]
+pub use optional::OptionalUpgrade;
 
 /// Types serving as protocol names.
 ///

core/src/upgrade/optional.rs

@@ -18,13 +18,18 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#![allow(deprecated)]
+
 use crate::upgrade::{InboundUpgrade, OutboundUpgrade, UpgradeInfo};
 
 /// Upgrade that can be disabled at runtime.
 ///
 /// Wraps around an `Option<T>` and makes it available or not depending on whether it contains or
 /// not an upgrade.
 #[derive(Debug, Clone)]
+#[deprecated(
+    note = "Will be removed without replacement because it is not used within rust-libp2p."
+)]
 pub struct OptionalUpgrade<T>(Option<T>);
 
 impl<T> OptionalUpgrade<T> {