tor-error: Put OnionService errors behind a new experimental-api feature

crates/tor-error/Cargo.toml

@@ -13,6 +13,7 @@ categories = ["rust-patterns"]
 
 [features]
 default = ["backtrace"]
+experimental-api = []
 
 [dependencies]
 backtrace = { version = "0.3.39", optional = true }

crates/tor-error/README.md

@@ -12,4 +12,15 @@ crates higher up the dependency stack.
 
 `backtrace` -- Enable backtraces in internal errors.  (On by default.)
 
+### Experimental and unstable features
+
+Note that the APIs enabled by these features are NOT covered by
+semantic versioning[^1] guarantees: we might break them or remove
+them between patch versions.
+
+* `experimental-api`: Add additional non-stable APIs to our public
+  interfaces.
+
+License: MIT OR Apache-2.0
+
 License: MIT OR Apache-2.0

crates/tor-error/src/lib.rs

@@ -462,6 +462,7 @@ pub enum ErrorKind {
     /// This probably means that the hidden service is not running, or does not exist.
     /// (It might mean that the directory servers are faulty,
     /// and that the hidden service was unable to publish its descriptor.)
+    #[cfg(feature = "experimental-api")]
     #[display(fmt = "Onion Service not found")]
     OnionServiceNotFound,
 
@@ -469,6 +470,7 @@ pub enum ErrorKind {
     ///
     /// Something unexpected happened when trying to connect to the selected hidden service.
     /// It seems to have been due to the hidden service violating the Tor protocols somehow.
+    #[cfg(feature = "experimental-api")]
     #[display(fmt = "Onion Service protocol failed (apprently due to service behaviour)")]
     OnionServiceProtocolFailed,