tokio-xmpp: Error out when both tls-native and tls-rust features are enabled

If the user enables the tls-rust feature and forgets to disable
default-features (which includes tls-native), tell them and bail out.

The code was made to work anyway when both are enabled, and here it
defaults to tls-native. It does seem better to have one explicitely
choose one though hence the compile_error! message.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

tokio-xmpp/src/lib.rs

@@ -2,6 +2,9 @@
 
 #![deny(unsafe_code, missing_docs, bare_trait_objects)]
 
+#[cfg(all(feature = "tls-native", feature = "tls-rust"))]
+compile_error!("Both tls-native and tls-rust features can't be enabled at the same time.");
+
 mod starttls;
 mod stream_start;
 mod xmpp_codec;