xmpp-parsers: Make DiscoInfoResult::features a BTreeSet

Features must never be duplicated, so we can use a BTreeSet instead of a
Vec to be nicer for users.

It makes the internal API for computing caps and ecaps2 a bit worse,
because it was expecting a slice directly, so for now let’s collect the
BTreeSet into a Vec when computing the thing.  A refactor to use
Iterator might make it better eventually, but I won’t work on that
before profiling it.

parsers/ChangeLog

@@ -13,7 +13,8 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         - Remove the unnecessary SaslMechanisms struct, to directly extract
           SASL mechanisms in a Vec<String> in StreamFeatures.
         - The type of disco::DiscoInfoResult::features changed from
-          Vec<Feature> to Vec<String>, this lets us remove Feature as well.
+          Vec<Feature> to BTreeSet<String>, this lets us remove Feature as
+          well.
         - bind::BindFeature::required is now a bool, thanks to xso’s flag.
         - jingle_rtp::Description::rtcp_mux is now a bool flag too.
         - stream_limits::Limits now extract directly to NonZeroU32.

parsers/src/caps.rs

@@ -125,8 +125,11 @@ fn compute_extensions(extensions: &[DataForm]) -> Vec<u8> {
 /// [ecaps2](../ecaps2/index.html) instead, see [this
 /// email](https://mail.jabber.org/pipermail/security/2009-July/000812.html).
 pub fn compute_disco(disco: &DiscoInfoResult) -> Vec<u8> {
+    // TODO: Figure out a way to remove the clones here.
+    let features: Vec<_> = disco.features.iter().cloned().collect();
+
     let identities_string = compute_identities(&disco.identities);
-    let features_string = compute_features(&disco.features);
+    let features_string = compute_features(&features);
     let extensions_string = compute_extensions(&disco.extensions);
 
     let mut final_string = vec![];

parsers/src/disco.rs

@@ -10,6 +10,7 @@ use crate::data_forms::DataForm;
 use crate::iq::{IqGetPayload, IqResultPayload};
 use crate::ns;
 use crate::rsm::{SetQuery, SetResult};
+use alloc::collections::BTreeSet;
 use jid::Jid;
 
 /// Structure representing a `<query xmlns='http://jabber.org/protocol/disco#info'/>` element.
@@ -98,7 +99,7 @@ pub struct DiscoInfoResult {
 
     /// List of features supported by this entity.
     #[xml(extract(n = .., name = "feature", fields(attribute(name = "var", type_ = String))))]
-    pub features: Vec<String>,
+    pub features: BTreeSet<String>,
 
     /// List of extensions reported by this entity.
     #[xml(child(n = ..))]

parsers/src/ecaps2.rs

@@ -124,7 +124,10 @@ fn compute_extensions(extensions: &[DataForm]) -> Result<Vec<u8>, Error> {
 /// XEP-0390](https://xmpp.org/extensions/xep-0390.html#algorithm-input) on a
 /// [disco#info query element](../disco/struct.DiscoInfoResult.html).
 pub fn compute_disco(disco: &DiscoInfoResult) -> Result<Vec<u8>, Error> {
-    let features_string = compute_features(&disco.features);
+    // TODO: Figure out a way to remove the clones here.
+    let features: Vec<_> = disco.features.iter().cloned().collect();
+
+    let features_string = compute_features(&features);
     let identities_string = compute_identities(&disco.identities);
     let extensions_string = compute_extensions(&disco.extensions)?;