identifiers-validation: Allow + in the localpart of user IDs

According to MSC4009

crates/ruma-identifiers-validation/CHANGELOG.md

@@ -1,9 +1,15 @@
 # [unreleased]
 
+Bug fixes:
+
 - Don't consider empty localpart of a user ID as valid
   - It is accepted under the `compat-user-id` feature, but not considered
     fully-conforming
 
+Improvements:
+
+- Allow `+` in the localpart of user IDs according to MSC4009
+
 # 0.9.1
 
 Improvements:

crates/ruma-identifiers-validation/src/user_id.rs

@@ -20,9 +20,9 @@ pub fn validate(s: &str) -> Result<(), Error> {
 pub fn localpart_is_fully_conforming(localpart: &str) -> Result<bool, Error> {
     // See https://spec.matrix.org/latest/appendices/#user-identifiers
     let is_fully_conforming = !localpart.is_empty()
-        && localpart
-            .bytes()
-            .all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'z' | b'-' | b'.' | b'=' | b'_' | b'/'));
+        && localpart.bytes().all(
+            |b| matches!(b, b'0'..=b'9' | b'a'..=b'z' | b'-' | b'.' | b'=' | b'_' | b'/' | b'+'),
+        );
 
     if !is_fully_conforming {
         // If it's not fully conforming, check if it contains characters that are also disallowed