Add user ID localpart emptiness check

crates/ruma-identifiers-validation/CHANGELOG.md

@@ -1,5 +1,9 @@
 # [unreleased]
 
+- 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
+
 # 0.9.1
 
 Improvements:

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

@@ -19,16 +19,18 @@ pub fn validate(s: &str) -> Result<(), Error> {
 /// version; see [MSC2828](https://github.com/matrix-org/matrix-spec-proposals/pull/2828).
 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
-        .bytes()
-        .all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'z' | b'-' | b'.' | b'=' | b'_' | b'/'));
+    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'/'));
 
     if !is_fully_conforming {
         // If it's not fully conforming, check if it contains characters that are also disallowed
-        // for historical user IDs. If there are, return an error.
+        // for historical user IDs, or is empty. If that's the case, return an error.
         // See https://spec.matrix.org/latest/appendices/#historical-user-ids
         #[cfg(not(feature = "compat-user-id"))]
-        let is_invalid = localpart.bytes().any(|b| b < 0x21 || b == b':' || b > 0x7E);
+        let is_invalid =
+            localpart.is_empty() || localpart.bytes().any(|b| b < 0x21 || b == b':' || b > 0x7E);
 
         // In compat mode, allow anything except `:` to match Synapse. The `:` check is only needed
         // because this function can be called through `UserId::parse_with_servername`, otherwise