Move server event filtering logic to rust (#17928)

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
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  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
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* [X] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

1 files changed, 86 insertions, 0 deletions

diff --git a/rust/src/identifier.rs b/rust/src/identifier.rs
new file mode 100644

index 0000000000..b199c5838e
--- /dev/null
+++ b/rust/src/identifier.rs
@@ -0,0 +1,86 @@
+/*
+ * This file is licensed under the Affero General Public License (AGPL) version 3.
+ *
+ * Copyright (C) 2024 New Vector, Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * See the GNU Affero General Public License for more details:
+ * <https://www.gnu.org/licenses/agpl-3.0.html>.
+ */
+
+//! # Matrix Identifiers
+//!
+//! This module contains definitions and utilities for working with matrix identifiers.
+
+use std::{fmt, ops::Deref};
+
+/// Errors that can occur when parsing a matrix identifier.
+#[derive(Clone, Debug, PartialEq)]
+pub enum IdentifierError {
+    IncorrectSigil,
+    MissingColon,
+}
+
+impl fmt::Display for IdentifierError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
+/// A Matrix user_id.
+#[derive(Clone, Debug, PartialEq)]
+pub struct UserID(String);
+
+impl UserID {
+    /// Returns the `localpart` of the user_id.
+    pub fn localpart(&self) -> &str {
+        &self[1..self.colon_pos()]
+    }
+
+    /// Returns the `server_name` / `domain` of the user_id.
+    pub fn server_name(&self) -> &str {
+        &self[self.colon_pos() + 1..]
+    }
+
+    /// Returns the position of the ':' inside of the user_id.
+    /// Used when splitting the user_id into it's respective parts.
+    fn colon_pos(&self) -> usize {
+        self.find(':').unwrap()
+    }
+}
+
+impl TryFrom<&str> for UserID {
+    type Error = IdentifierError;
+
+    /// Will try creating a `UserID` from the provided `&str`.
+    /// Can fail if the user_id is incorrectly formatted.
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        if !s.starts_with('@') {
+            return Err(IdentifierError::IncorrectSigil);
+        }
+
+        if s.find(':').is_none() {
+            return Err(IdentifierError::MissingColon);
+        }
+
+        Ok(UserID(s.to_string()))
+    }
+}
+
+impl Deref for UserID {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl fmt::Display for UserID {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}