feat: Add ListUsers operation

src/db/user.rs

@@ -1,13 +1,49 @@
+use serde::Serialize;
+
 use std::fmt;
 
 use crate::db::{Operation, Role};
 
+/// A requested user/domain combo for login, lowercased.
+///
+/// Domain may be empty, but a value with more than one
+/// `@` is considered invalid.
 #[derive(Clone, Debug)]
 pub struct UserRef {
     pub username: String,
     pub domain: Option<String>,
 }
 
+#[derive(Clone, Debug)]
+pub struct InvalidUserRef(pub String);
+
+impl std::error::Error for InvalidUserRef {}
+
+impl fmt::Display for InvalidUserRef {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Invalid username: {}", self.0)
+    }
+}
+
+impl UserRef {
+    pub fn from_user_maybe_domain(value: &str) -> Result<Self, InvalidUserRef> {
+        let value = value.to_lowercase();
+
+        let mut parts = value.split('@');
+        let username = parts.next().unwrap();
+        let domain = parts.next();
+
+        if parts.next().is_some() {
+            return Err(InvalidUserRef(value.clone()));
+        }
+
+        Ok(Self {
+            username: username.to_string(),
+            domain: domain.map(Into::into),
+        })
+    }
+}
+
 impl fmt::Display for UserRef {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if let Some(domain) = &self.domain {
@@ -18,7 +54,7 @@ impl fmt::Display for UserRef {
     }
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize)]
 pub struct User {
     /// Username, without the domain part. Once set, cannot be edited.
     pub username: String,
@@ -41,7 +77,6 @@ impl User {
         }
     }
 
-    #[expect(unused)]
     pub fn can_perform(&self, operation: &Operation) -> bool {
         self.role.can_perform(operation)
     }