feat: try only the full email address if username is unspecified

Previously Delta Chat tried to use local part of email address as well.
This configuration is very uncommon,
but trying it doubled the time of configuration try
in the worst case, e.g. when the password is typed in incorrectly.

src/configure/server_params.rs

@@ -29,24 +29,14 @@ pub(crate) struct ServerParams {
 
 impl ServerParams {
     fn expand_usernames(self, addr: &str) -> Vec<ServerParams> {
-        let mut res = Vec::new();
-
         if self.username.is_empty() {
-            res.push(Self {
+            vec![Self {
                 username: addr.to_string(),
                 ..self.clone()
-            });
-
-            if let Some(at) = addr.find('@') {
-                res.push(Self {
-                    username: addr.split_at(at).0.to_string(),
-                    ..self
-                });
-            }
+            }]
         } else {
-            res.push(self)
+            vec![self]
         }
-        res
     }
 
     fn expand_hostnames(self, param_domain: &str) -> Vec<ServerParams> {
@@ -155,8 +145,7 @@ pub(crate) fn expand_param_vector(
     v.into_iter()
         // The order of expansion is important.
         //
-        // Ports are expanded the last, so they are changed the first.  Username is only changed if
-        // default value (address with domain) didn't work for all available hosts and ports.
+        // Ports are expanded the last, so they are changed the first.
         .flat_map(|params| params.expand_strict_tls().into_iter())
         .flat_map(|params| params.expand_usernames(addr).into_iter())
         .flat_map(|params| params.expand_hostnames(domain).into_iter())
@@ -333,5 +322,47 @@ mod tests {
                 },
             ],
         );
+
+        // Test that email address is used as the default username.
+        // We do not try other usernames
+        // such as the local part of the address
+        // as this is very uncommon configuration
+        // and not worth doubling the number of candidates to try.
+        // If such configuration is used, email provider
+        // should provide XML autoconfig or
+        // be added to the provider database as an exception.
+        let v = expand_param_vector(
+            vec![ServerParams {
+                protocol: Protocol::Imap,
+                hostname: "example.net".to_string(),
+                port: 0,
+                socket: Socket::Automatic,
+                username: "".to_string(),
+                strict_tls: Some(true),
+            }],
+            "foobar@example.net",
+            "example.net",
+        );
+        assert_eq!(
+            v,
+            vec![
+                ServerParams {
+                    protocol: Protocol::Imap,
+                    hostname: "example.net".to_string(),
+                    port: 993,
+                    socket: Socket::Ssl,
+                    username: "foobar@example.net".to_string(),
+                    strict_tls: Some(true)
+                },
+                ServerParams {
+                    protocol: Protocol::Imap,
+                    hostname: "example.net".to_string(),
+                    port: 143,
+                    socket: Socket::Starttls,
+                    username: "foobar@example.net".to_string(),
+                    strict_tls: Some(true)
+                },
+            ],
+        );
     }
 }