Fixed build on nightly.

The fix for rust-lang/rust#51117 exposed some other
double mutable borrows, which are difficult to work around without enabling NLL.

src/main.rs

@@ -1,6 +1,6 @@
 //! A CLI tool to manage Factorio mods.
 
-#![feature(catch_expr, exhaustive_patterns, generators, never_type, proc_macro, proc_macro_non_items, proc_macro_path_invoc)]
+#![feature(catch_expr, exhaustive_patterns, generators, never_type, nll, proc_macro, proc_macro_non_items, proc_macro_path_invoc)]
 
 #![cfg_attr(feature = "cargo-clippy", deny(clippy, clippy_pedantic))]
 #![cfg_attr(feature = "cargo-clippy", allow(

src/solve.rs

@@ -8,7 +8,7 @@ use ::ResultExt;
 pub fn compute_and_apply_diff<'a>(
 	local_api: &'a ::factorio_mods_local::API,
 	web_api: &'a ::factorio_mods_web::API,
-	mut config: ::config::Config,
+	config: ::config::Config, // TODO: Should be `mut` but triggers warning due to https://github.com/rust-lang/rust/issues/50897
 	prompt_override: Option<bool>,
 ) -> impl Future<Item = (), Error = ::Error> + 'a {
 	::async_block! {

src/util.rs

@@ -46,20 +46,11 @@ pub fn ensure_user_credentials<'a>(
 			Ok(user_credentials) =>
 				return Ok(user_credentials),
 
-			Err(err) => {
-				let existing_username = if let ::factorio_mods_local::ErrorKind::IncompleteUserCredentials(ref existing_username) = *err.kind() {
-					Some(existing_username.clone())
-				}
-				else {
-					None
-				};
-
-				if let Some(existing_username) = existing_username {
-					existing_username
-				}
-				else {
-					return Err(err).chain_err(|| "Could not read user credentials");
-				}
+			Err(err) => if let ::factorio_mods_local::ErrorKind::IncompleteUserCredentials(existing_username) = err.kind() {
+				existing_username.clone()
+			}
+			else {
+				return Err(err).chain_err(|| "Could not read user credentials");
 			},
 		};