Avoid unnecessary argument mutation in fluent_bundle

compiler/rustc_error_messages/src/lib.rs

@@ -107,8 +107,8 @@ impl From<Vec<FluentError>> for TranslationBundleError {
 /// (overriding any conflicting messages).
 #[instrument(level = "trace")]
 pub fn fluent_bundle(
-    mut user_provided_sysroot: PathBuf,
-    mut sysroot_candidates: Vec<PathBuf>,
+    user_provided_sysroot: PathBuf,
+    sysroot_candidates: Vec<PathBuf>,
     requested_locale: Option<LanguageIdentifier>,
     additional_ftl_path: Option<&Path>,
     with_directionality_markers: bool,
@@ -142,8 +142,8 @@ pub fn fluent_bundle(
     // If the user requests the default locale then don't try to load anything.
     if let Some(requested_locale) = requested_locale {
         let mut found_resources = false;
-        for sysroot in
-            Some(&mut user_provided_sysroot).into_iter().chain(sysroot_candidates.iter_mut())
+        for mut sysroot in
+            Some(user_provided_sysroot).into_iter().chain(sysroot_candidates.into_iter())
         {
             sysroot.push("share");
             sysroot.push("locale");