Add testing for prefs

Also, spelling fix in opts.rs

Fix order of imports to satisfy the lint
Fix split line in test
Fix multi-line string

components/util/opts.rs

@@ -801,7 +801,7 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
 
     // These must happen after setting the default options, since the prefs rely on
     // on the resource path.
-    // Note that command line preferences have the highest precedent
+    // Note that command line preferences have the highest precedence
     if get().profile_dir.is_some() {
         prefs::add_user_prefs();
     }

components/util/prefs.rs

@@ -2,9 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use opts;
 use resource_files::resources_dir_path;
 use rustc_serialize::json::{Json, ToJson};
-use opts;
 use std::borrow::ToOwned;
 use std::collections::HashMap;
 use std::fs::File;

tests/unit/util/lib.rs

@@ -16,3 +16,4 @@ extern crate util;
 #[cfg(test)] mod opts;
 #[cfg(test)] mod str;
 #[cfg(test)] mod thread;
+#[cfg(test)] mod prefs;

tests/unit/util/prefs.rs

@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use util::prefs::{PrefValue, extend_prefs, read_prefs_from_file, get_pref, set_pref, reset_pref, reset_all_prefs};
+
+#[test]
+fn test_create_pref() {
+    let json_str = "{\
+  \"layout.writing-mode.enabled\": true,\
+  \"net.mime.sniff\": false,\
+  \"shell.homepage\": \"http://servo.org\"\
+}";
+
+    let prefs = read_prefs_from_file(json_str.as_bytes());
+    assert!(prefs.is_ok());
+    let prefs = prefs.unwrap();
+
+    assert_eq!(prefs.len(), 3);
+}
+
+#[test]
+fn test_get_set_reset_extend() {
+    let json_str = "{\
+  \"layout.writing-mode.enabled\": true,\
+  \"extra.stuff\": false,\
+  \"shell.homepage\": \"https://google.com\"\
+}";
+
+    assert_eq!(*get_pref("test"), PrefValue::Missing);
+    set_pref("test", PrefValue::String("hi".to_owned()));
+    assert_eq!(*get_pref("test"), PrefValue::String("hi".to_owned()));
+    assert_eq!(*get_pref("shell.homepage"), PrefValue::String("http://servo.org".to_owned()));
+    set_pref("shell.homepage", PrefValue::Boolean(true));
+    assert_eq!(*get_pref("shell.homepage"), PrefValue::Boolean(true));
+    reset_pref("shell.homepage");
+    assert_eq!(*get_pref("shell.homepage"), PrefValue::String("http://servo.org".to_owned()));
+
+    let extension = read_prefs_from_file(json_str.as_bytes()).unwrap();
+    extend_prefs(extension);
+    assert_eq!(*get_pref("shell.homepage"), PrefValue::String("https://google.com".to_owned()));
+    assert_eq!(*get_pref("layout.writing-mode.enabled"), PrefValue::Boolean(true));
+    assert_eq!(*get_pref("extra.stuff"), PrefValue::Boolean(false));
+}