Index: wp-admin/plugin-editor.php
===================================================================
--- wp-admin/plugin-editor.php	(revision 4810)
+++ wp-admin/plugin-editor.php	(working copy)
@@ -30,6 +30,15 @@
 		$f = fopen($real_file, 'w+');
 		fwrite($f, $newcontent);
 		fclose($f);
+
+		// Deactivate so we can test it.
+		$current = get_option('active_plugins');
+		if ( in_array($file, $current) ) {
+			array_splice($current, array_search( $file, $current), 1 ); // Array-fu!
+			update_option('active_plugins', $current);
+			wp_redirect(add_query_arg('_wpnonce', wp_create_nonce('edit-plugin-test_' . $file), "plugin-editor.php?file=$file&liveupdate=1"));
+			exit();
+		}
 		wp_redirect("plugin-editor.php?file=$file&a=te");
 	} else {
 		wp_redirect("plugin-editor.php?file=$file");
@@ -44,6 +53,24 @@
 	if ( !current_user_can('edit_plugins') )
 		wp_die('<p>'.__('You do not have sufficient permissions to edit plugins for this blog.').'</p>');
 
+	if ( $_GET['liveupdate'] ) {
+		check_admin_referer('edit-plugin-test_' . $file);
+		$current = get_option('active_plugins');
+		$plugin = $file;
+		if ( validate_file($plugin) )
+			wp_die(__('Invalid plugin.'));
+		if ( ! file_exists(ABSPATH . PLUGINDIR . '/' . $plugin) )
+			wp_die(__('Plugin file does not exist.'));
+		if (!in_array($plugin, $current)) {
+			wp_redirect("plugin-editor.php?file=$file&phperror=1"); // we'll override this later if the plugin can be included without fatal error
+			@include(ABSPATH . PLUGINDIR . '/' . $plugin);
+			$current[] = $plugin;
+			sort($current);
+			update_option('active_plugins', $current);
+		}
+		wp_redirect("plugin-editor.php?file=$file&a=te");
+	}
+
 	require_once('admin-header.php');
 
 	update_recently_edited(PLUGINDIR . "/$file");
@@ -60,13 +87,23 @@
 	?>
 <?php if (isset($_GET['a'])) : ?>
  <div id="message" class="updated fade"><p><?php _e('File edited successfully.') ?></p></div>
+<?php elseif (isset($_GET['phperror'])) : ?>
+ <div id="message" class="updated fade"><p><?php _e('This plugin has been deactivated because your changes resulted in a <strong>fatal error</strong>.') ?></p></div>
 <?php endif; ?>
  <div class="wrap">
 	<?php
-	if (is_writeable($real_file)) {
-		echo '<h2>' . sprintf(__('Editing <strong>%s</strong>'), $file) . '</h2>';
+	if ( in_array($file, (array) get_option('active_plugins')) ) {
+		if (is_writeable($real_file)) {
+			echo '<h2>' . sprintf(__('Editing <strong>%s</strong> (active)'), $file) . '</h2>';
+		} else {
+		echo '<h2>' . sprintf(__('Browsing <strong>%s</strong> (active)'), $file) . '</h2>';
+		}
 	} else {
-		echo '<h2>' . sprintf(__('Browsing <strong>%s</strong>'), $file) . '</h2>';
+		if (is_writeable($real_file)) {
+			echo '<h2>' . sprintf(__('Editing <strong>%s</strong> (inactive)'), $file) . '</h2>';
+		} else {
+		echo '<h2>' . sprintf(__('Browsing <strong>%s</strong> (inactive)'), $file) . '</h2>';
+		}
 	}
 	?>
 	<div id="templateside">
@@ -90,6 +127,9 @@
 		<input type="hidden" name="file" value="<?php echo $file ?>" />
 		</div>
 <?php if ( is_writeable($real_file) ) : ?>
+	<?php if ( in_array($file, (array) get_option('active_plugins')) ) { ?>
+		<p><?php _e('<strong>Warning:</strong> Making changes to active plugins is not recommended.  If your changes cause a fatal error, the plugin will be automatically deactivated.'); ?></p>
+	<?php } ?>
 	<p class="submit">
 	<?php
 		echo "<input type='submit' name='submit' value='	" . __('Update File &raquo;') . "' tabindex='2' />";