rustdoc: clean up main.js and src-script.js

* Run the querySelector for the toggleLabel only once, and store
  the result.
* Use querySelector to find the resizer and sidebar.
* Add comments to main.js sections.

Author: notriddle

src/librustdoc/html/static/js/main.js

@@ -1273,6 +1273,7 @@ href="https://doc.rust-lang.org/${channel}/rustdoc/how-to-read-rustdoc.html\
     searchState.setup();
 }());
 
+// This section handles sidebar resizing
 (function() {
     const sidebarButton = document.getElementById("sidebar-button");
     if (sidebarButton) {
@@ -1283,8 +1284,8 @@ href="https://doc.rust-lang.org/${channel}/rustdoc/how-to-read-rustdoc.html\
         });
     }
     let currentPointerId = null;
-    const resizer = document.getElementsByClassName("sidebar-resizer")[0];
-    const sidebar = document.getElementsByClassName("sidebar")[0];
+    const resizer = document.querySelector(".sidebar-resizer");
+    const sidebar = document.querySelector(".sidebar");
     if (!resizer || !sidebar) {
         return;
     }
@@ -1379,6 +1380,7 @@ href="https://doc.rust-lang.org/${channel}/rustdoc/how-to-read-rustdoc.html\
     resizer.addEventListener("pointerdown", initResize, false);
 }());
 
+// This section handles the copy button that appears next to the path breadcrumbs
 (function() {
     let reset_button_timeout = null;
 

src/librustdoc/html/static/js/src-script.js

@@ -71,17 +71,22 @@ function createDirEntry(elem, parent, fullPath, hasFoundFile) {
     return hasFoundFile;
 }
 
+let toggleLabel;
+
+function getToggleLabel() {
+    toggleLabel = toggleLabel || document.querySelector("#src-sidebar-toggle button");
+    return toggleLabel;
+}
+
 window.rustdocCloseSourceSidebar = () => {
-    const toggleLabel = document.querySelector("#src-sidebar-toggle button");
     removeClass(document.documentElement, "src-sidebar-expanded");
-    toggleLabel.innerText = ">";
+    getToggleLabel().innerText = ">";
     updateLocalStorage("source-sidebar-show", "false");
 };
 
 window.rustdocShowSourceSidebar = () => {
-    const toggleLabel = document.querySelector("#src-sidebar-toggle button");
     addClass(document.documentElement, "src-sidebar-expanded");
-    toggleLabel.innerText = "<";
+    getToggleLabel().innerText = "<";
     updateLocalStorage("source-sidebar-show", "true");
 };