Auto merge of #56005 - GuillaumeGomez:speedup-doc-render, r=QuietMisdreavus

Greatly improve rustdoc rendering speed issues

Fixes #55900.

So a few improvements here:

* we're switching to `DOMTokenList` API when available providing a replacement if it isn't (should only happen on safari and IE I think...)
* hide doc sections by default to allow the whole HTML generation to happen in the background to avoid triggering DOM redraw all the times (which killed the performances)

r? @QuietMisdreavus

src/librustdoc/html/static/noscript.css

@@ -0,0 +1,19 @@
+/**
+ * Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+ * file at the top-level directory of this distribution and at
+ * http://rust-lang.org/COPYRIGHT.
+ *
+ * Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+ * http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+ * <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+ * option. This file may not be copied, modified, or distributed
+ * except according to those terms.
+ */
+
+#main > h2 + div, #main > h2 + h3, #main > h3 + div {
+	display: block;
+}
+
+.loading-content {
+	display: none;
+}

src/librustdoc/html/static/rustdoc.css

@@ -368,6 +368,10 @@ body:not(.source) .example-wrap > pre {
 #main > .docblock h2 { font-size: 1.15em; }
 #main > .docblock h3, #main > .docblock h4, #main > .docblock h5 { font-size: 1em; }
 
+#main > h2 + div, #main > h2 + h3, #main > h3 + div {
+	display: none;
+}
+
 .docblock h1 { font-size: 1em; }
 .docblock h2 { font-size: 0.95em; }
 .docblock h3, .docblock h4, .docblock h5 { font-size: 0.9em; }

src/librustdoc/html/static/storage.js

@@ -19,55 +19,38 @@ var mainTheme = document.getElementById("mainThemeStyle");
 var savedHref = [];
 
 function hasClass(elem, className) {
-    if (elem && className && elem.className) {
-        var elemClass = elem.className;
-        var start = elemClass.indexOf(className);
-        if (start === -1) {
-            return false;
-        } else if (elemClass.length === className.length) {
-            return true;
-        } else {
-            if (start > 0 && elemClass[start - 1] !== ' ') {
-                return false;
-            }
-            var end = start + className.length;
-            return !(end < elemClass.length && elemClass[end] !== ' ');
-        }
-    }
-    return false;
+    return elem && elem.classList && elem.classList.contains(className);
 }
 
 function addClass(elem, className) {
-    if (elem && className && !hasClass(elem, className)) {
-        if (elem.className && elem.className.length > 0) {
-            elem.className += ' ' + className;
-        } else {
-            elem.className = className;
-        }
+    if (!elem || !elem.classList) {
+        return;
     }
+    elem.classList.add(className);
 }
 
 function removeClass(elem, className) {
-    if (elem && className && elem.className) {
-        elem.className = (" " + elem.className + " ").replace(" " + className + " ", " ")
-                                                     .trim();
+    if (!elem || !elem.classList) {
+        return;
     }
+    elem.classList.remove(className);
 }
 
 function isHidden(elem) {
-    return (elem.offsetParent === null)
+    return elem.offsetParent === null;
 }
 
 function onEach(arr, func, reversed) {
     if (arr && arr.length > 0 && func) {
+        var length = arr.length;
         if (reversed !== true) {
-            for (var i = 0; i < arr.length; ++i) {
+            for (var i = 0; i < length; ++i) {
                 if (func(arr[i]) === true) {
                     return true;
                 }
             }
         } else {
-            for (var i = arr.length - 1; i >= 0; --i) {
+            for (var i = length - 1; i >= 0; --i) {
                 if (func(arr[i]) === true) {
                     return true;
                 }
@@ -77,6 +60,13 @@ function onEach(arr, func, reversed) {
     return false;
 }
 
+function onEachLazy(lazyArray, func, reversed) {
+    return onEach(
+        Array.prototype.slice.call(lazyArray),
+        func,
+        reversed);
+}
+
 function usableLocalStorage() {
     // Check if the browser supports localStorage at all:
     if (typeof(Storage) === "undefined") {
@@ -133,8 +123,8 @@ function switchTheme(styleElem, mainStyleElem, newTheme) {
     });
     if (found === true) {
         styleElem.href = newHref;
-        updateLocalStorage('rustdoc-theme', newTheme);
+        updateLocalStorage("rustdoc-theme", newTheme);
     }
 }
 
-switchTheme(currentTheme, mainTheme, getCurrentValue('rustdoc-theme') || 'light');
+switchTheme(currentTheme, mainTheme, getCurrentValue("rustdoc-theme") || "light");