Make invisible closed toggles that haven't been used.

This reduces clutter on the page, while still supporting the "auto hide
X" settings and the "collapse all docs" button.

Author: jsha

src/librustdoc/html/static/main.js

@@ -862,6 +862,12 @@ function hideThemeButtonState() {
         displayHelp(true, ev);
     });
 
+    onEachLazy(document.getElementsByTagName("summary"), function(el) {
+        el.addEventListener("click", function(ev) {
+           addClass(el, "used");
+        });
+    })
+
     onEachLazy(document.getElementsByTagName("a"), function(el) {
         // For clicks on internal links (<A> tags with a hash property), we expand the section we're
         // jumping to *before* jumping there. We can't do this in onHashChange, because it changes

src/librustdoc/html/static/rustdoc.css

@@ -1487,11 +1487,21 @@ details.rustdoc-toggle[open] > summary.hideme > span {
 	display: none;
 }
 
-details.rustdoc-toggle[open] > summary::before {
+/* If a toggle has been used to open a section, we should make sure */
+/* there's a [-] so it can be closed again. But if a toggle is open */
+/* and has never been used, don't show it. This minimizes clutter on */
+/* the page while still supporting the "auto hide <X>" settings and */
+/* the expand/collapse all functionality. */
+details.rustdoc-toggle[open] > summary.used::before {
 	content: "[−]";
 	display: inline;
 }
 
+details.rustdoc-toggle[open] > summary::before {
+	content: "";
+	display: inline;
+}
+
 details.undocumented > summary::before {
 	content: "[+] Show hidden undocumented items";
 	cursor: pointer;