Improve page load performance in rustdoc.

Add font-display: swap. Per https://web.dev/font-display/, this prevents
"flash of invisible text" during load by using a system font until the
custom font is available. I've noticed this flash of invisible text
occasionally when reading Rust docs.

Add an explicit height to icons (which already had an explicit width)
to allow browsers to lay out the page more accurately before the icons
have been loaded. https://web.dev/optimize-cls/.

Add min-width: 115px to the crate search dropdown. When the HTML first
loads, this dropdown includes only the text "All crates." Later, JS
loads the items underneath it, some of which are wider. That causes
the dropdown to get wider, causing a distracting reflow. This sets a
min-width based on the size that the dropdown eventually becomes based
on the crates on doc.rust-lang.org, reducing page movement during load.

Author: jsha

src/doc/rust.css

@@ -3,30 +3,35 @@
 	font-style: normal;
 	font-weight: 400;
 	src: local('Fira Sans'), url("FiraSans-Regular.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Fira Sans';
 	font-style: normal;
 	font-weight: 500;
 	src: local('Fira Sans Medium'), url("FiraSans-Medium.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: normal;
 	font-weight: 400;
 	src: local('Source Serif Pro'), url("SourceSerifPro-Regular.ttf.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: italic;
 	font-weight: 400;
 	src: url("SourceSerifPro-It.ttf.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: normal;
 	font-weight: 700;
 	src: local('Source Serif Pro Bold'), url("SourceSerifPro-Bold.ttf.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Code Pro';
@@ -35,6 +40,7 @@
 	/* Avoid using locally installed font because bad versions are in circulation:
 	 * see https://github.com/rust-lang/rust/issues/24355 */
 	src: url("SourceCodePro-Regular.woff") format('woff');
+	font-display: swap;
 }
 
 *:not(body) {

src/librustdoc/html/layout.rs

@@ -82,7 +82,7 @@ crate fn render<T: Print, S: Print>(
     <div class=\"theme-picker\">\
         <button id=\"theme-picker\" aria-label=\"Pick another theme!\" aria-haspopup=\"menu\">\
             <img src=\"{static_root_path}brush{suffix}.svg\" \
-                 width=\"18\" \
+                 width=\"18\" height=\"18\" \
                  alt=\"Pick another theme!\">\
         </button>\
         <div id=\"theme-choices\" role=\"menu\"></div>\
@@ -102,7 +102,7 @@ crate fn render<T: Print, S: Print>(
                 <button type=\"button\" class=\"help-button\">?</button>
                 <a id=\"settings-menu\" href=\"{root_path}settings.html\">\
                     <img src=\"{static_root_path}wheel{suffix}.svg\" \
-                         width=\"18\" \
+                         width=\"18\" height=\"18\" \
                          alt=\"Change settings\">\
                 </a>\
             </div>\

src/librustdoc/html/static/rustdoc.css

@@ -6,6 +6,7 @@
 	src: local('Fira Sans'),
 		url("FiraSans-Regular.woff2") format("woff2"),
 		url("FiraSans-Regular.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Fira Sans';
@@ -14,6 +15,7 @@
 	src: local('Fira Sans Medium'),
 		url("FiraSans-Medium.woff2") format("woff2"),
 		url("FiraSans-Medium.woff") format('woff');
+	font-display: swap;
 }
 
 /* See SourceSerifPro-LICENSE.txt for the Source Serif Pro license. */
@@ -22,18 +24,21 @@
 	font-style: normal;
 	font-weight: 400;
 	src: local('Source Serif Pro'), url("SourceSerifPro-Regular.ttf.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: italic;
 	font-weight: 400;
 	src: local('Source Serif Pro Italic'), url("SourceSerifPro-It.ttf.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: normal;
 	font-weight: 700;
 	src: local('Source Serif Pro Bold'), url("SourceSerifPro-Bold.ttf.woff") format('woff');
+	font-display: swap;
 }
 
 /* See SourceCodePro-LICENSE.txt for the Source Code Pro license. */
@@ -44,12 +49,14 @@
 	/* Avoid using locally installed font because bad versions are in circulation:
 	 * see https://github.com/rust-lang/rust/issues/24355 */
 	src: url("SourceCodePro-Regular.woff") format('woff');
+	font-display: swap;
 }
 @font-face {
 	font-family: 'Source Code Pro';
 	font-style: normal;
 	font-weight: 600;
 	src: url("SourceCodePro-Semibold.woff") format('woff');
+	font-display: swap;
 }
 
 * {
@@ -684,6 +691,7 @@ a {
 	width: calc(100% - 63px);
 }
 #crate-search {
+	min-width: 115px;
 	margin-top: 5px;
 	padding: 6px;
 	padding-right: 19px;