Add warning block support in rustdoc

Author: GuillaumeGomez

src/librustdoc/html/static/css/rustdoc.css

@@ -270,15 +270,15 @@ ul ul, ol ul, ul ol, ol ol {
 	margin-bottom: .625em;
 }
 
-p {
+p, .docblock > .warning {
 	/* Paragraph spacing at least 1.5 times line spacing per Web Content Accessibility Guidelines.
 	   Line-height is 1.5rem, so line spacing is .5rem; .75em is 1.5 times that.
 	   https://www.w3.org/WAI/WCAG21/Understanding/visual-presentation.html */
 	margin: 0 0 .75em 0;
 }
 /* For the last child of a div, the margin will be taken care of
 	by the margin-top of the next item. */
-p:last-child {
+p:last-child, .docblock > .warning:last-child {
 	margin: 0;
 }
 
@@ -1096,7 +1096,7 @@ pre.rust .doccomment {
 }
 
 .example-wrap.ignore .tooltip {
-	color:  var(--codeblock-ignore-color);
+	color: var(--codeblock-ignore-color);
 }
 
 .example-wrap.compile_fail:hover .tooltip,
@@ -1124,6 +1124,26 @@ pre.rust .doccomment {
 	font-size: 1.25rem;
 }
 
+/* This class only exists for users who want to draw attention to a particular element in their
+documentation. */
+.content .docblock .warning {
+	border-left: 2px solid var(--warning-border-color);
+	padding: 14px;
+	position: relative;
+	/* The "!important" part is required because the rule is otherwise overruled in this CSS
+	selector: ".docblock > :not(.more-examples-toggle):not(.example-wrap)" */
+	overflow-x: visible !important;
+}
+.content .docblock .warning::before {
+	color: var(--warning-border-color);
+	content: "ⓘ";
+	position: absolute;
+	left: -25px;
+	top: 5px;
+	font-weight: bold;
+	font-size: 1.25rem;
+}
+
 a.test-arrow {
 	visibility: hidden;
 	position: absolute;

src/librustdoc/html/static/css/themes/ayu.css

@@ -31,6 +31,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--codeblock-error-color: rgba(255, 0, 0, .5);
 	--codeblock-ignore-hover-color: rgb(255, 142, 0);
 	--codeblock-ignore-color: rgba(255, 142, 0, .6);
+	--warning-border-color: rgb(255, 142, 0);
 	--type-link-color: #ffa0a5;
 	--trait-link-color: #39afd7;
 	--assoc-item-link-color: #39afd7;

src/librustdoc/html/static/css/themes/dark.css

@@ -26,6 +26,7 @@
 	--codeblock-error-color: rgba(255, 0, 0, .5);
 	--codeblock-ignore-hover-color: rgb(255, 142, 0);
 	--codeblock-ignore-color: rgba(255, 142, 0, .6);
+	--warning-border-color: rgb(255, 142, 0);
 	--type-link-color: #2dbfb8;
 	--trait-link-color: #b78cf2;
 	--assoc-item-link-color: #d2991d;

src/librustdoc/html/static/css/themes/light.css

@@ -26,6 +26,7 @@
 	--codeblock-error-color: rgba(255, 0, 0, .5);
 	--codeblock-ignore-hover-color: rgb(255, 142, 0);
 	--codeblock-ignore-color: rgba(255, 142, 0, .6);
+	--warning-border-color: rgb(255, 142, 0);
 	--type-link-color: #ad378a;
 	--trait-link-color: #6e4fc9;
 	--assoc-item-link-color: #3873ad;