rustdoc font links only emit crossorigin when needed

The `crossorigin` attribute may cause issues when the href is not actuall across origins. Specifically, the tag causes the browser to send a preflight OPTIONS request to the href even if it is same-origin.
Some tempermental servers may reject all CORS preflect requests even if they're actually same-origin, which causes a CORS error and prevents the fonts from loading, even later on.

This commit fixes that problem by not emitting `crossorigin` if the url looks like a domain-relative url.

Co-authored-by: Guillaume Gomez <[email protected]>

Author: MingweiSamuel

src/librustdoc/html/layout.rs

@@ -68,6 +68,18 @@ struct PageLayout<'a> {
     display_krate_version_extra: &'a str,
 }
 
+impl PageLayout<'_> {
+    /// Conservatively determines if [`Self::static_root_path`] is relative to the current origin,
+    /// so that `crossorigin` may be safely removed from `<link>` elements.
+    fn static_root_path_may_remove_crossorigin(&self) -> bool {
+        let href = &*self.static_root_path;
+        // Everything up to the first `/`, or the whole str.
+        let first_part = href.split_once('/').map_or(href, |(first, _)| first);
+        // Reject protocol-relative URLs (`//example.com/`) or anything with a protocol (`http:`, etc.)
+        !(href.starts_with("//") || first_part.contains(':'))
+    }
+}
+
 pub(crate) use crate::html::render::sidebar::filters;
 
 pub(crate) fn render<T: Display, S: Display>(

src/librustdoc/html/templates/page.html

@@ -7,7 +7,7 @@
     <meta name="description" content="{{page.description}}"> {# #}
     <title>{{page.title}}</title> {# #}
     <script>if(window.location.protocol!=="file:") {# Hack to skip preloading fonts locally - see #98769 #}
-    document.head.insertAdjacentHTML("beforeend","{{files.source_serif_4_regular}},{{files.fira_sans_italic}},{{files.fira_sans_regular}},{{files.fira_sans_medium_italic}},{{files.fira_sans_medium}},{{files.source_code_pro_regular}},{{files.source_code_pro_semibold}}".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path|safe}}${f}">`).join("")) {# #}
+    document.head.insertAdjacentHTML("beforeend","{{files.source_serif_4_regular}},{{files.fira_sans_italic}},{{files.fira_sans_regular}},{{files.fira_sans_medium_italic}},{{files.fira_sans_medium}},{{files.source_code_pro_regular}},{{files.source_code_pro_semibold}}".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"{% if !static_root_path_may_remove_crossorigin() %} crossorigin{% endif %} href="{{static_root_path|safe}}${f}">`).join("")) {# #}
     </script> {# #}
     <link rel="stylesheet" {#+ #}
           href="{{static_root_path|safe}}{{files.normalize_css}}"> {# #}