fixup! rustdoc font links only emit crossorigin when needed

Author: MingweiSamuel

src/librustdoc/html/layout.rs

@@ -73,10 +73,20 @@ impl PageLayout<'_> {
     /// 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(':'))
+        // Reject scheme-relative URLs (`//example.com/`).
+        if href.starts_with("//") {
+            return false;
+        }
+        // URL is interpreted as having a scheme iff: it starts with an ascii alpha, and only
+        // contains ascii alphanumeric or `+` `-` `.` up to the `:`.
+        // https://url.spec.whatwg.org/#url-parsing
+        let has_scheme = href.split_once(':').is_some_and(|(scheme, _rest)| {
+            let mut chars = scheme.chars();
+            scheme.next().is_some_and(|c| c.is_ascii_alphabetic())
+                && chars.all(|c| c.is_ascii_alphanumeric() || c == '+' || c == '-' || c == '.')
+        });
+        // Reject anything with a scheme (`http:`, etc.).
+        !has_scheme
     }
 }