source_span_for_markdown_range: fix utf8 violation

Author: lolbinarycat

compiler/rustc_resolve/src/rustdoc.rs

@@ -570,7 +570,12 @@ pub fn source_span_for_markdown_range(
             {
                 // If there is either a match in a previous fragment, or
                 // multiple matches in this fragment, there is ambiguity.
-                if match_data.is_none() && !snippet[match_start + 1..].contains(pat) {
+                // the snippet cannot be zero-sized,
+                // because it matches the pattern,
+                // which is checked to not be zero sized.
+                if match_data.is_none()
+                    && !snippet.as_bytes().windows(pat.len()).any(|s| s == pat.as_bytes())
+                {
                     match_data = Some((i, match_start));
                 } else {
                     // Heirustic produced ambiguity, return nothing.