Clone ID map instead of keeping them split

Author: GuillaumeGomez

src/librustdoc/html/markdown.rs

@@ -1950,16 +1950,7 @@ impl IdMap {
 
     pub(crate) fn derive<S: AsRef<str> + ToString>(&mut self, candidate: S) -> String {
         let id = match self.map.get_mut(candidate.as_ref()) {
-            None => {
-                let candidate = candidate.to_string();
-                if self.defined_ids.contains(candidate.as_str()) {
-                    let id = format!("{}-{}", candidate, 1);
-                    self.map.insert(candidate.into(), 2);
-                    id
-                } else {
-                    candidate.to_string()
-                }
-            }
+            None => candidate.to_string(),
             Some(a) => {
                 let id = format!("{}-{}", candidate.as_ref(), *a);
                 *a += 1;
@@ -1981,7 +1972,7 @@ impl IdMap {
     }
 
     pub(crate) fn clear(&mut self) {
-        self.map.clear();
+        self.map = self.defined_ids.clone();
         self.existing_footnotes = 0;
     }
 }