librustdoc: also stabilize iteration order here

Author: durin42

src/librustdoc/html/markdown.rs

@@ -1726,6 +1726,7 @@ pub(crate) fn markdown_links<'md, R>(
     md: &'md str,
     preprocess_link: impl Fn(MarkdownLink) -> Option<R>,
 ) -> Vec<R> {
+    use itertools::Itertools;
     if md.is_empty() {
         return vec![];
     }
@@ -1884,7 +1885,7 @@ pub(crate) fn markdown_links<'md, R>(
     let mut links = Vec::new();
 
     let mut refdefs = FxIndexMap::default();
-    for (label, refdef) in event_iter.reference_definitions().iter() {
+    for (label, refdef) in event_iter.reference_definitions().iter().sorted_by_key(|x| x.0) {
         refdefs.insert(label.to_string(), (false, refdef.dest.to_string(), refdef.span.clone()));
     }