use stable sort to sort multipart diagnostics

Author: folkertdev

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1124,8 +1124,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, '_, 'infcx, 'tcx> {
                     err.multipart_suggestion(
                         "consider moving the expression out of the loop so it is only moved once",
                         vec![
-                            (parent.span, "value".to_string()),
                             (span.shrink_to_lo(), format!("let mut value = {value};{indent}")),
+                            (parent.span, "value".to_string()),
                         ],
                         Applicability::MaybeIncorrect,
                     );

compiler/rustc_errors/src/diagnostic.rs

@@ -920,8 +920,8 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         applicability: Applicability,
         style: SuggestionStyle,
     ) -> &mut Self {
-        suggestion.sort_unstable();
-        suggestion.dedup_by(|(s1, m1), (s2, m2)| s1.source_equal(*s2) && m1 == m2);
+        let mut seen = crate::FxHashSet::default();
+        suggestion.retain(|(span, msg)| seen.insert((span.lo(), span.hi(), msg.clone())));
 
         let parts = suggestion
             .into_iter()