don't use partial ordering on types that support total ordering

compiler/rustc_resolve/src/diagnostics.rs

@@ -1139,7 +1139,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         });
 
         // Make sure error reporting is deterministic.
-        suggestions.sort_by(|a, b| a.candidate.as_str().partial_cmp(b.candidate.as_str()).unwrap());
+        suggestions.sort_by(|a, b| a.candidate.as_str().cmp(b.candidate.as_str()));
 
         match find_best_match_for_name(
             &suggestions.iter().map(|suggestion| suggestion.candidate).collect::<Vec<Symbol>>(),
@@ -2357,7 +2357,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         // 2) `std` suggestions before `core` suggestions.
         let mut extern_crate_names =
             self.extern_prelude.keys().map(|ident| ident.name).collect::<Vec<_>>();
-        extern_crate_names.sort_by(|a, b| b.as_str().partial_cmp(a.as_str()).unwrap());
+        extern_crate_names.sort_by(|a, b| b.as_str().cmp(a.as_str()));
 
         for name in extern_crate_names.into_iter() {
             // Replace first ident with a crate name and check if that is valid.