Use ordinal number in argument error

Author: long-long-float

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1014,6 +1014,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .peek()
             .is_some_and(|first| matches!(first, Error::Extra(arg_idx) if arg_idx.index() == 0));
         let mut suggestions = vec![];
+
+        // Convert the given number into the corresponding ordinal
+        // This is copied from rustc_resolve::diagnostics::ordinalize because it is too much to link rustc_resolve for this small function.
+        fn ordinalize(v: usize) -> String {
+            let suffix = match ((11..=13).contains(&(v % 100)), v % 10) {
+                (false, 1) => "st",
+                (false, 2) => "nd",
+                (false, 3) => "rd",
+                _ => "th",
+            };
+            format!("{v}{suffix}")
+        }
+
         while let Some(error) = errors.next() {
             only_extras_so_far &= matches!(error, Error::Extra(_));
 
@@ -1055,7 +1068,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     } else {
                         "".to_string()
                     };
-                    labels.push((provided_span, format!("unexpected argument{provided_ty_name}")));
+                    labels.push((provided_span, format!("unexpected {} argument{provided_ty_name}", ordinalize(arg_idx.as_usize() + 1))));
                     let mut span = provided_span;
                     if span.can_be_used_for_suggestions()
                         && error_span.can_be_used_for_suggestions()
@@ -1136,7 +1149,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             } else {
                                 "".to_string()
                             };
-                            labels.push((span, format!("an argument{rendered} is missing")));
+                            labels.push((span, format!("{} argument{rendered} is missing", ordinalize(expected_idx.as_usize() + 1))));
+
                             suggestion_text = match suggestion_text {
                                 SuggestionText::None => SuggestionText::Provide(false),
                                 SuggestionText::Provide(_) => SuggestionText::Provide(true),