Emit warning when named arguments are used positionally in format

Addresses Issue 98466 by emitting a warning if a named argument
is used like a position argument (i.e. the name is not used in
the string to be formatted).

Fixes rust-lang#98466

compiler/rustc_lint/src/context.rs

@@ -857,6 +857,18 @@ pub trait LintContext: Sized {
                         Applicability::MachineApplicable,
                     );
                 },
+                BuiltinLintDiagnostics::NamedArgumentUsedPositionally(positional_arg, named_arg, name) => {
+                    db.span_label(named_arg, "this named argument is only referred to by position in formatting string");
+                    let msg = format!("this formatting argument uses named argument `{}` by position", name);
+                    db.span_label(positional_arg, msg);
+                    db.span_suggestion_verbose(
+                        positional_arg,
+                        "use the named argument by name to avoid ambiguity",
+                        format!("{{{}}}", name),
+                        Applicability::MaybeIncorrect,
+                    );
+
+                }
             }
             // Rewrap `db`, and pass control to the user.
             decorate(LintDiagnosticBuilder::new(db));