Suggest fix

Author: edolstra

src/libexpr/eval-settings.cc

@@ -85,12 +85,15 @@ bool EvalSettings::isPseudoUrl(std::string_view s)
 std::string EvalSettings::resolvePseudoUrl(std::string_view url)
 {
     if (hasPrefix(url, "channel:")) {
+        auto realUrl = "https://nixos.org/channels/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
         static bool haveWarned = false;
         warnOnce(haveWarned,
             "Channels are deprecated in favor of flakes in Determinate Nix. "
+            "Instead of '%s', use '%s'. "
             "For a guide on Nix flakes, see: https://zero-to-nix.com/. "
-            "For details and to offer feedback on the deprecation process, see: https://github.com/DeterminateSystems/nix-src/issues/34.");
-        return "https://nixos.org/channels/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
+            "For details and to offer feedback on the deprecation process, see: https://github.com/DeterminateSystems/nix-src/issues/34.",
+            url, realUrl);
+        return realUrl;
     } else
         return std::string(url);
 }