Merge "libexpr: improve error reporting if NIX_PATH item cannot be downloaded" into main

doc/manual/rl-next/improve-nix_path-error.md

@@ -0,0 +1,21 @@
+---
+synopsis: Show error when item from NIX_PATH cannot be downloaded
+issues: []
+cls: []
+category: Fixes
+credits: [ma27]
+---
+
+For e.g. `nix-instantiate -I https://example.com/404`, you'd only get a warning if the download failed, such as
+
+    warning: Nix search path entry 'https://example.com/404' cannot be downloaded, ignoring
+
+Now, the full error that caused the download failure is displayed with a note that the search
+path entry is ignored, e.g.
+
+    warning:
+         … while downloading https://example.com/404 to satisfy NIX_PATH lookup, ignoring search path entry
+
+         warning: unable to download 'https://example.com/404': HTTP error 404 ()
+
+         response body: […]

lix/libexpr/eval.cc

@@ -2,6 +2,7 @@
 #include "lix/libexpr/eval-settings.hh"
 #include "lix/libutil/archive.hh"
 #include "lix/libutil/async.hh"
+#include "lix/libutil/error.hh"
 #include "lix/libutil/hash.hh"
 #include "lix/libexpr/primops.hh"
 #include "lix/libexpr/print-options.hh"
@@ -2827,9 +2828,8 @@ try {
                 store, EvalSettings::resolvePseudoUrl(value), "source", false)).tree.storePath;
             res = { store->toRealPath(storePath) };
         } catch (FileTransferError & e) {
-            logWarning({
-                .msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
-            });
+            e.addTrace(nullptr, "while downloading %s to satisfy NIX_PATH lookup, ignoring search path entry", value);
+            logWarning(e.info());
             res = std::nullopt;
         }
     }