nix flake info: Show resolved URL

This is useful for finding out what a registry lookup resolves to, e.g

  $ nix flake info patchelf
  Resolved URL:  github:NixOS/patchelf
  Locked URL:    github:NixOS/patchelf/cd7955af31698c571c30b7a0f78e59fd624d0229

3 files changed, 13 insertions, 6 deletions

diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index f23c53a82..86bb05b5d 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -46,7 +46,7 @@ static FlakeRef lookupInFlakeCache(
     return flakeRef;
 }
 
-static std::pair<fetchers::Tree, FlakeRef> fetchOrSubstituteTree(
+static std::tuple<fetchers::Tree, FlakeRef, FlakeRef> fetchOrSubstituteTree(
     EvalState & state,
     const FlakeRef & originalRef,
     std::optional<TreeInfo> treeInfo,
@@ -76,6 +76,7 @@ static std::pair<fetchers::Tree, FlakeRef> fetchOrSubstituteTree(
                     .storePath = std::move(storePath),
                     .info = *treeInfo,
                 },
+                originalRef,
                 originalRef
             };
         } catch (Error & e) {
@@ -101,7 +102,7 @@ static std::pair<fetchers::Tree, FlakeRef> fetchOrSubstituteTree(
     if (treeInfo)
         assert(tree.storePath == treeInfo->computeStorePath(*state.store));
 
-    return {std::move(tree), lockedRef};
+    return {std::move(tree), resolvedRef, lockedRef};
 }
 
 static void expectType(EvalState & state, ValueType type,
@@ -206,7 +207,7 @@ static Flake getFlake(
     bool allowLookup,
     FlakeCache & flakeCache)
 {
-    auto [sourceInfo, lockedRef] = fetchOrSubstituteTree(
+    auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
         state, originalRef, treeInfo, allowLookup, flakeCache);
 
     // Guard against symlink attacks.
@@ -217,6 +218,7 @@ static Flake getFlake(
 
     Flake flake {
         .originalRef = originalRef,
+        .resolvedRef = resolvedRef,
         .lockedRef = lockedRef,
         .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
     };
@@ -490,7 +492,7 @@ LockedFlake lockFlake(
                 }
 
                 else {
-                    auto [sourceInfo, lockedRef] = fetchOrSubstituteTree(
+                    auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
                         state, input.ref, {}, lockFlags.useRegistries, flakeCache);
                     node->inputs.insert_or_assign(id,
                         std::make_shared<LockedNode>(lockedRef, input.ref, sourceInfo.info, false));
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 88e386be0..7ee9bbe76 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -28,6 +28,7 @@ struct FlakeInput
 struct Flake
 {
     FlakeRef originalRef;
+    FlakeRef resolvedRef;
     FlakeRef lockedRef;
     std::optional<std::string> description;
     std::shared_ptr<const fetchers::Tree> sourceInfo;
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 03bb9fe37..e79f4129a 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -80,7 +80,8 @@ struct CmdFlakeList : EvalCommand
 
 static void printFlakeInfo(const Store & store, const Flake & flake)
 {
-    std::cout << fmt("URL:           %s\n", flake.lockedRef.to_string());
+    std::cout << fmt("Resolved URL:  %s\n", flake.resolvedRef.to_string());
+    std::cout << fmt("Locked URL:    %s\n", flake.lockedRef.to_string());
     std::cout << fmt("Edition:       %s\n", flake.edition);
     if (flake.description)
         std::cout << fmt("Description:   %s\n", *flake.description);
@@ -100,8 +101,11 @@ static nlohmann::json flakeToJson(const Store & store, const Flake & flake)
     if (flake.description)
         j["description"] = *flake.description;
     j["edition"] = flake.edition;
-    j["url"] = flake.lockedRef.to_string();
+    j["originalUrl"] = flake.originalRef.to_string();
     j["original"] = attrsToJson(flake.originalRef.toAttrs());
+    j["resolvedUrl"] = flake.resolvedRef.to_string();
+    j["resolved"] = attrsToJson(flake.resolvedRef.toAttrs());
+    j["url"] = flake.lockedRef.to_string(); // FIXME: rename to lockedUrl
     j["locked"] = attrsToJson(flake.lockedRef.toAttrs());
     j["info"] = flake.sourceInfo->info.toJson();
     if (auto rev = flake.lockedRef.input->getRev())