Add nix registry resolve command

src/nix/registry-resolve.md

@@ -0,0 +1,28 @@
+R""(
+
+# Examples
+
+* Resolve the `nixpkgs` and `blender-bin` flakerefs:
+
+  ```console
+  # nix registry resolve nixpkgs blender-bin
+  github:NixOS/nixpkgs/nixpkgs-unstable
+  github:edolstra/nix-warez?dir=blender
+  ```
+
+* Resolve an indirect flakeref with a branch override:
+
+  ```console
+  # nix registry resolve nixpkgs/25.05
+  github:NixOS/nixpkgs/25.05
+  ```
+
+# Description
+
+This command resolves indirect flakerefs (e.g. `nixpkgs`) to direct flakerefs (e.g. `github:NixOS/nixpkgs`) using the flake registries. It looks up each provided flakeref in all available registries (flag, user, system, and global) and returns the resolved direct flakeref on a separate line on standard output. It does not fetch any flakes.
+
+The resolution process may apply multiple redirections if necessary until a direct flakeref is found. If an indirect flakeref cannot be found in any registry, an error will be thrown.
+
+See the [`nix registry` manual page](./nix3-registry.md) for more details on the registry.
+
+)""

src/nix/registry.cc

@@ -202,6 +202,40 @@ struct CmdRegistryPin : RegistryCommand, EvalCommand
     }
 };
 
+struct CmdRegistryResolve : StoreCommand
+{
+    std::vector<std::string> urls;
+
+    std::string description() override
+    {
+        return "resolve flake references using the registry";
+    }
+
+    std::string doc() override
+    {
+        return
+#include "registry-resolve.md"
+            ;
+    }
+
+    CmdRegistryResolve()
+    {
+        expectArgs({
+            .label = "flake-refs",
+            .handler = {&urls},
+        });
+    }
+
+    void run(nix::ref<nix::Store> store) override
+    {
+        for (auto & url : urls) {
+            auto ref = parseFlakeRef(fetchSettings, url);
+            auto resolved = ref.resolve(fetchSettings, *store);
+            logger->cout("%s", resolved.to_string());
+        }
+    }
+};
+
 struct CmdRegistry : NixMultiCommand
 {
     CmdRegistry()
@@ -212,6 +246,7 @@ struct CmdRegistry : NixMultiCommand
                   {"add", []() { return make_ref<CmdRegistryAdd>(); }},
                   {"remove", []() { return make_ref<CmdRegistryRemove>(); }},
                   {"pin", []() { return make_ref<CmdRegistryPin>(); }},
+                  {"resolve", []() { return make_ref<CmdRegistryResolve>(); }},
               })
     {
     }

tests/functional/flakes/flakes.sh

@@ -252,15 +252,17 @@ nix flake lock "$flake3Dir"
 nix flake update --flake "$flake3Dir" --override-flake flake2 nixpkgs
 [[ -n $(git -C "$flake3Dir" diff master || echo failed) ]]
 
-# Testing the nix CLI
+# Test `nix registry` commands.
 nix registry add flake1 flake3
 [[ $(nix registry list | wc -l) == 5 ]]
+[[ $(nix registry resolve flake1) = "git+file://$percentEncodedFlake3Dir" ]]
 nix registry pin flake1
 [[ $(nix registry list | wc -l) == 5 ]]
 nix registry pin flake1 flake3
 [[ $(nix registry list | wc -l) == 5 ]]
 nix registry remove flake1
 [[ $(nix registry list | wc -l) == 4 ]]
+[[ $(nix registry resolve flake1) = "git+file://$flake1Dir" ]]
 
 # Test 'nix registry list' with a disabled global registry.
 nix registry add user-flake1 git+file://"$flake1Dir"