Fix `nix build --dry-run` with CA derivations

Don’t try and assume that we know the output paths when we’ve just built
with `--dry-run`. Instead make `--dry-run` follow a different code path
that won’t assume the knowledge of the output paths at all.

Fix #6275

1 files changed, 14 insertions, 3 deletions

diff --git a/src/nix/build.cc b/src/nix/build.cc
index 680db1c60..840c7ca38 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -52,15 +52,26 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 
     void run(ref<Store> store) override
     {
+        if (dryRun) {
+            std::vector<DerivedPath> pathsToBuild;
+
+            for (auto & i : installables) {
+                auto b = i->toDerivedPaths();
+                pathsToBuild.insert(pathsToBuild.end(), b.begin(), b.end());
+            }
+            printMissing(store, pathsToBuild, lvlError);
+            if (json)
+                logger->cout("%s", derivedPathsToJSON(pathsToBuild, store).dump());
+            return;
+        }
+
         auto buildables = Installable::build(
             getEvalStore(), store,
-            dryRun ? Realise::Derivation : Realise::Outputs,
+            Realise::Outputs,
             installables, buildMode);
 
         if (json) logger->cout("%s", derivedPathsWithHintsToJSON(buildables, store).dump());
 
-        if (dryRun) return;
-
         if (outLink != "")
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
                 for (const auto & [_i, buildable] : enumerate(buildables)) {