Require (new) computed-derivations experimental feature for ! installable

src/libcmd/installables.cc

@@ -740,6 +740,7 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
                     result.push_back(std::make_shared<InstallableIndexedStorePath>(
                         store,
                         DerivedPath::Built::parse(*store, s)));
+                    settings.requireExperimentalFeature(Xp::ComputedDerivations);
                     continue;
                 } catch (BadStorePath &) {
                 } catch (...) {

src/libutil/experimental-features.cc

@@ -13,6 +13,7 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::RecursiveNix, "recursive-nix" },
     { Xp::NoUrlLiterals, "no-url-literals" },
     { Xp::FetchClosure, "fetch-closure" },
+    { Xp::ComputedDerivations, "computed-derivations" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -22,6 +22,7 @@ enum struct ExperimentalFeature
     RecursiveNix,
     NoUrlLiterals,
     FetchClosure,
+    ComputedDerivations, // RFC 92
 };
 
 /**

tests/build-explicit-output.sh

@@ -1,5 +1,8 @@
 source common.sh
 
+enableFeatures "computed-derivations"
+restartDaemon
+
 drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
 if nix build "$drv!not-an-output" --json; then
     fail "'not-an-output' should fail to build"