nix: Add --derivation flag to operate on .drv paths

For instance, 'nix why-depends --use-derivation nixpkgs#hello
nixpkgs#glibc' shows why hello's .drv depends on glibc's .drv.

src/nix/command.cc

@@ -63,7 +63,7 @@ void StorePathsCommand::run(ref<Store> store)
     }
 
     else {
-        for (auto & p : toStorePaths(store, realiseMode, installables))
+        for (auto & p : toStorePaths(store, realiseMode, operateOn, installables))
             storePaths.push_back(p);
 
         if (recursive) {
@@ -80,7 +80,7 @@ void StorePathsCommand::run(ref<Store> store)
 
 void StorePathCommand::run(ref<Store> store)
 {
-    auto storePaths = toStorePaths(store, Realise::Nothing, installables);
+    auto storePaths = toStorePaths(store, Realise::Nothing, operateOn, installables);
 
     if (storePaths.size() != 1)
         throw UsageError("this command requires exactly one store path");

src/nix/command.hh

@@ -50,11 +50,22 @@ struct MixFlakeOptions : virtual Args, EvalCommand
     { return {}; }
 };
 
+/* How to handle derivations in commands that operate on store paths. */
+enum class OperateOn {
+    /* Operate on the output path. */
+    Output,
+    /* Operate on the .drv path. */
+    Derivation
+};
+
 struct SourceExprCommand : virtual Args, MixFlakeOptions
 {
     std::optional<Path> file;
     std::optional<std::string> expr;
 
+    // FIXME: move this; not all commands (e.g. 'nix run') use it.
+    OperateOn operateOn = OperateOn::Output;
+
     SourceExprCommand();
 
     std::vector<std::shared_ptr<Installable>> parseInstallables(
@@ -176,10 +187,12 @@ static RegisterCommand registerCommand(const std::string & name)
 Buildables build(ref<Store> store, Realise mode,
     std::vector<std::shared_ptr<Installable>> installables);
 
-std::set<StorePath> toStorePaths(ref<Store> store, Realise mode,
+std::set<StorePath> toStorePaths(ref<Store> store,
+    Realise mode, OperateOn operateOn,
     std::vector<std::shared_ptr<Installable>> installables);
 
-StorePath toStorePath(ref<Store> store, Realise mode,
+StorePath toStorePath(ref<Store> store,
+    Realise mode, OperateOn operateOn,
     std::shared_ptr<Installable> installable);
 
 std::set<StorePath> toDerivations(ref<Store> store,

src/nix/develop.cc

@@ -321,7 +321,8 @@ struct CmdDevelop : Common, MixEnvironment
                 Strings{"legacyPackages." + settings.thisSystem.get() + "."},
                 lockFlags);
 
-            shell = state->store->printStorePath(toStorePath(state->store, Realise::Outputs, bashInstallable)) + "/bin/bash";
+            shell = state->store->printStorePath(
+                toStorePath(state->store, Realise::Outputs, OperateOn::Output, bashInstallable)) + "/bin/bash";
         } catch (Error &) {
             ignoreException();
         }

src/nix/installables.cc

@@ -128,6 +128,12 @@ SourceExprCommand::SourceExprCommand()
         .labels = {"expr"},
         .handler = {&expr}
     });
+
+    addFlag({
+        .longName ="derivation",
+        .description = "operate on the store derivation rather than its outputs",
+        .handler = {&operateOn, OperateOn::Derivation},
+    });
 }
 
 Strings SourceExprCommand::getDefaultFlakeAttrPaths()
@@ -667,22 +673,34 @@ Buildables build(ref<Store> store, Realise mode,
     return buildables;
 }
 
-StorePathSet toStorePaths(ref<Store> store, Realise mode,
+StorePathSet toStorePaths(ref<Store> store,
+    Realise mode, OperateOn operateOn,
     std::vector<std::shared_ptr<Installable>> installables)
 {
     StorePathSet outPaths;
 
-    for (auto & b : build(store, mode, installables))
+    if (operateOn == OperateOn::Output) {
-        for (auto & output : b.outputs)
+        for (auto & b : build(store, mode, installables))
-            outPaths.insert(output.second);
+            for (auto & output : b.outputs)
+                outPaths.insert(output.second);
+    } else {
+        if (mode == Realise::Nothing)
+            settings.readOnlyMode = true;
+
+        for (auto & i : installables)
+            for (auto & b : i->toBuildables())
+                if (b.drvPath)
+                    outPaths.insert(*b.drvPath);
+    }
 
     return outPaths;
 }
 
-StorePath toStorePath(ref<Store> store, Realise mode,
+StorePath toStorePath(ref<Store> store,
+    Realise mode, OperateOn operateOn,
     std::shared_ptr<Installable> installable)
 {
-    auto paths = toStorePaths(store, mode, {installable});
+    auto paths = toStorePaths(store, mode, operateOn, {installable});
 
     if (paths.size() != 1)
         throw Error("argument '%s' should evaluate to one store path", installable->what());

src/nix/run.cc

@@ -104,7 +104,7 @@ struct CmdShell : InstallablesCommand, RunCommon, MixEnvironment
 
     void run(ref<Store> store) override
     {
-        auto outPaths = toStorePaths(store, Realise::Outputs, installables);
+        auto outPaths = toStorePaths(store, Realise::Outputs, OperateOn::Output, installables);
 
         auto accessor = store->getFSAccessor();
 

src/nix/why-depends.cc

@@ -73,9 +73,9 @@ struct CmdWhyDepends : SourceExprCommand
     void run(ref<Store> store) override
     {
         auto package = parseInstallable(store, _package);
-        auto packagePath = toStorePath(store, Realise::Outputs, package);
+        auto packagePath = toStorePath(store, Realise::Outputs, operateOn, package);
         auto dependency = parseInstallable(store, _dependency);
-        auto dependencyPath = toStorePath(store, Realise::Derivation, dependency);
+        auto dependencyPath = toStorePath(store, Realise::Derivation, operateOn, dependency);
         auto dependencyPathHash = dependencyPath.hashPart();
 
         StorePathSet closure;