Move value-only methods to `InstallableValue`

These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.

Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.

Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/nix/fmt.cc b/src/nix/fmt.cc
index 6f6a4a632..c85eacded 100644
--- a/src/nix/fmt.cc
+++ b/src/nix/fmt.cc
@@ -1,4 +1,5 @@
 #include "command.hh"
+#include "installable-value.hh"
 #include "run.hh"
 
 using namespace nix;
@@ -31,8 +32,9 @@ struct CmdFmt : SourceExprCommand {
         auto evalState = getEvalState();
         auto evalStore = getEvalStore();
 
-        auto installable = parseInstallable(store, ".");
-        auto app = installable->toApp(*evalState).resolve(evalStore, store);
+        auto installable_ = parseInstallable(store, ".");
+        auto & installable = InstallableValue::require(*installable_);
+        auto app = installable.toApp(*evalState).resolve(evalStore, store);
 
         Strings programArgs{app.program};