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.

src/libcmd/command-installable-value.cc

@@ -0,0 +1,11 @@
+#include "command-installable-value.hh"
+
+namespace nix {
+
+void InstallableValueCommand::run(ref<Store> store, ref<Installable> installable)
+{
+	auto installableValue = InstallableValue::require(installable);
+	run(store, installableValue);
+}
+
+}

src/libcmd/command-installable-value.hh

@@ -0,0 +1,13 @@
+#include "installable-value.hh"
+#include "command.hh"
+
+namespace nix {
+
+struct InstallableValueCommand : InstallableCommand
+{
+    virtual void run(ref<Store> store, ref<InstallableValue> installable) = 0;
+
+    void run(ref<Store> store, ref<Installable> installable) override;
+};
+
+}

src/libcmd/installable-value.cc

@@ -0,0 +1,44 @@
+#include "installable-value.hh"
+#include "eval-cache.hh"
+
+namespace nix {
+
+std::vector<ref<eval_cache::AttrCursor>>
+InstallableValue::getCursors(EvalState & state)
+{
+    auto evalCache =
+        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
+            [&]() { return toValue(state).first; });
+    return {evalCache->getRoot()};
+}
+
+ref<eval_cache::AttrCursor>
+InstallableValue::getCursor(EvalState & state)
+{
+    /* Although getCursors should return at least one element, in case it doesn't,
+       bound check to avoid an undefined behavior for vector[0] */
+    return getCursors(state).at(0);
+}
+
+static UsageError nonValueInstallable(Installable & installable)
+{
+	return UsageError("installable '%s' does not correspond to a Nix language value", installable.what());
+}
+
+InstallableValue & InstallableValue::require(Installable & installable)
+{
+    auto * castedInstallable = dynamic_cast<InstallableValue *>(&installable);
+    if (!castedInstallable)
+        throw nonValueInstallable(installable);
+    return *castedInstallable;
+}
+
+ref<InstallableValue> InstallableValue::require(ref<Installable> installable)
+{
+    auto castedInstallable = installable.dynamic_pointer_cast<InstallableValue>();
+    if (!castedInstallable)
+        throw nonValueInstallable(*installable);
+    return ref { castedInstallable };
+}
+
+}

src/libcmd/installable-value.hh

@@ -4,11 +4,41 @@
 
 namespace nix {
 
+struct App
+{
+    std::vector<DerivedPath> context;
+    Path program;
+    // FIXME: add args, sandbox settings, metadata, ...
+};
+
+struct UnresolvedApp
+{
+    App unresolved;
+    App resolve(ref<Store> evalStore, ref<Store> store);
+};
+
 struct InstallableValue : Installable
 {
     ref<EvalState> state;
 
     InstallableValue(ref<EvalState> state) : state(state) {}
+
+    virtual std::pair<Value *, PosIdx> toValue(EvalState & state) = 0;
+
+    /* Get a cursor to each value this Installable could refer to. However
+       if none exists, throw exception instead of returning empty vector. */
+    virtual std::vector<ref<eval_cache::AttrCursor>>
+    getCursors(EvalState & state);
+
+    /* Get the first and most preferred cursor this Installable could refer
+       to, or throw an exception if none exists. */
+    virtual ref<eval_cache::AttrCursor>
+    getCursor(EvalState & state);
+
+    UnresolvedApp toApp(EvalState & state);
+
+    static InstallableValue & require(Installable & installable);
+    static ref<InstallableValue> require(ref<Installable> installable);
 };
 
 }

src/libcmd/installables.cc

@@ -364,23 +364,6 @@ DerivedPathWithInfo Installable::toDerivedPath()
     return std::move(buildables[0]);
 }
 
-std::vector<ref<eval_cache::AttrCursor>>
-Installable::getCursors(EvalState & state)
-{
-    auto evalCache =
-        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
-            [&]() { return toValue(state).first; });
-    return {evalCache->getRoot()};
-}
-
-ref<eval_cache::AttrCursor>
-Installable::getCursor(EvalState & state)
-{
-    /* Although getCursors should return at least one element, in case it doesn't,
-       bound check to avoid an undefined behavior for vector[0] */
-    return getCursors(state).at(0);
-}
-
 static StorePath getDeriver(
     ref<Store> store,
     const Installable & i,

src/libcmd/installables.hh

@@ -18,19 +18,6 @@ struct SourceExprCommand;
 
 namespace eval_cache { class EvalCache; class AttrCursor; }
 
-struct App
-{
-    std::vector<DerivedPath> context;
-    Path program;
-    // FIXME: add args, sandbox settings, metadata, ...
-};
-
-struct UnresolvedApp
-{
-    App unresolved;
-    App resolve(ref<Store> evalStore, ref<Store> store);
-};
-
 enum class Realise {
     /* Build the derivation. Postcondition: the
        derivation outputs exist. */
@@ -92,13 +79,6 @@ struct Installable
 
     DerivedPathWithInfo toDerivedPath();
 
-    UnresolvedApp toApp(EvalState & state);
-
-    virtual std::pair<Value *, PosIdx> toValue(EvalState & state)
-    {
-        throw Error("argument '%s' cannot be evaluated", what());
-    }
-
     /* Return a value only if this installable is a store path or a
        symlink to it. */
     virtual std::optional<StorePath> getStorePath()
@@ -106,16 +86,6 @@ struct Installable
         return {};
     }
 
-    /* Get a cursor to each value this Installable could refer to. However
-       if none exists, throw exception instead of returning empty vector. */
-    virtual std::vector<ref<eval_cache::AttrCursor>>
-    getCursors(EvalState & state);
-
-    /* Get the first and most preferred cursor this Installable could refer
-       to, or throw an exception if none exists. */
-    virtual ref<eval_cache::AttrCursor>
-    getCursor(EvalState & state);
-
     virtual FlakeRef nixpkgsFlakeRef() const
     {
         return FlakeRef::fromAttrs({{"type","indirect"}, {"id", "nixpkgs"}});