Stratify `ExtraPathInfo` along `Installable` hierarchy

Instead of having a bunch of optional fields, have a few subclasses
which can have mandatory fields.

Additionally, the new `getExtraPathInfo`, and `nixpkgsFlakeRef`, are
moved to `InstallableValue`.

I did these things because https://github.com/NixOS/rfcs/pull/134 ; with
these things moved to `InstallableValue`, the base `Installable` no
longer depends on libexpr! This is a major step towards that.

Also, add a bunch of doc comments for sake of the internal API docs.

1 files changed, 67 insertions, 4 deletions

diff --git a/src/libcmd/installable-value.hh b/src/libcmd/installable-value.hh
index 682c8d942..9e076cb10 100644
--- a/src/libcmd/installable-value.hh
+++ b/src/libcmd/installable-value.hh
@@ -1,9 +1,15 @@
 #pragma once
 
 #include "installables.hh"
+#include "flake/flake.hh"
 
 namespace nix {
 
+struct DrvInfo;
+struct SourceExprCommand;
+
+namespace eval_cache { class EvalCache; class AttrCursor; }
+
 struct App
 {
     std::vector<DerivedPath> context;
@@ -17,26 +23,83 @@ struct UnresolvedApp
     App resolve(ref<Store> evalStore, ref<Store> store);
 };
 
+/**
+ * Extra info about a \ref DerivedPath "derived path" that ultimately
+ * come from a Nix language value.
+ *
+ * Invariant: every ExtraPathInfo gotten from an InstallableValue should
+ * be possible to downcast to an ExtraPathInfoValue.
+ */
+struct ExtraPathInfoValue : ExtraPathInfo
+{
+    /**
+     * Extra struct to get around C++ designated initializer limitations
+     */
+    struct Value {
+        /**
+         * An optional priority for use with "build envs". See Package
+         */
+        std::optional<NixInt> priority;
+
+        /**
+         * The attribute path associated with this value. The idea is
+         * that an installable referring to a value typically refers to
+         * a larger value, from which we project a smaller value out
+         * with this.
+         */
+        std::string attrPath;
+
+        /**
+         * \todo merge with DerivedPath's 'outputs' field?
+         */
+        ExtendedOutputsSpec extendedOutputsSpec;
+    };
+
+    Value value;
+
+    ExtraPathInfoValue(Value && v)
+        : value(v)
+    { }
+
+    virtual ~ExtraPathInfoValue() = default;
+};
+
+/**
+ * An Installable which corresponds a Nix langauge value, in addition to
+ * a collection of \ref DerivedPath "derived paths".
+ */
 struct InstallableValue : Installable
 {
     ref<EvalState> state;
 
     InstallableValue(ref<EvalState> state) : state(state) {}
 
+    virtual ~InstallableValue() { }
+
     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. */
+    /**
+     * 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. */
+    /**
+     * 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);
 
+    virtual FlakeRef nixpkgsFlakeRef() const
+    {
+        return FlakeRef::fromAttrs({{"type","indirect"}, {"id", "nixpkgs"}});
+    }
+
     static InstallableValue & require(Installable & installable);
     static ref<InstallableValue> require(ref<Installable> installable);
 };