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 NixOS/rfcs#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.

src/libcmd/command.hh

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "installables.hh"
+#include "installable-value.hh"
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"

src/libcmd/installable-attr-path.cc

@@ -87,6 +87,10 @@ DerivedPathsWithInfo InstallableAttrPath::toDerivedPaths()
                 .drvPath = drvPath,
                 .outputs = outputs,
             },
+            .info = make_ref<ExtraPathInfoValue>(ExtraPathInfoValue::Value {
+                /* FIXME: reconsider backwards compatibility above
+                   so we can fill in this info. */
+            }),
         });
 
     return res;

src/libcmd/installable-derived-path.cc

@@ -10,7 +10,10 @@ std::string InstallableDerivedPath::what() const
 
 DerivedPathsWithInfo InstallableDerivedPath::toDerivedPaths()
 {
-    return {{.path = derivedPath, .info = {} }};
+    return {{
+        .path = derivedPath,
+        .info = make_ref<ExtraPathInfo>(),
+    }};
 }
 
 std::optional<StorePath> InstallableDerivedPath::getStorePath()

src/libcmd/installable-flake.cc

@@ -101,7 +101,8 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
             return {{
                 .path = DerivedPath::Opaque {
                     .path = std::move(storePath),
-                }
+                },
+                .info = make_ref<ExtraPathInfo>(),
             }};
         }
 
@@ -113,7 +114,8 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
                 return {{
                     .path = DerivedPath::Opaque {
                         .path = std::move(*storePath),
-                    }
+                    },
+                    .info = make_ref<ExtraPathInfo>(),
                 }};
             } else
                 throw Error("flake output attribute '%s' evaluates to the string '%s' which is not a store path", attrPath, s);
@@ -160,13 +162,16 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
                 },
             }, extendedOutputsSpec.raw()),
         },
-        .info = {
-            .priority = priority,
-            .originalRef = flakeRef,
-            .resolvedRef = getLockedFlake()->flake.lockedRef,
-            .attrPath = attrPath,
-            .extendedOutputsSpec = extendedOutputsSpec,
-        }
+        .info = make_ref<ExtraPathInfoFlake>(
+            ExtraPathInfoValue::Value {
+                .priority = priority,
+                .attrPath = attrPath,
+                .extendedOutputsSpec = extendedOutputsSpec,
+            },
+            ExtraPathInfoFlake::Flake {
+                .originalRef = flakeRef,
+                .resolvedRef = getLockedFlake()->flake.lockedRef,
+            }),
     }};
 }
 
@@ -212,6 +217,7 @@ std::shared_ptr<flake::LockedFlake> InstallableFlake::getLockedFlake() const
 {
     if (!_lockedFlake) {
         flake::LockFlags lockFlagsApplyConfig = lockFlags;
+        // FIXME why this side effect?
         lockFlagsApplyConfig.applyNixConfig = true;
         _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state, flakeRef, lockFlagsApplyConfig));
     }
@@ -229,7 +235,7 @@ FlakeRef InstallableFlake::nixpkgsFlakeRef() const
         }
     }
 
-    return Installable::nixpkgsFlakeRef();
+    return InstallableValue::nixpkgsFlakeRef();
 }
 
 }

src/libcmd/installable-flake.hh

@@ -4,6 +4,30 @@
 
 namespace nix {
 
+/**
+ * Extra info about a \ref DerivedPath "derived path" that ultimately
+ * come from a Flake.
+ *
+ * Invariant: every ExtraPathInfo gotten from an InstallableFlake should
+ * be possible to downcast to an ExtraPathInfoFlake.
+ */
+struct ExtraPathInfoFlake : ExtraPathInfoValue
+{
+    /**
+     * Extra struct to get around C++ designated initializer limitations
+     */
+    struct Flake {
+        FlakeRef originalRef;
+        FlakeRef resolvedRef;
+    };
+
+    Flake flake;
+
+    ExtraPathInfoFlake(Value && v, Flake && f)
+        : ExtraPathInfoValue(std::move(v)), flake(f)
+    { }
+};
+
 struct InstallableFlake : InstallableValue
 {
     FlakeRef flakeRef;
@@ -33,8 +57,10 @@ struct InstallableFlake : InstallableValue
 
     std::pair<Value *, PosIdx> toValue(EvalState & state) override;
 
-    /* Get a cursor to every attrpath in getActualAttrPaths()
-       that exists. However if none exists, throw an exception. */
+    /**
+     * Get a cursor to every attrpath in getActualAttrPaths() that
+     * exists. However if none exists, throw an exception.
+     */
     std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state) override;
 

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);
 };

src/libcmd/installables.cc

@@ -517,7 +517,7 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
 
     struct Aux
     {
-        ExtraPathInfo info;
+        ref<ExtraPathInfo> info;
         ref<Installable> installable;
     };