Make the Derived Path family of types inductive for dynamic derivations

We want to be able to write down `foo.drv^bar.drv^baz`:
`foo.drv^bar.drv` is the dynamic derivation (since it is itself a
derivation output, `bar.drv` from `foo.drv`).

To that end, we create `Single{Derivation,BuiltPath}` types, that are
very similar except instead of having multiple outputs (in a set or
map), they have a single one. This is for everything to the left of the
rightmost `^`.

`NixStringContextElem` has an analogous change, and now can reuse
`SingleDerivedPath` at the top level. In fact, if we ever get rid of
`DrvDeep`, `NixStringContextElem` could be replaced with
`SingleDerivedPath` entirely!

Important note: some JSON formats have changed.

We already can *produce* dynamic derivations, but we can't refer to them
directly. Today, we can merely express building or example at the top
imperatively over time by building `foo.drv^bar.drv`, and then with a
second nix invocation doing `<result-from-first>^baz`, but this is not
declarative. The ethos of Nix of being able to write down the full plan
everything you want to do, and then execute than plan with a single
command, and for that we need the new inductive form of these types.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

1 files changed, 46 insertions, 4 deletions

diff --git a/src/libcmd/built-path.hh b/src/libcmd/built-path.hh
index 744e8090b..747bcc440 100644
--- a/src/libcmd/built-path.hh
+++ b/src/libcmd/built-path.hh
@@ -3,19 +3,60 @@
 
 namespace nix {
 
+struct SingleBuiltPath;
+
+struct SingleBuiltPathBuilt {
+    ref<SingleBuiltPath> drvPath;
+    std::pair<std::string, StorePath> output;
+
+    std::string to_string(const Store & store) const;
+    static SingleBuiltPathBuilt parse(const Store & store, std::string_view, std::string_view);
+    nlohmann::json toJSON(const Store & store) const;
+
+    DECLARE_CMP(SingleBuiltPathBuilt);
+};
+
+using _SingleBuiltPathRaw = std::variant<
+    DerivedPathOpaque,
+    SingleBuiltPathBuilt
+>;
+
+struct SingleBuiltPath : _SingleBuiltPathRaw {
+    using Raw = _SingleBuiltPathRaw;
+    using Raw::Raw;
+
+    using Opaque = DerivedPathOpaque;
+    using Built = SingleBuiltPathBuilt;
+
+    inline const Raw & raw() const {
+        return static_cast<const Raw &>(*this);
+    }
+
+    StorePath outPath() const;
+
+    static SingleBuiltPath parse(const Store & store, std::string_view);
+    nlohmann::json toJSON(const Store & store) const;
+};
+
+static inline ref<SingleBuiltPath> staticDrv(StorePath drvPath)
+{
+    return make_ref<SingleBuiltPath>(SingleBuiltPath::Opaque { drvPath });
+}
+
 /**
  * A built derived path with hints in the form of optional concrete output paths.
  *
  * See 'BuiltPath' for more an explanation.
  */
 struct BuiltPathBuilt {
-    StorePath drvPath;
+    ref<SingleBuiltPath> drvPath;
     std::map<std::string, StorePath> outputs;
 
-    nlohmann::json toJSON(ref<Store> store) const;
-    static BuiltPathBuilt parse(const Store & store, std::string_view);
+    std::string to_string(const Store & store) const;
+    static BuiltPathBuilt parse(const Store & store, std::string_view, std::string_view);
+    nlohmann::json toJSON(const Store & store) const;
 
-    GENERATE_CMP(BuiltPathBuilt, me->drvPath, me->outputs);
+    DECLARE_CMP(BuiltPathBuilt);
 };
 
 using _BuiltPathRaw = std::variant<
@@ -41,6 +82,7 @@ struct BuiltPath : _BuiltPathRaw {
     StorePathSet outPaths() const;
     RealisedPath::Set toRealisedPaths(Store & store) const;
 
+    nlohmann::json toJSON(const Store & store) const;
 };
 
 typedef std::vector<BuiltPath> BuiltPaths;