Allow dynamic derivation deps in `inputDrvs`

We use the same nested map representation we used for goals, again in
order to save space. We might someday want to combine with `inputDrvs`,
by doing `V = bool` instead of `V = std::set<OutputName>`, but we are
not doing that yet for sake of a smaller diff.

The ATerm format for Derivations also needs to be extended, in addition
to the in-memory format. To accomodate this, we added a new basic
versioning scheme, so old versions of Nix will get nice errors. (And
going forward, if the ATerm format changes again the errors will be even
better.)

`parsedStrings`, an internal function used as part of parsing
derivations in A-Term format, used to consume the final `]` but expect
the initial `[` to already be consumed. This made for what looked like
unbalanced brackets at callsites, which was confusing. Now it consumes
both which is hopefully less confusing.

As part of testing, we also created a unit test for the A-Term format for
regular non-experimental derivations too.

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

Apply suggestions from code review

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

2 files changed, 26 insertions, 0 deletions

diff --git a/src/libcmd/built-path.cc b/src/libcmd/built-path.cc
index c6cc7fa9c..9a2dce806 100644
--- a/src/libcmd/built-path.cc
+++ b/src/libcmd/built-path.cc
@@ -58,6 +58,28 @@ StorePathSet BuiltPath::outPaths() const
     );
 }
 
+SingleDerivedPath::Built SingleBuiltPath::Built::discardOutputPath() const
+{
+    return SingleDerivedPath::Built {
+        .drvPath = make_ref<SingleDerivedPath>(drvPath->discardOutputPath()),
+        .output = output.first,
+    };
+}
+
+SingleDerivedPath SingleBuiltPath::discardOutputPath() const
+{
+    return std::visit(
+        overloaded{
+            [](const SingleBuiltPath::Opaque & p) -> SingleDerivedPath {
+                return p;
+            },
+            [](const SingleBuiltPath::Built & b) -> SingleDerivedPath {
+                return b.discardOutputPath();
+            },
+        }, raw()
+    );
+}
+
 nlohmann::json BuiltPath::Built::toJSON(const Store & store) const
 {
     nlohmann::json res;
diff --git a/src/libcmd/built-path.hh b/src/libcmd/built-path.hh
index 747bcc440..e677bc810 100644
--- a/src/libcmd/built-path.hh
+++ b/src/libcmd/built-path.hh
@@ -9,6 +9,8 @@ struct SingleBuiltPathBuilt {
     ref<SingleBuiltPath> drvPath;
     std::pair<std::string, StorePath> output;
 
+    SingleDerivedPathBuilt discardOutputPath() const;
+
     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;
@@ -34,6 +36,8 @@ struct SingleBuiltPath : _SingleBuiltPathRaw {
 
     StorePath outPath() const;
 
+    SingleDerivedPath discardOutputPath() const;
+
     static SingleBuiltPath parse(const Store & store, std::string_view);
     nlohmann::json toJSON(const Store & store) const;
 };