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, 5 insertions, 10 deletions

diff --git a/src/libcmd/installable-derived-path.cc b/src/libcmd/installable-derived-path.cc
index 6ecf54b7c..b45641e8a 100644
--- a/src/libcmd/installable-derived-path.cc
+++ b/src/libcmd/installable-derived-path.cc
@@ -18,14 +18,7 @@ DerivedPathsWithInfo InstallableDerivedPath::toDerivedPaths()
 
 std::optional<StorePath> InstallableDerivedPath::getStorePath()
 {
-    return std::visit(overloaded {
-        [&](const DerivedPath::Built & bfd) {
-            return bfd.drvPath;
-        },
-        [&](const DerivedPath::Opaque & bo) {
-            return bo.path;
-        },
-    }, derivedPath.raw());
+    return derivedPath.getBaseStorePath();
 }
 
 InstallableDerivedPath InstallableDerivedPath::parse(
@@ -42,7 +35,7 @@ InstallableDerivedPath InstallableDerivedPath::parse(
             // Remove this prior to stabilizing the new CLI.
             if (storePath.isDerivation()) {
                 auto oldDerivedPath = DerivedPath::Built {
-                    .drvPath = storePath,
+                    .drvPath = makeConstantStorePathRef(storePath),
                     .outputs = OutputsSpec::All { },
                 };
                 warn(
@@ -55,8 +48,10 @@ InstallableDerivedPath InstallableDerivedPath::parse(
         },
         // If the user did use ^, we just do exactly what is written.
         [&](const ExtendedOutputsSpec::Explicit & outputSpec) -> DerivedPath {
+            auto drv = make_ref<SingleDerivedPath>(SingleDerivedPath::parse(*store, prefix));
+            drvRequireExperiment(*drv);
             return DerivedPath::Built {
-                .drvPath = store->parseStorePath(prefix),
+                .drvPath = std::move(drv),
                 .outputs = outputSpec,
             };
         },