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, 6 insertions, 3 deletions

diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index 6510df8f0..66f319c3e 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -393,7 +393,7 @@ static void main_nix_build(int argc, char * * argv)
 
                 auto bashDrv = drv->requireDrvPath();
                 pathsToBuild.push_back(DerivedPath::Built {
-                    .drvPath = bashDrv,
+                    .drvPath = makeConstantStorePathRef(bashDrv),
                     .outputs = OutputsSpec::Names {"out"},
                 });
                 pathsToCopy.insert(bashDrv);
@@ -417,7 +417,7 @@ static void main_nix_build(int argc, char * * argv)
                     }))
             {
                 pathsToBuild.push_back(DerivedPath::Built {
-                    .drvPath = inputDrv,
+                    .drvPath = makeConstantStorePathRef(inputDrv),
                     .outputs = OutputsSpec::Names { inputOutputs },
                 });
                 pathsToCopy.insert(inputDrv);
@@ -590,7 +590,10 @@ static void main_nix_build(int argc, char * * argv)
             if (outputName == "")
                 throw Error("derivation '%s' lacks an 'outputName' attribute", store->printStorePath(drvPath));
 
-            pathsToBuild.push_back(DerivedPath::Built{drvPath, OutputsSpec::Names{outputName}});
+            pathsToBuild.push_back(DerivedPath::Built{
+                .drvPath = makeConstantStorePathRef(drvPath),
+                .outputs = OutputsSpec::Names{outputName},
+            });
             pathsToBuildOrdered.push_back({drvPath, {outputName}});
             drvsToCopy.insert(drvPath);