Create `outputOf` primop.

In the Nix language, given a drv path, we should be able to construct
another string referencing to one of its output. We can do this today
with `(import drvPath).output`, but this only works for derivations we
already have.

With dynamic derivations, however, that doesn't work well because the
`drvPath` isn't yet built: importing it like would need to trigger IFD,
when the whole point of this feature is to do "dynamic build graph"
without IFD!

Instead, what we want to do is create a placeholder value with the right
string context to refer to the output of the as-yet unbuilt derivation.
A new primop in the language, analogous to `builtins.placeholder` can be
used to create one. This will achieve all the right properties. The
placeholder machinery also will match out the `outPath` attribute for CA
derivations works.

In 60b7121d2c6d4322b7c2e8e7acfec7b701b2d3a1 we added that type of
placeholder, and the derived path and string holder changes necessary to
support it. Then in the previous commit we cleaned up the code
(inspiration finally hit me!) to deduplicate the code and expose exactly
what we need. Now, we can wire up the primop trivally!

Part of RFC 92: dynamic derivations (tracking issue #6316)

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

2 files changed, 40 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index be78bca02..283f99a48 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1838,6 +1838,45 @@ static RegisterPrimOp primop_readDir({
     .fun = prim_readDir,
 });
 
+/* Extend single element string context with another output. */
+static void prim_outputOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    SingleDerivedPath drvPath = state.coerceToSingleDerivedPath(pos, *args[0], "while evaluating the first argument to builtins.outputOf");
+
+    std::string_view outputName = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument to builtins.outputOf");
+
+    state.mkSingleDerivedPathString(
+        SingleDerivedPath::Built {
+            .drvPath = make_ref<SingleDerivedPath>(drvPath),
+            .output = std::string { outputName },
+        },
+        v);
+}
+
+static RegisterPrimOp primop_outputOf({
+    .name = "__outputOf",
+    .args = {"derivation-reference", "output-name"},
+    .doc = R"(
+      Return the output path of a derivation, literally or using a placeholder if needed.
+
+      If the derivation has a statically-known output path (i.e. the derivation output is input-addressed, or fixed content-addresed), the output path will just be returned.
+      But if the derivation is content-addressed or if the derivation is itself not-statically produced (i.e. is the output of another derivation), a placeholder will be returned instead.
+
+      *`derivation reference`* must be a string that may contain a regular store path to a derivation, or may be a placeholder reference. If the derivation is produced by a derivation, you must explicitly select `drv.outPath`.
+      This primop can be chained arbitrarily deeply.
+      For instance,
+      ```nix
+      builtins.outputOf
+        (builtins.outputOf myDrv "out)
+        "out"
+      ```
+      will return a placeholder for the output of the output of `myDrv`.
+
+      This primop corresponds to the `^` sigil for derivable paths, e.g. as part of installable syntax on the command line.
+    )",
+    .fun = prim_outputOf,
+    .experimentalFeature = Xp::DynamicDerivations,
+});
 
 /*************************************************************
  * Creating files
diff --git a/src/nix/main.cc b/src/nix/main.cc
index c5a9c8b33..d05bac68e 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -359,6 +359,7 @@ void mainWrapped(int argc, char * * argv)
         experimentalFeatureSettings.experimentalFeatures = {
             Xp::Flakes,
             Xp::FetchClosure,
+            Xp::DynamicDerivations,
         };
         evalSettings.pureEval = false;
         EvalState state({}, openStore("dummy://"));