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 60b7121 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 NixOS#6316)

src/libexpr/primops.cc

@@ -1838,6 +1838,38 @@ 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 teh 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 = {"drv path", "output name"},
+    .doc = R"(
+      Return path (actually placeholder path) to the output of the given drv.
+      The drv path may itself be a placeholder, which permits chaining this primop.
+
+      For instance, `builtins.outputOf (builtins.outputOf myDrv "out) "out"`
+      will return a placeholder for the output of the output of `myDrv`,
+      interpreted as a derivation.
+
+      It may help to compare to the `->` operator in C, which can also by
+      chained. E.g. compare the above example to `drv->out->out`.
+    )",
+    .fun = prim_outputOf,
+    .experimentalFeature = Xp::DynamicDerivations,
+});
 
 /*************************************************************
  * Creating files

tests/dyn-drv/dep-built-drv.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+out1=$(nix-build ./text-hashed-output.nix -A hello --no-out-link)
+
+clearStore
+
+expectStderr 1 nix-build ./text-hashed-output.nix -A wrapper --no-out-link | grepQuiet "Dependencies on the outputs of dynamic derivations are not yet supported"

tests/dyn-drv/eval-outputOf.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+# Without the dynamic-derivations XP feature, we don't have the builtin.
+nix --experimental-features 'nix-command' eval --impure  --expr \
+    'assert ! (builtins ? outputOf); ""'
+
+# Test with a regular old input-addresed derivation: works without
+# ca-derivations and doesn't create a placeholder but just returns the
+# output path.
+nix --experimental-features 'nix-command dynamic-derivations' eval --impure --expr \
+    'let
+       item = import ../dependencies.nix;
+       a = item.outPath;
+	   b = builtins.outputOf (builtins.unsafeDiscardOutputDependency item.drvPath) "out";
+     in builtins.trace a
+       (builtins.trace b
+         (assert a == b; null))'
+
+# Test with content addressed derivation.
+nix eval --impure --expr \
+    'with (import ./text-hashed-output.nix); let
+       a = hello.outPath;
+	   b = builtins.outputOf (builtins.unsafeDiscardOutputDependency hello.drvPath) "out";
+     in builtins.trace a
+       (builtins.trace b
+         (assert a == b; null))'
+
+# Test with derivation-producing derivation.
+nix eval --impure --expr \
+    'with (import ./text-hashed-output.nix); let
+       a = producingDrv.outPath;
+	   b = builtins.outputOf (builtins.builtins.unsafeDiscardOutputDependency producingDrv.drvPath) "out";
+     in builtins.trace a
+       (builtins.trace b
+         (assert a == b; null))'
+
+# Test with unbuilt output of derivation-producing derivation.
+nix eval --impure --expr \
+    'with (import ./text-hashed-output.nix); let
+       a = builtins.outputOf producingDrv.out.outPath "out";
+	   b = builtins.outputOf (builtins.outputOf (builtins.unsafeDiscardOutputDependency producingDrv.drvPath) "out") "out";
+     in builtins.trace a
+       (builtins.trace b
+         (assert a == b; null))'

tests/dyn-drv/local.mk

@@ -1,7 +1,9 @@
 dyn-drv-tests := \
   $(d)/text-hashed-output.sh \
   $(d)/recursive-mod-json.sh \
-  $(d)/build-built-drv.sh
+  $(d)/build-built-drv.sh \
+  $(d)/eval-outputOf.sh \
+  $(d)/dep-built-drv.sh
 
 install-tests-groups += dyn-drv
 

tests/dyn-drv/text-hashed-output.nix

@@ -26,4 +26,14 @@ rec {
     outputHashMode = "text";
     outputHashAlgo = "sha256";
   };
+  wrapper = mkDerivation {
+    name = "put-it-all-together";
+    buildCommand = ''
+      echo "Copying the output of the dynamic derivation"
+      cp -r ${builtins.outputOf producingDrv.out.outPath "out"} $out
+    '';
+    __contentAddressed = true;
+    outputHashMode = "recursive";
+    outputHashAlgo = "sha256";
+  };
 }