Upgrade downstreamPlaceholder to a type with methods

This gets us ready for dynamic derivation dependencies (part of RFC 92).

src/libstore/tests/downstream-placeholder.cc

@@ -1,16 +1,33 @@
 #include <gtest/gtest.h>
 
-#include "derivations.hh"
+#include "downstream-placeholder.hh"
 
 namespace nix {
 
-TEST(Derivation, downstreamPlaceholder) {
+TEST(DownstreamPlaceholder, unknownCaOutput) {
     ASSERT_EQ(
-        downstreamPlaceholder(
-            (const Store &)*(const Store *)nullptr, // argument is unused
+        DownstreamPlaceholder::unknownCaOutput(
             StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv" },
-            "out"),
+            "out").render(),
         "/0c6rn30q4frawknapgwq386zq358m8r6msvywcvc89n6m5p2dgbz");
 }
 
+TEST(DownstreamPlaceholder, unknownDerivation) {
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "dynamic-derivations ca-derivations");
+
+    ASSERT_EQ(
+        DownstreamPlaceholder::unknownDerivation(
+            DownstreamPlaceholder::unknownCaOutput(
+                StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv.drv" },
+                "out"),
+            "out",
+            mockXpSettings).render(),
+        "/0gn6agqxjyyalf0dpihgyf49xq5hqxgw100f0wydnj6yqrhqsb3w");
+}
+
 }