Low level <drvPath>^<outputName> installable syntax to match existing <highLevelInstallable>^<outputNames> syntax

doc/manual/src/release-notes/rl-next.md

@@ -1 +1,4 @@
 # Release X.Y (202?-??-??)
+
+* Add experimental *indexed store derivations* installable syntax, part of the
+  the `computed-derivations` experimental feature.

src/libcmd/installables.cc

@@ -431,6 +431,26 @@ struct InstallableStorePath : Installable
     }
 };
 
+struct InstallableIndexedStorePath : Installable
+{
+    ref<Store> store;
+    DerivedPath::Built req;
+
+    InstallableIndexedStorePath(ref<Store> store, DerivedPath::Built && req)
+        : store(store), req(std::move(req))
+    { }
+
+    std::string what() const override
+    {
+        return req.to_string(*store);
+    }
+
+    DerivedPaths toDerivedPaths() override
+    {
+        return { req };
+    }
+};
+
 DerivedPaths InstallableValue::toDerivedPaths()
 {
     DerivedPaths res;
@@ -731,6 +751,20 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
         for (auto & s : ss) {
             std::exception_ptr ex;
 
+            if (s.rfind('!') != std::string::npos) {
+                try {
+                    result.push_back(std::make_shared<InstallableIndexedStorePath>(
+                        store,
+                        DerivedPath::Built::parse(*store, s)));
+                    settings.requireExperimentalFeature(Xp::ComputedDerivations);
+                    continue;
+                } catch (BadStorePath &) {
+                } catch (...) {
+                    if (!ex)
+                        ex = std::current_exception();
+                }
+            }
+
             if (s.find('/') != std::string::npos) {
                 try {
                     result.push_back(std::make_shared<InstallableStorePath>(store, store->followLinksToStorePath(s)));

src/libutil/experimental-features.cc

@@ -13,6 +13,7 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::RecursiveNix, "recursive-nix" },
     { Xp::NoUrlLiterals, "no-url-literals" },
     { Xp::FetchClosure, "fetch-closure" },
+    { Xp::ComputedDerivations, "computed-derivations" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -22,6 +22,7 @@ enum struct ExperimentalFeature
     RecursiveNix,
     NoUrlLiterals,
     FetchClosure,
+    ComputedDerivations, // RFC 92
 };
 
 /**

src/nix/nix.md

@@ -130,6 +130,18 @@ the Nix store. Here are the recognised types of installables:
   If you want to operate on the store derivation itself, pass the
   `--derivation` flag.
 
+* **Indexed store derivations**: `/nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv!out`
+
+  *(Experimental, part of by the `computed-derivations` experimental feature.)*
+
+  Store derivations can be indexed with a specific output name. This
+  allows finer control versus just specifying a derivation (without
+  `--derivation`) and getting all the outputs.
+
+  This is especially useful for (currently unstable) floating content
+  addressed derivations, which do not have precomputed output paths that
+  can be used instead.
+
 * **Nix attributes**: `--file /path/to/nixpkgs hello`
 
   When the `-f` / `--file` *path* option is given, installables are

tests/build-explicit-output.sh

@@ -0,0 +1,18 @@
+source common.sh
+
+enableFeatures "computed-derivations"
+restartDaemon
+
+drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
+if nix build "$drv!not-an-output" --json; then
+    fail "'not-an-output' should fail to build"
+fi
+
+nix build "$drv!first" --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 1) and
+      (.first | match(".*multiple-outputs-a-first")) and
+      (has("second") | not)))
+'

tests/build.sh

@@ -13,13 +13,15 @@ nix build -f multiple-outputs.nix --json a --no-link | jq --exit-status '
 nix build -f multiple-outputs.nix --json a.all b.all --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-a.drv")) and
-    (.outputs | keys | length == 2) and
-    (.outputs.first | match(".*multiple-outputs-a-first")) and
-    (.outputs.second | match(".*multiple-outputs-a-second")))
+    (.outputs |
+      (keys | length == 2) and
+      (.first | match(".*multiple-outputs-a-first")) and
+      (.second | match(".*multiple-outputs-a-second"))))
   and (.[1] |
     (.drvPath | match(".*multiple-outputs-b.drv")) and
-    (.outputs | keys | length == 1) and
-    (.outputs.out | match(".*multiple-outputs-b")))
+    (.outputs |
+      (keys | length == 1) and
+      (.out | match(".*multiple-outputs-b"))))
 '
 
 testNormalization () {

tests/local.mk

@@ -91,6 +91,7 @@ nix_tests = \
   ssh-relay.sh \
   plugins.sh \
   build.sh \
+  build-explicit-output.sh \
   ca/nix-run.sh \
   db-migration.sh \
   bash-profile.sh \