New "indexed" installable syntax: <drvPath>!<outputName>

Being conservative and only doing a single output name for now.

src/libcmd/installables.cc

@@ -349,6 +349,31 @@ 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() override
+    {
+        return req.to_string(*store);
+    }
+
+    DerivedPathsWithHints toDerivedPathsWithHints() override
+    {
+        std::map<std::string, std::optional<StorePath>> outputs;
+        for (auto & output : req.outputs)
+            outputs.insert_or_assign(output, std::nullopt);
+        return {
+            DerivedPathWithHints { DerivedPathWithHints::Built { req.drvPath, std::move(outputs) } }
+        };
+    }
+};
+
 DerivedPathsWithHints InstallableValue::toDerivedPathsWithHints()
 {
     DerivedPathsWithHints res;
@@ -638,7 +663,22 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
                 ex = std::current_exception();
             }
 
-            if (s.find('/') != std::string::npos) {
+            auto found = s.rfind('!');
+            if (found != std::string::npos) {
+                try {
+                    result.push_back(std::make_shared<InstallableIndexedStorePath>(
+                        store,
+                        DerivedPath::Built::parse(*store, s)));
+                    continue;
+                } catch (BadStorePath &) {
+                } catch (...) {
+                    if (!ex)
+                        ex = std::current_exception();
+                }
+            }
+
+            found = s.find('/');
+            if (found != std::string::npos) {
                 try {
                     result.push_back(std::make_shared<InstallableStorePath>(store, store->followLinksToStorePath(s)));
                     continue;

src/nix/nix.md

@@ -94,6 +94,16 @@ 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`
+
+  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 +1,17 @@
+source common.sh
+
+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 |
+      .first and
+	  (has("second") | not)))
+'
+# TODO use
+#      (.first | match(".*multiple-outputs-a-first")) and
+# once we make it put the result paths in the buildables.

tests/build.sh

@@ -4,8 +4,9 @@ expectedJSONRegex='\[\{"drvPath":".*multiple-outputs-a.drv","outputs":\{"first":
 nix build -f multiple-outputs.nix --json a.all b.all | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-a.drv")) and
-    (.outputs.first | match(".*multiple-outputs-a-first")) and
-    (.outputs.second | match(".*multiple-outputs-a-second")))
+    (.outputs |
+      (.first | match(".*multiple-outputs-a-first")) and
+      (.second | match(".*multiple-outputs-a-second"))))
   and (.[1] |
     (.drvPath | match(".*multiple-outputs-b.drv")) and
     (.outputs.out | match(".*multiple-outputs-b")))

tests/local.mk

@@ -41,6 +41,7 @@ nix_tests = \
   describe-stores.sh \
   flakes.sh \
   build.sh \
+  build-explicit-output.sh \
   compute-levels.sh \
   ca/build.sh \
   ca/substitute.sh \