4 files changed, 48 insertions, 0 deletions

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 74c8a6df5..483bc1421 100644
--- a/src/libcmd/installables.cc
+++ b/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)));
diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
index e033a4116..c1e574c0d 100644
--- a/src/libutil/experimental-features.cc
+++ b/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)
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
index 266e41a22..e6213bf86 100644
--- a/src/libutil/experimental-features.hh
+++ b/src/libutil/experimental-features.hh
@@ -22,6 +22,7 @@ enum struct ExperimentalFeature
     RecursiveNix,
     NoUrlLiterals,
     FetchClosure,
+    ComputedDerivations, // RFC 92
 };
 
 /**
diff --git a/src/nix/nix.md b/src/nix/nix.md
index 0dacadee6..691aa137b 100644
--- a/src/nix/nix.md
+++ b/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