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

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

@@ -14,3 +14,14 @@
       '<nixpkgs>' -A hello
   # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
   ```
+
+* Allow explicitly selecting outputs in a store derivation installable, just like we can do with other sorts of installables.
+  For example,
+  ```shell-session
+  $ nix-build /nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev`
+  ```
+  now works just as
+  ```shell-session
+  $ nix-build glibc^dev`
+  ```
+  does already.

src/libcmd/installables.cc

@@ -399,44 +399,56 @@ static StorePath getDeriver(
 struct InstallableStorePath : Installable
 {
     ref<Store> store;
-    StorePath storePath;
+    DerivedPath req;
 
     InstallableStorePath(ref<Store> store, StorePath && storePath)
-        : store(store), storePath(std::move(storePath)) { }
+        : store(store),
+          req(storePath.isDerivation()
+              ? (DerivedPath) DerivedPath::Built {
+                  .drvPath = std::move(storePath),
+                  .outputs = {},
+              }
+              : (DerivedPath) DerivedPath::Opaque {
+                  .path = std::move(storePath),
+              })
+    { }
+
+    InstallableStorePath(ref<Store> store, DerivedPath && req)
+        : store(store), req(std::move(req))
+    { }
 
-    std::string what() const override { return store->printStorePath(storePath); }
+    std::string what() const override
+    {
+        return req.to_string(*store);
+    }
 
     DerivedPaths toDerivedPaths() override
     {
-        if (storePath.isDerivation()) {
-            auto drv = store->readDerivation(storePath);
-            return {
-                DerivedPath::Built {
-                    .drvPath = storePath,
-                    .outputs = drv.outputNames(),
-                }
-            };
-        } else {
-            return {
-                DerivedPath::Opaque {
-                    .path = storePath,
-                }
-            };
-        }
+        return { req };
     }
 
     StorePathSet toDrvPaths(ref<Store> store) override
     {
-        if (storePath.isDerivation()) {
-            return {storePath};
-        } else {
-            return {getDeriver(store, *this, storePath)};
-        }
+        return std::visit(overloaded {
+            [&](const DerivedPath::Built & bfd) -> StorePathSet {
+                return { bfd.drvPath };
+            },
+            [&](const DerivedPath::Opaque & bo) -> StorePathSet {
+                return { getDeriver(store, *this, bo.path) };
+            },
+        }, req.raw());
     }
 
     std::optional<StorePath> getStorePath() override
     {
-        return storePath;
+        return std::visit(overloaded {
+            [&](const DerivedPath::Built & bfd) {
+                return bfd.drvPath;
+            },
+            [&](const DerivedPath::Opaque & bo) {
+                return bo.path;
+            },
+        }, req.raw());
     }
 };
 
@@ -803,7 +815,22 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
         for (auto & s : ss) {
             std::exception_ptr ex;
 
-            if (s.find('/') != std::string::npos) {
+            auto found = s.rfind('^');
+            if (found != std::string::npos) {
+                try {
+                    result.push_back(std::make_shared<InstallableStorePath>(
+                        store,
+                        DerivedPath::Built::parse(*store, s.substr(0, found), s.substr(found + 1))));
+                    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/libstore/derived-path.cc

@@ -78,15 +78,16 @@ DerivedPath::Opaque DerivedPath::Opaque::parse(const Store & store, std::string_
     return {store.parseStorePath(s)};
 }
 
-DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_view s)
+DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_view drvS, std::string_view outputsS)
 {
-    size_t n = s.find("!");
-    assert(n != s.npos);
-    auto drvPath = store.parseStorePath(s.substr(0, n));
-    auto outputsS = s.substr(n + 1);
+    auto drvPath = store.parseStorePath(drvS);
     std::set<std::string> outputs;
-    if (outputsS != "*")
+    if (outputsS != "*") {
         outputs = tokenizeString<std::set<std::string>>(outputsS, ",");
+        if (outputs.empty())
+            throw Error(
+                 "Explicit list of wanted outputs '%s' must not be empty. Consider using '*' as a wildcard meaning all outputs if no output in particular is wanted.", outputsS);
+	}
     return {drvPath, outputs};
 }
 
@@ -95,7 +96,7 @@ DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
     size_t n = s.find("!");
     return n == s.npos
         ? (DerivedPath) DerivedPath::Opaque::parse(store, s)
-        : (DerivedPath) DerivedPath::Built::parse(store, s);
+        : (DerivedPath) DerivedPath::Built::parse(store, s.substr(0, n), s.substr(n + 1));
 }
 
 RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const

src/libstore/derived-path.hh

@@ -47,7 +47,7 @@ struct DerivedPathBuilt {
     std::set<std::string> outputs;
 
     std::string to_string(const Store & store) const;
-    static DerivedPathBuilt parse(const Store & store, std::string_view);
+    static DerivedPathBuilt parse(const Store & store, std::string_view, std::string_view);
     nlohmann::json toJSON(ref<Store> store) const;
 
     bool operator < (const DerivedPathBuilt & b) const

src/nix/nix.md

@@ -164,6 +164,13 @@ operate are determined as follows:
   …
   ```
 
+  and likewise, using a store path to a "drv" file to specify the derivation:
+
+  ```console
+  # nix build '/nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev,static'
+  …
+  ```
+
 * You can also specify that *all* outputs should be used using the
   syntax *installable*`^*`. For example, the following shows the size
   of all outputs of the `glibc` package in the binary cache:
@@ -177,6 +184,12 @@ operate are determined as follows:
   /nix/store/q6580lr01jpcsqs4r5arlh4ki2c1m9rv-glibc-2.33-123-dev             44200560
   ```
 
+  and likewise, using a store path to a "drv" file to specify the derivation:
+
+  ```console
+  # nix path-info -S '/nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^*'
+  …
+  ```
 * If you didn't specify the desired outputs, but the derivation has an
   attribute `meta.outputsToInstall`, Nix will use those outputs. For
   example, since the package `nixpkgs#libxml2` has this attribute:
@@ -189,6 +202,9 @@ operate are determined as follows:
   a command like `nix shell nixpkgs#libxml2` will provide only those
   two outputs by default.
 
+  Note that a store derivation (given by `.drv` file store path) doesn't have
+  any attributes like `meta`, and thus this case doesn't apply to it.
+
 * Otherwise, Nix will use all outputs of the derivation.
 
 # Nix stores

tests/build.sh

@@ -8,13 +8,15 @@ set -o pipefail
 nix build -f multiple-outputs.nix --json a b --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"))))
 '
 
 # Test output selection using the '^' syntax.
@@ -56,6 +58,48 @@ nix build -f multiple-outputs.nix --json 'e^*' --no-link | jq --exit-status '
     (.outputs | keys == ["a", "b", "c"]))
 '
 
+# Test building from raw store path to drv not expression.
+
+drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
+if nix build "$drv^not-an-output" --no-link --json; then
+    fail "'not-an-output' should fail to build"
+fi
+
+if nix build "$drv^" --no-link --json; then
+    fail "'empty outputs list' should fail to build"
+fi
+
+if nix build "$drv^*nope" --no-link --json; then
+    fail "'* must be entire string' should fail to build"
+fi
+
+nix build "$drv^first" --no-link --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 1) and
+      (.first | match(".*multiple-outputs-a-first")) and
+      (has("second") | not)))
+'
+
+nix build "$drv^first,second" --no-link --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 2) and
+      (.first | match(".*multiple-outputs-a-first")) and
+	  (.second | match(".*multiple-outputs-a-second"))))
+'
+
+nix build "$drv^*" --no-link --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 2) and
+      (.first | match(".*multiple-outputs-a-first")) and
+	  (.second | match(".*multiple-outputs-a-second"))))
+'
+
 # Make sure that `--impure` works (regression test for https://github.com/NixOS/nix/issues/6488)
 nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status '
   (.[0] |