WIP: Allow for null packages and formatters

This is useful for frameworks (such as flake-parts) to avoid unnecessary
evaluation. See discussion here:
https://github.com/hercules-ci/flake-parts/issues/288#issuecomment-2912459614

While digging into this, I discovered that `nix fmt` already handles
`null` formatters identically to undefined formatters. I added a couple
of tests to demonstrate this behavior.

`nix flake show` needs some reworking to avoid crashing with a "error:
expected a derivation" when it encounters a `null` formatter or package.
My changes here avoid the crash, but has some cosmetic issues, which is why
I've labeled this PR as a draft.

Cosmetic issues
===============

With the following `flake.nix`:

```nix
{
  outputs = _: {
    packages.x86_64-linux.default = null;
  };
}
```

`nix flake show` shows a weird empty system:

```
$ nix flake show
path:/tmp/tmp.uL0iGuNwXB?lastModified=1748472558&narHash=sha256-tC%2BhdXAyoeFvWHNllJbois8X%2B7wpZ6CJzEzbcaGQxtM%3D
└───packages
    └───x86_64-linux
```

Similarly, `nix flake show --json` includes an empty object:

```
$ nix flake show --json
{
  "packages": {
    "x86_64-linux": {
      "default": null
    }
  }
}
```

`nix build` crashes:

```
$ nix build .#default
error: expected flake output attribute 'packages.x86_64-linux.default' to be a derivation or path but found null: null
```

src/libexpr/eval-cache.cc

@@ -1,3 +1,4 @@
+#include "nix/expr/value.hh"
 #include "nix/util/users.hh"
 #include "nix/expr/eval-cache.hh"
 #include "nix/store/sqlite.hh"
@@ -762,6 +763,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
     return attrs;
 }
 
+bool AttrCursor::isNull()
+{
+    // <<< TODO: caching? >>>
+    auto & v = forceValue();
+
+    return v.type() == nNull;
+}
+
 bool AttrCursor::isDerivation()
 {
     auto aType = maybeGetAttr("type");

src/libexpr/include/nix/expr/eval-cache.hh

@@ -153,6 +153,8 @@ public:
 
     bool isDerivation();
 
+    bool isNull();
+
     Value & forceValue();
 
     /**

src/nix/flake.cc

@@ -1340,10 +1340,14 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                         }
                     } else {
                         try {
-                            if (visitor.isDerivation())
+                            if (visitor.isNull()) {
-                                showDerivation();
+                                j = nullptr;
-                            else
+                            } else{
-                                throw Error("expected a derivation");
+                                if (visitor.isDerivation())
+                                    showDerivation();
+                                else
+                                    throw Error("expected a derivation");
+                            }
                         } catch (IFDError & e) {
                             if (!json) {
                                 logger->cout(fmt("%s " ANSI_WARNING "omitted due to use of import from derivation" ANSI_NORMAL, headerPrefix));

tests/functional/flakes/common.sh

@@ -16,6 +16,7 @@ writeSimpleFlake() {
       foo = import ./simple.nix;
       fooScript = (import ./shell.nix {}).foo;
       default = foo;
+      noPackage = null;
     };
     packages.someOtherSystem = rec {
       foo = import ./simple.nix;

tests/functional/flakes/show.sh

@@ -53,7 +53,7 @@ cat >flake.nix <<EOF
     packages.$system = { };
     packages.someOtherSystem = { };
 
-    formatter = { };
+    formatter.x86_64-linux = null;
     nixosConfigurations = { };
     nixosModules = { };
   };
@@ -63,7 +63,7 @@ nix flake show --json --all-systems > show-output.json
 nix eval --impure --expr '
 let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
 in
-assert show_output == { };
+assert show_output == { formatter = { x86_64-linux = null; }; };
 true
 '
 

tests/functional/formatter.sh

@@ -85,3 +85,40 @@ rm ./my-result
 # Flake outputs check.
 nix flake check
 nix flake show | grep -P "package 'formatter'"
+
+function expectFailWithOutputMatching() {
+    outputMustMatch=$1
+
+    if output=$(nix fmt 2>&1); then
+        echo >&2 "nix fmt unexpectedly succeeded"
+        exit 1
+    fi
+
+    if ! echo "$output" | grep "$outputMustMatch"; then
+        echo >&2 "Expected nix fmt output to match:"
+        echo >&2 "$outputMustMatch"
+        echo >&2 "However, the actual output was:"
+        echo >&2 "$output"
+        exit 1
+    fi
+}
+
+# Try a flake with no formatter.
+cat << EOF > flake.nix
+{
+  outputs = _: {};
+}
+EOF
+expectFailWithOutputMatching "does not provide attribute 'formatter.$system'"
+# Confirm that a null formatter is treated as if there is no formatter.
+cat << EOF > flake.nix
+{
+  outputs = _: {
+    formatter.$system = null;
+  };
+}
+EOF
+if nix fmt | grep "does not provide attribute 'formatter.$system'"; then
+    echo >&2 "nix fmt unexpectedly succeeded"
+    exit 1
+fi