Add an external-builders experimental feature

src/libstore/include/nix/store/globals.hh

@@ -1309,7 +1309,25 @@ public:
               "tmpDirInSandbox": "/build",
               "topTmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0"
             }
-        )"
+        )",
+        {}, // aliases
+        true, // document default
+        // NOTE(cole-h): even though we can make the experimental feature required here, the errors
+        // are not as good (it just becomes a warning if you try to use this setting without the
+        // experimental feature)
+        //
+        // With this commented out:
+        //
+        // error: experimental Nix feature 'external-builders' is disabled; add '--extra-experimental-features external-builders' to enable it
+        //
+        // With this uncommented:
+        //
+        // warning: Ignoring setting 'external-builders' because experimental feature 'external-builders' is not enabled
+        // error: Cannot build '/nix/store/vwsp4qd8a62jqa36p26d15hin4xnj949-opentofu-1.10.2.drv'.
+        //        Reason: required system or feature not available
+        //        Required system: 'aarch64-linux' with features {}
+        //        Current system: 'aarch64-darwin' with features {apple-virt, benchmark, big-parallel, nixos-test}
+        // Xp::ExternalBuilders
     };
 };
 

src/libstore/unix/build/external-derivation-builder.cc

@@ -17,6 +17,7 @@ struct ExternalDerivationBuilder : DerivationBuilderImpl
         : DerivationBuilderImpl(store, std::move(miscMethods), std::move(params))
         , externalBuilder(std::move(externalBuilder))
     {
+        experimentalFeatureSettings.require(Xp::ExternalBuilders);
     }
 
     static std::unique_ptr<ExternalDerivationBuilder> newIfSupported(

src/libutil/experimental-features.cc

@@ -288,6 +288,14 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
         .trackingUrl = "https://github.com/NixOS/nix/milestone/55",
     },
+    {
+        .tag = Xp::ExternalBuilders,
+        .name = "external-builders",
+        .description = R"(
+            Enables support for external builders / sandbox providers.
+        )",
+        .trackingUrl = "",
+    },
     {
         .tag = Xp::BLAKE3Hashes,
         .name = "blake3-hashes",

src/libutil/include/nix/util/experimental-features.hh

@@ -35,6 +35,7 @@ enum struct ExperimentalFeature
     MountedSSHStore,
     VerifiedFetches,
     PipeOperators,
+    ExternalBuilders,
     BLAKE3Hashes,
 };