chore(tvix): move filterRustCrateSrc to utils.nix

Change-Id: Ib6bae1ea0457d3309a6ec1e08b9e4b320523c161
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11857
Autosubmit: Ilan Joselevich <personal@ilanjoselevich.com>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/utils.nix

@@ -20,4 +20,23 @@
         );
       })
       (powerset features));
+
+  # Filters the given source, only keeping files related to the build, preventing unnecessary rebuilds.
+  # Includes src in the root, all other .rs files, as well as Cargo.toml.
+  # Additional files to be included can be specified in extraFileset.
+  filterRustCrateSrc =
+    { root # The original src
+    , extraFileset ? null # Additional filesets to include (e.g. fileFilter for proto files)
+    }:
+    lib.fileset.toSource {
+      inherit root;
+      fileset = (lib.fileset.intersection
+        (lib.fileset.fromSource root) # We build our final fileset from the original src
+        (lib.fileset.unions ([
+          (root + "/src")
+          (lib.fileset.fileFilter (f: f.hasExt "rs") root)
+          # We assume that every Rust crate will at a minimum have .rs files and a Cargo.toml
+          (lib.fileset.fileFilter (f: f.name == "Cargo.toml") root)
+        ] ++ lib.optional (extraFileset != null) extraFileset)));
+    };
 }