Speed up implementation indexing with bitmask pre-filtering

.github/workflows/test.yaml

@@ -128,9 +128,20 @@ jobs:
             | jq -e --arg f "${{ matrix.expect_file }}" \
                 'any(.documents[]; .relative_path == $f)'
 
+  fingerprint-sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-go@v6
+        with:
+          go-version: stable
+          cache-dependency-path: go.sum
+      - run: ./internal/implementations/fingerprint/sync.sh
+      - run: git diff --exit-code internal/implementations/fingerprint/fingerprint.go
+
   ci-test:
     if: always()
-    needs: [build, docker, index]
+    needs: [build, docker, index, fingerprint-sync]
     runs-on: ubuntu-latest
     steps:
       - if: >-

flake.nix

@@ -37,7 +37,8 @@
             nativeCheckInputs = [ pkgs.git ];
           };
           default = self.packages.${system}.scip-go;
-
+        }
+        // pkgs.lib.optionalAttrs pkgs.stdenv.isLinux {
           docker = pkgs.dockerTools.buildLayeredImage {
             name = "scip-go";
             tag = version;

internal/implementations/fingerprint/README.md

@@ -0,0 +1,24 @@
+# fingerprint
+
+This package is a verbatim copy of
+[`golang.org/x/tools/gopls/internal/util/fingerprint`], which is not importable
+because it lives under an `internal/` path.
+
+It is used by the implementation-indexing code to produce canonical
+method-signature strings (fingerprints) for the 64-bit bitmask filter that
+quickly rejects non-matching type/interface pairs before calling the expensive
+`types.Implements`.
+
+## Keeping in sync
+
+The copy is pinned to the `golang.org/x/tools` version declared in `go.mod`
+(currently **v0.43.0**). Run the sync script whenever you bump that dependency:
+
+``` bash
+./internal/implementations/fingerprint/sync.sh
+```
+
+The script fetches the file at the matching tag and overwrites the local copy.
+Any diff will show up in `git status`.
+
+  [`golang.org/x/tools/gopls/internal/util/fingerprint`]: https://github.com/golang/tools/tree/master/gopls/internal/util/fingerprint