tools: add a script for checking workspace dependencies

buck run -v0 tools/scripts:unused_workspace_deps

Signed-off-by: Austin Seipp <aseipp@pobox.com>

tools/scripts/BUILD

@@ -0,0 +1,6 @@
+
+python_bootstrap_binary(
+    name = 'unused_workspace_deps',
+    main = 'unused_workspace_deps.py',
+    visibility = [],
+)

tools/scripts/unused_workspace_deps.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+
+# tools like cargo-udeps will only check if the dependencies listed in a
+# crate's Cargo.toml file are actually used; it doesn't cross reference the
+# workspace cargo file to find unused workspace deps. this script will do that
+
+import tomllib
+
+CRATE_TOML_FILES = [
+    'cli/Cargo.toml',
+    'lib/Cargo.toml',
+    'lib/proc-macros/Cargo.toml',
+    'lib/gen-protos/Cargo.toml',
+    'lib/testutils/Cargo.toml',
+]
+
+def check_unused_deps():
+    all_deps = None
+    with open("Cargo.toml", "rb") as f:
+        dat = tomllib.load(f)
+        all_deps = dat["workspace"]["dependencies"]
+
+    total_deps = len(all_deps)
+    print(f"Found {total_deps} top-level dependencies in workspace Cargo.toml")
+
+    # now, iterate over all the crate.toml files and check for unused dependencies
+    # by deleting entries from all_deps, if they exist
+    deleted_deps = 0
+    for crate_toml in CRATE_TOML_FILES:
+        with open(crate_toml, "rb") as f:
+            dat = tomllib.load(f)
+            deps = dat["dependencies"]
+
+            if "build-dependencies" in dat:
+                for x, v in dat["build-dependencies"].items():
+                    deps[x] = v
+
+            if "dev-dependencies" in dat:
+                for x, v in dat["dev-dependencies"].items():
+                    deps[x] = v
+
+            if "target" in dat:
+                for target in dat["target"]:
+                    if target.startswith("cfg("):
+                        for x, v in dat["target"][target]["dependencies"].items():
+                            deps[x] = v
+
+            for x in deps.keys():
+                if x in all_deps:
+                    del all_deps[x]
+                    deleted_deps += 1
+
+    print(f'Found {deleted_deps} unique dependencies among {len(CRATE_TOML_FILES)} Cargo.toml files')
+    if len(all_deps) > 0:
+        print(f"Found {len(all_deps)} unused dependencies:")
+        for x in all_deps.keys():
+            print(f"  {x}")
+
+if __name__ == '__main__':
+    check_unused_deps()