Fix dead-code detector incorrectly flagging library functions (#2905)

* fix: exclude library functions from dead-code detector

The dead-code detector incorrectly flagged library functions as dead code.
This fix updates the type annotation and set comprehension to properly
track library function canonical names.

Also adds test case and snapshot for library function usage.

Fixes #1265

* fix: address PR review feedback

- Remove unused Contract import from dead_code.py
- Fix misleading comment in test file that claimed unused library
  function should be flagged (library functions are excluded from
  dead-code detection)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

---------

Co-authored-by: ep0chzer0 <[email protected]>
Co-authored-by: Dan Guido <[email protected]>
Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: 3 people

slither/detectors/functions/dead_code.py

@@ -2,7 +2,7 @@
 Module detecting dead code
 """
 
-from slither.core.declarations import Function, FunctionContract, Contract
+from slither.core.declarations import Function, FunctionContract
 from slither.detectors.abstract_detector import (
     AbstractDetector,
     DetectorClassification,
@@ -53,12 +53,10 @@ def _detect(self) -> list[Output]:
                 f.canonical_name for f in all_functions_called if isinstance(f, Function)
             }
             all_libss_called = [f.all_library_calls() for f in contract.functions_entry_points]
-            all_libs_called: list[tuple[Contract, Function]] = [
+            all_libs_called: list[Function] = [
                 item.function for sublist in all_libss_called for item in sublist
             ]
-            functions_used |= {
-                lib[1].canonical_name for lib in all_libs_called if isinstance(lib, tuple)
-            }
+            functions_used |= {f.canonical_name for f in all_libs_called if isinstance(f, Function)}
         for function in sorted(self.compilation_unit.functions, key=lambda x: x.canonical_name):
             if (
                 function.visibility in ["public", "external"]

tests/e2e/detectors/snapshots/detectors__detector_DeadCode_0_8_0_dead_code_library_sol__0.txt

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+// Library with internal functions
+library MyLibrary {
+    struct Data {
+        uint256 value;
+    }
+
+    // This function IS used via `using MyLibrary for Data`
+    // Should NOT be flagged as dead code
+    function getValue(Data storage self) internal view returns (uint256) {
+        return self.value;
+    }
+
+    // This function IS used via `using MyLibrary for Data`
+    // Should NOT be flagged as dead code
+    function setValue(Data storage self, uint256 newValue) internal {
+        self.value = newValue;
+    }
+
+    // This function is NOT used anywhere but library functions are excluded
+    // from dead-code detection, so it should NOT be flagged
+    function unusedLibraryFunction(Data storage self) internal pure returns (uint256) {
+        return 42;
+    }
+}
+
+contract UsingLibrary {
+    using MyLibrary for MyLibrary.Data;
+
+    MyLibrary.Data private data;
+
+    function store(uint256 value) external {
+        data.setValue(value);
+    }
+
+    function retrieve() external view returns (uint256) {
+        return data.getValue();
+    }
+}

tests/e2e/detectors/test_data/dead-code/0.8.0/dead-code-library.sol

@@ -1361,6 +1361,11 @@ def id_test(test_item: Test):
         "dead-code.sol",
         "0.8.0",
     ),
+    Test(
+        all_detectors.DeadCode,
+        "dead-code-library.sol",
+        "0.8.0",
+    ),
     Test(
         all_detectors.WriteAfterWrite,
         "write-after-write.sol",