Respect maybe_unused in unused-variable queries

Exclude variables annotated with the standard maybe_unused attribute from the unused-local and unused-static-variable queries, and update the regression expectations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql

@@ -57,5 +57,6 @@ where
   not declarationHasSideEffects(v) and
   not exists(AsmStmt s | f = s.getEnclosingFunction()) and
   not v.getAnAttribute().getName() = "unused" and
+  not v.getAnAttribute().getName() = "maybe_unused" and
   not f.hasErrors() // Unextracted expressions may use `v`
 select v, "Variable " + v.getName() + " is not used."

cpp/ql/src/Best Practices/Unused Entities/UnusedStaticVariables.ql

@@ -26,5 +26,6 @@ where
   not v instanceof MemberVariable and
   not declarationHasSideEffects(v) and
   not v.getAnAttribute().hasName("used") and
-  not v.getAnAttribute().hasName("unused")
+  not v.getAnAttribute().hasName("unused") and
+  not v.getAnAttribute().hasName("maybe_unused")
 select v, "Static variable " + v.getName() + " is never read."

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/UnusedLocals.expected

@@ -6,7 +6,6 @@
 | code2.cpp:64:7:64:8 | v3 | Variable v3 is not used. |
 | code2.cpp:108:11:108:12 | v2 | Variable v2 is not used. |
 | code2.cpp:128:9:128:9 | b | Variable b is not used. |
-| code2.cpp:182:23:182:28 | unused | Variable unused is not used. |
 | code.c:10:18:10:18 | y | Variable y is not used. |
 | code.c:11:18:11:18 | z | Variable z is not used. |
 | code.c:18:7:18:7 | x | Variable x is not used. |

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp

@@ -179,5 +179,5 @@ void test_switch_initializer()
 
 void test_maybe_unused()
 {
-	[[maybe_unused]] int unused; // $ SPURIOUS: Alert // GOOD: false positive for maybe_unused
+	[[maybe_unused]] int unused; // GOOD: explicitly marked as maybe_unused
 }

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/UnusedStaticVariables.expected

@@ -2,4 +2,3 @@
 | test.cpp:8:12:8:21 | staticVar6 | Static variable staticVar6 is never read. |
 | test.cpp:10:11:10:19 | constVar8 | Static variable constVar8 is never read. |
 | test.cpp:12:12:12:22 | staticVar10 | Static variable staticVar10 is never read. |
-| test.cpp:13:29:13:39 | staticVar11 | Static variable staticVar11 is never read. |

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -10,7 +10,7 @@ static __attribute__((__unused__)) int staticVar7; // GOOD (unused but this is e
 const int constVar8 = 8; // $ Alert // BAD (const defaults to static)
 extern const int constVar9 = 9; // GOOD
 static int staticVar10 = 10; // $ SPURIOUS: Alert // GOOD [FALSE POSITIVE] (referenced in a never instantiated template)
-[[maybe_unused]] static int staticVar11; // $ SPURIOUS: Alert // GOOD (false positive for maybe_unused)
+[[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as maybe_unused)
 
 void f()
 {