Fixes false positives for M0-1-3.

Author: rak3-sh

change_notes/2023-07-26-unused-local-variable.md

@@ -0,0 +1 @@
+ - Consider constexpr variables used in template instantiations as "used".

cpp/autosar/src/rules/M0-1-3/UnusedLocalVariable.ql

@@ -18,9 +18,37 @@ import cpp
 import codingstandards.cpp.autosar
 import codingstandards.cpp.deadcode.UnusedVariables
 
+/** Gets the constant value of a constexpr variable. */
+private string getConstExprValue(Variable v) {
+  result = v.getInitializer().getExpr().getValue() and
+  v.isConstexpr()
+}
+
+// This predicate is similar to getUseCount for M0-1-4 except that it also
+// considers static_asserts. This was created to cater for M0-1-3 specifically
+// and hence, doesn't attempt to reuse the M0-1-4 specific predicate
+// - getUseCount()
+int getUseCountConservatively(Variable v) {
+  result =
+        count(VariableAccess access | access = v.getAnAccess() and not access.isCompilerGenerated())
+        + count(UserProvidedConstructorFieldInit cfi | cfi.getTarget() = v) +
+        // For constexpr variables used as template arguments, we don't see accesses (just the
+        // appropriate literals). We therefore take a conservative approach and count the number of
+        // template instantiations that use the given constant, and consider each one to be a use
+        // of the variable
+        count(ClassTemplateInstantiation cti |
+          cti.getTemplateArgument(_).(Expr).getValue() = getConstExprValue(v)
+        )
+        // For static asserts too, check if there is a child which has the same value
+        // as the constexpr variable.
+        + count(StaticAssert s |
+         s.getCondition().getAChild*().getValue() = getConstExprValue(v))
+}
+
 from PotentiallyUnusedLocalVariable v
 where
   not isExcluded(v, DeadCodePackage::unusedLocalVariableQuery()) and
   // Local variable is never accessed
   not exists(v.getAnAccess())
+  and getUseCountConservatively(v) = 0
 select v, "Local variable " + v.getName() + " in " + v.getFunction().getName() + " is not used."

cpp/autosar/test/rules/M0-1-3/test.cpp

@@ -44,4 +44,24 @@ void test_side_effect_init() {
   LA a; // NON_COMPLIANT - no constructor called
   LC c; // COMPLIANT - constructor called which is considered to potentially
         // have side effects
-}
+}
+
+#include <cstdio>
+template <int t>
+class CharBuffer
+{
+  public:
+  int member[t];
+  CharBuffer():member{0}{}
+};
+
+int foo()
+{
+  constexpr int line_length = 1024U;
+  CharBuffer<line_length> buffer{};
+  constexpr std::size_t max_stack_size_usage = 64 * 1024;
+  static_assert(
+      (sizeof(buffer) + sizeof(line_length)) <= max_stack_size_usage,
+        "assert");
+  return buffer.member[0];
+}

rule_packages/cpp/Templates.json

@@ -190,6 +190,33 @@
           }
       ],
       "title": "In a class template with a dependent base, any name that may be found in that dependent base shall be referred to using a qualified-id or this->."
+    },
+    "M0-1-3": {
+      "properties": {
+        "allocated-target": [
+          "implementation"
+        ],
+        "enforcement": "automated",
+        "obligation": "required"
+      },
+      "queries": [
+        {
+          "description": "Unused variables complicate the program and can indicate a possible mistake on the part of the programmer.",
+          "kind": "problem",
+          "name": "A project shall not contain unused local variables",
+          "precision": "very-high",
+          "severity": "warning",
+          "short_name": "UnusedLocalVariable",
+          "tags": [
+            "maintainability",
+            "readability"
+          ],
+          "implementation_scope": {
+            "description": "In limited cases, this query can raise false-positives for variables that are defined as constexpr and used in an expression to instantiate a template."
+          }
+        }
+      ],
+      "title": "A project shall not contain unused local variables."
     }
   }
-}
+}