[clang-tidy] Suggest using reinterpret_cast in bugprone-casting-thro… (#106784)

…ugh-void

reinterpret_cast is the equivalent construct, and more clearly expresses
intent.

Co-authored-by: Carlos Gálvez <[email protected]>

Author: carlosgalvezp

clang-tools-extra/clang-tidy/bugprone/CastingThroughVoidCheck.cpp

@@ -38,7 +38,9 @@ void CastingThroughVoidCheck::check(const MatchFinder::MatchResult &Result) {
   const auto ST = *Result.Nodes.getNodeAs<QualType>("source_type");
   const auto VT = *Result.Nodes.getNodeAs<QualType>("void_type");
   const auto *CE = Result.Nodes.getNodeAs<ExplicitCastExpr>("cast");
-  diag(CE->getExprLoc(), "do not cast %0 to %1 through %2") << ST << TT << VT;
+  diag(CE->getExprLoc(),
+       "do not cast %0 to %1 through %2; use reinterpret_cast instead")
+      << ST << TT << VT;
 }
 
 } // namespace clang::tidy::bugprone

clang-tools-extra/docs/ReleaseNotes.rst

@@ -104,6 +104,10 @@ New check aliases
 Changes in existing checks
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Improved :doc:`bugprone-casting-through-void
+  <clang-tidy/checks/bugprone/casting-through-void>` check to suggest replacing
+  the offending code with ``reinterpret_cast``, to more clearly express intent.
+
 - Improved :doc:`modernize-use-std-format
   <clang-tidy/checks/modernize/use-std-format>` check to support replacing
   member function calls too.

clang-tools-extra/docs/clang-tidy/checks/bugprone/casting-through-void.rst

@@ -3,7 +3,9 @@
 bugprone-casting-through-void
 =============================
 
-Detects unsafe or redundant two-step casting operations involving ``void*``.
+Detects unsafe or redundant two-step casting operations involving ``void*``,
+which is equivalent to ``reinterpret_cast`` as per the
+`C++ Standard <https://eel.is/c++draft/expr.reinterpret.cast#7>`_.
 
 Two-step type conversions via ``void*`` are discouraged for several reasons.
 
@@ -16,7 +18,17 @@ Two-step type conversions via ``void*`` are discouraged for several reasons.
 
 In summary, avoiding two-step type conversions through ``void*`` ensures clearer code,
 maintains essential compiler warnings, and prevents ambiguity and potential runtime
-errors, particularly in complex inheritance scenarios.
+errors, particularly in complex inheritance scenarios. If such a cast is wanted,
+it shall be done via ``reinterpret_cast``, to express the intent more clearly.
+
+Note: it is expected that, after applying the suggested fix and using
+``reinterpret_cast``, the check :doc:`cppcoreguidelines-pro-type-reinterpret-cast
+<../cppcoreguidelines/pro-type-reinterpret-cast>` will emit a warning.
+This is intentional: ``reinterpret_cast`` is a dangerous operation that can
+easily break the strict aliasing rules when dereferencing the casted pointer,
+invoking Undefined Behavior. The warning is there to prompt users to carefuly
+analyze whether the usage of ``reinterpret_cast`` is safe, in which case the
+warning may be suppressed.
 
 Examples:
 
@@ -29,3 +41,8 @@ Examples:
    reinterpret_cast<IntegerPointer>(reinterpret_cast<void *>(ptr)); // WRONG
    (IntegerPointer)(void *)ptr; // WRONG
    IntegerPointer(static_cast<void *>(ptr)); // WRONG
+
+   reinterpret_cast<IntegerPointer>(ptr); // OK, clearly expresses intent.
+                                          // NOTE: dereferencing this pointer violates
+                                          // the strict aliasing rules, invoking
+                                          // Undefined Behavior.

clang-tools-extra/test/clang-tidy/checkers/bugprone/casting-through-void.cpp

@@ -10,54 +10,54 @@ const double cd = 100;
 
 void normal_test() {
   static_cast<int *>(static_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<int *>(static_cast<V>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'V' (aka 'void *') [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'V' (aka 'void *'); use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<int *>(static_cast<void *>(&i));
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'int *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'int *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<void *>(static_cast<void *>(&i));
 }
 
 void const_pointer_test() {
   static_cast<int *const>(static_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *const' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *const' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<int *const>(static_cast<V>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *const' through 'V' (aka 'void *') [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *const' through 'V' (aka 'void *'); use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<int *const>(static_cast<void *>(&i));
-  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'int *' to 'int *const' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'int *' to 'int *const' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<void *const>(static_cast<void *>(&i));
 }
 
 void const_test() {
   static_cast<const int *>(static_cast<const void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'double *' to 'const int *' through 'const void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'double *' to 'const int *' through 'const void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<const int *>(static_cast<const V>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'double *' to 'const int *' through 'const V' (aka 'void *const') [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'double *' to 'const int *' through 'const V' (aka 'void *const'); use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<const int *>(static_cast<const void *>(&i));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'int *' to 'const int *' through 'const void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'int *' to 'const int *' through 'const void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<const void *>(static_cast<const void *>(&i));
 
   static_cast<const int *>(static_cast<const void *>(&cd));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const double *' to 'const int *' through 'const void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const double *' to 'const int *' through 'const void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<const int *>(static_cast<const CV>(&cd));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const double *' to 'const int *' through 'const CV' (aka 'const void *const') [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const double *' to 'const int *' through 'const CV' (aka 'const void *const'); use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<const int *>(static_cast<const void *>(&ci));
-  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const int *' to 'const int *' through 'const void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:28: warning: do not cast 'const int *' to 'const int *' through 'const void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<const void *>(static_cast<const void *>(&ci));
 }
 
 
 void reinterpret_cast_test() {
   static_cast<int *>(reinterpret_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   reinterpret_cast<int *>(static_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   reinterpret_cast<int *>(reinterpret_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:27: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<void *>(reinterpret_cast<void *>(&i));
   reinterpret_cast<void *>(reinterpret_cast<void *>(&i));
@@ -66,11 +66,11 @@ void reinterpret_cast_test() {
 
 void c_style_cast_test() {
   static_cast<int *>((void *)&d);
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   (int *)(void *)&d;
-  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
   static_cast<int *>((void *)&d);
-  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:22: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 
   static_cast<void *>((void *)&i);
 }
@@ -82,12 +82,12 @@ using I = int *;
 void cxx_functional_cast() {
   A(static_cast<void*>(&d));
   I(static_cast<void*>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: do not cast 'double *' to 'I' (aka 'int *') through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: do not cast 'double *' to 'I' (aka 'int *') through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 }
 
 void bit_cast() {
   __builtin_bit_cast(int *, static_cast<void *>(&d));
-  // CHECK-MESSAGES: :[[@LINE-1]]:29: warning: do not cast 'double *' to 'int *' through 'void *' [bugprone-casting-through-void]
+  // CHECK-MESSAGES: :[[@LINE-1]]:29: warning: do not cast 'double *' to 'int *' through 'void *'; use reinterpret_cast instead [bugprone-casting-through-void]
 }
 
 namespace PR87069 {