Prevent implicit array casts to pointers with higher alignment. #1237

releasenotes.md

@@ -18,6 +18,7 @@
 - Max number of members in a struct is limited to 65535.
 - The maximum number of parameters in a call is now 255, up from 127.
 - Array comparison now uses built-in memcmp on LLVM to enable optimizations.
+- Prevent implicit array casts to pointers with higher alignment #1237.
 
 ### Fixes
 - Error with unsigned compare in `@ensure` when early returning 0 #1207.

src/compiler/compiler_internal.h

@@ -2496,6 +2496,7 @@ typedef enum
 	TYPE_MISMATCH = 0,
 	TYPE_SAME = 1,
 	TYPE_SAME_INT_SIZE = 2,
+	TYPE_ALIGNMENT_INCREASE = 3,
 	TYPE_ERROR = -1,
 } TypeCmpResult;
 

src/compiler/sema_casts.c

@@ -682,6 +682,17 @@ static bool rule_ptr_to_ptr(CastContext *cc, bool is_explicit, bool is_silent)
 			return true;
 		case TYPE_ERROR:
 			return false;
+		case TYPE_ALIGNMENT_INCREASE:
+			if (is_explicit) return false;
+			assert(!is_explicit);
+			RETURN_CAST_ERROR(cc->expr,
+			                  "Implicitly casting %s (alignment %d) to %s (alignment %d) is not permitted, "
+							  "it would require an explicit cast. Before using an explicit cast, please make "
+							  "sure you understand the ramifications as the explicit cast might crash your program if used incorrectly.",
+			                  type_quoted_error_string(type_no_optional(cc->expr->type)),
+			                  type_abi_alignment(type_get_indexed_type(cc->expr->type)),
+							  type_quoted_error_string(cc->to),
+							  type_abi_alignment(type_get_indexed_type(cc->to)));
 		case TYPE_MISMATCH:
 		case TYPE_SAME_INT_SIZE:
 			return sema_cast_error(cc, true, is_silent);

src/compiler/types.c

@@ -1661,6 +1661,8 @@ RETRY:
 
 	if (to_pointee->type_kind != from_pointee->type_kind)
 	{
+		TypeCmpResult res_current = TYPE_SAME;
+		if (type_abi_alignment(to_pointee) > type_abi_alignment(from_pointee)) res_current = TYPE_ALIGNMENT_INCREASE;
 		if (type_is_matching_int(to_pointee, from_pointee)) return TYPE_SAME_INT_SIZE;
 
 		if (type_is_any_arraylike(from_pointee))
@@ -1669,10 +1671,11 @@ RETRY:
 			if (type_is_any_arraylike(to_pointee))
 			{
 				TypeCmpResult res = type_array_is_equivalent(context, to_pointee, from_pointee, flatten_distinct);
-				if (res != TYPE_MISMATCH) return res;
+				if (res != TYPE_MISMATCH) return res == TYPE_SAME ? res_current : res;
 			}
 			// A possible int[4]* -> int* decay?
-			return type_is_pointer_equivalent(context, type_get_ptr(from_pointee->array.base), to_pointer, flatten_distinct);
+			TypeCmpResult res = type_is_pointer_equivalent(context, type_get_ptr(from_pointee->array.base), to_pointer, flatten_distinct);
+			return res == TYPE_SAME ? res_current : res;
 		}
 		// Not arraylike and no array decay. Failure.
 		return TYPE_MISMATCH;

test/test_suite/expressions/casts/cast_vector_fail.c3

@@ -0,0 +1,17 @@
+module foo;
+import std;
+fn void test(float[<4>]* x)
+{}
+fn void test2(float[4]* x)
+{}
+
+fn int main(String[] args)
+{
+	float[4] a;
+	float[<4>] b;
+	test(&a); // #error: Implicitly casting
+	test(&b);
+	test2(&a);
+	test2(&b);
+	return 1;
+}