ompile time dereference of a constant slice was too generous #2821

releasenotes.md

@@ -120,7 +120,8 @@
 - Generating typeid from function gives incorrect typeid #2816
 - Recursive definitions not discovered when initializer is access on other const #2817
 - Slice overrun detected late hit codegen assert #2822
- 
+- Compile time dereference of a constant slice was too generous #2821
+
 ### Stdlib changes
 - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.
 - Add `@in` compile-time macro to check for a value in a variable list of constants. #2662

src/compiler/sema_expr.c

@@ -8921,8 +8921,18 @@ static inline bool sema_expr_analyse_deref(SemaContext *context, Expr *expr, boo
 				break;
 			case CONST_BYTES:
 			case CONST_STRING:
-				expr_rewrite_const_int(expr, type_get_indexed_type(inner->type), inner->const_expr.bytes.len ? inner->const_expr.bytes.ptr[0] : 0);
+			{
+				if (inner->type->type_kind != TYPE_POINTER) break;
+				Type *inner_type = type_get_indexed_type(inner->type);
+				if (!type_kind_is_any_integer(type_flatten(inner_type)->type_kind)) break;
+				if (!inner->const_expr.bytes.len)
+				{
+					RETURN_SEMA_ERROR(inner, "You cannot dereference an empty constant array or slice.");
+				}
+				expr_rewrite_const_int(expr, inner_type, inner->const_expr.bytes.ptr[0]);
 				return true;
+
+			}
 			default:
 				UNREACHABLE
 		}

test/test_suite/expressions/deref_const_string.c3t

@@ -4,13 +4,11 @@ import std;
 
 const char *VOWS = "aeiou";
 const char *CONS = "bcdfghjklmnpqrstvwxyz";
-const char *TEST = "";
 
 fn int main (String[] args)
 {
 	char a = *VOWS;
 	char b = *CONS;
-	char c = *TEST;
 	return 0;
 }
 
@@ -18,12 +16,9 @@ fn int main (String[] args)
 
 @.str = private unnamed_addr constant [6 x i8] c"aeiou\00", align 1
 @test.VOWS = local_unnamed_addr constant ptr @.str, align 8
-@.str.3 = private unnamed_addr constant [22 x i8] c"bcdfghjklmnpqrstvwxyz\00", align 1
-@test.CONS = local_unnamed_addr constant ptr @.str.3, align 8
-@.str.4 = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
-@test.TEST = local_unnamed_addr constant ptr @.str.4, align 8
+@.str.2 = private unnamed_addr constant [22 x i8] c"bcdfghjklmnpqrstvwxyz\00", align 1
+@test.CONS = local_unnamed_addr constant ptr @.str.2, align 8
 
   store i8 97, ptr %a, align 1
   store i8 98, ptr %b, align 1
-  store i8 0, ptr %c, align 1
-  ret i32 0
+  ret i32 0

test/test_suite/slices/deref_const_slice_cast.c3t

@@ -0,0 +1,17 @@
+// #target: macos-x64
+module test;
+fn int main()
+{
+	char[*] b = *(char[5]*)"";
+	return 0;
+}
+/* #expect: test.ll
+
+@.str = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
+
+define i32 @main() #0 {
+entry:
+  %b = alloca [5 x i8], align 1
+  call void @llvm.memcpy.p0.p0.i32(ptr align 1 %b, ptr align 1 @.str, i32 5, i1 false)
+  ret i32 0
+}