ir: Avoid constant-folding ptr to sentinels

Constant-folding the pointers to the expected sentinel value have some
big problems: it hides the real content of the array, makes the pointer
to the sentinel point to a completely different memory region and treats
it like a const value even when the underlying array is mutable.

Fixes #4840

Author: LemonBoy

src/ir.cpp

@@ -20880,13 +20880,8 @@ static IrInstGen *ir_analyze_instruction_elem_ptr(IrAnalyze *ira, IrInstSrcElemP
     if (instr_is_comptime(casted_elem_index)) {
         uint64_t index = bigint_as_u64(&casted_elem_index->value->data.x_bigint);
         if (array_type->id == ZigTypeIdArray) {
-            uint64_t array_len = array_type->data.array.len;
-            if (index == array_len && array_type->data.array.sentinel != nullptr) {
-                ZigType *elem_type = array_type->data.array.child_type;
-                IrInstGen *sentinel_elem = ir_const(ira, &elem_ptr_instruction->base.base, elem_type);
-                copy_const_val(ira->codegen, sentinel_elem->value, array_type->data.array.sentinel);
-                return ir_get_ref(ira, &elem_ptr_instruction->base.base, sentinel_elem, true, false);
-            }
+            uint64_t array_len = array_type->data.array.len +
+                (array_type->data.array.sentinel != nullptr);
             if (index >= array_len) {
                 ir_add_error_node(ira, elem_ptr_instruction->base.base.source_node,
                     buf_sprintf("index %" ZIG_PRI_u64 " outside array of size %" ZIG_PRI_u64,

test/stage1/behavior/array.zig

@@ -31,14 +31,23 @@ fn getArrayLen(a: []const u32) usize {
 test "array with sentinels" {
     const S = struct {
         fn doTheTest(is_ct: bool) void {
-            var zero_sized: [0:0xde]u8 = [_:0xde]u8{};
-            expectEqual(@as(u8, 0xde), zero_sized[0]);
-            // Disabled at runtime because of
-            // https://github.com/ziglang/zig/issues/4372
             if (is_ct) {
+                var zero_sized: [0:0xde]u8 = [_:0xde]u8{};
+                // Disabled at runtime because of
+                // https://github.com/ziglang/zig/issues/4372
+                expectEqual(@as(u8, 0xde), zero_sized[0]);
                 var reinterpreted = @ptrCast(*[1]u8, &zero_sized);
                 expectEqual(@as(u8, 0xde), reinterpreted[0]);
             }
+            var arr: [3:0x55]u8 = undefined;
+            // Make sure the sentinel pointer is pointing after the last element
+            if (!is_ct) {
+                const sentinel_ptr = @ptrToInt(&arr[3]);
+                const last_elem_ptr = @ptrToInt(&arr[2]);
+                expectEqual(@as(usize, 1), sentinel_ptr - last_elem_ptr);
+            }
+            // Make sure the sentinel is writeable
+            arr[3] = 0x55;
         }
     };
 
@@ -372,7 +381,7 @@ test "access the null element of a null terminated array" {
     const S = struct {
         fn doTheTest() void {
             var array: [4:0]u8 = .{ 'a', 'o', 'e', 'u' };
-            comptime expect(array[4] == 0);
+            expect(array[4] == 0);
             var len: usize = 4;
             expect(array[len] == 0);
         }