stage1: Fix bug in internal string slicing (#6843)

Closes #6456

1 files changed, 11 insertions, 5 deletions

diff --git a/src/stage1/ir.cpp b/src/stage1/ir.cpp
index c1d89444a3..62d466fa88 100644
--- a/src/stage1/ir.cpp
+++ b/src/stage1/ir.cpp
@@ -25975,24 +25975,30 @@ static Error get_const_field_buf(IrAnalyze *ira, AstNode *source_node, ZigValue
     ZigValue *ptr = slice->data.x_struct.fields[slice_ptr_index];
     ZigValue *len = slice->data.x_struct.fields[slice_len_index];
     assert(ptr->data.x_ptr.special == ConstPtrSpecialBaseArray);
-    assert(ptr->data.x_ptr.data.base_array.elem_index == 0);
     ZigValue *arr = ptr->data.x_ptr.data.base_array.array_val;
     assert(arr->special == ConstValSpecialStatic);
+
+    const size_t start_value = ptr->data.x_ptr.data.base_array.elem_index;
+    const size_t len_value = bigint_as_usize(&len->data.x_bigint);
+
     switch (arr->data.x_array.special) {
         case ConstArraySpecialUndef:
             return ErrorSemanticAnalyzeFail;
         case ConstArraySpecialNone: {
+            assert(start_value <= arr->type->data.array.len);
+            assert(start_value + len_value <= arr->type->data.array.len);
             buf_resize(out, 0);
-            size_t count = bigint_as_usize(&len->data.x_bigint);
-            for (size_t j = 0; j < count; j++) {
-                ZigValue *ch_val = &arr->data.x_array.data.s_none.elements[j];
+            for (size_t j = 0; j < len_value; j++) {
+                ZigValue *ch_val = &arr->data.x_array.data.s_none.elements[start_value + j];
                 unsigned ch = bigint_as_u32(&ch_val->data.x_bigint);
                 buf_append_char(out, ch);
             }
             break;
         }
         case ConstArraySpecialBuf:
-            buf_init_from_buf(out, arr->data.x_array.data.s_buf);
+            assert(start_value <= buf_len(arr->data.x_array.data.s_buf));
+            assert(start_value + len_value <= buf_len(arr->data.x_array.data.s_buf));
+            buf_init_from_mem(out, buf_ptr(arr->data.x_array.data.s_buf) + start_value, len_value);
             break;
     }
     return ErrorNone;