fix comptime slicing not preserving comptime mutability

 * fix comptime slice of slice not preserving mutatibility
   of the comptime data
 * fix comptime slice of pointer not preserving mutability
   of the comptime data

closes #826

src/ir.cpp

@@ -15995,6 +15995,10 @@ static TypeTableEntry *ir_analyze_instruction_slice(IrAnalyze *ira, IrInstructio
             init_const_ptr_array(ira->codegen, ptr_val, array_val, index, is_const);
             if (array_type->id == TypeTableEntryIdArray) {
                 ptr_val->data.x_ptr.mut = ptr_ptr->value.data.x_ptr.mut;
+            } else if (is_slice(array_type)) {
+                ptr_val->data.x_ptr.mut = parent_ptr->data.x_ptr.mut;
+            } else if (array_type->id == TypeTableEntryIdPointer) {
+                ptr_val->data.x_ptr.mut = parent_ptr->data.x_ptr.mut;
             }
         } else if (ptr_is_undef) {
             ptr_val->type = get_pointer_to_type(ira->codegen, parent_ptr->type->data.pointer.child_type,

test/cases/eval.zig

@@ -469,3 +469,20 @@ fn doesAlotT(comptime T: type, value: usize) T {
 test "@setEvalBranchQuota at same scope as generic function call" {
     assert(doesAlotT(u32, 2) == 2);
 }
+
+test "comptime slice of slice preserves comptime var" {
+    comptime {
+        var buff: [10]u8 = undefined;
+        buff[0..][0..][0] = 1;
+        assert(buff[0..][0..][0] == 1);
+    }
+}
+
+test "comptime slice of pointer preserves comptime var" {
+    comptime {
+        var buff: [10]u8 = undefined;
+        var a = &buff[0];
+        a[0..1][0] = 1;
+        assert(buff[0..][0..][0] == 1);
+    }
+}