fix error related to RustPython#746 but for list.__delitem__

Author: Ryex

tests/snippets/list.py

@@ -201,3 +201,11 @@ def bad_del_1():
 def bad_del_2():
   del ['a', 'b'][2]
 assert_raises(IndexError, bad_del_2)
+
+# is step != 1 and start or stop of slice == -1
+x = list(range(10))
+del x[-1:-5:-1]
+assert x == [0, 1, 2, 3, 4, 5]
+x = list(range(10))
+del x[-5:-1:-1]
+assert x == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

vm/src/obj/objlist.rs

@@ -3,7 +3,7 @@ use std::fmt;
 
 use std::ops::Range;
 
-use num_bigint::BigInt;
+use num_bigint::{BigInt, ToBigInt};
 use num_traits::{One, Signed, ToPrimitive, Zero};
 
 use crate::function::{OptionalArg, PyFuncArgs};
@@ -411,8 +411,20 @@ impl PyListRef {
         } else {
             // calculate the range for the reverse slice, first the bounds needs to be made
             // exclusive around stop, the lower number
-            let start = start.as_ref().map(|x| x + 1);
-            let stop = stop.as_ref().map(|x| x + 1);
+            let start = start.as_ref().map(|x| {
+                if *x == (-1).to_bigint().unwrap() {
+                    self.get_len() + BigInt::one() //.to_bigint().unwrap()
+                } else {
+                    x + 1
+                }
+            });
+            let stop = stop.as_ref().map(|x| {
+                if *x == (-1).to_bigint().unwrap() {
+                    self.get_len().to_bigint().unwrap()
+                } else {
+                    x + 1
+                }
+            });
             let range = self.get_slice_range(&stop, &start);
             if range.start < range.end {
                 match (-step).to_i32() {