core: Fix size_hint for signed integer Range<T> iterators

There was an overflow bug in .size_hint() for signed iterators, which
produced an hilariously incorrect size or an overflow panic.

Incorrect size is a serious bug since the iterators are marked
ExactSizeIterator. (And leads to abort() on (-1i8..127).collect() when
the collection tries to preallocate too much).

> (-1i8..127).size_hint()
(18446744073709551488, Some(18446744073709551488))

Bug found using quickcheck.

Fixes #24851

Author: Ulrik Sverdrup

src/libcore/iter.rs

@@ -2407,6 +2407,8 @@ pub trait Step: PartialOrd {
     /// `start` should always be less than `end`, so the result should never
     /// be negative.
     ///
+    /// `by` must be > 0.
+    ///
     /// Returns `None` if it is not possible to calculate steps_between
     /// without overflow.
     fn steps_between(start: &Self, end: &Self, by: &Self) -> Option<usize>;
@@ -2423,7 +2425,11 @@ macro_rules! step_impl {
             #[allow(trivial_numeric_casts)]
             fn steps_between(start: &$t, end: &$t, by: &$t) -> Option<usize> {
                 if *start <= *end {
-                    Some(((*end - *start) / *by) as usize)
+                    // Note: We assume $t <= usize/isize here
+                    Some(
+                        ((*end as isize).wrapping_sub(*start as isize) as usize
+                        / (*by as usize))
+                    )
                 } else {
                     Some(0)
                 }

src/libcoretest/iter.rs

@@ -11,6 +11,7 @@
 use core::iter::*;
 use core::iter::order::*;
 use core::iter::MinMaxResult::*;
+use core::i16;
 use core::usize;
 use core::cmp;
 
@@ -758,6 +759,11 @@ fn test_range() {
     assert_eq!((usize::MAX - 1..usize::MAX).size_hint(), (1, Some(1)));
     assert_eq!((-10..-1).size_hint(), (9, Some(9)));
     assert_eq!((-1..-10).size_hint(), (0, Some(0)));
+
+    assert_eq!((-70..58i8).size_hint(), (128, Some(128)));
+    assert_eq!((-128..127i8).size_hint(), (255, Some(255)));
+    let m16 = i16::MAX;
+    assert_eq!((-2..m16).size_hint(), (m16 as usize + 2, Some(m16 as usize + 2)));
 }
 
 #[test]