fix silent overflows on Step impls

r? @eddyb

src/libcore/iter/range.rs

@@ -95,11 +95,13 @@ macro_rules! step_impl_unsigned {
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn add_one(&self) -> Self {
                 *self + 1
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn sub_one(&self) -> Self {
                 *self - 1
             }
@@ -166,11 +168,13 @@ macro_rules! step_impl_signed {
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn add_one(&self) -> Self {
                 *self + 1
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn sub_one(&self) -> Self {
                 *self - 1
             }
@@ -215,11 +219,13 @@ macro_rules! step_impl_no_between {
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn add_one(&self) -> Self {
                 *self + 1
             }
 
             #[inline]
+            #[rustc_inherit_overflow_checks]
             fn sub_one(&self) -> Self {
                 *self - 1
             }

src/libcoretest/iter.rs

@@ -915,6 +915,20 @@ fn test_range_step() {
     assert_eq!((isize::MIN..isize::MAX).step_by(1).size_hint(), (usize::MAX, Some(usize::MAX)));
 }
 
+#[test]
+#[should_panic]
+fn test_range_overflow_unsigned() {
+    let mut it = u8::MAX..;
+    it.next();
+}
+
+#[test]
+#[should_panic]
+fn test_range_overflow_signed() {
+    let mut it = i8::MAX..;
+    it.next();
+}
+
 #[test]
 fn test_repeat() {
     let mut it = repeat(42);