Add min specialisation for RangeFrom

Calling `min` on `RangeFrom` currently causes an infinite loop.
Although other methods such as `max` also result in an infinite loop,
it is strictly incorrect in the case of `min`. Adding a specialisation
fixes this.

Separated from rust-lang#47180 because
this technically changes behaviour; it’s not just an optimisation, so
it’s a little different.

Author: varkor

src/libcore/iter/range.rs

@@ -320,6 +320,11 @@ impl<A: Step> Iterator for ops::RangeFrom<A> {
         self.start = plus_n.add_one();
         Some(plus_n)
     }
+
+    #[inline]
+    fn min(mut self) -> Option<A> {
+        self.next()
+    }
 }
 
 #[unstable(feature = "fused", issue = "35602")]

src/libcore/tests/iter.rs

@@ -1397,6 +1397,12 @@ fn test_range_inclusive_min() {
     assert_eq!(r.min(), None);
 }
 
+#[test]
+fn test_range_from_min() {
+    assert_eq!((0..).min(), Some(0));
+    assert_eq!((-20..).min(), Some(-20));
+}
+
 #[test]
 fn test_repeat() {
     let mut it = repeat(42);