Step::replace_one should put a one, not a zero (Issue #41492)

Turns out all six of these impls are incorrect.

2 files changed, 40 insertions, 0 deletions

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index 08442f9bcbf..001fa304cd0 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1082,3 +1082,41 @@ fn test_chain_fold() {
     assert_eq!(&[2, 3, 1, 2, 0], &result[..]);
 }
 
+#[test]
+fn test_step_replace_unsigned() {
+    let mut x = 4u32;
+    let y = x.replace_zero();
+    assert_eq!(x, 0);
+    assert_eq!(y, 4);
+
+    x = 5;
+    let y = x.replace_one();
+    assert_eq!(x, 1);
+    assert_eq!(y, 5);
+}
+
+#[test]
+fn test_step_replace_signed() {
+    let mut x = 4i32;
+    let y = x.replace_zero();
+    assert_eq!(x, 0);
+    assert_eq!(y, 4);
+
+    x = 5;
+    let y = x.replace_one();
+    assert_eq!(x, 1);
+    assert_eq!(y, 5);
+}
+
+#[test]
+fn test_step_replace_no_between() {
+    let mut x = 4u128;
+    let y = x.replace_zero();
+    assert_eq!(x, 0);
+    assert_eq!(y, 4);
+
+    x = 5;
+    let y = x.replace_one();
+    assert_eq!(x, 1);
+    assert_eq!(y, 5);
+}
\ No newline at end of file
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 528ab3bc845..f0c46a6f194 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -20,6 +20,7 @@
 #![feature(fixed_size_array)]
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
+#![feature(i128_type)]
 #![feature(iter_rfind)]
 #![feature(libc)]
 #![feature(nonzero)]
@@ -30,6 +31,7 @@
 #![feature(sort_internals)]
 #![feature(sort_unstable)]
 #![feature(step_by)]
+#![feature(step_trait)]
 #![feature(test)]
 #![feature(try_from)]
 #![feature(unicode)]