Add libcore test

Author: fee1-dead

library/core/src/iter/adapters/zip.rs

@@ -375,7 +375,8 @@ where
                 if A::MAY_HAVE_SIDE_EFFECT && sz_a > self.len {
                     // FIXME(const_trait_impl): replace with `for`
                     let mut i = 0;
-                    while i < sz_a - self.len {
+                    let upper_bound = sz_a - self.len;
+                    while i < upper_bound {
                         // since next_back() may panic we increment the counters beforehand
                         // to keep Zip's state in sync with the underlying iterator source
                         self.a_len -= 1;
@@ -401,7 +402,7 @@ where
                 if B::MAY_HAVE_SIDE_EFFECT && sz_b > self.len {
                     // FIXME(const_trait_impl): replace with `for`
                     let mut i = 0;
-                    while i < sz_a - self.len {
+                    while i < sz_b - self.len {
                         self.b.next_back();
                         i += 1;
                     }

library/core/tests/iter/mod.rs

@@ -100,3 +100,18 @@ pub fn extend_for_unit() {
     }
     assert_eq!(x, 5);
 }
+
+#[test]
+fn test_const_iter() {
+    const X: usize = {
+        let mut n = 0;
+        #[allow(for_loops_over_fallibles)]
+        for a in Some(1) {
+            n = a;
+        }
+        n
+    };
+
+    const _: () = assert!(X == 1);
+    assert_eq!(1, X);
+}

library/core/tests/lib.rs

@@ -13,7 +13,9 @@
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
 #![feature(const_hash)]
+#![feature(const_for)]
 #![feature(const_heap)]
+#![feature(const_iter)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init_read)]
 #![feature(const_nonnull_new)]