Benchmark for impl FromIterator for Result.

(This is an attempt to ensure we do not regress performance here,
since the performance of this operation varied pretty wildly of the
course of rust-lang#11084.)

Author: pnkfelix

src/libcore/benches/iter.rs

@@ -344,3 +344,15 @@ fn bench_partial_cmp(b: &mut Bencher) {
 fn bench_lt(b: &mut Bencher) {
     b.iter(|| (0..100000).map(black_box).lt((0..100000).map(black_box)))
 }
+
+// rust-lang/rust#11084: benchmark how built-in `FromIterator` for
+// `Result` performs, to avoid unknowingly regressing its performance.
+#[bench]
+fn bench_result_from_iter_into_vec(b: &mut Bencher) {
+    let expected = vec![1; 1000];
+    let v: Vec<Result<isize, String>> = vec![Ok(1); 1000];
+    b.iter(|| {
+        let result: Result<Vec<isize>, String> = FromIterator::from_iter(v.iter().cloned());
+        assert_eq!(result.unwrap(), expected);
+    });
+}