Add transpose conversions for Option and Result

These impls are useful when working with combinator
methods that expect an option or a result, but you
have a Result<Option<T>, E> instead of an Option<Result<T, E>>
or vice versa.

src/test/run-pass/result-opt-conversions.rs

@@ -0,0 +1,57 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(transpose_result)]
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+struct BadNumErr;
+
+fn try_num(x: i32) -> Result<i32, BadNumErr> {
+    if x <= 5 {
+        Ok(x + 1)
+    } else {
+        Err(BadNumErr)
+    }
+}
+
+type ResOpt = Result<Option<i32>, BadNumErr>;
+type OptRes = Option<Result<i32, BadNumErr>>;
+
+fn main() {
+    let mut x: ResOpt = Ok(Some(5));
+    let mut y: OptRes = Some(Ok(5));
+    assert_eq!(x, y.transpose());
+    assert_eq!(x.transpose(), y);
+
+    x = Ok(None);
+    y = None;
+    assert_eq!(x, y.transpose());
+    assert_eq!(x.transpose(), y);
+
+    x = Err(BadNumErr);
+    y = Some(Err(BadNumErr));
+    assert_eq!(x, y.transpose());
+    assert_eq!(x.transpose(), y);
+
+    let res: Result<Vec<i32>, BadNumErr> =
+        (0..10)
+            .map(|x| {
+                let y = try_num(x)?;
+                Ok(if y % 2 == 0 {
+                    Some(y - 1)
+                } else {
+                    None
+                })
+            })
+            .filter_map(Result::transpose)
+            .collect();
+
+    assert_eq!(res, Err(BadNumErr))
+}