Fix switch_stdout_to on Windows7

The switch_stdout_to test was broken on Windows7, as the test
infrastructure would refuse to delete the temporary test folder because
the switch-stdout-output file we redirected the stdout to was still
opened.

To fix this issue, we make switch_stdout_to return the previous handle,
and add a new switch_stdout_to call at the end of the test to return the
stdio handles to their original state. The handle the second
switch_stdout_to returns will be automatically closed, which should
allow the temporary test folder to be deleted properly.

library/std/tests/switch-stdout.rs

@@ -5,32 +5,48 @@ use std::io::{Read, Write};
 
 mod common;
 
+#[cfg(windows)]
+use std::os::windows::io::OwnedHandle;
+
 #[cfg(unix)]
-fn switch_stdout_to(file: File) {
+use std::os::fd::OwnedFd;
+
+#[cfg(unix)]
+fn switch_stdout_to(file: OwnedFd) -> OwnedFd {
     use std::os::unix::prelude::*;
 
     extern "C" {
+        fn dup(old: i32) -> i32;
         fn dup2(old: i32, new: i32) -> i32;
     }
 
     unsafe {
+        let orig_fd = dup(1);
+        assert_ne!(orig_fd, -1);
+        let res = OwnedFd::from_raw_fd(orig_fd);
         assert_eq!(dup2(file.as_raw_fd(), 1), 1);
+        res
     }
 }
 
 #[cfg(windows)]
-fn switch_stdout_to(file: File) {
+fn switch_stdout_to(file: OwnedHandle) -> OwnedHandle {
     use std::os::windows::prelude::*;
 
     extern "system" {
+        fn GetStdHandle(nStdHandle: u32) -> *mut u8;
         fn SetStdHandle(nStdHandle: u32, handle: *mut u8) -> i32;
     }
 
     const STD_OUTPUT_HANDLE: u32 = (-11i32) as u32;
+    const INVALID_HANDLE_VALUE: *mut u8 = !0 as *mut u8;
 
     unsafe {
+        let orig_hdl = GetStdHandle(STD_OUTPUT_HANDLE);
+        assert!(!orig_hdl.is_null() && orig_hdl != INVALID_HANDLE_VALUE);
         let rc = SetStdHandle(STD_OUTPUT_HANDLE, file.into_raw_handle() as *mut _);
         assert!(rc != 0);
+        OwnedHandle::from_raw_handle(orig_hdl as _)
     }
 }
 
@@ -43,10 +59,12 @@ fn switch_stdout() {
     let mut stdout = std::io::stdout();
     stdout.write(b"foo\n").unwrap();
     stdout.flush().unwrap();
-    switch_stdout_to(f);
+    let orig_hdl = switch_stdout_to(f.into());
     stdout.write(b"bar\n").unwrap();
     stdout.flush().unwrap();
 
+    switch_stdout_to(orig_hdl);
+
     let mut contents = String::new();
     File::open(&path).unwrap().read_to_string(&mut contents).unwrap();
     assert_eq!(contents, "bar\n");