Auto merge of #40009 - clarcharr:box_to_buf, r=alexcrichton

Leftovers from #39594; From<Box> impls

These are a few more impls that follow the same reasoning as those from #39594.

What's included:
* `From<Box<str>> for String`
* `From<Box<[T]>> for Vec<T>`
* `From<Box<CStr>> for CString`
* `From<Box<OsStr>> for OsString`
* `From<Box<Path>> for PathBuf`
* `Into<Box<str>> for String`
* `Into<Box<[T]>> for Vec<T>`
* `Into<Box<CStr>> for CString`
* `Into<Box<OsStr>> for OsString`
* `Into<Box<Path>> for PathBuf`
* `<Box<CStr>>::into_c_string`
* `<Box<OsStr>>::into_os_string`
* `<Box<Path>>::into_path_buf`
* Tracking issue for latter three methods + three from previous PR.

Currently, the opposite direction isn't doable with `From` (only `Into`) because of the separation between `liballoc` and `libcollections`. I'm holding off on those for a later PR.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libstd/sys/windows/os_str.rs b/src/libstd/sys/windows/os_str.rs
index b02b06e1ef2..f401e7b35c8 100644
--- a/src/libstd/sys/windows/os_str.rs
+++ b/src/libstd/sys/windows/os_str.rs
@@ -97,6 +97,12 @@ impl Buf {
     pub fn into_box(self) -> Box<Slice> {
         unsafe { mem::transmute(self.inner.into_box()) }
     }
+
+    #[inline]
+    pub fn from_box(boxed: Box<Slice>) -> Buf {
+        let inner: Box<Wtf8> = unsafe { mem::transmute(boxed) };
+        Buf { inner: Wtf8Buf::from_box(inner) }
+    }
 }
 
 impl Slice {