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_common/wtf8.rs b/src/libstd/sys_common/wtf8.rs
index b486d4ffda3..79aaf34ce2e 100644
--- a/src/libstd/sys_common/wtf8.rs
+++ b/src/libstd/sys_common/wtf8.rs
@@ -351,6 +351,12 @@ impl Wtf8Buf {
     pub fn into_box(self) -> Box<Wtf8> {
         unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
     }
+
+    /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
+    pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
+        let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
+        Wtf8Buf { bytes: bytes.into_vec() }
+    }
 }
 
 /// Create a new WTF-8 string from an iterator of code points.