Add methods for handing CStrings back and forth to C

Author: shepmaster

Diff for: src/libstd/ffi/c_str.rs

@@ -11,7 +11,7 @@
 #![unstable(feature = "std_misc")]
 
 use borrow::{Cow, ToOwned};
-use boxed::Box;
+use boxed::{self, Box};
 use clone::Clone;
 use convert::{Into, From};
 use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
@@ -202,6 +202,34 @@ impl CString {
         CString { inner: v.into_boxed_slice() }
     }
 
+    /// Retakes ownership of a CString that was transferred to C.
+    ///
+    /// The only appropriate argument is a pointer obtained by calling
+    /// `into_ptr`.
+    #[unstable(feature = "c_str_memory", reason = "recently added")]
+    pub unsafe fn from_ptr(ptr: *const libc::c_char) -> CString {
+        let len = libc::strlen(ptr);
+        let len_with_nul = len as usize + 1;
+        let vec = Vec::from_raw_parts(ptr as *mut u8, len_with_nul, len_with_nul);
+        CString { inner: vec.into_boxed_slice() }
+    }
+
+    /// Transfers ownership of the string to a C caller.
+    ///
+    /// The pointer must be returned to Rust and reconstituted using
+    /// `from_ptr` to be properly deallocated. Specifically, one
+    /// should *not* use the standard C `free` function to deallocate
+    /// this string.
+    ///
+    /// Failure to call `from_ptr` will lead to a memory leak.
+    #[unstable(feature = "c_str_memory", reason = "recently added")]
+    pub fn into_ptr(self) -> *const libc::c_char {
+        // It is important that the bytes be sized to fit - we need
+        // the capacity to be determinable from the string length, and
+        // shrinking to fit is the only way to be sure.
+        boxed::into_raw(self.inner) as *const libc::c_char
+    }
+
     /// Returns the contents of this `CString` as a slice of bytes.
     ///
     /// The returned slice does **not** contain the trailing nul separator and