Add methods for handing allocated strings back to C

src/libstd/ffi/c_str.rs

@@ -200,6 +200,28 @@ impl CString {
         CString { inner: v }
     }
 
+    /// Retakes ownership from a Rust string that has been returned to C.
+    pub unsafe fn from_raw_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::from_vec_unchecked(vec)
+    }
+
+    /// Transfers ownership from Rust to C. Be warned that the string
+    /// must be returned to Rust and reconstituted via `from_raw_ptr`
+    /// to be properly deallocated.
+    pub fn into_raw_ptr(self) -> *const libc::c_char {
+        // Resize the vector to fit - we need the capacity to be
+        // determinable from the string length, and shrinking to fit
+        // is the only way to be sure.
+        let mut vec = self.inner;
+        vec.shrink_to_fit();
+        let ptr = vec.as_ptr() as *const libc::c_char;
+        mem::forget(vec);
+        ptr
+    }
+
     /// Returns the contents of this `CString` as a slice of bytes.
     ///
     /// The returned slice does **not** contain the trailing nul separator and