Stabilize `std::convert` and related code

* Marks `#[stable]` the contents of the `std::convert` module.

* Added methods `PathBuf::as_path`, `OsString::as_os_str`,
  `String::as_str`, `Vec::{as_slice, as_mut_slice}`.

* Deprecates `OsStr::from_str` in favor of a new, stable, and more
  general `OsStr::new`.

* Adds unstable methods `OsString::from_bytes` and `OsStr::{to_bytes,
  to_cstring}` for ergonomic FFI usage.

[breaking-change]

3 files changed, 4 insertions, 11 deletions

diff --git a/src/libstd/sys/unix/ext.rs b/src/libstd/sys/unix/ext.rs
index 0805949d560..d197d04c0a4 100644
--- a/src/libstd/sys/unix/ext.rs
+++ b/src/libstd/sys/unix/ext.rs
@@ -138,7 +138,7 @@ pub mod io {
 /// Unix-specific extension to the primitives in the `std::ffi` module
 #[stable(feature = "rust1", since = "1.0.0")]
 pub mod ffi {
-    use ffi::{CString, NulError, OsStr, OsString};
+    use ffi::{OsStr, OsString};
     use mem;
     use prelude::v1::*;
     use sys::os_str::Buf;
@@ -175,10 +175,6 @@ pub mod ffi {
         /// Get the underlying byte view of the `OsStr` slice.
         #[stable(feature = "rust1", since = "1.0.0")]
         fn as_bytes(&self) -> &[u8];
-
-        /// Convert the `OsStr` slice into a `CString`.
-        #[stable(feature = "rust1", since = "1.0.0")]
-        fn to_cstring(&self) -> Result<CString, NulError>;
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -189,9 +185,6 @@ pub mod ffi {
         fn as_bytes(&self) -> &[u8] {
             &self.as_inner().inner
         }
-        fn to_cstring(&self) -> Result<CString, NulError> {
-            CString::new(self.as_bytes())
-        }
     }
 }
 
diff --git a/src/libstd/sys/unix/fs2.rs b/src/libstd/sys/unix/fs2.rs
index 202e5ddaec4..c7bfb8604bc 100644
--- a/src/libstd/sys/unix/fs2.rs
+++ b/src/libstd/sys/unix/fs2.rs
@@ -276,8 +276,8 @@ impl File {
 }
 
 fn cstr(path: &Path) -> io::Result<CString> {
-    let cstring = try!(path.as_os_str().to_cstring());
-    Ok(cstring)
+    path.as_os_str().to_cstring().ok_or(
+        io::Error::new(io::ErrorKind::InvalidInput, "path contained a null", None))
 }
 
 pub fn mkdir(p: &Path) -> io::Result<()> {
diff --git a/src/libstd/sys/unix/process2.rs b/src/libstd/sys/unix/process2.rs
index 20c409154b8..c2a8b26aef4 100644
--- a/src/libstd/sys/unix/process2.rs
+++ b/src/libstd/sys/unix/process2.rs
@@ -54,7 +54,7 @@ impl Command {
         self.args.push(arg.to_cstring().unwrap())
     }
     pub fn args<'a, I: Iterator<Item = &'a OsStr>>(&mut self, args: I) {
-        self.args.extend(args.map(|s| OsStrExt::to_cstring(s).unwrap()))
+        self.args.extend(args.map(|s| s.to_cstring().unwrap()))
     }
     fn init_env_map(&mut self) {
         if self.env.is_none() {