mv std libs to library/

3 files changed, 3333 insertions, 0 deletions

diff --git a/library/std/src/ffi/c_str.rs b/library/std/src/ffi/c_str.rs
new file mode 100644
index 00000000000..da25a0ede72
--- /dev/null
+++ b/library/std/src/ffi/c_str.rs
@@ -0,0 +1,1788 @@
+use crate::ascii;
+use crate::borrow::{Borrow, Cow};
+use crate::cmp::Ordering;
+use crate::error::Error;
+use crate::fmt::{self, Write};
+use crate::io;
+use crate::mem;
+use crate::memchr;
+use crate::num::NonZeroU8;
+use crate::ops;
+use crate::os::raw::c_char;
+use crate::ptr;
+use crate::rc::Rc;
+use crate::slice;
+use crate::str::{self, Utf8Error};
+use crate::sync::Arc;
+use crate::sys;
+
+/// A type representing an owned, C-compatible, nul-terminated string with no nul bytes in the
+/// middle.
+///
+/// This type serves the purpose of being able to safely generate a
+/// C-compatible string from a Rust byte slice or vector. An instance of this
+/// type is a static guarantee that the underlying bytes contain no interior 0
+/// bytes ("nul characters") and that the final byte is 0 ("nul terminator").
+///
+/// `CString` is to [`&CStr`] as [`String`] is to [`&str`]: the former
+/// in each pair are owned strings; the latter are borrowed
+/// references.
+///
+/// # Creating a `CString`
+///
+/// A `CString` is created from either a byte slice or a byte vector,
+/// or anything that implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>` (for
+/// example, you can build a `CString` straight out of a [`String`] or
+/// a [`&str`], since both implement that trait).
+///
+/// The [`new`] method will actually check that the provided `&[u8]`
+/// does not have 0 bytes in the middle, and return an error if it
+/// finds one.
+///
+/// # Extracting a raw pointer to the whole C string
+///
+/// `CString` implements a [`as_ptr`] method through the [`Deref`]
+/// trait. This method will give you a `*const c_char` which you can
+/// feed directly to extern functions that expect a nul-terminated
+/// string, like C's `strdup()`. Notice that [`as_ptr`] returns a
+/// read-only pointer; if the C code writes to it, that causes
+/// undefined behavior.
+///
+/// # Extracting a slice of the whole C string
+///
+/// Alternatively, you can obtain a `&[`[`u8`]`]` slice from a
+/// `CString` with the [`as_bytes`] method. Slices produced in this
+/// way do *not* contain the trailing nul terminator. This is useful
+/// when you will be calling an extern function that takes a `*const
+/// u8` argument which is not necessarily nul-terminated, plus another
+/// argument with the length of the string — like C's `strndup()`.
+/// You can of course get the slice's length with its
+/// [`len`][slice.len] method.
+///
+/// If you need a `&[`[`u8`]`]` slice *with* the nul terminator, you
+/// can use [`as_bytes_with_nul`] instead.
+///
+/// Once you have the kind of slice you need (with or without a nul
+/// terminator), you can call the slice's own
+/// [`as_ptr`][slice.as_ptr] method to get a read-only raw pointer to pass to
+/// extern functions. See the documentation for that function for a
+/// discussion on ensuring the lifetime of the raw pointer.
+///
+/// [`Into`]: ../convert/trait.Into.html
+/// [`Vec`]: ../vec/struct.Vec.html
+/// [`String`]: ../string/struct.String.html
+/// [`&str`]: ../primitive.str.html
+/// [`u8`]: ../primitive.u8.html
+/// [`new`]: #method.new
+/// [`as_bytes`]: #method.as_bytes
+/// [`as_bytes_with_nul`]: #method.as_bytes_with_nul
+/// [`as_ptr`]: #method.as_ptr
+/// [slice.as_ptr]: ../primitive.slice.html#method.as_ptr
+/// [slice.len]: ../primitive.slice.html#method.len
+/// [`Deref`]: ../ops/trait.Deref.html
+/// [`CStr`]: struct.CStr.html
+/// [`&CStr`]: struct.CStr.html
+///
+/// # Examples
+///
+/// ```ignore (extern-declaration)
+/// # fn main() {
+/// use std::ffi::CString;
+/// use std::os::raw::c_char;
+///
+/// extern {
+///     fn my_printer(s: *const c_char);
+/// }
+///
+/// // We are certain that our string doesn't have 0 bytes in the middle,
+/// // so we can .expect()
+/// let c_to_print = CString::new("Hello, world!").expect("CString::new failed");
+/// unsafe {
+///     my_printer(c_to_print.as_ptr());
+/// }
+/// # }
+/// ```
+///
+/// # Safety
+///
+/// `CString` is intended for working with traditional C-style strings
+/// (a sequence of non-nul bytes terminated by a single nul byte); the
+/// primary use case for these kinds of strings is interoperating with C-like
+/// code. Often you will need to transfer ownership to/from that external
+/// code. It is strongly recommended that you thoroughly read through the
+/// documentation of `CString` before use, as improper ownership management
+/// of `CString` instances can lead to invalid memory accesses, memory leaks,
+/// and other memory errors.
+
+#[derive(PartialEq, PartialOrd, Eq, Ord, Hash, Clone)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct CString {
+    // Invariant 1: the slice ends with a zero byte and has a length of at least one.
+    // Invariant 2: the slice contains only one zero byte.
+    // Improper usage of unsafe function can break Invariant 2, but not Invariant 1.
+    inner: Box<[u8]>,
+}
+
+/// Representation of a borrowed C string.
+///
+/// This type represents a borrowed reference to a nul-terminated
+/// array of bytes. It can be constructed safely from a `&[`[`u8`]`]`
+/// slice, or unsafely from a raw `*const c_char`. It can then be
+/// converted to a Rust [`&str`] by performing UTF-8 validation, or
+/// into an owned [`CString`].
+///
+/// `&CStr` is to [`CString`] as [`&str`] is to [`String`]: the former
+/// in each pair are borrowed references; the latter are owned
+/// strings.
+///
+/// Note that this structure is **not** `repr(C)` and is not recommended to be
+/// placed in the signatures of FFI functions. Instead, safe wrappers of FFI
+/// functions may leverage the unsafe [`from_ptr`] constructor to provide a safe
+/// interface to other consumers.
+///
+/// # Examples
+///
+/// Inspecting a foreign C string:
+///
+/// ```ignore (extern-declaration)
+/// use std::ffi::CStr;
+/// use std::os::raw::c_char;
+///
+/// extern { fn my_string() -> *const c_char; }
+///
+/// unsafe {
+///     let slice = CStr::from_ptr(my_string());
+///     println!("string buffer size without nul terminator: {}", slice.to_bytes().len());
+/// }
+/// ```
+///
+/// Passing a Rust-originating C string:
+///
+/// ```ignore (extern-declaration)
+/// use std::ffi::{CString, CStr};
+/// use std::os::raw::c_char;
+///
+/// fn work(data: &CStr) {
+///     extern { fn work_with(data: *const c_char); }
+///
+///     unsafe { work_with(data.as_ptr()) }
+/// }
+///
+/// let s = CString::new("data data data data").expect("CString::new failed");
+/// work(&s);
+/// ```
+///
+/// Converting a foreign C string into a Rust [`String`]:
+///
+/// ```ignore (extern-declaration)
+/// use std::ffi::CStr;
+/// use std::os::raw::c_char;
+///
+/// extern { fn my_string() -> *const c_char; }
+///
+/// fn my_string_safe() -> String {
+///     unsafe {
+///         CStr::from_ptr(my_string()).to_string_lossy().into_owned()
+///     }
+/// }
+///
+/// println!("string: {}", my_string_safe());
+/// ```
+///
+/// [`u8`]: ../primitive.u8.html
+/// [`&str`]: ../primitive.str.html
+/// [`String`]: ../string/struct.String.html
+/// [`CString`]: struct.CString.html
+/// [`from_ptr`]: #method.from_ptr
+#[derive(Hash)]
+#[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies
+// on `CStr` being layout-compatible with `[u8]`.
+// When attribute privacy is implemented, `CStr` should be annotated as `#[repr(transparent)]`.
+// Anyway, `CStr` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
+pub struct CStr {
+    // FIXME: this should not be represented with a DST slice but rather with
+    //        just a raw `c_char` along with some form of marker to make
+    //        this an unsized type. Essentially `sizeof(&CStr)` should be the
+    //        same as `sizeof(&c_char)` but `CStr` should be an unsized type.
+    inner: [c_char],
+}
+
+/// An error indicating that an interior nul byte was found.
+///
+/// While Rust strings may contain nul bytes in the middle, C strings
+/// can't, as that byte would effectively truncate the string.
+///
+/// This error is created by the [`new`][`CString::new`] method on
+/// [`CString`]. See its documentation for more.
+///
+/// [`CString`]: struct.CString.html
+/// [`CString::new`]: struct.CString.html#method.new
+///
+/// # Examples
+///
+/// ```
+/// use std::ffi::{CString, NulError};
+///
+/// let _: NulError = CString::new(b"f\0oo".to_vec()).unwrap_err();
+/// ```
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct NulError(usize, Vec<u8>);
+
+/// An error indicating that a nul byte was not in the expected position.
+///
+/// The slice used to create a [`CStr`] must have one and only one nul byte,
+/// positioned at the end.
+///
+/// This error is created by the [`from_bytes_with_nul`] method on [`CStr`].
+/// See its documentation for more.
+///
+/// [`CStr`]: struct.CStr.html
+/// [`from_bytes_with_nul`]: struct.CStr.html#method.from_bytes_with_nul
+///
+/// # Examples
+///
+/// ```
+/// use std::ffi::{CStr, FromBytesWithNulError};
+///
+/// let _: FromBytesWithNulError = CStr::from_bytes_with_nul(b"f\0oo").unwrap_err();
+/// ```
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[stable(feature = "cstr_from_bytes", since = "1.10.0")]
+pub struct FromBytesWithNulError {
+    kind: FromBytesWithNulErrorKind,
+}
+
+/// An error indicating that a nul byte was not in the expected position.
+///
+/// The vector used to create a [`CString`] must have one and only one nul byte,
+/// positioned at the end.
+///
+/// This error is created by the [`from_vec_with_nul`] method on [`CString`].
+/// See its documentation for more.
+///
+/// [`CString`]: struct.CString.html
+/// [`from_vec_with_nul`]: struct.CString.html#method.from_vec_with_nul
+///
+/// # Examples
+///
+/// ```
+/// #![feature(cstring_from_vec_with_nul)]
+/// use std::ffi::{CString, FromVecWithNulError};
+///
+/// let _: FromVecWithNulError = CString::from_vec_with_nul(b"f\0oo".to_vec()).unwrap_err();
+/// ```
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+pub struct FromVecWithNulError {
+    error_kind: FromBytesWithNulErrorKind,
+    bytes: Vec<u8>,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug)]
+enum FromBytesWithNulErrorKind {
+    InteriorNul(usize),
+    NotNulTerminated,
+}
+
+impl FromBytesWithNulError {
+    fn interior_nul(pos: usize) -> FromBytesWithNulError {
+        FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
+    }
+    fn not_nul_terminated() -> FromBytesWithNulError {
+        FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
+    }
+}
+
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl FromVecWithNulError {
+    /// Returns a slice of [`u8`]s bytes that were attempted to convert to a [`CString`].
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    ///
+    /// // Some invalid bytes in a vector
+    /// let bytes = b"f\0oo".to_vec();
+    ///
+    /// let value = CString::from_vec_with_nul(bytes.clone());
+    ///
+    /// assert_eq!(&bytes[..], value.unwrap_err().as_bytes());
+    /// ```
+    ///
+    /// [`CString`]: struct.CString.html
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes[..]
+    }
+
+    /// Returns the bytes that were attempted to convert to a [`CString`].
+    ///
+    /// This method is carefully constructed to avoid allocation. It will
+    /// consume the error, moving out the bytes, so that a copy of the bytes
+    /// does not need to be made.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    ///
+    /// // Some invalid bytes in a vector
+    /// let bytes = b"f\0oo".to_vec();
+    ///
+    /// let value = CString::from_vec_with_nul(bytes.clone());
+    ///
+    /// assert_eq!(bytes, value.unwrap_err().into_bytes());
+    /// ```
+    ///
+    /// [`CString`]: struct.CString.html
+    pub fn into_bytes(self) -> Vec<u8> {
+        self.bytes
+    }
+}
+
+/// An error indicating invalid UTF-8 when converting a [`CString`] into a [`String`].
+///
+/// `CString` is just a wrapper over a buffer of bytes with a nul
+/// terminator; [`into_string`][`CString::into_string`] performs UTF-8
+/// validation on those bytes and may return this error.
+///
+/// This `struct` is created by the
+/// [`into_string`][`CString::into_string`] method on [`CString`]. See
+/// its documentation for more.
+///
+/// [`String`]: ../string/struct.String.html
+/// [`CString`]: struct.CString.html
+/// [`CString::into_string`]: struct.CString.html#method.into_string
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[stable(feature = "cstring_into", since = "1.7.0")]
+pub struct IntoStringError {
+    inner: CString,
+    error: Utf8Error,
+}
+
+impl CString {
+    /// Creates a new C-compatible string from a container of bytes.
+    ///
+    /// This function will consume the provided data and use the
+    /// underlying bytes to construct a new string, ensuring that
+    /// there is a trailing 0 byte. This trailing 0 byte will be
+    /// appended by this function; the provided data should *not*
+    /// contain any 0 bytes in it.
+    ///
+    /// # Examples
+    ///
+    /// ```ignore (extern-declaration)
+    /// use std::ffi::CString;
+    /// use std::os::raw::c_char;
+    ///
+    /// extern { fn puts(s: *const c_char); }
+    ///
+    /// let to_print = CString::new("Hello!").expect("CString::new failed");
+    /// unsafe {
+    ///     puts(to_print.as_ptr());
+    /// }
+    /// ```
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if the supplied bytes contain an
+    /// internal 0 byte. The [`NulError`] returned will contain the bytes as well as
+    /// the position of the nul byte.
+    ///
+    /// [`NulError`]: struct.NulError.html
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new<T: Into<Vec<u8>>>(t: T) -> Result<CString, NulError> {
+        trait SpecIntoVec {
+            fn into_vec(self) -> Vec<u8>;
+        }
+        impl<T: Into<Vec<u8>>> SpecIntoVec for T {
+            default fn into_vec(self) -> Vec<u8> {
+                self.into()
+            }
+        }
+        // Specialization for avoiding reallocation.
+        impl SpecIntoVec for &'_ [u8] {
+            fn into_vec(self) -> Vec<u8> {
+                let mut v = Vec::with_capacity(self.len() + 1);
+                v.extend(self);
+                v
+            }
+        }
+        impl SpecIntoVec for &'_ str {
+            fn into_vec(self) -> Vec<u8> {
+                let mut v = Vec::with_capacity(self.len() + 1);
+                v.extend(self.as_bytes());
+                v
+            }
+        }
+
+        Self::_new(SpecIntoVec::into_vec(t))
+    }
+
+    fn _new(bytes: Vec<u8>) -> Result<CString, NulError> {
+        match memchr::memchr(0, &bytes) {
+            Some(i) => Err(NulError(i, bytes)),
+            None => Ok(unsafe { CString::from_vec_unchecked(bytes) }),
+        }
+    }
+
+    /// Creates a C-compatible string by consuming a byte vector,
+    /// without checking for interior 0 bytes.
+    ///
+    /// This method is equivalent to [`new`] except that no runtime assertion
+    /// is made that `v` contains no 0 bytes, and it requires an actual
+    /// byte vector, not anything that can be converted to one with Into.
+    ///
+    /// [`new`]: #method.new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let raw = b"foo".to_vec();
+    /// unsafe {
+    ///     let c_string = CString::from_vec_unchecked(raw);
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub unsafe fn from_vec_unchecked(mut v: Vec<u8>) -> CString {
+        v.reserve_exact(1);
+        v.push(0);
+        CString { inner: v.into_boxed_slice() }
+    }
+
+    /// Retakes ownership of a `CString` that was transferred to C via [`into_raw`].
+    ///
+    /// Additionally, the length of the string will be recalculated from the pointer.
+    ///
+    /// # Safety
+    ///
+    /// This should only ever be called with a pointer that was earlier
+    /// obtained by calling [`into_raw`] on a `CString`. Other usage (e.g., trying to take
+    /// ownership of a string that was allocated by foreign code) is likely to lead
+    /// to undefined behavior or allocator corruption.
+    ///
+    /// It should be noted that the length isn't just "recomputed," but that
+    /// the recomputed length must match the original length from the
+    /// [`into_raw`] call. This means the [`into_raw`]/`from_raw` methods
+    /// should not be used when passing the string to C functions that can
+    /// modify the string's length.
+    ///
+    /// > **Note:** If you need to borrow a string that was allocated by
+    /// > foreign code, use [`CStr`]. If you need to take ownership of
+    /// > a string that was allocated by foreign code, you will need to
+    /// > make your own provisions for freeing it appropriately, likely
+    /// > with the foreign code's API to do that.
+    ///
+    /// [`into_raw`]: #method.into_raw
+    /// [`CStr`]: struct.CStr.html
+    ///
+    /// # Examples
+    ///
+    /// Creates a `CString`, pass ownership to an `extern` function (via raw pointer), then retake
+    /// ownership with `from_raw`:
+    ///
+    /// ```ignore (extern-declaration)
+    /// use std::ffi::CString;
+    /// use std::os::raw::c_char;
+    ///
+    /// extern {
+    ///     fn some_extern_function(s: *mut c_char);
+    /// }
+    ///
+    /// let c_string = CString::new("Hello!").expect("CString::new failed");
+    /// let raw = c_string.into_raw();
+    /// unsafe {
+    ///     some_extern_function(raw);
+    ///     let c_string = CString::from_raw(raw);
+    /// }
+    /// ```
+    #[stable(feature = "cstr_memory", since = "1.4.0")]
+    pub unsafe fn from_raw(ptr: *mut c_char) -> CString {
+        let len = sys::strlen(ptr) + 1; // Including the NUL byte
+        let slice = slice::from_raw_parts_mut(ptr, len as usize);
+        CString { inner: Box::from_raw(slice as *mut [c_char] as *mut [u8]) }
+    }
+
+    /// Consumes the `CString` and transfers ownership of the string to a C caller.
+    ///
+    /// The pointer which this function returns must be returned to Rust and reconstituted using
+    /// [`from_raw`] to be properly deallocated. Specifically, one
+    /// should *not* use the standard C `free()` function to deallocate
+    /// this string.
+    ///
+    /// Failure to call [`from_raw`] will lead to a memory leak.
+    ///
+    /// The C side must **not** modify the length of the string (by writing a
+    /// `NULL` somewhere inside the string or removing the final one) before
+    /// it makes it back into Rust using [`from_raw`]. See the safety section
+    /// in [`from_raw`].
+    ///
+    /// [`from_raw`]: #method.from_raw
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new("foo").expect("CString::new failed");
+    ///
+    /// let ptr = c_string.into_raw();
+    ///
+    /// unsafe {
+    ///     assert_eq!(b'f', *ptr as u8);
+    ///     assert_eq!(b'o', *ptr.offset(1) as u8);
+    ///     assert_eq!(b'o', *ptr.offset(2) as u8);
+    ///     assert_eq!(b'\0', *ptr.offset(3) as u8);
+    ///
+    ///     // retake pointer to free memory
+    ///     let _ = CString::from_raw(ptr);
+    /// }
+    /// ```
+    #[inline]
+    #[stable(feature = "cstr_memory", since = "1.4.0")]
+    pub fn into_raw(self) -> *mut c_char {
+        Box::into_raw(self.into_inner()) as *mut c_char
+    }
+
+    /// Converts the `CString` into a [`String`] if it contains valid UTF-8 data.
+    ///
+    /// On failure, ownership of the original `CString` is returned.
+    ///
+    /// [`String`]: ../string/struct.String.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let valid_utf8 = vec![b'f', b'o', b'o'];
+    /// let cstring = CString::new(valid_utf8).expect("CString::new failed");
+    /// assert_eq!(cstring.into_string().expect("into_string() call failed"), "foo");
+    ///
+    /// let invalid_utf8 = vec![b'f', 0xff, b'o', b'o'];
+    /// let cstring = CString::new(invalid_utf8).expect("CString::new failed");
+    /// let err = cstring.into_string().err().expect("into_string().err() failed");
+    /// assert_eq!(err.utf8_error().valid_up_to(), 1);
+    /// ```
+
+    #[stable(feature = "cstring_into", since = "1.7.0")]
+    pub fn into_string(self) -> Result<String, IntoStringError> {
+        String::from_utf8(self.into_bytes()).map_err(|e| IntoStringError {
+            error: e.utf8_error(),
+            inner: unsafe { CString::from_vec_unchecked(e.into_bytes()) },
+        })
+    }
+
+    /// Consumes the `CString` and returns the underlying byte buffer.
+    ///
+    /// The returned buffer does **not** contain the trailing nul
+    /// terminator, and it is guaranteed to not have any interior nul
+    /// bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new("foo").expect("CString::new failed");
+    /// let bytes = c_string.into_bytes();
+    /// assert_eq!(bytes, vec![b'f', b'o', b'o']);
+    /// ```
+    #[stable(feature = "cstring_into", since = "1.7.0")]
+    pub fn into_bytes(self) -> Vec<u8> {
+        let mut vec = self.into_inner().into_vec();
+        let _nul = vec.pop();
+        debug_assert_eq!(_nul, Some(0u8));
+        vec
+    }
+
+    /// Equivalent to the [`into_bytes`] function except that the returned vector
+    /// includes the trailing nul terminator.
+    ///
+    /// [`into_bytes`]: #method.into_bytes
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new("foo").expect("CString::new failed");
+    /// let bytes = c_string.into_bytes_with_nul();
+    /// assert_eq!(bytes, vec![b'f', b'o', b'o', b'\0']);
+    /// ```
+    #[stable(feature = "cstring_into", since = "1.7.0")]
+    pub fn into_bytes_with_nul(self) -> Vec<u8> {
+        self.into_inner().into_vec()
+    }
+
+    /// Returns the contents of this `CString` as a slice of bytes.
+    ///
+    /// The returned slice does **not** contain the trailing nul
+    /// terminator, and it is guaranteed to not have any interior nul
+    /// bytes. If you need the nul terminator, use
+    /// [`as_bytes_with_nul`] instead.
+    ///
+    /// [`as_bytes_with_nul`]: #method.as_bytes_with_nul
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new("foo").expect("CString::new failed");
+    /// let bytes = c_string.as_bytes();
+    /// assert_eq!(bytes, &[b'f', b'o', b'o']);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.inner[..self.inner.len() - 1]
+    }
+
+    /// Equivalent to the [`as_bytes`] function except that the returned slice
+    /// includes the trailing nul terminator.
+    ///
+    /// [`as_bytes`]: #method.as_bytes
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new("foo").expect("CString::new failed");
+    /// let bytes = c_string.as_bytes_with_nul();
+    /// assert_eq!(bytes, &[b'f', b'o', b'o', b'\0']);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn as_bytes_with_nul(&self) -> &[u8] {
+        &self.inner
+    }
+
+    /// Extracts a [`CStr`] slice containing the entire string.
+    ///
+    /// [`CStr`]: struct.CStr.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{CString, CStr};
+    ///
+    /// let c_string = CString::new(b"foo".to_vec()).expect("CString::new failed");
+    /// let cstr = c_string.as_c_str();
+    /// assert_eq!(cstr,
+    ///            CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed"));
+    /// ```
+    #[inline]
+    #[stable(feature = "as_c_str", since = "1.20.0")]
+    pub fn as_c_str(&self) -> &CStr {
+        &*self
+    }
+
+    /// Converts this `CString` into a boxed [`CStr`].
+    ///
+    /// [`CStr`]: struct.CStr.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{CString, CStr};
+    ///
+    /// let c_string = CString::new(b"foo".to_vec()).expect("CString::new failed");
+    /// let boxed = c_string.into_boxed_c_str();
+    /// assert_eq!(&*boxed,
+    ///            CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed"));
+    /// ```
+    #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
+    pub fn into_boxed_c_str(self) -> Box<CStr> {
+        unsafe { Box::from_raw(Box::into_raw(self.into_inner()) as *mut CStr) }
+    }
+
+    /// Bypass "move out of struct which implements [`Drop`] trait" restriction.
+    ///
+    /// [`Drop`]: ../ops/trait.Drop.html
+    fn into_inner(self) -> Box<[u8]> {
+        // Rationale: `mem::forget(self)` invalidates the previous call to `ptr::read(&self.inner)`
+        // so we use `ManuallyDrop` to ensure `self` is not dropped.
+        // Then we can return the box directly without invalidating it.
+        // See https://github.com/rust-lang/rust/issues/62553.
+        let this = mem::ManuallyDrop::new(self);
+        unsafe { ptr::read(&this.inner) }
+    }
+
+    /// Converts a `Vec` of `u8` to a `CString` without checking the invariants
+    /// on the given `Vec`.
+    ///
+    /// # Safety
+    ///
+    /// The given `Vec` **must** have one nul byte as its last element.
+    /// This means it cannot be empty nor have any other nul byte anywhere else.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    /// assert_eq!(
+    ///     unsafe { CString::from_vec_with_nul_unchecked(b"abc\0".to_vec()) },
+    ///     unsafe { CString::from_vec_unchecked(b"abc".to_vec()) }
+    /// );
+    /// ```
+    #[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+    pub unsafe fn from_vec_with_nul_unchecked(v: Vec<u8>) -> Self {
+        Self { inner: v.into_boxed_slice() }
+    }
+
+    /// Attempts to converts a `Vec` of `u8` to a `CString`.
+    ///
+    /// Runtime checks are present to ensure there is only one nul byte in the
+    /// `Vec`, its last element.
+    ///
+    /// # Errors
+    ///
+    /// If a nul byte is present and not the last element or no nul bytes
+    /// is present, an error will be returned.
+    ///
+    /// # Examples
+    ///
+    /// A successful conversion will produce the same result as [`new`] when
+    /// called without the ending nul byte.
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    /// assert_eq!(
+    ///     CString::from_vec_with_nul(b"abc\0".to_vec())
+    ///         .expect("CString::from_vec_with_nul failed"),
+    ///     CString::new(b"abc".to_vec()).expect("CString::new failed")
+    /// );
+    /// ```
+    ///
+    /// A incorrectly formatted vector will produce an error.
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::{CString, FromVecWithNulError};
+    /// // Interior nul byte
+    /// let _: FromVecWithNulError = CString::from_vec_with_nul(b"a\0bc".to_vec()).unwrap_err();
+    /// // No nul byte
+    /// let _: FromVecWithNulError = CString::from_vec_with_nul(b"abc".to_vec()).unwrap_err();
+    /// ```
+    ///
+    /// [`new`]: #method.new
+    #[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+    pub fn from_vec_with_nul(v: Vec<u8>) -> Result<Self, FromVecWithNulError> {
+        let nul_pos = memchr::memchr(0, &v);
+        match nul_pos {
+            Some(nul_pos) if nul_pos + 1 == v.len() => {
+                // SAFETY: We know there is only one nul byte, at the end
+                // of the vec.
+                Ok(unsafe { Self::from_vec_with_nul_unchecked(v) })
+            }
+            Some(nul_pos) => Err(FromVecWithNulError {
+                error_kind: FromBytesWithNulErrorKind::InteriorNul(nul_pos),
+                bytes: v,
+            }),
+            None => Err(FromVecWithNulError {
+                error_kind: FromBytesWithNulErrorKind::NotNulTerminated,
+                bytes: v,
+            }),
+        }
+    }
+}
+
+// Turns this `CString` into an empty string to prevent
+// memory-unsafe code from working by accident. Inline
+// to prevent LLVM from optimizing it away in debug builds.
+#[stable(feature = "cstring_drop", since = "1.13.0")]
+impl Drop for CString {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe {
+            *self.inner.get_unchecked_mut(0) = 0;
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl ops::Deref for CString {
+    type Target = CStr;
+
+    #[inline]
+    fn deref(&self) -> &CStr {
+        unsafe { CStr::from_bytes_with_nul_unchecked(self.as_bytes_with_nul()) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for CString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
+#[stable(feature = "cstring_into", since = "1.7.0")]
+impl From<CString> for Vec<u8> {
+    /// Converts a [`CString`] into a [`Vec`]`<u8>`.
+    ///
+    /// The conversion consumes the [`CString`], and removes the terminating NUL byte.
+    ///
+    /// [`Vec`]: ../vec/struct.Vec.html
+    /// [`CString`]: ../ffi/struct.CString.html
+    #[inline]
+    fn from(s: CString) -> Vec<u8> {
+        s.into_bytes()
+    }
+}
+
+#[stable(feature = "cstr_debug", since = "1.3.0")]
+impl fmt::Debug for CStr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "\"")?;
+        for byte in self.to_bytes().iter().flat_map(|&b| ascii::escape_default(b)) {
+            f.write_char(byte as char)?;
+        }
+        write!(f, "\"")
+    }
+}
+
+#[stable(feature = "cstr_default", since = "1.10.0")]
+impl Default for &CStr {
+    fn default() -> Self {
+        const SLICE: &[c_char] = &[0];
+        unsafe { CStr::from_ptr(SLICE.as_ptr()) }
+    }
+}
+
+#[stable(feature = "cstr_default", since = "1.10.0")]
+impl Default for CString {
+    /// Creates an empty `CString`.
+    fn default() -> CString {
+        let a: &CStr = Default::default();
+        a.to_owned()
+    }
+}
+
+#[stable(feature = "cstr_borrow", since = "1.3.0")]
+impl Borrow<CStr> for CString {
+    #[inline]
+    fn borrow(&self) -> &CStr {
+        self
+    }
+}
+
+#[stable(feature = "cstring_from_cow_cstr", since = "1.28.0")]
+impl<'a> From<Cow<'a, CStr>> for CString {
+    #[inline]
+    fn from(s: Cow<'a, CStr>) -> Self {
+        s.into_owned()
+    }
+}
+
+#[stable(feature = "box_from_c_str", since = "1.17.0")]
+impl From<&CStr> for Box<CStr> {
+    fn from(s: &CStr) -> Box<CStr> {
+        let boxed: Box<[u8]> = Box::from(s.to_bytes_with_nul());
+        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
+    }
+}
+
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, CStr>> for Box<CStr> {
+    #[inline]
+    fn from(cow: Cow<'_, CStr>) -> Box<CStr> {
+        match cow {
+            Cow::Borrowed(s) => Box::from(s),
+            Cow::Owned(s) => Box::from(s),
+        }
+    }
+}
+
+#[stable(feature = "c_string_from_box", since = "1.18.0")]
+impl From<Box<CStr>> for CString {
+    /// Converts a [`Box`]`<CStr>` into a [`CString`] without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`CString`]: ../ffi/struct.CString.html
+    #[inline]
+    fn from(s: Box<CStr>) -> CString {
+        s.into_c_string()
+    }
+}
+
+#[stable(feature = "cstring_from_vec_of_nonzerou8", since = "1.43.0")]
+impl From<Vec<NonZeroU8>> for CString {
+    /// Converts a [`Vec`]`<`[`NonZeroU8`]`>` into a [`CString`] without
+    /// copying nor checking for inner null bytes.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`NonZeroU8`]: ../num/struct.NonZeroU8.html
+    /// [`Vec`]: ../vec/struct.Vec.html
+    #[inline]
+    fn from(v: Vec<NonZeroU8>) -> CString {
+        unsafe {
+            // Transmute `Vec<NonZeroU8>` to `Vec<u8>`.
+            let v: Vec<u8> = {
+                // Safety:
+                //   - transmuting between `NonZeroU8` and `u8` is sound;
+                //   - `alloc::Layout<NonZeroU8> == alloc::Layout<u8>`.
+                let (ptr, len, cap): (*mut NonZeroU8, _, _) = Vec::into_raw_parts(v);
+                Vec::from_raw_parts(ptr.cast::<u8>(), len, cap)
+            };
+            // Safety: `v` cannot contain null bytes, given the type-level
+            // invariant of `NonZeroU8`.
+            CString::from_vec_unchecked(v)
+        }
+    }
+}
+
+#[stable(feature = "more_box_slice_clone", since = "1.29.0")]
+impl Clone for Box<CStr> {
+    #[inline]
+    fn clone(&self) -> Self {
+        (**self).into()
+    }
+}
+
+#[stable(feature = "box_from_c_string", since = "1.20.0")]
+impl From<CString> for Box<CStr> {
+    /// Converts a [`CString`] into a [`Box`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Box`]: ../boxed/struct.Box.html
+    #[inline]
+    fn from(s: CString) -> Box<CStr> {
+        s.into_boxed_c_str()
+    }
+}
+
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<CString> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: CString) -> Cow<'a, CStr> {
+        Cow::Owned(s)
+    }
+}
+
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<&'a CStr> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: &'a CStr) -> Cow<'a, CStr> {
+        Cow::Borrowed(s)
+    }
+}
+
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<&'a CString> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: &'a CString) -> Cow<'a, CStr> {
+        Cow::Borrowed(s.as_c_str())
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<CString> for Arc<CStr> {
+    /// Converts a [`CString`] into a [`Arc`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Arc`]: ../sync/struct.Arc.html
+    #[inline]
+    fn from(s: CString) -> Arc<CStr> {
+        let arc: Arc<[u8]> = Arc::from(s.into_inner());
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const CStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<&CStr> for Arc<CStr> {
+    #[inline]
+    fn from(s: &CStr) -> Arc<CStr> {
+        let arc: Arc<[u8]> = Arc::from(s.to_bytes_with_nul());
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const CStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<CString> for Rc<CStr> {
+    /// Converts a [`CString`] into a [`Rc`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Rc`]: ../rc/struct.Rc.html
+    #[inline]
+    fn from(s: CString) -> Rc<CStr> {
+        let rc: Rc<[u8]> = Rc::from(s.into_inner());
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const CStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<&CStr> for Rc<CStr> {
+    #[inline]
+    fn from(s: &CStr) -> Rc<CStr> {
+        let rc: Rc<[u8]> = Rc::from(s.to_bytes_with_nul());
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const CStr) }
+    }
+}
+
+#[stable(feature = "default_box_extra", since = "1.17.0")]
+impl Default for Box<CStr> {
+    fn default() -> Box<CStr> {
+        let boxed: Box<[u8]> = Box::from([0]);
+        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
+    }
+}
+
+impl NulError {
+    /// Returns the position of the nul byte in the slice that caused
+    /// [`CString::new`] to fail.
+    ///
+    /// [`CString::new`]: struct.CString.html#method.new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let nul_error = CString::new("foo\0bar").unwrap_err();
+    /// assert_eq!(nul_error.nul_position(), 3);
+    ///
+    /// let nul_error = CString::new("foo bar\0").unwrap_err();
+    /// assert_eq!(nul_error.nul_position(), 7);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn nul_position(&self) -> usize {
+        self.0
+    }
+
+    /// Consumes this error, returning the underlying vector of bytes which
+    /// generated the error in the first place.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let nul_error = CString::new("foo\0bar").unwrap_err();
+    /// assert_eq!(nul_error.into_vec(), b"foo\0bar");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn into_vec(self) -> Vec<u8> {
+        self.1
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Error for NulError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "nul byte found in data"
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Display for NulError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "nul byte found in provided data at position: {}", self.0)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<NulError> for io::Error {
+    /// Converts a [`NulError`] into a [`io::Error`].
+    ///
+    /// [`NulError`]: ../ffi/struct.NulError.html
+    /// [`io::Error`]: ../io/struct.Error.html
+    fn from(_: NulError) -> io::Error {
+        io::Error::new(io::ErrorKind::InvalidInput, "data provided contains a nul byte")
+    }
+}
+
+#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
+impl Error for FromBytesWithNulError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        match self.kind {
+            FromBytesWithNulErrorKind::InteriorNul(..) => {
+                "data provided contains an interior nul byte"
+            }
+            FromBytesWithNulErrorKind::NotNulTerminated => "data provided is not nul terminated",
+        }
+    }
+}
+
+#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
+impl fmt::Display for FromBytesWithNulError {
+    #[allow(deprecated, deprecated_in_future)]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.description())?;
+        if let FromBytesWithNulErrorKind::InteriorNul(pos) = self.kind {
+            write!(f, " at byte pos {}", pos)?;
+        }
+        Ok(())
+    }
+}
+
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl Error for FromVecWithNulError {}
+
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl fmt::Display for FromVecWithNulError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.error_kind {
+            FromBytesWithNulErrorKind::InteriorNul(pos) => {
+                write!(f, "data provided contains an interior nul byte at pos {}", pos)
+            }
+            FromBytesWithNulErrorKind::NotNulTerminated => {
+                write!(f, "data provided is not nul terminated")
+            }
+        }
+    }
+}
+
+impl IntoStringError {
+    /// Consumes this error, returning original [`CString`] which generated the
+    /// error.
+    ///
+    /// [`CString`]: struct.CString.html
+    #[stable(feature = "cstring_into", since = "1.7.0")]
+    pub fn into_cstring(self) -> CString {
+        self.inner
+    }
+
+    /// Access the underlying UTF-8 error that was the cause of this error.
+    #[stable(feature = "cstring_into", since = "1.7.0")]
+    pub fn utf8_error(&self) -> Utf8Error {
+        self.error
+    }
+}
+
+#[stable(feature = "cstring_into", since = "1.7.0")]
+impl Error for IntoStringError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "C string contained non-utf8 bytes"
+    }
+
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        Some(&self.error)
+    }
+}
+
+#[stable(feature = "cstring_into", since = "1.7.0")]
+impl fmt::Display for IntoStringError {
+    #[allow(deprecated, deprecated_in_future)]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.description().fmt(f)
+    }
+}
+
+impl CStr {
+    /// Wraps a raw C string with a safe C string wrapper.
+    ///
+    /// This function will wrap the provided `ptr` with a `CStr` wrapper, which
+    /// allows inspection and interoperation of non-owned C strings. The total
+    /// size of the raw C string must be smaller than `isize::MAX` **bytes**
+    /// in memory due to calling the `slice::from_raw_parts` function.
+    /// This method is unsafe for a number of reasons:
+    ///
+    /// * There is no guarantee to the validity of `ptr`.
+    /// * The returned lifetime is not guaranteed to be the actual lifetime of
+    ///   `ptr`.
+    /// * There is no guarantee that the memory pointed to by `ptr` contains a
+    ///   valid nul terminator byte at the end of the string.
+    /// * It is not guaranteed that the memory pointed by `ptr` won't change
+    ///   before the `CStr` has been destroyed.
+    ///
+    /// > **Note**: This operation is intended to be a 0-cost cast but it is
+    /// > currently implemented with an up-front calculation of the length of
+    /// > the string. This is not guaranteed to always be the case.
+    ///
+    /// # Examples
+    ///
+    /// ```ignore (extern-declaration)
+    /// # fn main() {
+    /// use std::ffi::CStr;
+    /// use std::os::raw::c_char;
+    ///
+    /// extern {
+    ///     fn my_string() -> *const c_char;
+    /// }
+    ///
+    /// unsafe {
+    ///     let slice = CStr::from_ptr(my_string());
+    ///     println!("string returned: {}", slice.to_str().unwrap());
+    /// }
+    /// # }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
+        let len = sys::strlen(ptr);
+        let ptr = ptr as *const u8;
+        CStr::from_bytes_with_nul_unchecked(slice::from_raw_parts(ptr, len as usize + 1))
+    }
+
+    /// Creates a C string wrapper from a byte slice.
+    ///
+    /// This function will cast the provided `bytes` to a `CStr`
+    /// wrapper after ensuring that the byte slice is nul-terminated
+    /// and does not contain any interior nul bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"hello\0");
+    /// assert!(cstr.is_ok());
+    /// ```
+    ///
+    /// Creating a `CStr` without a trailing nul terminator is an error:
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"hello");
+    /// assert!(cstr.is_err());
+    /// ```
+    ///
+    /// Creating a `CStr` with an interior nul byte is an error:
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"he\0llo\0");
+    /// assert!(cstr.is_err());
+    /// ```
+    #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
+    pub fn from_bytes_with_nul(bytes: &[u8]) -> Result<&CStr, FromBytesWithNulError> {
+        let nul_pos = memchr::memchr(0, bytes);
+        if let Some(nul_pos) = nul_pos {
+            if nul_pos + 1 != bytes.len() {
+                return Err(FromBytesWithNulError::interior_nul(nul_pos));
+            }
+            Ok(unsafe { CStr::from_bytes_with_nul_unchecked(bytes) })
+        } else {
+            Err(FromBytesWithNulError::not_nul_terminated())
+        }
+    }
+
+    /// Unsafely creates a C string wrapper from a byte slice.
+    ///
+    /// This function will cast the provided `bytes` to a `CStr` wrapper without
+    /// performing any sanity checks. The provided slice **must** be nul-terminated
+    /// and not contain any interior nul bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{CStr, CString};
+    ///
+    /// unsafe {
+    ///     let cstring = CString::new("hello").expect("CString::new failed");
+    ///     let cstr = CStr::from_bytes_with_nul_unchecked(cstring.to_bytes_with_nul());
+    ///     assert_eq!(cstr, &*cstring);
+    /// }
+    /// ```
+    #[inline]
+    #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
+    #[rustc_const_unstable(feature = "const_cstr_unchecked", issue = "none")]
+    pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
+        &*(bytes as *const [u8] as *const CStr)
+    }
+
+    /// Returns the inner pointer to this C string.
+    ///
+    /// The returned pointer will be valid for as long as `self` is, and points
+    /// to a contiguous region of memory terminated with a 0 byte to represent
+    /// the end of the string.
+    ///
+    /// **WARNING**
+    ///
+    /// The returned pointer is read-only; writing to it (including passing it
+    /// to C code that writes to it) causes undefined behavior.
+    ///
+    /// It is your responsibility to make sure that the underlying memory is not
+    /// freed too early. For example, the following code will cause undefined
+    /// behavior when `ptr` is used inside the `unsafe` block:
+    ///
+    /// ```no_run
+    /// # #![allow(unused_must_use)]
+    /// use std::ffi::CString;
+    ///
+    /// let ptr = CString::new("Hello").expect("CString::new failed").as_ptr();
+    /// unsafe {
+    ///     // `ptr` is dangling
+    ///     *ptr;
+    /// }
+    /// ```
+    ///
+    /// This happens because the pointer returned by `as_ptr` does not carry any
+    /// lifetime information and the [`CString`] is deallocated immediately after
+    /// the `CString::new("Hello").expect("CString::new failed").as_ptr()` expression is evaluated.
+    /// To fix the problem, bind the `CString` to a local variable:
+    ///
+    /// ```no_run
+    /// # #![allow(unused_must_use)]
+    /// use std::ffi::CString;
+    ///
+    /// let hello = CString::new("Hello").expect("CString::new failed");
+    /// let ptr = hello.as_ptr();
+    /// unsafe {
+    ///     // `ptr` is valid because `hello` is in scope
+    ///     *ptr;
+    /// }
+    /// ```
+    ///
+    /// This way, the lifetime of the `CString` in `hello` encompasses
+    /// the lifetime of `ptr` and the `unsafe` block.
+    ///
+    /// [`CString`]: struct.CString.html
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_str_as_ptr", since = "1.32.0")]
+    pub const fn as_ptr(&self) -> *const c_char {
+        self.inner.as_ptr()
+    }
+
+    /// Converts this C string to a byte slice.
+    ///
+    /// The returned slice will **not** contain the trailing nul terminator that this C
+    /// string has.
+    ///
+    /// > **Note**: This method is currently implemented as a constant-time
+    /// > cast, but it is planned to alter its definition in the future to
+    /// > perform the length calculation whenever this method is called.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    /// assert_eq!(cstr.to_bytes(), b"foo");
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_bytes(&self) -> &[u8] {
+        let bytes = self.to_bytes_with_nul();
+        &bytes[..bytes.len() - 1]
+    }
+
+    /// Converts this C string to a byte slice containing the trailing 0 byte.
+    ///
+    /// This function is the equivalent of [`to_bytes`] except that it will retain
+    /// the trailing nul terminator instead of chopping it off.
+    ///
+    /// > **Note**: This method is currently implemented as a 0-cost cast, but
+    /// > it is planned to alter its definition in the future to perform the
+    /// > length calculation whenever this method is called.
+    ///
+    /// [`to_bytes`]: #method.to_bytes
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    /// assert_eq!(cstr.to_bytes_with_nul(), b"foo\0");
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_bytes_with_nul(&self) -> &[u8] {
+        unsafe { &*(&self.inner as *const [c_char] as *const [u8]) }
+    }
+
+    /// Yields a [`&str`] slice if the `CStr` contains valid UTF-8.
+    ///
+    /// If the contents of the `CStr` are valid UTF-8 data, this
+    /// function will return the corresponding [`&str`] slice. Otherwise,
+    /// it will return an error with details of where UTF-8 validation failed.
+    ///
+    /// [`&str`]: ../primitive.str.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed");
+    /// assert_eq!(cstr.to_str(), Ok("foo"));
+    /// ```
+    #[stable(feature = "cstr_to_str", since = "1.4.0")]
+    pub fn to_str(&self) -> Result<&str, str::Utf8Error> {
+        // N.B., when `CStr` is changed to perform the length check in `.to_bytes()`
+        // instead of in `from_ptr()`, it may be worth considering if this should
+        // be rewritten to do the UTF-8 check inline with the length calculation
+        // instead of doing it afterwards.
+        str::from_utf8(self.to_bytes())
+    }
+
+    /// Converts a `CStr` into a [`Cow`]`<`[`str`]`>`.
+    ///
+    /// If the contents of the `CStr` are valid UTF-8 data, this
+    /// function will return a [`Cow`]`::`[`Borrowed`]`(`[`&str`]`)`
+    /// with the corresponding [`&str`] slice. Otherwise, it will
+    /// replace any invalid UTF-8 sequences with
+    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD] and return a
+    /// [`Cow`]`::`[`Owned`]`(`[`String`]`)` with the result.
+    ///
+    /// [`Cow`]: ../borrow/enum.Cow.html
+    /// [`Borrowed`]: ../borrow/enum.Cow.html#variant.Borrowed
+    /// [`Owned`]: ../borrow/enum.Cow.html#variant.Owned
+    /// [`str`]: ../primitive.str.html
+    /// [`String`]: ../string/struct.String.html
+    /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
+    ///
+    /// # Examples
+    ///
+    /// Calling `to_string_lossy` on a `CStr` containing valid UTF-8:
+    ///
+    /// ```
+    /// use std::borrow::Cow;
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"Hello World\0")
+    ///                  .expect("CStr::from_bytes_with_nul failed");
+    /// assert_eq!(cstr.to_string_lossy(), Cow::Borrowed("Hello World"));
+    /// ```
+    ///
+    /// Calling `to_string_lossy` on a `CStr` containing invalid UTF-8:
+    ///
+    /// ```
+    /// use std::borrow::Cow;
+    /// use std::ffi::CStr;
+    ///
+    /// let cstr = CStr::from_bytes_with_nul(b"Hello \xF0\x90\x80World\0")
+    ///                  .expect("CStr::from_bytes_with_nul failed");
+    /// assert_eq!(
+    ///     cstr.to_string_lossy(),
+    ///     Cow::Owned(String::from("Hello �World")) as Cow<'_, str>
+    /// );
+    /// ```
+    #[stable(feature = "cstr_to_str", since = "1.4.0")]
+    pub fn to_string_lossy(&self) -> Cow<'_, str> {
+        String::from_utf8_lossy(self.to_bytes())
+    }
+
+    /// Converts a [`Box`]`<CStr>` into a [`CString`] without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`CString`]: struct.CString.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::CString;
+    ///
+    /// let c_string = CString::new(b"foo".to_vec()).expect("CString::new failed");
+    /// let boxed = c_string.into_boxed_c_str();
+    /// assert_eq!(boxed.into_c_string(), CString::new("foo").expect("CString::new failed"));
+    /// ```
+    #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
+    pub fn into_c_string(self: Box<CStr>) -> CString {
+        let raw = Box::into_raw(self) as *mut [u8];
+        CString { inner: unsafe { Box::from_raw(raw) } }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq for CStr {
+    fn eq(&self, other: &CStr) -> bool {
+        self.to_bytes().eq(other.to_bytes())
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Eq for CStr {}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd for CStr {
+    fn partial_cmp(&self, other: &CStr) -> Option<Ordering> {
+        self.to_bytes().partial_cmp(&other.to_bytes())
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Ord for CStr {
+    fn cmp(&self, other: &CStr) -> Ordering {
+        self.to_bytes().cmp(&other.to_bytes())
+    }
+}
+
+#[stable(feature = "cstr_borrow", since = "1.3.0")]
+impl ToOwned for CStr {
+    type Owned = CString;
+
+    fn to_owned(&self) -> CString {
+        CString { inner: self.to_bytes_with_nul().into() }
+    }
+
+    fn clone_into(&self, target: &mut CString) {
+        let mut b = Vec::from(mem::take(&mut target.inner));
+        self.to_bytes_with_nul().clone_into(&mut b);
+        target.inner = b.into_boxed_slice();
+    }
+}
+
+#[stable(feature = "cstring_asref", since = "1.7.0")]
+impl From<&CStr> for CString {
+    fn from(s: &CStr) -> CString {
+        s.to_owned()
+    }
+}
+
+#[stable(feature = "cstring_asref", since = "1.7.0")]
+impl ops::Index<ops::RangeFull> for CString {
+    type Output = CStr;
+
+    #[inline]
+    fn index(&self, _index: ops::RangeFull) -> &CStr {
+        self
+    }
+}
+
+#[stable(feature = "cstr_range_from", since = "1.47.0")]
+impl ops::Index<ops::RangeFrom<usize>> for CStr {
+    type Output = CStr;
+
+    fn index(&self, index: ops::RangeFrom<usize>) -> &CStr {
+        let bytes = self.to_bytes_with_nul();
+        // we need to manually check the starting index to account for the null
+        // byte, since otherwise we could get an empty string that doesn't end
+        // in a null.
+        if index.start < bytes.len() {
+            unsafe { CStr::from_bytes_with_nul_unchecked(&bytes[index.start..]) }
+        } else {
+            panic!(
+                "index out of bounds: the len is {} but the index is {}",
+                bytes.len(),
+                index.start
+            );
+        }
+    }
+}
+
+#[stable(feature = "cstring_asref", since = "1.7.0")]
+impl AsRef<CStr> for CStr {
+    #[inline]
+    fn as_ref(&self) -> &CStr {
+        self
+    }
+}
+
+#[stable(feature = "cstring_asref", since = "1.7.0")]
+impl AsRef<CStr> for CString {
+    #[inline]
+    fn as_ref(&self) -> &CStr {
+        self
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::borrow::Cow::{Borrowed, Owned};
+    use crate::collections::hash_map::DefaultHasher;
+    use crate::hash::{Hash, Hasher};
+    use crate::os::raw::c_char;
+    use crate::rc::Rc;
+    use crate::sync::Arc;
+
+    #[test]
+    fn c_to_rust() {
+        let data = b"123\0";
+        let ptr = data.as_ptr() as *const c_char;
+        unsafe {
+            assert_eq!(CStr::from_ptr(ptr).to_bytes(), b"123");
+            assert_eq!(CStr::from_ptr(ptr).to_bytes_with_nul(), b"123\0");
+        }
+    }
+
+    #[test]
+    fn simple() {
+        let s = CString::new("1234").unwrap();
+        assert_eq!(s.as_bytes(), b"1234");
+        assert_eq!(s.as_bytes_with_nul(), b"1234\0");
+    }
+
+    #[test]
+    fn build_with_zero1() {
+        assert!(CString::new(&b"\0"[..]).is_err());
+    }
+    #[test]
+    fn build_with_zero2() {
+        assert!(CString::new(vec![0]).is_err());
+    }
+
+    #[test]
+    fn build_with_zero3() {
+        unsafe {
+            let s = CString::from_vec_unchecked(vec![0]);
+            assert_eq!(s.as_bytes(), b"\0");
+        }
+    }
+
+    #[test]
+    fn formatted() {
+        let s = CString::new(&b"abc\x01\x02\n\xE2\x80\xA6\xFF"[..]).unwrap();
+        assert_eq!(format!("{:?}", s), r#""abc\x01\x02\n\xe2\x80\xa6\xff""#);
+    }
+
+    #[test]
+    fn borrowed() {
+        unsafe {
+            let s = CStr::from_ptr(b"12\0".as_ptr() as *const _);
+            assert_eq!(s.to_bytes(), b"12");
+            assert_eq!(s.to_bytes_with_nul(), b"12\0");
+        }
+    }
+
+    #[test]
+    fn to_str() {
+        let data = b"123\xE2\x80\xA6\0";
+        let ptr = data.as_ptr() as *const c_char;
+        unsafe {
+            assert_eq!(CStr::from_ptr(ptr).to_str(), Ok("123…"));
+            assert_eq!(CStr::from_ptr(ptr).to_string_lossy(), Borrowed("123…"));
+        }
+        let data = b"123\xE2\0";
+        let ptr = data.as_ptr() as *const c_char;
+        unsafe {
+            assert!(CStr::from_ptr(ptr).to_str().is_err());
+            assert_eq!(CStr::from_ptr(ptr).to_string_lossy(), Owned::<str>(format!("123\u{FFFD}")));
+        }
+    }
+
+    #[test]
+    fn to_owned() {
+        let data = b"123\0";
+        let ptr = data.as_ptr() as *const c_char;
+
+        let owned = unsafe { CStr::from_ptr(ptr).to_owned() };
+        assert_eq!(owned.as_bytes_with_nul(), data);
+    }
+
+    #[test]
+    fn equal_hash() {
+        let data = b"123\xE2\xFA\xA6\0";
+        let ptr = data.as_ptr() as *const c_char;
+        let cstr: &'static CStr = unsafe { CStr::from_ptr(ptr) };
+
+        let mut s = DefaultHasher::new();
+        cstr.hash(&mut s);
+        let cstr_hash = s.finish();
+        let mut s = DefaultHasher::new();
+        CString::new(&data[..data.len() - 1]).unwrap().hash(&mut s);
+        let cstring_hash = s.finish();
+
+        assert_eq!(cstr_hash, cstring_hash);
+    }
+
+    #[test]
+    fn from_bytes_with_nul() {
+        let data = b"123\0";
+        let cstr = CStr::from_bytes_with_nul(data);
+        assert_eq!(cstr.map(CStr::to_bytes), Ok(&b"123"[..]));
+        let cstr = CStr::from_bytes_with_nul(data);
+        assert_eq!(cstr.map(CStr::to_bytes_with_nul), Ok(&b"123\0"[..]));
+
+        unsafe {
+            let cstr = CStr::from_bytes_with_nul(data);
+            let cstr_unchecked = CStr::from_bytes_with_nul_unchecked(data);
+            assert_eq!(cstr, Ok(cstr_unchecked));
+        }
+    }
+
+    #[test]
+    fn from_bytes_with_nul_unterminated() {
+        let data = b"123";
+        let cstr = CStr::from_bytes_with_nul(data);
+        assert!(cstr.is_err());
+    }
+
+    #[test]
+    fn from_bytes_with_nul_interior() {
+        let data = b"1\023\0";
+        let cstr = CStr::from_bytes_with_nul(data);
+        assert!(cstr.is_err());
+    }
+
+    #[test]
+    fn into_boxed() {
+        let orig: &[u8] = b"Hello, world!\0";
+        let cstr = CStr::from_bytes_with_nul(orig).unwrap();
+        let boxed: Box<CStr> = Box::from(cstr);
+        let cstring = cstr.to_owned().into_boxed_c_str().into_c_string();
+        assert_eq!(cstr, &*boxed);
+        assert_eq!(&*boxed, &*cstring);
+        assert_eq!(&*cstring, cstr);
+    }
+
+    #[test]
+    fn boxed_default() {
+        let boxed = <Box<CStr>>::default();
+        assert_eq!(boxed.to_bytes_with_nul(), &[0]);
+    }
+
+    #[test]
+    fn test_c_str_clone_into() {
+        let mut c_string = CString::new("lorem").unwrap();
+        let c_ptr = c_string.as_ptr();
+        let c_str = CStr::from_bytes_with_nul(b"ipsum\0").unwrap();
+        c_str.clone_into(&mut c_string);
+        assert_eq!(c_str, c_string.as_c_str());
+        // The exact same size shouldn't have needed to move its allocation
+        assert_eq!(c_ptr, c_string.as_ptr());
+    }
+
+    #[test]
+    fn into_rc() {
+        let orig: &[u8] = b"Hello, world!\0";
+        let cstr = CStr::from_bytes_with_nul(orig).unwrap();
+        let rc: Rc<CStr> = Rc::from(cstr);
+        let arc: Arc<CStr> = Arc::from(cstr);
+
+        assert_eq!(&*rc, cstr);
+        assert_eq!(&*arc, cstr);
+
+        let rc2: Rc<CStr> = Rc::from(cstr.to_owned());
+        let arc2: Arc<CStr> = Arc::from(cstr.to_owned());
+
+        assert_eq!(&*rc2, cstr);
+        assert_eq!(&*arc2, cstr);
+    }
+
+    #[test]
+    fn cstr_const_constructor() {
+        const CSTR: &CStr = unsafe { CStr::from_bytes_with_nul_unchecked(b"Hello, world!\0") };
+
+        assert_eq!(CSTR.to_str().unwrap(), "Hello, world!");
+    }
+
+    #[test]
+    fn cstr_index_from() {
+        let original = b"Hello, world!\0";
+        let cstr = CStr::from_bytes_with_nul(original).unwrap();
+        let result = CStr::from_bytes_with_nul(&original[7..]).unwrap();
+
+        assert_eq!(&cstr[7..], result);
+    }
+
+    #[test]
+    #[should_panic]
+    fn cstr_index_from_empty() {
+        let original = b"Hello, world!\0";
+        let cstr = CStr::from_bytes_with_nul(original).unwrap();
+        let _ = &cstr[original.len()..];
+    }
+}
diff --git a/library/std/src/ffi/mod.rs b/library/std/src/ffi/mod.rs
new file mode 100644
index 00000000000..f442d7fde1a
--- /dev/null
+++ b/library/std/src/ffi/mod.rs
@@ -0,0 +1,180 @@
+//! Utilities related to FFI bindings.
+//!
+//! This module provides utilities to handle data across non-Rust
+//! interfaces, like other programming languages and the underlying
+//! operating system. It is mainly of use for FFI (Foreign Function
+//! Interface) bindings and code that needs to exchange C-like strings
+//! with other languages.
+//!
+//! # Overview
+//!
+//! Rust represents owned strings with the [`String`] type, and
+//! borrowed slices of strings with the [`str`] primitive. Both are
+//! always in UTF-8 encoding, and may contain nul bytes in the middle,
+//! i.e., if you look at the bytes that make up the string, there may
+//! be a `\0` among them. Both `String` and `str` store their length
+//! explicitly; there are no nul terminators at the end of strings
+//! like in C.
+//!
+//! C strings are different from Rust strings:
+//!
+//! * **Encodings** - Rust strings are UTF-8, but C strings may use
+//! other encodings. If you are using a string from C, you should
+//! check its encoding explicitly, rather than just assuming that it
+//! is UTF-8 like you can do in Rust.
+//!
+//! * **Character size** - C strings may use `char` or `wchar_t`-sized
+//! characters; please **note** that C's `char` is different from Rust's.
+//! The C standard leaves the actual sizes of those types open to
+//! interpretation, but defines different APIs for strings made up of
+//! each character type. Rust strings are always UTF-8, so different
+//! Unicode characters will be encoded in a variable number of bytes
+//! each. The Rust type [`char`] represents a '[Unicode scalar
+//! value]', which is similar to, but not the same as, a '[Unicode
+//! code point]'.
+//!
+//! * **Nul terminators and implicit string lengths** - Often, C
+//! strings are nul-terminated, i.e., they have a `\0` character at the
+//! end. The length of a string buffer is not stored, but has to be
+//! calculated; to compute the length of a string, C code must
+//! manually call a function like `strlen()` for `char`-based strings,
+//! or `wcslen()` for `wchar_t`-based ones. Those functions return
+//! the number of characters in the string excluding the nul
+//! terminator, so the buffer length is really `len+1` characters.
+//! Rust strings don't have a nul terminator; their length is always
+//! stored and does not need to be calculated. While in Rust
+//! accessing a string's length is a `O(1)` operation (because the
+//! length is stored); in C it is an `O(length)` operation because the
+//! length needs to be computed by scanning the string for the nul
+//! terminator.
+//!
+//! * **Internal nul characters** - When C strings have a nul
+//! terminator character, this usually means that they cannot have nul
+//! characters in the middle — a nul character would essentially
+//! truncate the string. Rust strings *can* have nul characters in
+//! the middle, because nul does not have to mark the end of the
+//! string in Rust.
+//!
+//! # Representations of non-Rust strings
+//!
+//! [`CString`] and [`CStr`] are useful when you need to transfer
+//! UTF-8 strings to and from languages with a C ABI, like Python.
+//!
+//! * **From Rust to C:** [`CString`] represents an owned, C-friendly
+//! string: it is nul-terminated, and has no internal nul characters.
+//! Rust code can create a [`CString`] out of a normal string (provided
+//! that the string doesn't have nul characters in the middle), and
+//! then use a variety of methods to obtain a raw `*mut `[`u8`] that can
+//! then be passed as an argument to functions which use the C
+//! conventions for strings.
+//!
+//! * **From C to Rust:** [`CStr`] represents a borrowed C string; it
+//! is what you would use to wrap a raw `*const `[`u8`] that you got from
+//! a C function. A [`CStr`] is guaranteed to be a nul-terminated array
+//! of bytes. Once you have a [`CStr`], you can convert it to a Rust
+//! [`&str`][`str`] if it's valid UTF-8, or lossily convert it by adding
+//! replacement characters.
+//!
+//! [`OsString`] and [`OsStr`] are useful when you need to transfer
+//! strings to and from the operating system itself, or when capturing
+//! the output of external commands. Conversions between [`OsString`],
+//! [`OsStr`] and Rust strings work similarly to those for [`CString`]
+//! and [`CStr`].
+//!
+//! * [`OsString`] represents an owned string in whatever
+//! representation the operating system prefers. In the Rust standard
+//! library, various APIs that transfer strings to/from the operating
+//! system use [`OsString`] instead of plain strings. For example,
+//! [`env::var_os()`] is used to query environment variables; it
+//! returns an [`Option`]`<`[`OsString`]`>`. If the environment variable
+//! exists you will get a [`Some`]`(os_string)`, which you can *then* try to
+//! convert to a Rust string. This yields a [`Result<>`], so that
+//! your code can detect errors in case the environment variable did
+//! not in fact contain valid Unicode data.
+//!
+//! * [`OsStr`] represents a borrowed reference to a string in a
+//! format that can be passed to the operating system. It can be
+//! converted into an UTF-8 Rust string slice in a similar way to
+//! [`OsString`].
+//!
+//! # Conversions
+//!
+//! ## On Unix
+//!
+//! On Unix, [`OsStr`] implements the
+//! `std::os::unix::ffi::`[`OsStrExt`][unix.OsStrExt] trait, which
+//! augments it with two methods, [`from_bytes`] and [`as_bytes`].
+//! These do inexpensive conversions from and to UTF-8 byte slices.
+//!
+//! Additionally, on Unix [`OsString`] implements the
+//! `std::os::unix::ffi::`[`OsStringExt`][unix.OsStringExt] trait,
+//! which provides [`from_vec`] and [`into_vec`] methods that consume
+//! their arguments, and take or produce vectors of [`u8`].
+//!
+//! ## On Windows
+//!
+//! On Windows, [`OsStr`] implements the
+//! `std::os::windows::ffi::`[`OsStrExt`][windows.OsStrExt] trait,
+//! which provides an [`encode_wide`] method. This provides an
+//! iterator that can be [`collect`]ed into a vector of [`u16`].
+//!
+//! Additionally, on Windows [`OsString`] implements the
+//! `std::os::windows:ffi::`[`OsStringExt`][windows.OsStringExt]
+//! trait, which provides a [`from_wide`] method. The result of this
+//! method is an [`OsString`] which can be round-tripped to a Windows
+//! string losslessly.
+//!
+//! [`String`]: ../string/struct.String.html
+//! [`str`]: ../primitive.str.html
+//! [`char`]: ../primitive.char.html
+//! [`u8`]: ../primitive.u8.html
+//! [`u16`]: ../primitive.u16.html
+//! [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
+//! [Unicode code point]: http://www.unicode.org/glossary/#code_point
+//! [`CString`]: struct.CString.html
+//! [`CStr`]: struct.CStr.html
+//! [`OsString`]: struct.OsString.html
+//! [`OsStr`]: struct.OsStr.html
+//! [`env::set_var()`]: ../env/fn.set_var.html
+//! [`env::var_os()`]: ../env/fn.var_os.html
+//! [`Result<>`]: ../result/enum.Result.html
+//! [unix.OsStringExt]: ../os/unix/ffi/trait.OsStringExt.html
+//! [`from_vec`]: ../os/unix/ffi/trait.OsStringExt.html#tymethod.from_vec
+//! [`into_vec`]: ../os/unix/ffi/trait.OsStringExt.html#tymethod.into_vec
+//! [unix.OsStrExt]: ../os/unix/ffi/trait.OsStrExt.html
+//! [`from_bytes`]: ../os/unix/ffi/trait.OsStrExt.html#tymethod.from_bytes
+//! [`as_bytes`]: ../os/unix/ffi/trait.OsStrExt.html#tymethod.as_bytes
+//! [`OsStrExt`]: ../os/unix/ffi/trait.OsStrExt.html
+//! [windows.OsStrExt]: ../os/windows/ffi/trait.OsStrExt.html
+//! [`encode_wide`]: ../os/windows/ffi/trait.OsStrExt.html#tymethod.encode_wide
+//! [`collect`]: ../iter/trait.Iterator.html#method.collect
+//! [windows.OsStringExt]: ../os/windows/ffi/trait.OsStringExt.html
+//! [`from_wide`]: ../os/windows/ffi/trait.OsStringExt.html#tymethod.from_wide
+//! [`Option`]: ../option/enum.Option.html
+//! [`Some`]: ../option/enum.Option.html#variant.Some
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[stable(feature = "cstr_from_bytes", since = "1.10.0")]
+pub use self::c_str::FromBytesWithNulError;
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+pub use self::c_str::FromVecWithNulError;
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::c_str::{CStr, CString, IntoStringError, NulError};
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::os_str::{OsStr, OsString};
+
+#[stable(feature = "core_c_void", since = "1.30.0")]
+pub use core::ffi::c_void;
+
+#[unstable(
+    feature = "c_variadic",
+    reason = "the `c_variadic` feature has not been properly tested on \
+              all supported platforms",
+    issue = "44930"
+)]
+pub use core::ffi::{VaList, VaListImpl};
+
+mod c_str;
+mod os_str;
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
new file mode 100644
index 00000000000..d1eaf3c583f
--- /dev/null
+++ b/library/std/src/ffi/os_str.rs
@@ -0,0 +1,1365 @@
+use crate::borrow::{Borrow, Cow};
+use crate::cmp;
+use crate::fmt;
+use crate::hash::{Hash, Hasher};
+use crate::ops;
+use crate::rc::Rc;
+use crate::str::FromStr;
+use crate::sync::Arc;
+
+use crate::sys::os_str::{Buf, Slice};
+use crate::sys_common::{AsInner, FromInner, IntoInner};
+
+/// A type that can represent owned, mutable platform-native strings, but is
+/// cheaply inter-convertible with Rust strings.
+///
+/// The need for this type arises from the fact that:
+///
+/// * On Unix systems, strings are often arbitrary sequences of non-zero
+///   bytes, in many cases interpreted as UTF-8.
+///
+/// * On Windows, strings are often arbitrary sequences of non-zero 16-bit
+///   values, interpreted as UTF-16 when it is valid to do so.
+///
+/// * In Rust, strings are always valid UTF-8, which may contain zeros.
+///
+/// `OsString` and [`OsStr`] bridge this gap by simultaneously representing Rust
+/// and platform-native string values, and in particular allowing a Rust string
+/// to be converted into an "OS" string with no cost if possible. A consequence
+/// of this is that `OsString` instances are *not* `NUL` terminated; in order
+/// to pass to e.g., Unix system call, you should create a [`CStr`].
+///
+/// `OsString` is to [`&OsStr`] as [`String`] is to [`&str`]: the former
+/// in each pair are owned strings; the latter are borrowed
+/// references.
+///
+/// Note, `OsString` and [`OsStr`] internally do not necessarily hold strings in
+/// the form native to the platform; While on Unix, strings are stored as a
+/// sequence of 8-bit values, on Windows, where strings are 16-bit value based
+/// as just discussed, strings are also actually stored as a sequence of 8-bit
+/// values, encoded in a less-strict variant of UTF-8. This is useful to
+/// understand when handling capacity and length values.
+///
+/// # Creating an `OsString`
+///
+/// **From a Rust string**: `OsString` implements
+/// [`From`]`<`[`String`]`>`, so you can use `my_string.from` to
+/// create an `OsString` from a normal Rust string.
+///
+/// **From slices:** Just like you can start with an empty Rust
+/// [`String`] and then [`push_str`][String.push_str] `&str`
+/// sub-string slices into it, you can create an empty `OsString` with
+/// the [`new`] method and then push string slices into it with the
+/// [`push`] method.
+///
+/// # Extracting a borrowed reference to the whole OS string
+///
+/// You can use the [`as_os_str`] method to get an `&`[`OsStr`] from
+/// an `OsString`; this is effectively a borrowed reference to the
+/// whole string.
+///
+/// # Conversions
+///
+/// See the [module's toplevel documentation about conversions][conversions] for a discussion on
+/// the traits which `OsString` implements for [conversions] from/to native representations.
+///
+/// [`OsStr`]: struct.OsStr.html
+/// [`&OsStr`]: struct.OsStr.html
+/// [`CStr`]: struct.CStr.html
+/// [`From`]: ../convert/trait.From.html
+/// [`String`]: ../string/struct.String.html
+/// [`&str`]: ../primitive.str.html
+/// [`u8`]: ../primitive.u8.html
+/// [`u16`]: ../primitive.u16.html
+/// [String.push_str]: ../string/struct.String.html#method.push_str
+/// [`new`]: #method.new
+/// [`push`]: #method.push
+/// [`as_os_str`]: #method.as_os_str
+/// [conversions]: index.html#conversions
+#[derive(Clone)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct OsString {
+    inner: Buf,
+}
+
+/// Borrowed reference to an OS string (see [`OsString`]).
+///
+/// This type represents a borrowed reference to a string in the operating system's preferred
+/// representation.
+///
+/// `&OsStr` is to [`OsString`] as [`&str`] is to [`String`]: the former in each pair are borrowed
+/// references; the latter are owned strings.
+///
+/// See the [module's toplevel documentation about conversions][conversions] for a discussion on
+/// the traits which `OsStr` implements for [conversions] from/to native representations.
+///
+/// [`OsString`]: struct.OsString.html
+/// [`&str`]: ../primitive.str.html
+/// [`String`]: ../string/struct.String.html
+/// [conversions]: index.html#conversions
+#[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `OsStr::from_inner` current implementation relies
+// on `OsStr` being layout-compatible with `Slice`.
+// When attribute privacy is implemented, `OsStr` should be annotated as `#[repr(transparent)]`.
+// Anyway, `OsStr` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
+pub struct OsStr {
+    inner: Slice,
+}
+
+impl OsString {
+    /// Constructs a new empty `OsString`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let os_string = OsString::new();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new() -> OsString {
+        OsString { inner: Buf::from_string(String::new()) }
+    }
+
+    /// Converts to an [`OsStr`] slice.
+    ///
+    /// [`OsStr`]: struct.OsStr.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsString, OsStr};
+    ///
+    /// let os_string = OsString::from("foo");
+    /// let os_str = OsStr::new("foo");
+    /// assert_eq!(os_string.as_os_str(), os_str);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn as_os_str(&self) -> &OsStr {
+        self
+    }
+
+    /// Converts the `OsString` into a [`String`] if it contains valid Unicode data.
+    ///
+    /// On failure, ownership of the original `OsString` is returned.
+    ///
+    /// [`String`]: ../../std/string/struct.String.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let os_string = OsString::from("foo");
+    /// let string = os_string.into_string();
+    /// assert_eq!(string, Ok(String::from("foo")));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn into_string(self) -> Result<String, OsString> {
+        self.inner.into_string().map_err(|buf| OsString { inner: buf })
+    }
+
+    /// Extends the string with the given [`&OsStr`] slice.
+    ///
+    /// [`&OsStr`]: struct.OsStr.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut os_string = OsString::from("foo");
+    /// os_string.push("bar");
+    /// assert_eq!(&os_string, "foobar");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn push<T: AsRef<OsStr>>(&mut self, s: T) {
+        self.inner.push_slice(&s.as_ref().inner)
+    }
+
+    /// Creates a new `OsString` with the given capacity.
+    ///
+    /// The string will be able to hold exactly `capacity` length units of other
+    /// OS strings without reallocating. If `capacity` is 0, the string will not
+    /// allocate.
+    ///
+    /// See main `OsString` documentation information about encoding.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut os_string = OsString::with_capacity(10);
+    /// let capacity = os_string.capacity();
+    ///
+    /// // This push is done without reallocating
+    /// os_string.push("foo");
+    ///
+    /// assert_eq!(capacity, os_string.capacity());
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn with_capacity(capacity: usize) -> OsString {
+        OsString { inner: Buf::with_capacity(capacity) }
+    }
+
+    /// Truncates the `OsString` to zero length.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut os_string = OsString::from("foo");
+    /// assert_eq!(&os_string, "foo");
+    ///
+    /// os_string.clear();
+    /// assert_eq!(&os_string, "");
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn clear(&mut self) {
+        self.inner.clear()
+    }
+
+    /// Returns the capacity this `OsString` can hold without reallocating.
+    ///
+    /// See `OsString` introduction for information about encoding.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let os_string = OsString::with_capacity(10);
+    /// assert!(os_string.capacity() >= 10);
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
+    /// Reserves capacity for at least `additional` more capacity to be inserted
+    /// in the given `OsString`.
+    ///
+    /// The collection may reserve more space to avoid frequent reallocations.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::new();
+    /// s.reserve(10);
+    /// assert!(s.capacity() >= 10);
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn reserve(&mut self, additional: usize) {
+        self.inner.reserve(additional)
+    }
+
+    /// Reserves the minimum capacity for exactly `additional` more capacity to
+    /// be inserted in the given `OsString`. Does nothing if the capacity is
+    /// already sufficient.
+    ///
+    /// Note that the allocator may give the collection more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer reserve if future insertions are expected.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::new();
+    /// s.reserve_exact(10);
+    /// assert!(s.capacity() >= 10);
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.inner.reserve_exact(additional)
+    }
+
+    /// Shrinks the capacity of the `OsString` to match its length.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("foo");
+    ///
+    /// s.reserve(100);
+    /// assert!(s.capacity() >= 100);
+    ///
+    /// s.shrink_to_fit();
+    /// assert_eq!(3, s.capacity());
+    /// ```
+    #[stable(feature = "osstring_shrink_to_fit", since = "1.19.0")]
+    pub fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit()
+    }
+
+    /// Shrinks the capacity of the `OsString` with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// Panics if the current capacity is smaller than the supplied
+    /// minimum capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(shrink_to)]
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("foo");
+    ///
+    /// s.reserve(100);
+    /// assert!(s.capacity() >= 100);
+    ///
+    /// s.shrink_to(10);
+    /// assert!(s.capacity() >= 10);
+    /// s.shrink_to(0);
+    /// assert!(s.capacity() >= 3);
+    /// ```
+    #[inline]
+    #[unstable(feature = "shrink_to", reason = "new API", issue = "56431")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.inner.shrink_to(min_capacity)
+    }
+
+    /// Converts this `OsString` into a boxed [`OsStr`].
+    ///
+    /// [`OsStr`]: struct.OsStr.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsString, OsStr};
+    ///
+    /// let s = OsString::from("hello");
+    ///
+    /// let b: Box<OsStr> = s.into_boxed_os_str();
+    /// ```
+    #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
+    pub fn into_boxed_os_str(self) -> Box<OsStr> {
+        let rw = Box::into_raw(self.inner.into_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<String> for OsString {
+    /// Converts a [`String`] into a [`OsString`].
+    ///
+    /// The conversion copies the data, and includes an allocation on the heap.
+    ///
+    /// [`OsString`]: ../../std/ffi/struct.OsString.html
+    fn from(s: String) -> OsString {
+        OsString { inner: Buf::from_string(s) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + AsRef<OsStr>> From<&T> for OsString {
+    fn from(s: &T) -> OsString {
+        s.as_ref().to_os_string()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl ops::Index<ops::RangeFull> for OsString {
+    type Output = OsStr;
+
+    #[inline]
+    fn index(&self, _index: ops::RangeFull) -> &OsStr {
+        OsStr::from_inner(self.inner.as_slice())
+    }
+}
+
+#[stable(feature = "mut_osstr", since = "1.44.0")]
+impl ops::IndexMut<ops::RangeFull> for OsString {
+    #[inline]
+    fn index_mut(&mut self, _index: ops::RangeFull) -> &mut OsStr {
+        OsStr::from_inner_mut(self.inner.as_mut_slice())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl ops::Deref for OsString {
+    type Target = OsStr;
+
+    #[inline]
+    fn deref(&self) -> &OsStr {
+        &self[..]
+    }
+}
+
+#[stable(feature = "mut_osstr", since = "1.44.0")]
+impl ops::DerefMut for OsString {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut OsStr {
+        &mut self[..]
+    }
+}
+
+#[stable(feature = "osstring_default", since = "1.9.0")]
+impl Default for OsString {
+    /// Constructs an empty `OsString`.
+    #[inline]
+    fn default() -> OsString {
+        OsString::new()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for OsString {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, formatter)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq for OsString {
+    fn eq(&self, other: &OsString) -> bool {
+        &**self == &**other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<str> for OsString {
+    fn eq(&self, other: &str) -> bool {
+        &**self == other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<OsString> for str {
+    fn eq(&self, other: &OsString) -> bool {
+        &**other == self
+    }
+}
+
+#[stable(feature = "os_str_str_ref_eq", since = "1.29.0")]
+impl PartialEq<&str> for OsString {
+    fn eq(&self, other: &&str) -> bool {
+        **self == **other
+    }
+}
+
+#[stable(feature = "os_str_str_ref_eq", since = "1.29.0")]
+impl<'a> PartialEq<OsString> for &'a str {
+    fn eq(&self, other: &OsString) -> bool {
+        **other == **self
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Eq for OsString {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd for OsString {
+    #[inline]
+    fn partial_cmp(&self, other: &OsString) -> Option<cmp::Ordering> {
+        (&**self).partial_cmp(&**other)
+    }
+    #[inline]
+    fn lt(&self, other: &OsString) -> bool {
+        &**self < &**other
+    }
+    #[inline]
+    fn le(&self, other: &OsString) -> bool {
+        &**self <= &**other
+    }
+    #[inline]
+    fn gt(&self, other: &OsString) -> bool {
+        &**self > &**other
+    }
+    #[inline]
+    fn ge(&self, other: &OsString) -> bool {
+        &**self >= &**other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd<str> for OsString {
+    #[inline]
+    fn partial_cmp(&self, other: &str) -> Option<cmp::Ordering> {
+        (&**self).partial_cmp(other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Ord for OsString {
+    #[inline]
+    fn cmp(&self, other: &OsString) -> cmp::Ordering {
+        (&**self).cmp(&**other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Hash for OsString {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        (&**self).hash(state)
+    }
+}
+
+impl OsStr {
+    /// Coerces into an `OsStr` slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &OsStr {
+        s.as_ref()
+    }
+
+    #[inline]
+    fn from_inner(inner: &Slice) -> &OsStr {
+        // Safety: OsStr is just a wrapper of Slice,
+        // therefore converting &Slice to &OsStr is safe.
+        unsafe { &*(inner as *const Slice as *const OsStr) }
+    }
+
+    #[inline]
+    fn from_inner_mut(inner: &mut Slice) -> &mut OsStr {
+        // Safety: OsStr is just a wrapper of Slice,
+        // therefore converting &mut Slice to &mut OsStr is safe.
+        // Any method that mutates OsStr must be careful not to
+        // break platform-specific encoding, in particular Wtf8 on Windows.
+        unsafe { &mut *(inner as *mut Slice as *mut OsStr) }
+    }
+
+    /// Yields a [`&str`] slice if the `OsStr` is valid Unicode.
+    ///
+    /// This conversion may entail doing a check for UTF-8 validity.
+    ///
+    /// [`&str`]: ../../std/primitive.str.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// assert_eq!(os_str.to_str(), Some("foo"));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_str(&self) -> Option<&str> {
+        self.inner.to_str()
+    }
+
+    /// Converts an `OsStr` to a [`Cow`]`<`[`str`]`>`.
+    ///
+    /// Any non-Unicode sequences are replaced with
+    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD].
+    ///
+    /// [`Cow`]: ../../std/borrow/enum.Cow.html
+    /// [`str`]: ../../std/primitive.str.html
+    /// [U+FFFD]: ../../std/char/constant.REPLACEMENT_CHARACTER.html
+    ///
+    /// # Examples
+    ///
+    /// Calling `to_string_lossy` on an `OsStr` with invalid unicode:
+    ///
+    /// ```
+    /// // Note, due to differences in how Unix and Windows represent strings,
+    /// // we are forced to complicate this example, setting up example `OsStr`s
+    /// // with different source data and via different platform extensions.
+    /// // Understand that in reality you could end up with such example invalid
+    /// // sequences simply through collecting user command line arguments, for
+    /// // example.
+    ///
+    /// #[cfg(any(unix, target_os = "redox"))] {
+    ///     use std::ffi::OsStr;
+    ///     use std::os::unix::ffi::OsStrExt;
+    ///
+    ///     // Here, the values 0x66 and 0x6f correspond to 'f' and 'o'
+    ///     // respectively. The value 0x80 is a lone continuation byte, invalid
+    ///     // in a UTF-8 sequence.
+    ///     let source = [0x66, 0x6f, 0x80, 0x6f];
+    ///     let os_str = OsStr::from_bytes(&source[..]);
+    ///
+    ///     assert_eq!(os_str.to_string_lossy(), "fo�o");
+    /// }
+    /// #[cfg(windows)] {
+    ///     use std::ffi::OsString;
+    ///     use std::os::windows::prelude::*;
+    ///
+    ///     // Here the values 0x0066 and 0x006f correspond to 'f' and 'o'
+    ///     // respectively. The value 0xD800 is a lone surrogate half, invalid
+    ///     // in a UTF-16 sequence.
+    ///     let source = [0x0066, 0x006f, 0xD800, 0x006f];
+    ///     let os_string = OsString::from_wide(&source[..]);
+    ///     let os_str = os_string.as_os_str();
+    ///
+    ///     assert_eq!(os_str.to_string_lossy(), "fo�o");
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_string_lossy(&self) -> Cow<'_, str> {
+        self.inner.to_string_lossy()
+    }
+
+    /// Copies the slice into an owned [`OsString`].
+    ///
+    /// [`OsString`]: struct.OsString.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsStr, OsString};
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// let os_string = os_str.to_os_string();
+    /// assert_eq!(os_string, OsString::from("foo"));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_os_string(&self) -> OsString {
+        OsString { inner: self.inner.to_owned() }
+    }
+
+    /// Checks whether the `OsStr` is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("");
+    /// assert!(os_str.is_empty());
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// assert!(!os_str.is_empty());
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.inner.inner.is_empty()
+    }
+
+    /// Returns the length of this `OsStr`.
+    ///
+    /// Note that this does **not** return the number of bytes in the string in
+    /// OS string form.
+    ///
+    /// The length returned is that of the underlying storage used by `OsStr`.
+    /// As discussed in the [`OsString`] introduction, [`OsString`] and `OsStr`
+    /// store strings in a form best suited for cheap inter-conversion between
+    /// native-platform and Rust string forms, which may differ significantly
+    /// from both of them, including in storage size and encoding.
+    ///
+    /// This number is simply useful for passing to other methods, like
+    /// [`OsString::with_capacity`] to avoid reallocations.
+    ///
+    /// [`OsString`]: struct.OsString.html
+    /// [`OsString::with_capacity`]: struct.OsString.html#method.with_capacity
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let os_str = OsStr::new("");
+    /// assert_eq!(os_str.len(), 0);
+    ///
+    /// let os_str = OsStr::new("foo");
+    /// assert_eq!(os_str.len(), 3);
+    /// ```
+    #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    pub fn len(&self) -> usize {
+        self.inner.inner.len()
+    }
+
+    /// Converts a [`Box`]`<OsStr>` into an [`OsString`] without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: struct.OsString.html
+    #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
+    pub fn into_os_string(self: Box<OsStr>) -> OsString {
+        let boxed = unsafe { Box::from_raw(Box::into_raw(self) as *mut Slice) };
+        OsString { inner: Buf::from_box(boxed) }
+    }
+
+    /// Gets the underlying byte representation.
+    ///
+    /// Note: it is *crucial* that this API is private, to avoid
+    /// revealing the internal, platform-specific encodings.
+    #[inline]
+    fn bytes(&self) -> &[u8] {
+        unsafe { &*(&self.inner as *const _ as *const [u8]) }
+    }
+
+    /// Converts this string to its ASCII lower case equivalent in-place.
+    ///
+    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To return a new lowercased value without modifying the existing one, use
+    /// [`to_ascii_lowercase`].
+    ///
+    /// [`to_ascii_lowercase`]: #method.to_ascii_lowercase
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("GRÜßE, JÜRGEN ❤");
+    ///
+    /// s.make_ascii_lowercase();
+    ///
+    /// assert_eq!("grÜße, jÜrgen ❤", s);
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn make_ascii_lowercase(&mut self) {
+        self.inner.make_ascii_lowercase()
+    }
+
+    /// Converts this string to its ASCII upper case equivalent in-place.
+    ///
+    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To return a new uppercased value without modifying the existing one, use
+    /// [`to_ascii_uppercase`].
+    ///
+    /// [`to_ascii_uppercase`]: #method.to_ascii_uppercase
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// s.make_ascii_uppercase();
+    ///
+    /// assert_eq!("GRüßE, JüRGEN ❤", s);
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn make_ascii_uppercase(&mut self) {
+        self.inner.make_ascii_uppercase()
+    }
+
+    /// Returns a copy of this string where each character is mapped to its
+    /// ASCII lower case equivalent.
+    ///
+    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To lowercase the value in-place, use [`make_ascii_lowercase`].
+    ///
+    /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    /// let s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// assert_eq!("grüße, jürgen ❤", s.to_ascii_lowercase());
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn to_ascii_lowercase(&self) -> OsString {
+        OsString::from_inner(self.inner.to_ascii_lowercase())
+    }
+
+    /// Returns a copy of this string where each character is mapped to its
+    /// ASCII upper case equivalent.
+    ///
+    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
+    /// but non-ASCII letters are unchanged.
+    ///
+    /// To uppercase the value in-place, use [`make_ascii_uppercase`].
+    ///
+    /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    /// let s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// assert_eq!("GRüßE, JüRGEN ❤", s.to_ascii_uppercase());
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn to_ascii_uppercase(&self) -> OsString {
+        OsString::from_inner(self.inner.to_ascii_uppercase())
+    }
+
+    /// Checks if all characters in this string are within the ASCII range.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    ///
+    /// let ascii = OsString::from("hello!\n");
+    /// let non_ascii = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// assert!(ascii.is_ascii());
+    /// assert!(!non_ascii.is_ascii());
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn is_ascii(&self) -> bool {
+        self.inner.is_ascii()
+    }
+
+    /// Checks that two strings are an ASCII case-insensitive match.
+    ///
+    /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,
+    /// but without allocating and copying temporaries.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstring_ascii)]
+    /// use std::ffi::OsString;
+    ///
+    /// assert!(OsString::from("Ferris").eq_ignore_ascii_case("FERRIS"));
+    /// assert!(OsString::from("Ferrös").eq_ignore_ascii_case("FERRöS"));
+    /// assert!(!OsString::from("Ferrös").eq_ignore_ascii_case("FERRÖS"));
+    /// ```
+    #[unstable(feature = "osstring_ascii", issue = "70516")]
+    pub fn eq_ignore_ascii_case<S: ?Sized + AsRef<OsStr>>(&self, other: &S) -> bool {
+        self.inner.eq_ignore_ascii_case(&other.as_ref().inner)
+    }
+}
+
+#[stable(feature = "box_from_os_str", since = "1.17.0")]
+impl From<&OsStr> for Box<OsStr> {
+    fn from(s: &OsStr) -> Box<OsStr> {
+        let rw = Box::into_raw(s.inner.into_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
+    }
+}
+
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, OsStr>> for Box<OsStr> {
+    #[inline]
+    fn from(cow: Cow<'_, OsStr>) -> Box<OsStr> {
+        match cow {
+            Cow::Borrowed(s) => Box::from(s),
+            Cow::Owned(s) => Box::from(s),
+        }
+    }
+}
+
+#[stable(feature = "os_string_from_box", since = "1.18.0")]
+impl From<Box<OsStr>> for OsString {
+    /// Converts a [`Box`]`<`[`OsStr`]`>` into a `OsString` without copying or
+    /// allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsStr`]: ../ffi/struct.OsStr.html
+    fn from(boxed: Box<OsStr>) -> OsString {
+        boxed.into_os_string()
+    }
+}
+
+#[stable(feature = "box_from_os_string", since = "1.20.0")]
+impl From<OsString> for Box<OsStr> {
+    /// Converts a [`OsString`] into a [`Box`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
+    fn from(s: OsString) -> Box<OsStr> {
+        s.into_boxed_os_str()
+    }
+}
+
+#[stable(feature = "more_box_slice_clone", since = "1.29.0")]
+impl Clone for Box<OsStr> {
+    #[inline]
+    fn clone(&self) -> Self {
+        self.to_os_string().into_boxed_os_str()
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<OsString> for Arc<OsStr> {
+    /// Converts a [`OsString`] into a [`Arc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Arc`]: ../sync/struct.Arc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
+    #[inline]
+    fn from(s: OsString) -> Arc<OsStr> {
+        let arc = s.inner.into_arc();
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const OsStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<&OsStr> for Arc<OsStr> {
+    #[inline]
+    fn from(s: &OsStr) -> Arc<OsStr> {
+        let arc = s.inner.into_arc();
+        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const OsStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<OsString> for Rc<OsStr> {
+    /// Converts a [`OsString`] into a [`Rc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Rc`]: ../rc/struct.Rc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
+    #[inline]
+    fn from(s: OsString) -> Rc<OsStr> {
+        let rc = s.inner.into_rc();
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const OsStr) }
+    }
+}
+
+#[stable(feature = "shared_from_slice2", since = "1.24.0")]
+impl From<&OsStr> for Rc<OsStr> {
+    #[inline]
+    fn from(s: &OsStr) -> Rc<OsStr> {
+        let rc = s.inner.into_rc();
+        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const OsStr) }
+    }
+}
+
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<OsString> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: OsString) -> Cow<'a, OsStr> {
+        Cow::Owned(s)
+    }
+}
+
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<&'a OsStr> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: &'a OsStr) -> Cow<'a, OsStr> {
+        Cow::Borrowed(s)
+    }
+}
+
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<&'a OsString> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: &'a OsString) -> Cow<'a, OsStr> {
+        Cow::Borrowed(s.as_os_str())
+    }
+}
+
+#[stable(feature = "osstring_from_cow_osstr", since = "1.28.0")]
+impl<'a> From<Cow<'a, OsStr>> for OsString {
+    #[inline]
+    fn from(s: Cow<'a, OsStr>) -> Self {
+        s.into_owned()
+    }
+}
+
+#[stable(feature = "box_default_extra", since = "1.17.0")]
+impl Default for Box<OsStr> {
+    fn default() -> Box<OsStr> {
+        let rw = Box::into_raw(Slice::empty_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
+    }
+}
+
+#[stable(feature = "osstring_default", since = "1.9.0")]
+impl Default for &OsStr {
+    /// Creates an empty `OsStr`.
+    #[inline]
+    fn default() -> Self {
+        OsStr::new("")
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq for OsStr {
+    #[inline]
+    fn eq(&self, other: &OsStr) -> bool {
+        self.bytes().eq(other.bytes())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<str> for OsStr {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        *self == *OsStr::new(other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<OsStr> for str {
+    #[inline]
+    fn eq(&self, other: &OsStr) -> bool {
+        *other == *OsStr::new(self)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Eq for OsStr {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd for OsStr {
+    #[inline]
+    fn partial_cmp(&self, other: &OsStr) -> Option<cmp::Ordering> {
+        self.bytes().partial_cmp(other.bytes())
+    }
+    #[inline]
+    fn lt(&self, other: &OsStr) -> bool {
+        self.bytes().lt(other.bytes())
+    }
+    #[inline]
+    fn le(&self, other: &OsStr) -> bool {
+        self.bytes().le(other.bytes())
+    }
+    #[inline]
+    fn gt(&self, other: &OsStr) -> bool {
+        self.bytes().gt(other.bytes())
+    }
+    #[inline]
+    fn ge(&self, other: &OsStr) -> bool {
+        self.bytes().ge(other.bytes())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialOrd<str> for OsStr {
+    #[inline]
+    fn partial_cmp(&self, other: &str) -> Option<cmp::Ordering> {
+        self.partial_cmp(OsStr::new(other))
+    }
+}
+
+// FIXME (#19470): cannot provide PartialOrd<OsStr> for str until we
+// have more flexible coherence rules.
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Ord for OsStr {
+    #[inline]
+    fn cmp(&self, other: &OsStr) -> cmp::Ordering {
+        self.bytes().cmp(other.bytes())
+    }
+}
+
+macro_rules! impl_cmp {
+    ($lhs:ty, $rhs: ty) => {
+        #[stable(feature = "cmp_os_str", since = "1.8.0")]
+        impl<'a, 'b> PartialEq<$rhs> for $lhs {
+            #[inline]
+            fn eq(&self, other: &$rhs) -> bool {
+                <OsStr as PartialEq>::eq(self, other)
+            }
+        }
+
+        #[stable(feature = "cmp_os_str", since = "1.8.0")]
+        impl<'a, 'b> PartialEq<$lhs> for $rhs {
+            #[inline]
+            fn eq(&self, other: &$lhs) -> bool {
+                <OsStr as PartialEq>::eq(self, other)
+            }
+        }
+
+        #[stable(feature = "cmp_os_str", since = "1.8.0")]
+        impl<'a, 'b> PartialOrd<$rhs> for $lhs {
+            #[inline]
+            fn partial_cmp(&self, other: &$rhs) -> Option<cmp::Ordering> {
+                <OsStr as PartialOrd>::partial_cmp(self, other)
+            }
+        }
+
+        #[stable(feature = "cmp_os_str", since = "1.8.0")]
+        impl<'a, 'b> PartialOrd<$lhs> for $rhs {
+            #[inline]
+            fn partial_cmp(&self, other: &$lhs) -> Option<cmp::Ordering> {
+                <OsStr as PartialOrd>::partial_cmp(self, other)
+            }
+        }
+    };
+}
+
+impl_cmp!(OsString, OsStr);
+impl_cmp!(OsString, &'a OsStr);
+impl_cmp!(Cow<'a, OsStr>, OsStr);
+impl_cmp!(Cow<'a, OsStr>, &'b OsStr);
+impl_cmp!(Cow<'a, OsStr>, OsString);
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Hash for OsStr {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.bytes().hash(state)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for OsStr {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.inner, formatter)
+    }
+}
+
+impl OsStr {
+    pub(crate) fn display(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.inner, formatter)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Borrow<OsStr> for OsString {
+    fn borrow(&self) -> &OsStr {
+        &self[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl ToOwned for OsStr {
+    type Owned = OsString;
+    fn to_owned(&self) -> OsString {
+        self.to_os_string()
+    }
+    fn clone_into(&self, target: &mut OsString) {
+        self.inner.clone_into(&mut target.inner)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl AsRef<OsStr> for OsStr {
+    fn as_ref(&self) -> &OsStr {
+        self
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl AsRef<OsStr> for OsString {
+    #[inline]
+    fn as_ref(&self) -> &OsStr {
+        self
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl AsRef<OsStr> for str {
+    #[inline]
+    fn as_ref(&self) -> &OsStr {
+        OsStr::from_inner(Slice::from_str(self))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl AsRef<OsStr> for String {
+    #[inline]
+    fn as_ref(&self) -> &OsStr {
+        (&**self).as_ref()
+    }
+}
+
+impl FromInner<Buf> for OsString {
+    fn from_inner(buf: Buf) -> OsString {
+        OsString { inner: buf }
+    }
+}
+
+impl IntoInner<Buf> for OsString {
+    fn into_inner(self) -> Buf {
+        self.inner
+    }
+}
+
+impl AsInner<Slice> for OsStr {
+    #[inline]
+    fn as_inner(&self) -> &Slice {
+        &self.inner
+    }
+}
+
+#[stable(feature = "osstring_from_str", since = "1.45.0")]
+impl FromStr for OsString {
+    type Err = core::convert::Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(OsString::from(s))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::sys_common::{AsInner, IntoInner};
+
+    use crate::rc::Rc;
+    use crate::sync::Arc;
+
+    #[test]
+    fn test_os_string_with_capacity() {
+        let os_string = OsString::with_capacity(0);
+        assert_eq!(0, os_string.inner.into_inner().capacity());
+
+        let os_string = OsString::with_capacity(10);
+        assert_eq!(10, os_string.inner.into_inner().capacity());
+
+        let mut os_string = OsString::with_capacity(0);
+        os_string.push("abc");
+        assert!(os_string.inner.into_inner().capacity() >= 3);
+    }
+
+    #[test]
+    fn test_os_string_clear() {
+        let mut os_string = OsString::from("abc");
+        assert_eq!(3, os_string.inner.as_inner().len());
+
+        os_string.clear();
+        assert_eq!(&os_string, "");
+        assert_eq!(0, os_string.inner.as_inner().len());
+    }
+
+    #[test]
+    fn test_os_string_capacity() {
+        let os_string = OsString::with_capacity(0);
+        assert_eq!(0, os_string.capacity());
+
+        let os_string = OsString::with_capacity(10);
+        assert_eq!(10, os_string.capacity());
+
+        let mut os_string = OsString::with_capacity(0);
+        os_string.push("abc");
+        assert!(os_string.capacity() >= 3);
+    }
+
+    #[test]
+    fn test_os_string_reserve() {
+        let mut os_string = OsString::new();
+        assert_eq!(os_string.capacity(), 0);
+
+        os_string.reserve(2);
+        assert!(os_string.capacity() >= 2);
+
+        for _ in 0..16 {
+            os_string.push("a");
+        }
+
+        assert!(os_string.capacity() >= 16);
+        os_string.reserve(16);
+        assert!(os_string.capacity() >= 32);
+
+        os_string.push("a");
+
+        os_string.reserve(16);
+        assert!(os_string.capacity() >= 33)
+    }
+
+    #[test]
+    fn test_os_string_reserve_exact() {
+        let mut os_string = OsString::new();
+        assert_eq!(os_string.capacity(), 0);
+
+        os_string.reserve_exact(2);
+        assert!(os_string.capacity() >= 2);
+
+        for _ in 0..16 {
+            os_string.push("a");
+        }
+
+        assert!(os_string.capacity() >= 16);
+        os_string.reserve_exact(16);
+        assert!(os_string.capacity() >= 32);
+
+        os_string.push("a");
+
+        os_string.reserve_exact(16);
+        assert!(os_string.capacity() >= 33)
+    }
+
+    #[test]
+    fn test_os_string_default() {
+        let os_string: OsString = Default::default();
+        assert_eq!("", &os_string);
+    }
+
+    #[test]
+    fn test_os_str_is_empty() {
+        let mut os_string = OsString::new();
+        assert!(os_string.is_empty());
+
+        os_string.push("abc");
+        assert!(!os_string.is_empty());
+
+        os_string.clear();
+        assert!(os_string.is_empty());
+    }
+
+    #[test]
+    fn test_os_str_len() {
+        let mut os_string = OsString::new();
+        assert_eq!(0, os_string.len());
+
+        os_string.push("abc");
+        assert_eq!(3, os_string.len());
+
+        os_string.clear();
+        assert_eq!(0, os_string.len());
+    }
+
+    #[test]
+    fn test_os_str_default() {
+        let os_str: &OsStr = Default::default();
+        assert_eq!("", os_str);
+    }
+
+    #[test]
+    fn into_boxed() {
+        let orig = "Hello, world!";
+        let os_str = OsStr::new(orig);
+        let boxed: Box<OsStr> = Box::from(os_str);
+        let os_string = os_str.to_owned().into_boxed_os_str().into_os_string();
+        assert_eq!(os_str, &*boxed);
+        assert_eq!(&*boxed, &*os_string);
+        assert_eq!(&*os_string, os_str);
+    }
+
+    #[test]
+    fn boxed_default() {
+        let boxed = <Box<OsStr>>::default();
+        assert!(boxed.is_empty());
+    }
+
+    #[test]
+    fn test_os_str_clone_into() {
+        let mut os_string = OsString::with_capacity(123);
+        os_string.push("hello");
+        let os_str = OsStr::new("bonjour");
+        os_str.clone_into(&mut os_string);
+        assert_eq!(os_str, os_string);
+        assert!(os_string.capacity() >= 123);
+    }
+
+    #[test]
+    fn into_rc() {
+        let orig = "Hello, world!";
+        let os_str = OsStr::new(orig);
+        let rc: Rc<OsStr> = Rc::from(os_str);
+        let arc: Arc<OsStr> = Arc::from(os_str);
+
+        assert_eq!(&*rc, os_str);
+        assert_eq!(&*arc, os_str);
+
+        let rc2: Rc<OsStr> = Rc::from(os_str.to_owned());
+        let arc2: Arc<OsStr> = Arc::from(os_str.to_owned());
+
+        assert_eq!(&*rc2, os_str);
+        assert_eq!(&*arc2, os_str);
+    }
+}