Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 1800 insertions, 0 deletions

diff --git a/library/std/src/ffi/mod.rs b/library/std/src/ffi/mod.rs
new file mode 100644
index 000000000..d987bf69b
--- /dev/null
+++ b/library/std/src/ffi/mod.rs
@@ -0,0 +1,174 @@
+//! 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 an *O*(1) operation (because the
+//! length is stored); in C it is an *O*(*n*) 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 <code>\*mut [u8]</code> 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 <code>\*const [u8]</code> 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
+//! <code>&[str]</code> 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`] losslessly represents an owned platform string. However, this
+//! representation is not necessarily in a form native to the platform.
+//! 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 <code>[Option]<[OsString]></code>. If the environment variable
+//! exists you will get a <code>[Some]\(os_string)</code>, 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`] losslessly represents a borrowed reference to a platform string.
+//! However, this representation is not necessarily in a form native to the platform.
+//! It can be converted into a UTF-8 Rust string slice in a similar way to
+//! [`OsString`].
+//!
+//! # Conversions
+//!
+//! ## On Unix
+//!
+//! On Unix, [`OsStr`] implements the
+//! <code>std::os::unix::ffi::[OsStrExt][unix.OsStrExt]</code> trait, which
+//! augments it with two methods, [`from_bytes`] and [`as_bytes`].
+//! These do inexpensive conversions from and to byte slices.
+//!
+//! Additionally, on Unix [`OsString`] implements the
+//! <code>std::os::unix::ffi::[OsStringExt][unix.OsStringExt]</code> trait,
+//! which provides [`from_vec`] and [`into_vec`] methods that consume
+//! their arguments, and take or produce vectors of [`u8`].
+//!
+//! ## On Windows
+//!
+//! An [`OsStr`] can be losslessly converted to a native Windows string. And
+//! a native Windows string can be losslessly converted to an [`OsString`].
+//!
+//! On Windows, [`OsStr`] implements the
+//! <code>std::os::windows::ffi::[OsStrExt][windows.OsStrExt]</code> trait,
+//! which provides an [`encode_wide`] method. This provides an
+//! iterator that can be [`collect`]ed into a vector of [`u16`]. After a nul
+//! characters is appended, this is the same as a native Windows string.
+//!
+//! Additionally, on Windows [`OsString`] implements the
+//! <code>std::os::windows:ffi::[OsStringExt][windows.OsStringExt]</code>
+//! trait, which provides a [`from_wide`] method to convert a native Windows
+//! string (without the terminating nul character) to an [`OsString`].
+//!
+//! [Unicode scalar value]: https://www.unicode.org/glossary/#unicode_scalar_value
+//! [Unicode code point]: https://www.unicode.org/glossary/#code_point
+//! [`env::set_var()`]: crate::env::set_var "env::set_var"
+//! [`env::var_os()`]: crate::env::var_os "env::var_os"
+//! [unix.OsStringExt]: crate::os::unix::ffi::OsStringExt "os::unix::ffi::OsStringExt"
+//! [`from_vec`]: crate::os::unix::ffi::OsStringExt::from_vec "os::unix::ffi::OsStringExt::from_vec"
+//! [`into_vec`]: crate::os::unix::ffi::OsStringExt::into_vec "os::unix::ffi::OsStringExt::into_vec"
+//! [unix.OsStrExt]: crate::os::unix::ffi::OsStrExt "os::unix::ffi::OsStrExt"
+//! [`from_bytes`]: crate::os::unix::ffi::OsStrExt::from_bytes "os::unix::ffi::OsStrExt::from_bytes"
+//! [`as_bytes`]: crate::os::unix::ffi::OsStrExt::as_bytes "os::unix::ffi::OsStrExt::as_bytes"
+//! [`OsStrExt`]: crate::os::unix::ffi::OsStrExt "os::unix::ffi::OsStrExt"
+//! [windows.OsStrExt]: crate::os::windows::ffi::OsStrExt "os::windows::ffi::OsStrExt"
+//! [`encode_wide`]: crate::os::windows::ffi::OsStrExt::encode_wide "os::windows::ffi::OsStrExt::encode_wide"
+//! [`collect`]: crate::iter::Iterator::collect "iter::Iterator::collect"
+//! [windows.OsStringExt]: crate::os::windows::ffi::OsStringExt "os::windows::ffi::OsStringExt"
+//! [`from_wide`]: crate::os::windows::ffi::OsStringExt::from_wide "os::windows::ffi::OsStringExt::from_wide"
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[stable(feature = "alloc_c_string", since = "1.64.0")]
+pub use alloc::ffi::{CString, FromVecWithNulError, IntoStringError, NulError};
+#[stable(feature = "core_c_str", since = "1.64.0")]
+pub use core::ffi::{CStr, FromBytesWithNulError};
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::os_str::{OsStr, OsString};
+
+#[stable(feature = "core_ffi_c", since = "1.64.0")]
+pub use core::ffi::{
+    c_char, c_double, c_float, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint,
+    c_ulong, c_ulonglong, c_ushort,
+};
+
+#[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 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 000000000..a0a5c003d
--- /dev/null
+++ b/library/std/src/ffi/os_str.rs
@@ -0,0 +1,1447 @@
+#[cfg(test)]
+mod tests;
+
+use crate::borrow::{Borrow, Cow};
+use crate::cmp;
+use crate::collections::TryReserveError;
+use crate::fmt;
+use crate::hash::{Hash, Hasher};
+use crate::iter::Extend;
+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 <code>&[OsStr]</code> as [`String`] is to <code>&[str]</code>: 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.
+///
+/// # Capacity of `OsString`
+///
+/// Capacity uses units of UTF-8 bytes for OS strings which were created from valid unicode, and
+/// uses units of bytes in an unspecified encoding for other contents. On a given target, all
+/// `OsString` and `OsStr` values use the same units for capacity, so the following will work:
+/// ```
+/// use std::ffi::{OsStr, OsString};
+///
+/// fn concat_os_strings(a: &OsStr, b: &OsStr) -> OsString {
+///     let mut ret = OsString::with_capacity(a.len() + b.len()); // This will allocate
+///     ret.push(a); // This will not allocate further
+///     ret.push(b); // This will not allocate further
+///     ret
+/// }
+/// ```
+///
+/// # Creating an `OsString`
+///
+/// **From a Rust string**: `OsString` implements
+/// <code>[From]<[String]></code>, so you can use <code>my_string.[into]\()</code> to
+/// create an `OsString` from a normal Rust string.
+///
+/// **From slices:** Just like you can start with an empty Rust
+/// [`String`] and then [`String::push_str`] some <code>&[str]</code>
+/// sub-string slices into it, you can create an empty `OsString` with
+/// the [`OsString::new`] method and then push string slices into it with the
+/// [`OsString::push`] method.
+///
+/// # Extracting a borrowed reference to the whole OS string
+///
+/// You can use the [`OsString::as_os_str`] method to get an <code>&[OsStr]</code> 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.
+///
+/// [`CStr`]: crate::ffi::CStr
+/// [conversions]: super#conversions
+/// [into]: Into::into
+#[cfg_attr(not(test), rustc_diagnostic_item = "OsString")]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct OsString {
+    inner: Buf,
+}
+
+/// Allows extension traits within `std`.
+#[unstable(feature = "sealed", issue = "none")]
+impl crate::sealed::Sealed for OsString {}
+
+/// 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 <code>&[str]</code> 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.
+///
+/// [conversions]: super#conversions
+#[cfg_attr(not(test), rustc_diagnostic_item = "OsStr")]
+#[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 details, are
+// not documented and must not be relied upon.
+pub struct OsStr {
+    inner: Slice,
+}
+
+/// Allows extension traits within `std`.
+#[unstable(feature = "sealed", issue = "none")]
+impl crate::sealed::Sealed for OsStr {}
+
+impl OsString {
+    /// Constructs a new empty `OsString`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let os_string = OsString::new();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[inline]
+    pub fn new() -> OsString {
+        OsString { inner: Buf::from_string(String::new()) }
+    }
+
+    /// Converts to an [`OsStr`] slice.
+    ///
+    /// # 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")]
+    #[must_use]
+    #[inline]
+    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.
+    ///
+    /// # 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")]
+    #[inline]
+    pub fn into_string(self) -> Result<String, OsString> {
+        self.inner.into_string().map_err(|buf| OsString { inner: buf })
+    }
+
+    /// Extends the string with the given <code>&[OsStr]</code> slice.
+    ///
+    /// # 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")]
+    #[inline]
+    pub fn push<T: AsRef<OsStr>>(&mut self, s: T) {
+        self.inner.push_slice(&s.as_ref().inner)
+    }
+
+    /// Creates a new `OsString` with at least the given capacity.
+    ///
+    /// The string will be able to hold at least `capacity` length units of other
+    /// OS strings without reallocating. This method is allowed to allocate for
+    /// more units than `capacity`. If `capacity` is 0, the string will not
+    /// allocate.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[must_use]
+    #[inline]
+    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")]
+    #[inline]
+    pub fn clear(&mut self) {
+        self.inner.clear()
+    }
+
+    /// Returns the capacity this `OsString` can hold without reallocating.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[must_use]
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
+    /// Reserves capacity for at least `additional` more capacity to be inserted
+    /// in the given `OsString`. Does nothing if the capacity is
+    /// already sufficient.
+    ///
+    /// The collection may reserve more space to speculatively avoid frequent reallocations.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        self.inner.reserve(additional)
+    }
+
+    /// Tries to reserve capacity for at least `additional` more length units
+    /// in the given `OsString`. The string may reserve more space to speculatively avoid
+    /// frequent reallocations. After calling `try_reserve`, capacity will be
+    /// greater than or equal to `self.len() + additional` if it returns `Ok(())`.
+    /// Does nothing if capacity is already sufficient.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsStr, OsString};
+    /// use std::collections::TryReserveError;
+    ///
+    /// fn process_data(data: &str) -> Result<OsString, TryReserveError> {
+    ///     let mut s = OsString::new();
+    ///
+    ///     // Pre-reserve the memory, exiting if we can't
+    ///     s.try_reserve(OsStr::new(data).len())?;
+    ///
+    ///     // Now we know this can't OOM in the middle of our complex work
+    ///     s.push(data);
+    ///
+    ///     Ok(s)
+    /// }
+    /// # process_data("123").expect("why is the test harness OOMing on 3 bytes?");
+    /// ```
+    #[stable(feature = "try_reserve_2", since = "1.63.0")]
+    #[inline]
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.inner.try_reserve(additional)
+    }
+
+    /// Reserves the minimum capacity for at least `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.
+    ///
+    /// [`reserve`]: OsString::reserve
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[inline]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.inner.reserve_exact(additional)
+    }
+
+    /// Tries to reserve the minimum capacity for at least `additional`
+    /// more length units in the given `OsString`. After calling
+    /// `try_reserve_exact`, capacity will be greater than or equal to
+    /// `self.len() + additional` if it returns `Ok(())`.
+    /// Does nothing if the capacity is already sufficient.
+    ///
+    /// Note that the allocator may give the `OsString` more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
+    ///
+    /// [`try_reserve`]: OsString::try_reserve
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # Errors
+    ///
+    /// If the capacity overflows, or the allocator reports a failure, then an error
+    /// is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsStr, OsString};
+    /// use std::collections::TryReserveError;
+    ///
+    /// fn process_data(data: &str) -> Result<OsString, TryReserveError> {
+    ///     let mut s = OsString::new();
+    ///
+    ///     // Pre-reserve the memory, exiting if we can't
+    ///     s.try_reserve_exact(OsStr::new(data).len())?;
+    ///
+    ///     // Now we know this can't OOM in the middle of our complex work
+    ///     s.push(data);
+    ///
+    ///     Ok(s)
+    /// }
+    /// # process_data("123").expect("why is the test harness OOMing on 3 bytes?");
+    /// ```
+    #[stable(feature = "try_reserve_2", since = "1.63.0")]
+    #[inline]
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.inner.try_reserve_exact(additional)
+    }
+
+    /// Shrinks the capacity of the `OsString` to match its length.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[inline]
+    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.
+    ///
+    /// If the current capacity is less than the lower limit, this is a no-op.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// 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]
+    #[stable(feature = "shrink_to", since = "1.56.0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.inner.shrink_to(min_capacity)
+    }
+
+    /// Converts this `OsString` into a boxed [`OsStr`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::{OsString, OsStr};
+    ///
+    /// let s = OsString::from("hello");
+    ///
+    /// let b: Box<OsStr> = s.into_boxed_os_str();
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[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 an [`OsString`].
+    ///
+    /// This conversion does not allocate or copy memory.
+    #[inline]
+    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 {
+    /// Copies any value implementing <code>[AsRef]&lt;[OsStr]&gt;</code>
+    /// into a newly allocated [`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 Clone for OsString {
+    #[inline]
+    fn clone(&self) -> Self {
+        OsString { inner: self.inner.clone() }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, source: &Self) {
+        self.inner.clone_from(&source.inner)
+    }
+}
+
+#[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 {
+    #[inline]
+    fn eq(&self, other: &OsString) -> bool {
+        &**self == &**other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<str> for OsString {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        &**self == other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl PartialEq<OsString> for str {
+    #[inline]
+    fn eq(&self, other: &OsString) -> bool {
+        &**other == self
+    }
+}
+
+#[stable(feature = "os_str_str_ref_eq", since = "1.29.0")]
+impl PartialEq<&str> for OsString {
+    #[inline]
+    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 {
+    #[inline]
+    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)
+    }
+}
+
+#[stable(feature = "os_string_fmt_write", since = "1.64.0")]
+impl fmt::Write for OsString {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.push(s);
+        Ok(())
+    }
+}
+
+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 <code>&[str]</code> slice if the `OsStr` is valid Unicode.
+    ///
+    /// This conversion may entail doing a check for UTF-8 validity.
+    ///
+    /// # 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")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
+    #[inline]
+    pub fn to_str(&self) -> Option<&str> {
+        self.inner.to_str()
+    }
+
+    /// Converts an `OsStr` to a <code>[Cow]<[str]></code>.
+    ///
+    /// Any non-Unicode sequences are replaced with
+    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD].
+    ///
+    /// [U+FFFD]: crate::char::REPLACEMENT_CHARACTER
+    ///
+    /// # 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(unix)] {
+    ///     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")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
+    #[inline]
+    pub fn to_string_lossy(&self) -> Cow<'_, str> {
+        self.inner.to_string_lossy()
+    }
+
+    /// Copies the slice into an owned [`OsString`].
+    ///
+    /// # 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")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
+    #[inline]
+    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")]
+    #[must_use]
+    #[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.
+    ///
+    /// See the main `OsString` documentation information about encoding and capacity units.
+    ///
+    /// # 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")]
+    #[must_use]
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.inner.inner.len()
+    }
+
+    /// Converts a <code>[Box]<[OsStr]></code> into an [`OsString`] without copying or allocating.
+    #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    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 not externally public, to avoid
+    /// revealing the internal, platform-specific encodings.
+    #[inline]
+    pub(crate) 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
+    /// [`OsStr::to_ascii_lowercase`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("GRÜßE, JÜRGEN ❤");
+    ///
+    /// s.make_ascii_lowercase();
+    ///
+    /// assert_eq!("grÜße, jÜrgen ❤", s);
+    /// ```
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    #[inline]
+    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
+    /// [`OsStr::to_ascii_uppercase`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    ///
+    /// let mut s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// s.make_ascii_uppercase();
+    ///
+    /// assert_eq!("GRüßE, JüRGEN ❤", s);
+    /// ```
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    #[inline]
+    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 [`OsStr::make_ascii_lowercase`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    /// let s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// assert_eq!("grüße, jürgen ❤", s.to_ascii_lowercase());
+    /// ```
+    #[must_use = "to lowercase the value in-place, use `make_ascii_lowercase`"]
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    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 [`OsStr::make_ascii_uppercase`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    /// let s = OsString::from("Grüße, Jürgen ❤");
+    ///
+    /// assert_eq!("GRüßE, JüRGEN ❤", s.to_ascii_uppercase());
+    /// ```
+    #[must_use = "to uppercase the value in-place, use `make_ascii_uppercase`"]
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    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
+    ///
+    /// ```
+    /// 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());
+    /// ```
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    #[must_use]
+    #[inline]
+    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
+    ///
+    /// ```
+    /// 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"));
+    /// ```
+    #[stable(feature = "osstring_ascii", since = "1.53.0")]
+    pub fn eq_ignore_ascii_case<S: 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> {
+    /// Copies the string into a newly allocated <code>[Box]&lt;[OsStr]&gt;</code>.
+    #[inline]
+    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> {
+    /// Converts a `Cow<'a, OsStr>` into a <code>[Box]&lt;[OsStr]&gt;</code>,
+    /// by copying the contents if they are borrowed.
+    #[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 <code>[Box]<[OsStr]></code> into an [`OsString`] without copying or
+    /// allocating.
+    #[inline]
+    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 an [`OsString`] into a <code>[Box]<[OsStr]></code> without copying or allocating.
+    #[inline]
+    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 an [`OsString`] into an <code>[Arc]<[OsStr]></code> by moving the [`OsString`]
+    /// data into a new [`Arc`] buffer.
+    #[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> {
+    /// Copies the string into a newly allocated <code>[Arc]&lt;[OsStr]&gt;</code>.
+    #[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 an [`OsString`] into an <code>[Rc]<[OsStr]></code> by moving the [`OsString`]
+    /// data into a new [`Rc`] buffer.
+    #[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> {
+    /// Copies the string into a newly allocated <code>[Rc]&lt;[OsStr]&gt;</code>.
+    #[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> {
+    /// Moves the string into a [`Cow::Owned`].
+    #[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> {
+    /// Converts the string reference into a [`Cow::Borrowed`].
+    #[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> {
+    /// Converts the string reference into a [`Cow::Borrowed`].
+    #[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 {
+    /// Converts a `Cow<'a, OsStr>` into an [`OsString`],
+    /// by copying the contents if they are borrowed.
+    #[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> {
+    #[inline]
+    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)
+    }
+}
+
+#[unstable(feature = "slice_concat_ext", issue = "27747")]
+impl<S: Borrow<OsStr>> alloc::slice::Join<&OsStr> for [S] {
+    type Output = OsString;
+
+    fn join(slice: &Self, sep: &OsStr) -> OsString {
+        let Some((first, suffix)) = slice.split_first() else {
+            return OsString::new();
+        };
+        let first_owned = first.borrow().to_owned();
+        suffix.iter().fold(first_owned, |mut a, b| {
+            a.push(sep);
+            a.push(b.borrow());
+            a
+        })
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Borrow<OsStr> for OsString {
+    #[inline]
+    fn borrow(&self) -> &OsStr {
+        &self[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl ToOwned for OsStr {
+    type Owned = OsString;
+    #[inline]
+    fn to_owned(&self) -> OsString {
+        self.to_os_string()
+    }
+    #[inline]
+    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 {
+    #[inline]
+    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 {
+    #[inline]
+    fn from_inner(buf: Buf) -> OsString {
+        OsString { inner: buf }
+    }
+}
+
+impl IntoInner<Buf> for OsString {
+    #[inline]
+    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;
+
+    #[inline]
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(OsString::from(s))
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl Extend<OsString> for OsString {
+    #[inline]
+    fn extend<T: IntoIterator<Item = OsString>>(&mut self, iter: T) {
+        for s in iter {
+            self.push(&s);
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> Extend<&'a OsStr> for OsString {
+    #[inline]
+    fn extend<T: IntoIterator<Item = &'a OsStr>>(&mut self, iter: T) {
+        for s in iter {
+            self.push(s);
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> Extend<Cow<'a, OsStr>> for OsString {
+    #[inline]
+    fn extend<T: IntoIterator<Item = Cow<'a, OsStr>>>(&mut self, iter: T) {
+        for s in iter {
+            self.push(&s);
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl FromIterator<OsString> for OsString {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = OsString>>(iter: I) -> Self {
+        let mut iterator = iter.into_iter();
+
+        // Because we're iterating over `OsString`s, we can avoid at least
+        // one allocation by getting the first string from the iterator
+        // and appending to it all the subsequent strings.
+        match iterator.next() {
+            None => OsString::new(),
+            Some(mut buf) => {
+                buf.extend(iterator);
+                buf
+            }
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> FromIterator<&'a OsStr> for OsString {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = &'a OsStr>>(iter: I) -> Self {
+        let mut buf = Self::new();
+        for s in iter {
+            buf.push(s);
+        }
+        buf
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> FromIterator<Cow<'a, OsStr>> for OsString {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = Cow<'a, OsStr>>>(iter: I) -> Self {
+        let mut iterator = iter.into_iter();
+
+        // Because we're iterating over `OsString`s, we can avoid at least
+        // one allocation by getting the first owned string from the iterator
+        // and appending to it all the subsequent strings.
+        match iterator.next() {
+            None => OsString::new(),
+            Some(Cow::Owned(mut buf)) => {
+                buf.extend(iterator);
+                buf
+            }
+            Some(Cow::Borrowed(buf)) => {
+                let mut buf = OsString::from(buf);
+                buf.extend(iterator);
+                buf
+            }
+        }
+    }
+}
diff --git a/library/std/src/ffi/os_str/tests.rs b/library/std/src/ffi/os_str/tests.rs
new file mode 100644
index 000000000..d7926749a
--- /dev/null
+++ b/library/std/src/ffi/os_str/tests.rs
@@ -0,0 +1,179 @@
+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_join() {
+    let strings = [OsStr::new("hello"), OsStr::new("dear"), OsStr::new("world")];
+    assert_eq!("hello", strings[..1].join(OsStr::new(" ")));
+    assert_eq!("hello dear world", strings.join(OsStr::new(" ")));
+    assert_eq!("hellodearworld", strings.join(OsStr::new("")));
+    assert_eq!("hello.\n dear.\n world", strings.join(OsStr::new(".\n ")));
+
+    assert_eq!("dear world", strings[1..].join(&OsString::from(" ")));
+
+    let strings_abc = [OsString::from("a"), OsString::from("b"), OsString::from("c")];
+    assert_eq!("a b c", strings_abc.join(OsStr::new(" ")));
+}
+
+#[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);
+}