Add display method to OsStr

Add `display` method to `OsStr` for lossy display of an `OsStr` which may contain invalid unicode.

Invalid Unicode sequences are replaced with `U+FFFD REPLACEMENT CHARACTER`.

This change also makes the `std::ffi::os_str` module public.

library/std/src/ffi/mod.rs

@@ -181,4 +181,5 @@ pub use core::ffi::c_void;
 )]
 pub use core::ffi::{VaList, VaListImpl};
 
-mod os_str;
+#[unstable(feature = "os_str_display", issue = "120048")]
+pub mod os_str;

library/std/src/ffi/os_str.rs

@@ -1,3 +1,5 @@
+//! The [`OsStr`] and [`OsString`] types and associated utilities.
+
 #[cfg(test)]
 mod tests;
 
@@ -1173,6 +1175,32 @@ impl OsStr {
     pub fn eq_ignore_ascii_case<S: AsRef<OsStr>>(&self, other: S) -> bool {
         self.inner.eq_ignore_ascii_case(&other.as_ref().inner)
     }
+
+    /// Returns an object that implements [`Display`] for safely printing an
+    /// [`OsStr`] that may contain non-Unicode data. This may perform lossy
+    /// conversion, depending on the platform.  If you would like an
+    /// implementation which escapes the [`OsStr`] please use [`Debug`]
+    /// instead.
+    ///
+    /// [`Display`]: fmt::Display
+    /// [`Debug`]: fmt::Debug
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsStr;
+    ///
+    /// let s = OsStr::new("Hello, world!");
+    ///
+    /// println!("{}", s.display());
+    /// ```
+    #[unstable(feature = "os_str_display", issue = "120048")]
+    #[must_use = "this does not display the osstr, \
+                  it returns an object that can be displayed"]
+    #[inline]
+    pub fn display(&self) -> Display<'_> {
+        Display { os_str: self }
+    }
 }
 
 #[stable(feature = "box_from_os_str", since = "1.17.0")]
@@ -1467,9 +1495,42 @@ impl fmt::Debug for OsStr {
     }
 }
 
-impl OsStr {
-    pub(crate) fn display(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&self.inner, formatter)
+/// Helper struct for safely printing an [`OsStr`] with [`format!`] and `{}`.
+///
+/// An [`OsStr`] might contain non-Unicode data. This `struct` implements the
+/// [`Display`] trait in a way that mitigates that. It is created by the
+/// [`display`](OsStr::display) method on [`OsStr`]. This may perform lossy
+/// conversion, depending on the platform. If you would like an implementation
+/// which escapes the [`OsStr`] please use [`Debug`] instead.
+///
+/// # Examples
+///
+/// ```
+/// use std::ffi::OsStr;
+///
+/// let s = OsStr::new("Hello, world!");
+///
+/// println!("{}", s.display());
+/// ```
+///
+/// [`Display`]: fmt::Display
+/// [`format!`]: crate::format
+#[unstable(feature = "os_str_display", issue = "120048")]
+pub struct Display<'a> {
+    os_str: &'a OsStr,
+}
+
+#[unstable(feature = "os_str_display", issue = "120048")]
+impl fmt::Debug for Display<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.os_str, f)
+    }
+}
+
+#[unstable(feature = "os_str_display", issue = "120048")]
+impl fmt::Display for Display<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.os_str.inner, f)
     }
 }
 

library/std/src/path.rs

@@ -84,7 +84,7 @@ use crate::rc::Rc;
 use crate::str::FromStr;
 use crate::sync::Arc;
 
-use crate::ffi::{OsStr, OsString};
+use crate::ffi::{os_str, OsStr, OsString};
 use crate::sys;
 use crate::sys::path::{is_sep_byte, is_verbatim_sep, parse_prefix, MAIN_SEP_STR};
 
@@ -2725,7 +2725,7 @@ impl Path {
                   it returns an object that can be displayed"]
     #[inline]
     pub fn display(&self) -> Display<'_> {
-        Display { path: self }
+        Display { inner: self.inner.display() }
     }
 
     /// Queries the file system to get information about a file, directory, etc.
@@ -3032,20 +3032,20 @@ impl fmt::Debug for Path {
 /// [`format!`]: crate::format
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Display<'a> {
-    path: &'a Path,
+    inner: os_str::Display<'a>,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for Display<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.path, f)
+        fmt::Debug::fmt(&self.inner, f)
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for Display<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.path.inner.display(f)
+        fmt::Display::fmt(&self.inner, f)
     }
 }