Add std::os::unix::fs::chroot to change the root directory of the current process

This is a straightforward wrapper that uses the existing helpers for C
string handling and errno handling.

Having this available is convenient for UNIX utility programs written in
Rust, and avoids having to call the unsafe `libc::chroot` directly and
handle errors manually, in a program that may otherwise be entirely safe
code.

Author: joshtriplett

library/std/src/sys/unix/ext/fs.rs

@@ -884,3 +884,27 @@ impl DirBuilderExt for fs::DirBuilder {
         self
     }
 }
+
+/// Change the root directory of the current process to the specified path.
+///
+/// This typically requires privileges, such as root or a specific capability.
+///
+/// This does not change the current working directory; you should call
+/// [`std::env::set_current_dir`][`crate::env::set_current_dir`] afterwards.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::os::unix::fs;
+///
+/// fn main() -> std::io::Result<()> {
+///     fs::chroot("/sandbox")?;
+///     std::env::set_current_dir("/")?;
+///     // continue working in sandbox
+///     Ok(())
+/// }
+/// ```
+#[unstable(feature = "unix_chroot", issue = "84715")]
+pub fn chroot<P: AsRef<Path>>(dir: P) -> io::Result<()> {
+    sys::fs::chroot(dir.as_ref())
+}

library/std/src/sys/unix/fs.rs

@@ -1328,3 +1328,9 @@ pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
     })?;
     Ok(bytes_copied as u64)
 }
+
+pub fn chroot(dir: &Path) -> io::Result<()> {
+    let dir = cstr(dir)?;
+    cvt(unsafe { libc::chroot(dir.as_ptr()) })?;
+    Ok(())
+}