x86_64/usr/lib/rustlib/src/rust/library/std/src/sys/windows/args.rs - manifest_repos/toolchain - Git at Google

 //! The Windows command line is just a string
 //! <https://docs.microsoft.com/en-us/archive/blogs/larryosterman/the-windows-command-line-is-just-a-string>
 //!
 //! This module implements the parsing necessary to turn that string into a list of arguments.

 #[cfg(test)]
 mod tests;

 use crate::ffi::OsString;
 use crate::fmt;
 use crate::io;
 use crate::num::NonZeroU16;
 use crate::os::windows::prelude::*;
 use crate::path::PathBuf;
 use crate::sys::c;
 use crate::sys::process::ensure_no_nuls;
 use crate::sys::windows::os::current_exe;
 use crate::sys_common::wstr::WStrUnits;
 use crate::vec;

 use crate::iter;

 /// This is the const equivalent to `NonZeroU16::new(n).unwrap()`
 ///
 /// FIXME: This can be removed once `Option::unwrap` is stably const.
 /// See the `const_option` feature (#67441).
 const fn non_zero_u16(n: u16) -> NonZeroU16 {
     match NonZeroU16::new(n) {
         Some(n) => n,
         None => panic!("called `unwrap` on a `None` value"),
     }
 }

 pub fn args() -> Args {
     // SAFETY: `GetCommandLineW` returns a pointer to a null terminated UTF-16
     // string so it's safe for `WStrUnits` to use.
     unsafe {
         let lp_cmd_line = c::GetCommandLineW();
         let parsed_args_list = parse_lp_cmd_line(WStrUnits::new(lp_cmd_line), || {
             current_exe().map(PathBuf::into_os_string).unwrap_or_else(|_| OsString::new())
         });

         Args { parsed_args_list: parsed_args_list.into_iter() }
     }
 }

 /// Implements the Windows command-line argument parsing algorithm.
 ///
 /// Microsoft's documentation for the Windows CLI argument format can be found at
 /// <https://docs.microsoft.com/en-us/cpp/cpp/main-function-command-line-args?view=msvc-160#parsing-c-command-line-arguments>
 ///
 /// A more in-depth explanation is here:
 /// <https://daviddeley.com/autohotkey/parameters/parameters.htm#WIN>
 ///
 /// Windows includes a function to do command line parsing in shell32.dll.
 /// However, this is not used for two reasons:
 ///
 /// 1. Linking with that DLL causes the process to be registered as a GUI application.
 /// GUI applications add a bunch of overhead, even if no windows are drawn. See
 /// <https://randomascii.wordpress.com/2018/12/03/a-not-called-function-can-cause-a-5x-slowdown/>.
 ///
 /// 2. It does not follow the modern C/C++ argv rules outlined in the first two links above.
 ///
 /// This function was tested for equivalence to the C/C++ parsing rules using an
 /// extensive test suite available at
 /// <https://github.com/ChrisDenton/winarg/tree/std>.
 fn parse_lp_cmd_line<'a, F: Fn() -> OsString>(
     lp_cmd_line: Option<WStrUnits<'a>>,
     exe_name: F,
 ) -> Vec<OsString> {
     const BACKSLASH: NonZeroU16 = non_zero_u16(b'\\' as u16);
     const QUOTE: NonZeroU16 = non_zero_u16(b'"' as u16);
     const TAB: NonZeroU16 = non_zero_u16(b'\t' as u16);
     const SPACE: NonZeroU16 = non_zero_u16(b' ' as u16);

     let mut ret_val = Vec::new();
     // If the cmd line pointer is null or it points to an empty string then
     // return the name of the executable as argv[0].
     if lp_cmd_line.as_ref().and_then(|cmd| cmd.peek()).is_none() {
         ret_val.push(exe_name());
         return ret_val;
     }
     let mut code_units = lp_cmd_line.unwrap();

     // The executable name at the beginning is special.
     let mut in_quotes = false;
     let mut cur = Vec::new();
     for w in &mut code_units {
         match w {
             // A quote mark always toggles `in_quotes` no matter what because
             // there are no escape characters when parsing the executable name.
             QUOTE => in_quotes = !in_quotes,
             // If not `in_quotes` then whitespace ends argv[0].
             SPACE | TAB if !in_quotes => break,
             // In all other cases the code unit is taken literally.
             _ => cur.push(w.get()),
         }
     }
     // Skip whitespace.
     code_units.advance_while(|w| w == SPACE || w == TAB);
     ret_val.push(OsString::from_wide(&cur));

     // Parse the arguments according to these rules:
     // * All code units are taken literally except space, tab, quote and backslash.
     // * When not `in_quotes`, space and tab separate arguments. Consecutive spaces and tabs are
     // treated as a single separator.
     // * A space or tab `in_quotes` is taken literally.
     // * A quote toggles `in_quotes` mode unless it's escaped. An escaped quote is taken literally.
     // * A quote can be escaped if preceded by an odd number of backslashes.
     // * If any number of backslashes is immediately followed by a quote then the number of
     // backslashes is halved (rounding down).
     // * Backslashes not followed by a quote are all taken literally.
     // * If `in_quotes` then a quote can also be escaped using another quote
     // (i.e. two consecutive quotes become one literal quote).
     let mut cur = Vec::new();
     let mut in_quotes = false;
     while let Some(w) = code_units.next() {
         match w {
             // If not `in_quotes`, a space or tab ends the argument.
             SPACE | TAB if !in_quotes => {
                 ret_val.push(OsString::from_wide(&cur[..]));
                 cur.truncate(0);

                 // Skip whitespace.
                 code_units.advance_while(|w| w == SPACE || w == TAB);
             }
             // Backslashes can escape quotes or backslashes but only if consecutive backslashes are followed by a quote.
             BACKSLASH => {
                 let backslash_count = code_units.advance_while(|w| w == BACKSLASH) + 1;
                 if code_units.peek() == Some(QUOTE) {
                     cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count / 2));
                     // The quote is escaped if there are an odd number of backslashes.
                     if backslash_count % 2 == 1 {
                         code_units.next();
                         cur.push(QUOTE.get());
                     }
                 } else {
                     // If there is no quote on the end then there is no escaping.
                     cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count));
                 }
             }
             // If `in_quotes` and not backslash escaped (see above) then a quote either
             // unsets `in_quote` or is escaped by another quote.
             QUOTE if in_quotes => match code_units.peek() {
                 // Two consecutive quotes when `in_quotes` produces one literal quote.
                 Some(QUOTE) => {
                     cur.push(QUOTE.get());
                     code_units.next();
                 }
                 // Otherwise set `in_quotes`.
                 Some(_) => in_quotes = false,
                 // The end of the command line.
                 // Push `cur` even if empty, which we do by breaking while `in_quotes` is still set.
                 None => break,
             },
             // If not `in_quotes` and not BACKSLASH escaped (see above) then a quote sets `in_quote`.
             QUOTE => in_quotes = true,
             // Everything else is always taken literally.
             _ => cur.push(w.get()),
         }
     }
     // Push the final argument, if any.
     if !cur.is_empty() || in_quotes {
         ret_val.push(OsString::from_wide(&cur[..]));
     }
     ret_val
 }

 pub struct Args {
     parsed_args_list: vec::IntoIter<OsString>,
 }

 impl fmt::Debug for Args {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.parsed_args_list.as_slice().fmt(f)
     }
 }

 impl Iterator for Args {
     type Item = OsString;
     fn next(&mut self) -> Option<OsString> {
         self.parsed_args_list.next()
     }
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.parsed_args_list.size_hint()
     }
 }

 impl DoubleEndedIterator for Args {
     fn next_back(&mut self) -> Option<OsString> {
         self.parsed_args_list.next_back()
     }
 }

 impl ExactSizeIterator for Args {
     fn len(&self) -> usize {
         self.parsed_args_list.len()
     }
 }

 #[derive(Debug)]
 pub(crate) enum Arg {
     /// Add quotes (if needed)
     Regular(OsString),
     /// Append raw string without quoting
     Raw(OsString),
 }

 enum Quote {
     // Every arg is quoted
     Always,
     // Whitespace and empty args are quoted
     Auto,
     // Arg appended without any changes (#29494)
     Never,
 }

 pub(crate) fn append_arg(cmd: &mut Vec<u16>, arg: &Arg, force_quotes: bool) -> io::Result<()> {
     let (arg, quote) = match arg {
         Arg::Regular(arg) => (arg, if force_quotes { Quote::Always } else { Quote::Auto }),
         Arg::Raw(arg) => (arg, Quote::Never),
     };

     // If an argument has 0 characters then we need to quote it to ensure
     // that it actually gets passed through on the command line or otherwise
     // it will be dropped entirely when parsed on the other end.
     ensure_no_nuls(arg)?;
     let arg_bytes = arg.bytes();
     let (quote, escape) = match quote {
         Quote::Always => (true, true),
         Quote::Auto => {
             (arg_bytes.iter().any(|c| *c == b' ' || *c == b'\t') || arg_bytes.is_empty(), true)
         }
         Quote::Never => (false, false),
     };
     if quote {
         cmd.push('"' as u16);
     }

     let mut backslashes: usize = 0;
     for x in arg.encode_wide() {
         if escape {
             if x == '\\' as u16 {
                 backslashes += 1;
             } else {
                 if x == '"' as u16 {
                     // Add n+1 backslashes to total 2n+1 before internal '"'.
                     cmd.extend((0..=backslashes).map(|_| '\\' as u16));
                 }
                 backslashes = 0;
             }
         }
         cmd.push(x);
     }

     if quote {
         // Add n backslashes to total 2n before ending '"'.
         cmd.extend((0..backslashes).map(|_| '\\' as u16));
         cmd.push('"' as u16);
     }
     Ok(())
 }

 pub(crate) fn make_bat_command_line(
     script: &[u16],
     args: &[Arg],
     force_quotes: bool,
 ) -> io::Result<Vec<u16>> {
     // Set the start of the command line to `cmd.exe /c "`
     // It is necessary to surround the command in an extra pair of quotes,
     // hence the trailing quote here. It will be closed after all arguments
     // have been added.
     let mut cmd: Vec<u16> = "cmd.exe /c \"".encode_utf16().collect();

     // Push the script name surrounded by its quote pair.
     cmd.push(b'"' as u16);
     // Windows file names cannot contain a `"` character or end with `\\`.
     // If the script name does then return an error.
     if script.contains(&(b'"' as u16)) || script.last() == Some(&(b'\\' as u16)) {
         return Err(io::const_io_error!(
             io::ErrorKind::InvalidInput,
             "Windows file names may not contain `\"` or end with `\\`"
         ));
     }
     cmd.extend_from_slice(script.strip_suffix(&[0]).unwrap_or(script));
     cmd.push(b'"' as u16);

     // Append the arguments.
     // FIXME: This needs tests to ensure that the arguments are properly
     // reconstructed by the batch script by default.
     for arg in args {
         cmd.push(' ' as u16);
         append_arg(&mut cmd, arg, force_quotes)?;
     }

     // Close the quote we left opened earlier.
     cmd.push(b'"' as u16);

     Ok(cmd)
 }

 /// Takes a path and tries to return a non-verbatim path.
 ///
 /// This is necessary because cmd.exe does not support verbatim paths.
 pub(crate) fn to_user_path(mut path: Vec<u16>) -> io::Result<Vec<u16>> {
     use crate::ptr;
     use crate::sys::windows::fill_utf16_buf;

     // UTF-16 encoded code points, used in parsing and building UTF-16 paths.
     // All of these are in the ASCII range so they can be cast directly to `u16`.
     const SEP: u16 = b'\\' as _;
     const QUERY: u16 = b'?' as _;
     const COLON: u16 = b':' as _;
     const U: u16 = b'U' as _;
     const N: u16 = b'N' as _;
     const C: u16 = b'C' as _;

     // Early return if the path is too long to remove the verbatim prefix.
     const LEGACY_MAX_PATH: usize = 260;
     if path.len() > LEGACY_MAX_PATH {
         return Ok(path);
     }

     match &path[..] {
         // `\\?\C:\...` => `C:\...`
         [SEP, SEP, QUERY, SEP, _, COLON, SEP, ..] => unsafe {
             let lpfilename = path[4..].as_ptr();
             fill_utf16_buf(
                 |buffer, size| c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()),
                 |full_path: &[u16]| {
                     if full_path == &path[4..path.len() - 1] { full_path.into() } else { path }
                 },
             )
         },
         // `\\?\UNC\...` => `\\...`
         [SEP, SEP, QUERY, SEP, U, N, C, SEP, ..] => unsafe {
             // Change the `C` in `UNC\` to `\` so we can get a slice that starts with `\\`.
             path[6] = b'\\' as u16;
             let lpfilename = path[6..].as_ptr();
             fill_utf16_buf(
                 |buffer, size| c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()),
                 |full_path: &[u16]| {
                     if full_path == &path[6..path.len() - 1] {
                         full_path.into()
                     } else {
                         // Restore the 'C' in "UNC".
                         path[6] = b'C' as u16;
                         path
                     }
                 },
             )
         },
         // For everything else, leave the path unchanged.
         _ => Ok(path),
     }
 }
	//! The Windows command line is just a string
	//! <https://docs.microsoft.com/en-us/archive/blogs/larryosterman/the-windows-command-line-is-just-a-string>
	//!
	//! This module implements the parsing necessary to turn that string into a list of arguments.

	#[cfg(test)]
	mod tests;

	use crate::ffi::OsString;
	use crate::fmt;
	use crate::io;
	use crate::num::NonZeroU16;
	use crate::os::windows::prelude::*;
	use crate::path::PathBuf;
	use crate::sys::c;
	use crate::sys::process::ensure_no_nuls;
	use crate::sys::windows::os::current_exe;
	use crate::sys_common::wstr::WStrUnits;
	use crate::vec;

	use crate::iter;

	/// This is the const equivalent to `NonZeroU16::new(n).unwrap()`
	///
	/// FIXME: This can be removed once `Option::unwrap` is stably const.
	/// See the `const_option` feature (#67441).
	const fn non_zero_u16(n: u16) -> NonZeroU16 {
	match NonZeroU16::new(n) {
	Some(n) => n,
	None => panic!("called `unwrap` on a `None` value"),
	}
	}

	pub fn args() -> Args {
	// SAFETY: `GetCommandLineW` returns a pointer to a null terminated UTF-16
	// string so it's safe for `WStrUnits` to use.
	unsafe {
	let lp_cmd_line = c::GetCommandLineW();
	let parsed_args_list = parse_lp_cmd_line(WStrUnits::new(lp_cmd_line), \|\| {
	current_exe().map(PathBuf::into_os_string).unwrap_or_else(\|_\| OsString::new())
	});

	Args { parsed_args_list: parsed_args_list.into_iter() }
	}
	}

	/// Implements the Windows command-line argument parsing algorithm.
	///
	/// Microsoft's documentation for the Windows CLI argument format can be found at
	/// <https://docs.microsoft.com/en-us/cpp/cpp/main-function-command-line-args?view=msvc-160#parsing-c-command-line-arguments>
	///
	/// A more in-depth explanation is here:
	/// <https://daviddeley.com/autohotkey/parameters/parameters.htm#WIN>
	///
	/// Windows includes a function to do command line parsing in shell32.dll.
	/// However, this is not used for two reasons:
	///
	/// 1. Linking with that DLL causes the process to be registered as a GUI application.
	/// GUI applications add a bunch of overhead, even if no windows are drawn. See
	/// <https://randomascii.wordpress.com/2018/12/03/a-not-called-function-can-cause-a-5x-slowdown/>.
	///
	/// 2. It does not follow the modern C/C++ argv rules outlined in the first two links above.
	///
	/// This function was tested for equivalence to the C/C++ parsing rules using an
	/// extensive test suite available at
	/// <https://github.com/ChrisDenton/winarg/tree/std>.
	fn parse_lp_cmd_line<'a, F: Fn() -> OsString>(
	lp_cmd_line: Option<WStrUnits<'a>>,
	exe_name: F,
	) -> Vec<OsString> {
	const BACKSLASH: NonZeroU16 = non_zero_u16(b'\\' as u16);
	const QUOTE: NonZeroU16 = non_zero_u16(b'"' as u16);
	const TAB: NonZeroU16 = non_zero_u16(b'\t' as u16);
	const SPACE: NonZeroU16 = non_zero_u16(b' ' as u16);

	let mut ret_val = Vec::new();
	// If the cmd line pointer is null or it points to an empty string then
	// return the name of the executable as argv[0].
	if lp_cmd_line.as_ref().and_then(\|cmd\| cmd.peek()).is_none() {
	ret_val.push(exe_name());
	return ret_val;
	}
	let mut code_units = lp_cmd_line.unwrap();

	// The executable name at the beginning is special.
	let mut in_quotes = false;
	let mut cur = Vec::new();
	for w in &mut code_units {
	match w {
	// A quote mark always toggles `in_quotes` no matter what because
	// there are no escape characters when parsing the executable name.
	QUOTE => in_quotes = !in_quotes,
	// If not `in_quotes` then whitespace ends argv[0].
	SPACE \| TAB if !in_quotes => break,
	// In all other cases the code unit is taken literally.
	_ => cur.push(w.get()),
	}
	}
	// Skip whitespace.
	code_units.advance_while(\|w\| w == SPACE \|\| w == TAB);
	ret_val.push(OsString::from_wide(&cur));

	// Parse the arguments according to these rules:
	// * All code units are taken literally except space, tab, quote and backslash.
	// * When not `in_quotes`, space and tab separate arguments. Consecutive spaces and tabs are
	// treated as a single separator.
	// * A space or tab `in_quotes` is taken literally.
	// * A quote toggles `in_quotes` mode unless it's escaped. An escaped quote is taken literally.
	// * A quote can be escaped if preceded by an odd number of backslashes.
	// * If any number of backslashes is immediately followed by a quote then the number of
	// backslashes is halved (rounding down).
	// * Backslashes not followed by a quote are all taken literally.
	// * If `in_quotes` then a quote can also be escaped using another quote
	// (i.e. two consecutive quotes become one literal quote).
	let mut cur = Vec::new();
	let mut in_quotes = false;
	while let Some(w) = code_units.next() {
	match w {
	// If not `in_quotes`, a space or tab ends the argument.
	SPACE \| TAB if !in_quotes => {
	ret_val.push(OsString::from_wide(&cur[..]));
	cur.truncate(0);

	// Skip whitespace.
	code_units.advance_while(\|w\| w == SPACE \|\| w == TAB);
	}
	// Backslashes can escape quotes or backslashes but only if consecutive backslashes are followed by a quote.
	BACKSLASH => {
	let backslash_count = code_units.advance_while(\|w\| w == BACKSLASH) + 1;
	if code_units.peek() == Some(QUOTE) {
	cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count / 2));
	// The quote is escaped if there are an odd number of backslashes.
	if backslash_count % 2 == 1 {
	code_units.next();
	cur.push(QUOTE.get());
	}
	} else {
	// If there is no quote on the end then there is no escaping.
	cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count));
	}
	}
	// If `in_quotes` and not backslash escaped (see above) then a quote either
	// unsets `in_quote` or is escaped by another quote.
	QUOTE if in_quotes => match code_units.peek() {
	// Two consecutive quotes when `in_quotes` produces one literal quote.
	Some(QUOTE) => {
	cur.push(QUOTE.get());
	code_units.next();
	}
	// Otherwise set `in_quotes`.
	Some(_) => in_quotes = false,
	// The end of the command line.
	// Push `cur` even if empty, which we do by breaking while `in_quotes` is still set.
	None => break,
	},
	// If not `in_quotes` and not BACKSLASH escaped (see above) then a quote sets `in_quote`.
	QUOTE => in_quotes = true,
	// Everything else is always taken literally.
	_ => cur.push(w.get()),
	}
	}
	// Push the final argument, if any.
	if !cur.is_empty() \|\| in_quotes {
	ret_val.push(OsString::from_wide(&cur[..]));
	}
	ret_val
	}

	pub struct Args {
	parsed_args_list: vec::IntoIter<OsString>,
	}

	impl fmt::Debug for Args {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	self.parsed_args_list.as_slice().fmt(f)
	}
	}

	impl Iterator for Args {
	type Item = OsString;
	fn next(&mut self) -> Option<OsString> {
	self.parsed_args_list.next()
	}
	fn size_hint(&self) -> (usize, Option<usize>) {
	self.parsed_args_list.size_hint()
	}
	}

	impl DoubleEndedIterator for Args {
	fn next_back(&mut self) -> Option<OsString> {
	self.parsed_args_list.next_back()
	}
	}

	impl ExactSizeIterator for Args {
	fn len(&self) -> usize {
	self.parsed_args_list.len()
	}
	}

	#[derive(Debug)]
	pub(crate) enum Arg {
	/// Add quotes (if needed)
	Regular(OsString),
	/// Append raw string without quoting
	Raw(OsString),
	}

	enum Quote {
	// Every arg is quoted
	Always,
	// Whitespace and empty args are quoted
	Auto,
	// Arg appended without any changes (#29494)
	Never,
	}

	pub(crate) fn append_arg(cmd: &mut Vec<u16>, arg: &Arg, force_quotes: bool) -> io::Result<()> {
	let (arg, quote) = match arg {
	Arg::Regular(arg) => (arg, if force_quotes { Quote::Always } else { Quote::Auto }),
	Arg::Raw(arg) => (arg, Quote::Never),
	};

	// If an argument has 0 characters then we need to quote it to ensure
	// that it actually gets passed through on the command line or otherwise
	// it will be dropped entirely when parsed on the other end.
	ensure_no_nuls(arg)?;
	let arg_bytes = arg.bytes();
	let (quote, escape) = match quote {
	Quote::Always => (true, true),
	Quote::Auto => {
	(arg_bytes.iter().any(\|c\| c == b' ' \|\| c == b'\t') \|\| arg_bytes.is_empty(), true)
	}
	Quote::Never => (false, false),
	};
	if quote {
	cmd.push('"' as u16);
	}

	let mut backslashes: usize = 0;
	for x in arg.encode_wide() {
	if escape {
	if x == '\\' as u16 {
	backslashes += 1;
	} else {
	if x == '"' as u16 {
	// Add n+1 backslashes to total 2n+1 before internal '"'.
	cmd.extend((0..=backslashes).map(\|_\| '\\' as u16));
	}
	backslashes = 0;
	}
	}
	cmd.push(x);
	}

	if quote {
	// Add n backslashes to total 2n before ending '"'.
	cmd.extend((0..backslashes).map(\|_\| '\\' as u16));
	cmd.push('"' as u16);
	}
	Ok(())
	}

	pub(crate) fn make_bat_command_line(
	script: &[u16],
	args: &[Arg],
	force_quotes: bool,
	) -> io::Result<Vec<u16>> {
	// Set the start of the command line to `cmd.exe /c "`
	// It is necessary to surround the command in an extra pair of quotes,
	// hence the trailing quote here. It will be closed after all arguments
	// have been added.
	let mut cmd: Vec<u16> = "cmd.exe /c \"".encode_utf16().collect();

	// Push the script name surrounded by its quote pair.
	cmd.push(b'"' as u16);
	// Windows file names cannot contain a `"` character or end with `\\`.
	// If the script name does then return an error.
	if script.contains(&(b'"' as u16)) \|\| script.last() == Some(&(b'\\' as u16)) {
	return Err(io::const_io_error!(
	io::ErrorKind::InvalidInput,
	"Windows file names may not contain `\"` or end with `\\`"
	));
	}
	cmd.extend_from_slice(script.strip_suffix(&[0]).unwrap_or(script));
	cmd.push(b'"' as u16);

	// Append the arguments.
	// FIXME: This needs tests to ensure that the arguments are properly
	// reconstructed by the batch script by default.
	for arg in args {
	cmd.push(' ' as u16);
	append_arg(&mut cmd, arg, force_quotes)?;
	}

	// Close the quote we left opened earlier.
	cmd.push(b'"' as u16);

	Ok(cmd)
	}

	/// Takes a path and tries to return a non-verbatim path.
	///
	/// This is necessary because cmd.exe does not support verbatim paths.
	pub(crate) fn to_user_path(mut path: Vec<u16>) -> io::Result<Vec<u16>> {
	use crate::ptr;
	use crate::sys::windows::fill_utf16_buf;

	// UTF-16 encoded code points, used in parsing and building UTF-16 paths.
	// All of these are in the ASCII range so they can be cast directly to `u16`.
	const SEP: u16 = b'\\' as _;
	const QUERY: u16 = b'?' as _;
	const COLON: u16 = b':' as _;
	const U: u16 = b'U' as _;
	const N: u16 = b'N' as _;
	const C: u16 = b'C' as _;

	// Early return if the path is too long to remove the verbatim prefix.
	const LEGACY_MAX_PATH: usize = 260;
	if path.len() > LEGACY_MAX_PATH {
	return Ok(path);
	}

	match &path[..] {
	// `\\?\C:\...` => `C:\...`
	[SEP, SEP, QUERY, SEP, _, COLON, SEP, ..] => unsafe {
	let lpfilename = path[4..].as_ptr();
	fill_utf16_buf(
	\|buffer, size\| c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()),
	\|full_path: &[u16]\| {
	if full_path == &path[4..path.len() - 1] { full_path.into() } else { path }
	},
	)
	},
	// `\\?\UNC\...` => `\\...`
	[SEP, SEP, QUERY, SEP, U, N, C, SEP, ..] => unsafe {
	// Change the `C` in `UNC\` to `\` so we can get a slice that starts with `\\`.
	path[6] = b'\\' as u16;
	let lpfilename = path[6..].as_ptr();
	fill_utf16_buf(
	\|buffer, size\| c::GetFullPathNameW(lpfilename, size, buffer, ptr::null_mut()),
	\|full_path: &[u16]\| {
	if full_path == &path[6..path.len() - 1] {
	full_path.into()
	} else {
	// Restore the 'C' in "UNC".
	path[6] = b'C' as u16;
	path
	}
	},
	)
	},
	// For everything else, leave the path unchanged.
	_ => Ok(path),
	}
	}