compiler/rustc_session/src/filesearch.rs - rust - Git at Google

 //! A module for searching for libraries

 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::{env, fs, iter};

 use rustc_fs_util::try_canonicalize;
 use rustc_target::spec::Target;

 use crate::search_paths::{PathKind, SearchPath};

 pub struct FileSearch {
     cli_search_paths: Vec<SearchPath>,
     tlib_path: SearchPath,
     use_implicit_sysroot_deps: bool,
     files: Vec<FileSearchCandidate>,
 }

 impl FileSearch {
     pub fn cli_search_paths<'b>(&'b self, kind: PathKind) -> impl Iterator<Item = &'b SearchPath> {
         self.cli_search_paths.iter().filter(move |sp| sp.kind.matches(kind))
     }

     pub fn search_paths<'b>(&'b self, kind: PathKind) -> impl Iterator<Item = &'b SearchPath> {
         // If the crate is `PathKind::Crate` (a top level dependency)
         // and `-Z implicit-sysroot-deps=false`, then don't include the sysroot in the search paths.
         let exclude_sysroot = kind.matches(PathKind::Crate) && !self.use_implicit_sysroot_deps;
         let maybe_tlib = (!exclude_sysroot).then_some(&self.tlib_path);

         self.cli_search_paths
             .iter()
             .filter(move |sp| sp.kind.matches(kind))
             .chain(maybe_tlib.into_iter())
     }

     /// Return files from the search dirs of this filesearch that match the given `prefix` and
     /// `suffix` and have the given `kind`.
     ///
     /// Note that this function only searches files that match lib/staticlib/dlllib prefixes, not
     /// all files from the search paths!
     /// Access `search_paths` directly if you want to scan all files within them.
     pub fn get_library_candidates<'b>(
         &'b self,
         prefix: &'b str,
         suffix: &'b str,
         kind: PathKind,
     ) -> impl Iterator<Item = (&'b str, PathBuf)> {
         let exclude_sysroot = kind.matches(PathKind::Crate) && !self.use_implicit_sysroot_deps;

         // The indices are clipped to have only a single iterator returned from this function, to
         // avoid allocating it.
         let start = self.files.partition_point(|v| *v.filename < *prefix).min(self.files.len());
         let end = self.files[start..].partition_point(|v| v.filename.starts_with(prefix));
         let prefixed_items = &self.files[start..][..end];

         prefixed_items
             .into_iter()
             .filter(move |c| {
                 c.kind.matches(kind)
                     && !(exclude_sysroot && c.from_sysroot)
                     && c.filename.ends_with(suffix)
             })
             .map(|c| (&c.filename[prefix.len()..c.filename.len() - suffix.len()], c.path()))
     }

     pub fn new(
         cli_search_paths: &[SearchPath],
         tlib_path: &SearchPath,
         target: &Target,
         use_implicit_sysroot_deps: bool,
     ) -> Self {
         // We keep a list of all found paths that look like libraries in `FileSearch`, to optimize
         // lookup in `get_library_candidates`.
         // These prefixes should be kept in sync with `CrateLocator::find_library_crate`.
         let prefixes = ["lib", &target.staticlib_prefix, &target.dll_prefix];

         // Load all files from all search paths, filter them by supported prefixes, and sort them,
         // so that we can efficiently look them up in `get_file_candidates` via binary search.
         let mut files: Vec<FileSearchCandidate> = Vec::with_capacity(cli_search_paths.len());
         for (search_path, is_sysroot) in
             cli_search_paths.iter().map(|path| (path, false)).chain(iter::once((tlib_path, true)))
         {
             let Ok(dir) = fs::read_dir(&search_path.dir) else {
                 continue;
             };
             files.extend(dir.filter_map(|entry| {
                 let entry = entry.ok()?;

                 let filename = entry.file_name();
                 let filename = filename.to_str()?;

                 if !prefixes.iter().any(|prefix| filename.starts_with(prefix)) {
                     return None;
                 }
                 Some(FileSearchCandidate {
                     dir: Arc::clone(&search_path.dir),
                     filename: filename.into(),
                     kind: search_path.kind,
                     from_sysroot: is_sysroot,
                 })
             }));
         }
         files.sort_unstable_by(|lhs, rhs| lhs.filename.cmp(&rhs.filename));

         FileSearch {
             cli_search_paths: cli_search_paths.to_owned(),
             tlib_path: tlib_path.clone(),
             use_implicit_sysroot_deps,
             files,
         }
     }
 }

 /// This type stores `Box<str>` instead of `PathBuf` for the filename, because getting the
 /// `file_name` of a `PathBuf` allocates, which is unnecessary. We have to go through the files
 /// a lot of times, so storing file name and the directory separately saves time and memory.
 ///
 /// The filename must be valid UTF-8. If it's not, the entry should be skipped, because all Rust
 /// output files are valid UTF-8, and so a non-UTF-8 filename couldn't be one we're looking for.
 #[derive(Debug)]
 struct FileSearchCandidate {
     dir: Arc<Path>,
     filename: Box<str>,
     kind: PathKind,
     /// Was this file added through the target sysroot?
     from_sysroot: bool,
 }

 impl FileSearchCandidate {
     /// Constructs the full path to the file.
     fn path(&self) -> PathBuf {
         self.dir.join(&*self.filename)
     }
 }

 pub fn make_target_lib_path(sysroot: &Path, target_triple: &str) -> PathBuf {
     let rustlib_path = rustc_target::relative_target_rustlib_path(sysroot, target_triple);
     sysroot.join(rustlib_path).join("lib")
 }

 /// Returns a path to the target's `bin` folder within its `rustlib` path in the sysroot. This is
 /// where binaries are usually installed, e.g. the self-contained linkers, lld-wrappers, LLVM tools,
 /// etc.
 pub fn make_target_bin_path(sysroot: &Path, target_triple: &str) -> PathBuf {
     let rustlib_path = rustc_target::relative_target_rustlib_path(sysroot, target_triple);
     sysroot.join(rustlib_path).join("bin")
 }

 #[cfg(unix)]
 fn current_dll_path() -> Result<PathBuf, String> {
     use std::sync::OnceLock;

     // This is somewhat expensive relative to other work when compiling `fn main() {}` as `dladdr`
     // needs to iterate over the symbol table of librustc_driver.so until it finds a match.
     // As such cache this to avoid recomputing if we try to get the sysroot in multiple places.
     static CURRENT_DLL_PATH: OnceLock<Result<PathBuf, String>> = OnceLock::new();
     CURRENT_DLL_PATH
         .get_or_init(|| {
             use std::ffi::{CStr, OsStr};
             use std::os::unix::prelude::*;

             #[cfg(not(target_os = "aix"))]
             unsafe {
                 let addr = current_dll_path as fn() -> Result<PathBuf, String> as *mut _;
                 let mut info = std::mem::zeroed();
                 if libc::dladdr(addr, &mut info) == 0 {
                     return Err("dladdr failed".into());
                 }
                 #[cfg(target_os = "cygwin")]
                 let fname_ptr = info.dli_fname.as_ptr();
                 #[cfg(not(target_os = "cygwin"))]
                 let fname_ptr = {
                     assert!(!info.dli_fname.is_null(), "dli_fname cannot be null");
                     info.dli_fname
                 };
                 let bytes = CStr::from_ptr(fname_ptr).to_bytes();
                 let os = OsStr::from_bytes(bytes);
                 try_canonicalize(Path::new(os)).map_err(|e| e.to_string())
             }

             #[cfg(target_os = "aix")]
             unsafe {
                 // On AIX, the symbol `current_dll_path` references a function descriptor.
                 // A function descriptor is consisted of (See https://reviews.llvm.org/D62532)
                 // * The address of the entry point of the function.
                 // * The TOC base address for the function.
                 // * The environment pointer.
                 // The function descriptor is in the data section.
                 let addr = current_dll_path as u64;
                 let mut buffer = vec![std::mem::zeroed::<libc::ld_info>(); 64];
                 loop {
                     if libc::loadquery(
                         libc::L_GETINFO,
                         buffer.as_mut_ptr() as *mut libc::c_void,
                         (size_of::<libc::ld_info>() * buffer.len()) as u32,
                     ) >= 0
                     {
                         break;
                     } else {
                         if std::io::Error::last_os_error().raw_os_error().unwrap() != libc::ENOMEM {
                             return Err("loadquery failed".into());
                         }
                         buffer.resize(buffer.len() * 2, std::mem::zeroed::<libc::ld_info>());
                     }
                 }
                 let mut current = buffer.as_mut_ptr() as *mut libc::ld_info;
                 loop {
                     let data_base = (*current).ldinfo_dataorg as u64;
                     let data_end = data_base + (*current).ldinfo_datasize;
                     if (data_base..data_end).contains(&addr) {
                         let bytes = CStr::from_ptr(&(*current).ldinfo_filename[0]).to_bytes();
                         let os = OsStr::from_bytes(bytes);
                         return try_canonicalize(Path::new(os)).map_err(|e| e.to_string());
                     }
                     if (*current).ldinfo_next == 0 {
                         break;
                     }
                     current = (current as *mut i8).offset((*current).ldinfo_next as isize)
                         as *mut libc::ld_info;
                 }
                 return Err(format!("current dll's address {} is not in the load map", addr));
             }
         })
         .clone()
 }

 #[cfg(windows)]
 fn current_dll_path() -> Result<PathBuf, String> {
     use std::ffi::OsString;
     use std::io;
     use std::os::windows::prelude::*;

     use windows::Win32::Foundation::HMODULE;
     use windows::Win32::System::LibraryLoader::{
         GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS, GetModuleFileNameW, GetModuleHandleExW,
     };
     use windows::core::PCWSTR;

     let mut module = HMODULE::default();
     unsafe {
         GetModuleHandleExW(
             GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
             PCWSTR(
                 current_dll_path as fn() -> Result<std::path::PathBuf, std::string::String>
                     as *mut u16,
             ),
             &mut module,
         )
     }
     .map_err(|e| e.to_string())?;

     let mut filename = vec![0; 1024];
     let n = unsafe { GetModuleFileNameW(Some(module), &mut filename) } as usize;
     if n == 0 {
         return Err(format!("GetModuleFileNameW failed: {}", io::Error::last_os_error()));
     }
     if n >= filename.capacity() {
         return Err(format!("our buffer was too small? {}", io::Error::last_os_error()));
     }

     filename.truncate(n);

     let path = try_canonicalize(OsString::from_wide(&filename)).map_err(|e| e.to_string())?;

     // See comments on this target function, but the gist is that
     // gcc chokes on verbatim paths which fs::canonicalize generates
     // so we try to avoid those kinds of paths.
     Ok(rustc_fs_util::fix_windows_verbatim_for_gcc(&path))
 }

 #[cfg(target_os = "wasi")]
 fn current_dll_path() -> Result<PathBuf, String> {
     Err("current_dll_path is not supported on WASI".to_string())
 }

 /// This function checks if sysroot is found using env::args().next(), and if it
 /// is not found, finds sysroot from current rustc_driver dll.
 pub(crate) fn default_sysroot() -> PathBuf {
     fn default_from_rustc_driver_dll() -> Result<PathBuf, String> {
         let dll = current_dll_path()?;

         // `dll` will be in one of the following two:
         // - compiler's libdir: $sysroot/lib/*.dll
         // - target's libdir: $sysroot/lib/rustlib/$target/lib/*.dll
         //
         // use `parent` twice to chop off the file name and then also the
         // directory containing the dll
         let dir = dll.parent().and_then(|p| p.parent()).ok_or_else(|| {
             format!("Could not move 2 levels upper using `parent()` on {}", dll.display())
         })?;

         // if `dir` points to target's dir, move up to the sysroot
         let mut sysroot_dir = if dir.ends_with(crate::config::host_tuple()) {
             dir.parent() // chop off `$target`
                 .and_then(|p| p.parent()) // chop off `rustlib`
                 .and_then(|p| p.parent()) // chop off `lib`
                 .map(|s| s.to_owned())
                 .ok_or_else(|| {
                     format!("Could not move 3 levels upper using `parent()` on {}", dir.display())
                 })?
         } else {
             dir.to_owned()
         };

         // On multiarch linux systems, there will be multiarch directory named
         // with the architecture(e.g `x86_64-linux-gnu`) under the `lib` directory.
         // Which cause us to mistakenly end up in the lib directory instead of the sysroot directory.
         if sysroot_dir.ends_with("lib") {
             sysroot_dir =
                 sysroot_dir.parent().map(|real_sysroot| real_sysroot.to_owned()).ok_or_else(
                     || format!("Could not move to parent path of {}", sysroot_dir.display()),
                 )?
         }

         Ok(sysroot_dir)
     }

     // Use env::args().next() to get the path of the executable without
     // following symlinks/canonicalizing any component. This makes the rustc
     // binary able to locate Rust libraries in systems using content-addressable
     // storage (CAS).
     fn from_env_args_next() -> Option<PathBuf> {
         let mut p = PathBuf::from(env::args_os().next()?);

         // Check if sysroot is found using env::args().next() only if the rustc in argv[0]
         // is a symlink (see #79253). We might want to change/remove it to conform with
         // https://www.gnu.org/prep/standards/standards.html#Finding-Program-Files in the
         // future.
         if fs::read_link(&p).is_err() {
             // Path is not a symbolic link or does not exist.
             return None;
         }

         // Pop off `bin/rustc`, obtaining the suspected sysroot.
         p.pop();
         p.pop();
         // Look for the target rustlib directory in the suspected sysroot.
         let mut rustlib_path = rustc_target::relative_target_rustlib_path(&p, "dummy");
         rustlib_path.pop(); // pop off the dummy target.
         rustlib_path.exists().then_some(p)
     }

     from_env_args_next()
         .unwrap_or_else(|| default_from_rustc_driver_dll().expect("Failed finding sysroot"))
 }
	//! A module for searching for libraries

	use std::path::{Path, PathBuf};
	use std::sync::Arc;
	use std::{env, fs, iter};

	use rustc_fs_util::try_canonicalize;
	use rustc_target::spec::Target;

	use crate::search_paths::{PathKind, SearchPath};

	pub struct FileSearch {
	cli_search_paths: Vec<SearchPath>,
	tlib_path: SearchPath,
	use_implicit_sysroot_deps: bool,
	files: Vec<FileSearchCandidate>,
	}

	impl FileSearch {
	pub fn cli_search_paths<'b>(&'b self, kind: PathKind) -> impl Iterator<Item = &'b SearchPath> {
	self.cli_search_paths.iter().filter(move \|sp\| sp.kind.matches(kind))
	}

	pub fn search_paths<'b>(&'b self, kind: PathKind) -> impl Iterator<Item = &'b SearchPath> {
	// If the crate is `PathKind::Crate` (a top level dependency)
	// and `-Z implicit-sysroot-deps=false`, then don't include the sysroot in the search paths.
	let exclude_sysroot = kind.matches(PathKind::Crate) && !self.use_implicit_sysroot_deps;
	let maybe_tlib = (!exclude_sysroot).then_some(&self.tlib_path);

	self.cli_search_paths
	.iter()
	.filter(move \|sp\| sp.kind.matches(kind))
	.chain(maybe_tlib.into_iter())
	}

	/// Return files from the search dirs of this filesearch that match the given `prefix` and
	/// `suffix` and have the given `kind`.
	///
	/// Note that this function only searches files that match lib/staticlib/dlllib prefixes, not
	/// all files from the search paths!
	/// Access `search_paths` directly if you want to scan all files within them.
	pub fn get_library_candidates<'b>(
	&'b self,
	prefix: &'b str,
	suffix: &'b str,
	kind: PathKind,
	) -> impl Iterator<Item = (&'b str, PathBuf)> {
	let exclude_sysroot = kind.matches(PathKind::Crate) && !self.use_implicit_sysroot_deps;

	// The indices are clipped to have only a single iterator returned from this function, to
	// avoid allocating it.
	let start = self.files.partition_point(\|v\| v.filename < prefix).min(self.files.len());
	let end = self.files[start..].partition_point(\|v\| v.filename.starts_with(prefix));
	let prefixed_items = &self.files[start..][..end];

	prefixed_items
	.into_iter()
	.filter(move \|c\| {
	c.kind.matches(kind)
	&& !(exclude_sysroot && c.from_sysroot)
	&& c.filename.ends_with(suffix)
	})
	.map(\|c\| (&c.filename[prefix.len()..c.filename.len() - suffix.len()], c.path()))
	}

	pub fn new(
	cli_search_paths: &[SearchPath],
	tlib_path: &SearchPath,
	target: &Target,
	use_implicit_sysroot_deps: bool,
	) -> Self {
	// We keep a list of all found paths that look like libraries in `FileSearch`, to optimize
	// lookup in `get_library_candidates`.
	// These prefixes should be kept in sync with `CrateLocator::find_library_crate`.
	let prefixes = ["lib", &target.staticlib_prefix, &target.dll_prefix];

	// Load all files from all search paths, filter them by supported prefixes, and sort them,
	// so that we can efficiently look them up in `get_file_candidates` via binary search.
	let mut files: Vec<FileSearchCandidate> = Vec::with_capacity(cli_search_paths.len());
	for (search_path, is_sysroot) in
	cli_search_paths.iter().map(\|path\| (path, false)).chain(iter::once((tlib_path, true)))
	{
	let Ok(dir) = fs::read_dir(&search_path.dir) else {
	continue;
	};
	files.extend(dir.filter_map(\|entry\| {
	let entry = entry.ok()?;

	let filename = entry.file_name();
	let filename = filename.to_str()?;

	if !prefixes.iter().any(\|prefix\| filename.starts_with(prefix)) {
	return None;
	}
	Some(FileSearchCandidate {
	dir: Arc::clone(&search_path.dir),
	filename: filename.into(),
	kind: search_path.kind,
	from_sysroot: is_sysroot,
	})
	}));
	}
	files.sort_unstable_by(\|lhs, rhs\| lhs.filename.cmp(&rhs.filename));

	FileSearch {
	cli_search_paths: cli_search_paths.to_owned(),
	tlib_path: tlib_path.clone(),
	use_implicit_sysroot_deps,
	files,
	}
	}
	}

	/// This type stores `Box<str>` instead of `PathBuf` for the filename, because getting the
	/// `file_name` of a `PathBuf` allocates, which is unnecessary. We have to go through the files
	/// a lot of times, so storing file name and the directory separately saves time and memory.
	///
	/// The filename must be valid UTF-8. If it's not, the entry should be skipped, because all Rust
	/// output files are valid UTF-8, and so a non-UTF-8 filename couldn't be one we're looking for.
	#[derive(Debug)]
	struct FileSearchCandidate {
	dir: Arc<Path>,
	filename: Box<str>,
	kind: PathKind,
	/// Was this file added through the target sysroot?
	from_sysroot: bool,
	}

	impl FileSearchCandidate {
	/// Constructs the full path to the file.
	fn path(&self) -> PathBuf {
	self.dir.join(&*self.filename)
	}
	}

	pub fn make_target_lib_path(sysroot: &Path, target_triple: &str) -> PathBuf {
	let rustlib_path = rustc_target::relative_target_rustlib_path(sysroot, target_triple);
	sysroot.join(rustlib_path).join("lib")
	}

	/// Returns a path to the target's `bin` folder within its `rustlib` path in the sysroot. This is
	/// where binaries are usually installed, e.g. the self-contained linkers, lld-wrappers, LLVM tools,
	/// etc.
	pub fn make_target_bin_path(sysroot: &Path, target_triple: &str) -> PathBuf {
	let rustlib_path = rustc_target::relative_target_rustlib_path(sysroot, target_triple);
	sysroot.join(rustlib_path).join("bin")
	}

	#[cfg(unix)]
	fn current_dll_path() -> Result<PathBuf, String> {
	use std::sync::OnceLock;

	// This is somewhat expensive relative to other work when compiling `fn main() {}` as `dladdr`
	// needs to iterate over the symbol table of librustc_driver.so until it finds a match.
	// As such cache this to avoid recomputing if we try to get the sysroot in multiple places.
	static CURRENT_DLL_PATH: OnceLock<Result<PathBuf, String>> = OnceLock::new();
	CURRENT_DLL_PATH
	.get_or_init(\|\| {
	use std::ffi::{CStr, OsStr};
	use std::os::unix::prelude::*;

	#[cfg(not(target_os = "aix"))]
	unsafe {
	let addr = current_dll_path as fn() -> Result<PathBuf, String> as *mut _;
	let mut info = std::mem::zeroed();
	if libc::dladdr(addr, &mut info) == 0 {
	return Err("dladdr failed".into());
	}
	#[cfg(target_os = "cygwin")]
	let fname_ptr = info.dli_fname.as_ptr();
	#[cfg(not(target_os = "cygwin"))]
	let fname_ptr = {
	assert!(!info.dli_fname.is_null(), "dli_fname cannot be null");
	info.dli_fname
	};
	let bytes = CStr::from_ptr(fname_ptr).to_bytes();
	let os = OsStr::from_bytes(bytes);
	try_canonicalize(Path::new(os)).map_err(\|e\| e.to_string())
	}

	#[cfg(target_os = "aix")]
	unsafe {
	// On AIX, the symbol `current_dll_path` references a function descriptor.
	// A function descriptor is consisted of (See https://reviews.llvm.org/D62532)
	// * The address of the entry point of the function.
	// * The TOC base address for the function.
	// * The environment pointer.
	// The function descriptor is in the data section.
	let addr = current_dll_path as u64;
	let mut buffer = vec![std::mem::zeroed::<libc::ld_info>(); 64];
	loop {
	if libc::loadquery(
	libc::L_GETINFO,
	buffer.as_mut_ptr() as *mut libc::c_void,
	(size_of::<libc::ld_info>() * buffer.len()) as u32,
	) >= 0
	{
	break;
	} else {
	if std::io::Error::last_os_error().raw_os_error().unwrap() != libc::ENOMEM {
	return Err("loadquery failed".into());
	}
	buffer.resize(buffer.len() * 2, std::mem::zeroed::<libc::ld_info>());
	}
	}
	let mut current = buffer.as_mut_ptr() as *mut libc::ld_info;
	loop {
	let data_base = (*current).ldinfo_dataorg as u64;
	let data_end = data_base + (*current).ldinfo_datasize;
	if (data_base..data_end).contains(&addr) {
	let bytes = CStr::from_ptr(&(*current).ldinfo_filename[0]).to_bytes();
	let os = OsStr::from_bytes(bytes);
	return try_canonicalize(Path::new(os)).map_err(\|e\| e.to_string());
	}
	if (*current).ldinfo_next == 0 {
	break;
	}
	current = (current as mut i8).offset((current).ldinfo_next as isize)
	as *mut libc::ld_info;
	}
	return Err(format!("current dll's address {} is not in the load map", addr));
	}
	})
	.clone()
	}

	#[cfg(windows)]
	fn current_dll_path() -> Result<PathBuf, String> {
	use std::ffi::OsString;
	use std::io;
	use std::os::windows::prelude::*;

	use windows::Win32::Foundation::HMODULE;
	use windows::Win32::System::LibraryLoader::{
	GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS, GetModuleFileNameW, GetModuleHandleExW,
	};
	use windows::core::PCWSTR;

	let mut module = HMODULE::default();
	unsafe {
	GetModuleHandleExW(
	GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
	PCWSTR(
	current_dll_path as fn() -> Result<std::path::PathBuf, std::string::String>
	as *mut u16,
	),
	&mut module,
	)
	}
	.map_err(\|e\| e.to_string())?;

	let mut filename = vec![0; 1024];
	let n = unsafe { GetModuleFileNameW(Some(module), &mut filename) } as usize;
	if n == 0 {
	return Err(format!("GetModuleFileNameW failed: {}", io::Error::last_os_error()));
	}
	if n >= filename.capacity() {
	return Err(format!("our buffer was too small? {}", io::Error::last_os_error()));
	}

	filename.truncate(n);

	let path = try_canonicalize(OsString::from_wide(&filename)).map_err(\|e\| e.to_string())?;

	// See comments on this target function, but the gist is that
	// gcc chokes on verbatim paths which fs::canonicalize generates
	// so we try to avoid those kinds of paths.
	Ok(rustc_fs_util::fix_windows_verbatim_for_gcc(&path))
	}

	#[cfg(target_os = "wasi")]
	fn current_dll_path() -> Result<PathBuf, String> {
	Err("current_dll_path is not supported on WASI".to_string())
	}

	/// This function checks if sysroot is found using env::args().next(), and if it
	/// is not found, finds sysroot from current rustc_driver dll.
	pub(crate) fn default_sysroot() -> PathBuf {
	fn default_from_rustc_driver_dll() -> Result<PathBuf, String> {
	let dll = current_dll_path()?;

	// `dll` will be in one of the following two:
	// - compiler's libdir: $sysroot/lib/*.dll
	// - target's libdir: $sysroot/lib/rustlib/$target/lib/*.dll
	//
	// use `parent` twice to chop off the file name and then also the
	// directory containing the dll
	let dir = dll.parent().and_then(\|p\| p.parent()).ok_or_else(\|\| {
	format!("Could not move 2 levels upper using `parent()` on {}", dll.display())
	})?;

	// if `dir` points to target's dir, move up to the sysroot
	let mut sysroot_dir = if dir.ends_with(crate::config::host_tuple()) {
	dir.parent() // chop off `$target`
	.and_then(\|p\| p.parent()) // chop off `rustlib`
	.and_then(\|p\| p.parent()) // chop off `lib`
	.map(\|s\| s.to_owned())
	.ok_or_else(\|\| {
	format!("Could not move 3 levels upper using `parent()` on {}", dir.display())
	})?
	} else {
	dir.to_owned()
	};

	// On multiarch linux systems, there will be multiarch directory named
	// with the architecture(e.g `x86_64-linux-gnu`) under the `lib` directory.
	// Which cause us to mistakenly end up in the lib directory instead of the sysroot directory.
	if sysroot_dir.ends_with("lib") {
	sysroot_dir =
	sysroot_dir.parent().map(\|real_sysroot\| real_sysroot.to_owned()).ok_or_else(
	\|\| format!("Could not move to parent path of {}", sysroot_dir.display()),
	)?
	}

	Ok(sysroot_dir)
	}

	// Use env::args().next() to get the path of the executable without
	// following symlinks/canonicalizing any component. This makes the rustc
	// binary able to locate Rust libraries in systems using content-addressable
	// storage (CAS).
	fn from_env_args_next() -> Option<PathBuf> {
	let mut p = PathBuf::from(env::args_os().next()?);

	// Check if sysroot is found using env::args().next() only if the rustc in argv[0]
	// is a symlink (see #79253). We might want to change/remove it to conform with
	// https://www.gnu.org/prep/standards/standards.html#Finding-Program-Files in the
	// future.
	if fs::read_link(&p).is_err() {
	// Path is not a symbolic link or does not exist.
	return None;
	}

	// Pop off `bin/rustc`, obtaining the suspected sysroot.
	p.pop();
	p.pop();
	// Look for the target rustlib directory in the suspected sysroot.
	let mut rustlib_path = rustc_target::relative_target_rustlib_path(&p, "dummy");
	rustlib_path.pop(); // pop off the dummy target.
	rustlib_path.exists().then_some(p)
	}

	from_env_args_next()
	.unwrap_or_else(\|\| default_from_rustc_driver_dll().expect("Failed finding sysroot"))
	}