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-rw-r--r--library/std/src/collections/hash/map.rs7
-rw-r--r--library/std/src/collections/mod.rs12
-rw-r--r--library/std/src/env.rs10
-rw-r--r--library/std/src/ffi/os_str.rs2
-rw-r--r--library/std/src/fs.rs24
-rw-r--r--library/std/src/io/buffered/bufreader/buffer.rs2
-rw-r--r--library/std/src/io/buffered/bufwriter.rs12
-rw-r--r--library/std/src/io/buffered/mod.rs7
-rw-r--r--library/std/src/io/copy.rs7
-rw-r--r--library/std/src/io/error.rs11
-rw-r--r--library/std/src/io/mod.rs9
-rw-r--r--library/std/src/lib.rs21
-rw-r--r--library/std/src/os/unix/net/stream.rs10
-rw-r--r--library/std/src/os/unix/process.rs12
-rw-r--r--library/std/src/os/windows/ffi.rs38
-rw-r--r--library/std/src/os/windows/io/socket.rs2
-rw-r--r--library/std/src/path.rs77
-rw-r--r--library/std/src/sync/lazy_lock.rs9
-rw-r--r--library/std/src/sync/mpmc/utils.rs8
-rw-r--r--library/std/src/sync/mpsc.rs52
-rw-r--r--library/std/src/sync/nonpoison.rs5
-rw-r--r--library/std/src/sync/nonpoison/mutex.rs2
-rw-r--r--library/std/src/sync/nonpoison/rwlock.rs1081
-rw-r--r--library/std/src/sync/poison.rs18
-rw-r--r--library/std/src/sync/poison/mutex.rs2
-rw-r--r--library/std/src/sync/poison/rwlock.rs699
-rw-r--r--library/std/src/sync/reentrant_lock.rs9
-rw-r--r--library/std/src/sys/alloc/mod.rs30
-rw-r--r--library/std/src/sys/alloc/unix.rs10
-rw-r--r--library/std/src/sys/alloc/wasm.rs17
-rw-r--r--library/std/src/sys/anonymous_pipe/mod.rs10
-rw-r--r--library/std/src/sys/args/mod.rs27
-rw-r--r--library/std/src/sys/backtrace.rs2
-rw-r--r--library/std/src/sys/cmath.rs111
-rw-r--r--library/std/src/sys/env/mod.rs31
-rw-r--r--library/std/src/sys/env/wasi.rs7
-rw-r--r--library/std/src/sys/exit_guard.rs7
-rw-r--r--library/std/src/sys/fd/mod.rs14
-rw-r--r--library/std/src/sys/fd/unix.rs6
-rw-r--r--library/std/src/sys/fs/mod.rs46
-rw-r--r--library/std/src/sys/fs/unix.rs54
-rw-r--r--library/std/src/sys/io/mod.rs29
-rw-r--r--library/std/src/sys/mod.rs2
-rw-r--r--library/std/src/sys/net/connection/sgx.rs7
-rw-r--r--library/std/src/sys/net/connection/socket.rs56
-rw-r--r--library/std/src/sys/net/connection/socket/unix.rs34
-rw-r--r--library/std/src/sys/net/mod.rs21
-rw-r--r--library/std/src/sys/os_str/mod.rs10
-rw-r--r--library/std/src/sys/os_str/wtf8.rs8
-rw-r--r--library/std/src/sys/pal/hermit/os.rs8
-rw-r--r--library/std/src/sys/pal/mod.rs54
-rw-r--r--library/std/src/sys/pal/sgx/mod.rs3
-rw-r--r--library/std/src/sys/pal/sgx/os.rs10
-rw-r--r--library/std/src/sys/pal/solid/os.rs8
-rw-r--r--library/std/src/sys/pal/teeos/os.rs8
-rw-r--r--library/std/src/sys/pal/uefi/os.rs3
-rw-r--r--library/std/src/sys/pal/uefi/time.rs13
-rw-r--r--library/std/src/sys/pal/unix/futex.rs10
-rw-r--r--library/std/src/sys/pal/unix/mod.rs82
-rw-r--r--library/std/src/sys/pal/unix/os.rs31
-rw-r--r--library/std/src/sys/pal/unix/pipe.rs9
-rw-r--r--library/std/src/sys/pal/unix/thread.rs81
-rw-r--r--library/std/src/sys/pal/unsupported/os.rs8
-rw-r--r--library/std/src/sys/pal/wasi/os.rs8
-rw-r--r--library/std/src/sys/pal/wasi/thread.rs28
-rw-r--r--library/std/src/sys/pal/wasm/mod.rs7
-rw-r--r--library/std/src/sys/pal/windows/c.rs12
-rw-r--r--library/std/src/sys/pal/windows/c/bindings.txt2
-rw-r--r--library/std/src/sys/pal/windows/c/windows_sys.rs2
-rw-r--r--library/std/src/sys/pal/windows/mod.rs20
-rw-r--r--library/std/src/sys/pal/windows/os.rs8
-rw-r--r--library/std/src/sys/pal/windows/thread.rs2
-rw-r--r--library/std/src/sys/pal/xous/os.rs8
-rw-r--r--library/std/src/sys/pal/zkvm/os.rs8
-rw-r--r--library/std/src/sys/path/mod.rs19
-rw-r--r--library/std/src/sys/personality/gcc.rs22
-rw-r--r--library/std/src/sys/personality/mod.rs15
-rw-r--r--library/std/src/sys/process/mod.rs13
-rw-r--r--library/std/src/sys/process/unix/common.rs17
-rw-r--r--library/std/src/sys/process/unix/mod.rs13
-rw-r--r--library/std/src/sys/process/unix/unix.rs19
-rw-r--r--library/std/src/sys/random/mod.rs68
-rw-r--r--library/std/src/sys/random/uefi.rs7
-rw-r--r--library/std/src/sys/stdio/mod.rs37
-rw-r--r--library/std/src/sys/sync/condvar/mod.rs26
-rw-r--r--library/std/src/sys/sync/mutex/mod.rs29
-rw-r--r--library/std/src/sys/sync/once/mod.rs14
-rw-r--r--library/std/src/sys/sync/rwlock/mod.rs20
-rw-r--r--library/std/src/sys/sync/thread_parking/mod.rs26
-rw-r--r--library/std/src/sys/thread_local/mod.rs60
-rw-r--r--library/std/src/sys_common/mod.rs14
-rw-r--r--library/std/src/sys_common/wtf8.rs1083
-rw-r--r--library/std/src/sys_common/wtf8/tests.rs751
-rw-r--r--library/std/src/thread/current.rs13
-rw-r--r--library/std/src/thread/local.rs8
-rw-r--r--library/std/src/thread/mod.rs14
-rw-r--r--library/std/src/time.rs7
97 files changed, 2470 insertions, 2945 deletions
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index 15a7a770d1a..fc0fef620e3 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -1875,12 +1875,7 @@ impl<'a, K: Debug, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
 }
 
 #[unstable(feature = "map_try_insert", issue = "82766")]
-impl<'a, K: fmt::Debug, V: fmt::Debug> Error for OccupiedError<'a, K, V> {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "key already exists"
-    }
-}
+impl<'a, K: Debug, V: Debug> Error for OccupiedError<'a, K, V> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> {
diff --git a/library/std/src/collections/mod.rs b/library/std/src/collections/mod.rs
index 889ed3c5380..6104a02c739 100644
--- a/library/std/src/collections/mod.rs
+++ b/library/std/src/collections/mod.rs
@@ -26,7 +26,7 @@
 //! should be considered. Detailed discussions of strengths and weaknesses of
 //! individual collections can be found on their own documentation pages.
 //!
-//! ### Use a `Vec` when:
+//! ### Use a [`Vec`] when:
 //! * You want to collect items up to be processed or sent elsewhere later, and
 //!   don't care about any properties of the actual values being stored.
 //! * You want a sequence of elements in a particular order, and will only be
@@ -35,25 +35,25 @@
 //! * You want a resizable array.
 //! * You want a heap-allocated array.
 //!
-//! ### Use a `VecDeque` when:
+//! ### Use a [`VecDeque`] when:
 //! * You want a [`Vec`] that supports efficient insertion at both ends of the
 //!   sequence.
 //! * You want a queue.
 //! * You want a double-ended queue (deque).
 //!
-//! ### Use a `LinkedList` when:
+//! ### Use a [`LinkedList`] when:
 //! * You want a [`Vec`] or [`VecDeque`] of unknown size, and can't tolerate
 //!   amortization.
 //! * You want to efficiently split and append lists.
 //! * You are *absolutely* certain you *really*, *truly*, want a doubly linked
 //!   list.
 //!
-//! ### Use a `HashMap` when:
+//! ### Use a [`HashMap`] when:
 //! * You want to associate arbitrary keys with an arbitrary value.
 //! * You want a cache.
 //! * You want a map, with no extra functionality.
 //!
-//! ### Use a `BTreeMap` when:
+//! ### Use a [`BTreeMap`] when:
 //! * You want a map sorted by its keys.
 //! * You want to be able to get a range of entries on-demand.
 //! * You're interested in what the smallest or largest key-value pair is.
@@ -65,7 +65,7 @@
 //! * There is no meaningful value to associate with your keys.
 //! * You just want a set.
 //!
-//! ### Use a `BinaryHeap` when:
+//! ### Use a [`BinaryHeap`] when:
 //!
 //! * You want to store a bunch of elements, but only ever want to process the
 //!   "biggest" or "most important" one at any given time.
diff --git a/library/std/src/env.rs b/library/std/src/env.rs
index 9f17ff76445..e457cd61c75 100644
--- a/library/std/src/env.rs
+++ b/library/std/src/env.rs
@@ -296,15 +296,7 @@ impl fmt::Display for VarError {
 }
 
 #[stable(feature = "env", since = "1.0.0")]
-impl Error for VarError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        match *self {
-            VarError::NotPresent => "environment variable not found",
-            VarError::NotUnicode(..) => "environment variable was not valid unicode",
-        }
-    }
-}
+impl Error for VarError {}
 
 /// Sets the environment variable `key` to the value `value` for the currently running
 /// process.
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index 8d7edc732af..1214490caad 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -137,7 +137,7 @@ impl OsString {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    #[rustc_const_unstable(feature = "const_pathbuf_osstring_new", issue = "141520")]
+    #[rustc_const_stable(feature = "const_pathbuf_osstring_new", since = "CURRENT_RUSTC_VERSION")]
     pub const fn new() -> OsString {
         OsString { inner: Buf::from_string(String::new()) }
     }
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index a220a3f56e9..d9c9606fc1c 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -3036,6 +3036,9 @@ pub fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// Entries for the current and parent directories (typically `.` and `..`) are
 /// skipped.
 ///
+/// The order in which `read_dir` returns entries can change between calls. If reproducible
+/// ordering is required, the entries should be explicitly sorted.
+///
 /// # Platform-specific behavior
 ///
 /// This function currently corresponds to the `opendir` function on Unix
@@ -3119,7 +3122,7 @@ pub fn read_dir<P: AsRef<Path>>(path: P) -> io::Result<ReadDir> {
 /// On UNIX-like systems, this function will update the permission bits
 /// of the file pointed to by the symlink.
 ///
-/// Note that this behavior can lead to privalage escalation vulnerabilities,
+/// Note that this behavior can lead to privilege escalation vulnerabilities,
 /// where the ability to create a symlink in one directory allows you to
 /// cause the permissions of another file or directory to be modified.
 ///
@@ -3156,6 +3159,25 @@ pub fn set_permissions<P: AsRef<Path>>(path: P, perm: Permissions) -> io::Result
     fs_imp::set_permissions(path.as_ref(), perm.0)
 }
 
+/// Set the permissions of a file, unless it is a symlink.
+///
+/// Note that the non-final path elements are allowed to be symlinks.
+///
+/// # Platform-specific behavior
+///
+/// Currently unimplemented on Windows.
+///
+/// On Unix platforms, this results in a [`FilesystemLoop`] error if the last element is a symlink.
+///
+/// This behavior may change in the future.
+///
+/// [`FilesystemLoop`]: crate::io::ErrorKind::FilesystemLoop
+#[doc(alias = "chmod", alias = "SetFileAttributes")]
+#[unstable(feature = "set_permissions_nofollow", issue = "141607")]
+pub fn set_permissions_nofollow<P: AsRef<Path>>(path: P, perm: Permissions) -> io::Result<()> {
+    fs_imp::set_permissions_nofollow(path.as_ref(), perm)
+}
+
 impl DirBuilder {
     /// Creates a new set of options with default mode/security settings for all
     /// platforms and also non-recursive.
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 574288e579e..9b600cd5575 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -122,7 +122,7 @@ impl Buffer {
 
     /// Remove bytes that have already been read from the buffer.
     pub fn backshift(&mut self) {
-        self.buf.copy_within(self.pos.., 0);
+        self.buf.copy_within(self.pos..self.filled, 0);
         self.filled -= self.pos;
         self.pos = 0;
     }
diff --git a/library/std/src/io/buffered/bufwriter.rs b/library/std/src/io/buffered/bufwriter.rs
index 574eb83dc56..d569fed24c5 100644
--- a/library/std/src/io/buffered/bufwriter.rs
+++ b/library/std/src/io/buffered/bufwriter.rs
@@ -492,23 +492,15 @@ impl WriterPanicked {
     pub fn into_inner(self) -> Vec<u8> {
         self.buf
     }
-
-    const DESCRIPTION: &'static str =
-        "BufWriter inner writer panicked, what data remains unwritten is not known";
 }
 
 #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
-impl error::Error for WriterPanicked {
-    #[allow(deprecated, deprecated_in_future)]
-    fn description(&self) -> &str {
-        Self::DESCRIPTION
-    }
-}
+impl error::Error for WriterPanicked {}
 
 #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
 impl fmt::Display for WriterPanicked {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", Self::DESCRIPTION)
+        "BufWriter inner writer panicked, what data remains unwritten is not known".fmt(f)
     }
 }
 
diff --git a/library/std/src/io/buffered/mod.rs b/library/std/src/io/buffered/mod.rs
index 475d877528f..e36f2d92108 100644
--- a/library/std/src/io/buffered/mod.rs
+++ b/library/std/src/io/buffered/mod.rs
@@ -179,12 +179,7 @@ impl<W> From<IntoInnerError<W>> for Error {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {
-    #[allow(deprecated, deprecated_in_future)]
-    fn description(&self) -> &str {
-        error::Error::description(self.error())
-    }
-}
+impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<W> fmt::Display for IntoInnerError<W> {
diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
index 15e962924ac..d060ad52897 100644
--- a/library/std/src/io/copy.rs
+++ b/library/std/src/io/copy.rs
@@ -63,10 +63,11 @@ where
     R: Read,
     W: Write,
 {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_os = "linux", target_os = "android"))] {
+    cfg_select! {
+        any(target_os = "linux", target_os = "android") => {
             crate::sys::kernel_copy::copy_spec(reader, writer)
-        } else {
+        }
+        _ => {
             generic_copy(reader, writer)
         }
     }
diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
index 562fdbf4ff7..57a980d6acd 100644
--- a/library/std/src/io/error.rs
+++ b/library/std/src/io/error.rs
@@ -18,7 +18,7 @@ use crate::{error, fmt, result, sys};
 /// This type is broadly used across [`std::io`] for any operation which may
 /// produce an error.
 ///
-/// This typedef is generally used to avoid writing out [`io::Error`] directly and
+/// This type alias is generally used to avoid writing out [`io::Error`] directly and
 /// is otherwise a direct mapping to [`Result`].
 ///
 /// While usual Rust style is to import types directly, aliases of [`Result`]
@@ -1049,15 +1049,6 @@ impl fmt::Display for Error {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl error::Error for Error {
-    #[allow(deprecated, deprecated_in_future)]
-    fn description(&self) -> &str {
-        match self.repr.data() {
-            ErrorData::Os(..) | ErrorData::Simple(..) => self.kind().as_str(),
-            ErrorData::SimpleMessage(msg) => msg.message,
-            ErrorData::Custom(c) => c.error.description(),
-        }
-    }
-
     #[allow(deprecated)]
     fn cause(&self) -> Option<&dyn error::Error> {
         match self.repr.data() {
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index d351ee5e739..ff0e29e04c2 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -2461,7 +2461,7 @@ pub trait BufRead: Read {
     /// delimiter or EOF is found.
     ///
     /// If successful, this function will return the total number of bytes read,
-    /// including the delimiter byte.
+    /// including the delimiter byte if found.
     ///
     /// This is useful for efficiently skipping data such as NUL-terminated strings
     /// in binary file formats without buffering.
@@ -2489,7 +2489,7 @@ pub trait BufRead: Read {
     /// ```
     /// use std::io::{self, BufRead};
     ///
-    /// let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0");
+    /// let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0!");
     ///
     /// // read name
     /// let mut name = Vec::new();
@@ -2509,6 +2509,11 @@ pub trait BufRead: Read {
     ///     .expect("reading from cursor won't fail");
     /// assert_eq!(num_bytes, 11);
     /// assert_eq!(animal, b"Crustacean\0");
+    ///
+    /// // reach EOF
+    /// let num_bytes = cursor.skip_until(b'\0')
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 1);
     /// ```
     #[stable(feature = "bufread_skip_until", since = "1.83.0")]
     fn skip_until(&mut self, byte: u8) -> Result<usize> {
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 07b38c65898..30a1b108817 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -15,7 +15,7 @@
 //!
 //! If you already know the name of what you are looking for, the fastest way to
 //! find it is to use the <a href="#" onclick="window.searchState.focus();">search
-//! bar</a> at the top of the page.
+//! button</a> at the top of the page.
 //!
 //! Otherwise, you may want to jump to one of these useful sections:
 //!
@@ -330,6 +330,7 @@
 #![feature(bstr)]
 #![feature(bstr_internals)]
 #![feature(cast_maybe_uninit)]
+#![feature(cfg_select)]
 #![feature(char_internals)]
 #![feature(clone_to_uninit)]
 #![feature(const_cmp)]
@@ -390,6 +391,7 @@
 #![feature(try_with_capacity)]
 #![feature(unique_rc_arc)]
 #![feature(vec_into_raw_parts)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Library features (unwind):
@@ -428,11 +430,15 @@
 //
 #![default_lib_allocator]
 
+// The Rust prelude
+// The compiler expects the prelude definition to be defined before it's use statement.
+pub mod prelude;
+
 // Explicitly import the prelude. The compiler uses this same unstable attribute
 // to import the prelude implicitly when building crates that depend on std.
 #[prelude_import]
 #[allow(unused)]
-use prelude::rust_2021::*;
+use prelude::rust_2024::*;
 
 // Access to Bencher, etc.
 #[cfg(test)]
@@ -483,9 +489,6 @@ mod macros;
 #[macro_use]
 pub mod rt;
 
-// The Rust prelude
-pub mod prelude;
-
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::any;
 #[stable(feature = "core_array", since = "1.35.0")]
@@ -736,6 +739,14 @@ pub use core::{
     unreachable, write, writeln,
 };
 
+// Re-export unstable derive macro defined through core.
+#[unstable(feature = "derive_from", issue = "144889")]
+/// Unstable module containing the unstable `From` derive macro.
+pub mod from {
+    #[unstable(feature = "derive_from", issue = "144889")]
+    pub use core::from::From;
+}
+
 // Include a number of private modules that exist solely to provide
 // the rustdoc documentation for primitive types. Using `include!`
 // because rustdoc only looks for these modules at the crate level.
diff --git a/library/std/src/os/unix/net/stream.rs b/library/std/src/os/unix/net/stream.rs
index 035768a6fab..768fa77a5f8 100644
--- a/library/std/src/os/unix/net/stream.rs
+++ b/library/std/src/os/unix/net/stream.rs
@@ -1,14 +1,16 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         target_os = "linux", target_os = "android",
         target_os = "hurd",
         target_os = "dragonfly", target_os = "freebsd",
         target_os = "openbsd", target_os = "netbsd",
         target_os = "solaris", target_os = "illumos",
         target_os = "haiku", target_os = "nto",
-        target_os = "cygwin"))] {
+        target_os = "cygwin",
+    ) => {
         use libc::MSG_NOSIGNAL;
-    } else {
+    }
+    _ => {
         const MSG_NOSIGNAL: core::ffi::c_int = 0x0;
     }
 }
diff --git a/library/std/src/os/unix/process.rs b/library/std/src/os/unix/process.rs
index 76e63a69e45..09429af06e3 100644
--- a/library/std/src/os/unix/process.rs
+++ b/library/std/src/os/unix/process.rs
@@ -4,8 +4,6 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use cfg_if::cfg_if;
-
 use crate::ffi::OsStr;
 use crate::os::unix::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::path::Path;
@@ -13,17 +11,19 @@ use crate::sealed::Sealed;
 use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 use crate::{io, process, sys};
 
-cfg_if! {
-    if #[cfg(any(target_os = "vxworks", target_os = "espidf", target_os = "horizon", target_os = "vita"))] {
+cfg_select! {
+    any(target_os = "vxworks", target_os = "espidf", target_os = "horizon", target_os = "vita") => {
         type UserId = u16;
         type GroupId = u16;
-    } else if #[cfg(target_os = "nto")] {
+    }
+    target_os = "nto" => {
         // Both IDs are signed, see `sys/target_nto.h` of the QNX Neutrino SDP.
         // Only positive values should be used, see e.g.
         // https://www.qnx.com/developers/docs/7.1/#com.qnx.doc.neutrino.lib_ref/topic/s/setuid.html
         type UserId = i32;
         type GroupId = i32;
-    } else {
+    }
+    _ => {
         type UserId = u32;
         type GroupId = u32;
     }
diff --git a/library/std/src/os/windows/ffi.rs b/library/std/src/os/windows/ffi.rs
index 496443dbbc3..345d5b74285 100644
--- a/library/std/src/os/windows/ffi.rs
+++ b/library/std/src/os/windows/ffi.rs
@@ -53,12 +53,13 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use alloc::wtf8::Wtf8Buf;
+
 use crate::ffi::{OsStr, OsString};
+use crate::fmt;
+use crate::iter::FusedIterator;
 use crate::sealed::Sealed;
 use crate::sys::os_str::Buf;
-#[stable(feature = "rust1", since = "1.0.0")]
-pub use crate::sys_common::wtf8::EncodeWide;
-use crate::sys_common::wtf8::Wtf8Buf;
 use crate::sys_common::{AsInner, FromInner};
 
 /// Windows-specific extensions to [`OsString`].
@@ -130,6 +131,35 @@ pub trait OsStrExt: Sealed {
 impl OsStrExt for OsStr {
     #[inline]
     fn encode_wide(&self) -> EncodeWide<'_> {
-        self.as_inner().inner.encode_wide()
+        EncodeWide { inner: self.as_inner().inner.encode_wide() }
+    }
+}
+
+/// Iterator returned by [`OsStrExt::encode_wide`].
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
+pub struct EncodeWide<'a> {
+    inner: alloc::wtf8::EncodeWide<'a>,
+}
+#[stable(feature = "encode_wide_debug", since = "CURRENT_RUSTC_VERSION")]
+impl fmt::Debug for EncodeWide<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.inner, f)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Iterator for EncodeWide<'_> {
+    type Item = u16;
+
+    #[inline]
+    fn next(&mut self) -> Option<u16> {
+        self.inner.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
     }
 }
+#[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
+impl FusedIterator for EncodeWide<'_> {}
diff --git a/library/std/src/os/windows/io/socket.rs b/library/std/src/os/windows/io/socket.rs
index 1c228914de9..28e972925e6 100644
--- a/library/std/src/os/windows/io/socket.rs
+++ b/library/std/src/os/windows/io/socket.rs
@@ -54,7 +54,7 @@ impl BorrowedSocket<'_> {
     ///
     /// # Safety
     ///
-    /// The resource pointed to by `raw` must remain open for the duration of
+    /// The resource pointed to by `socket` must remain open for the duration of
     /// the returned `BorrowedSocket`, and it must not have the value
     /// `INVALID_SOCKET`.
     #[inline]
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 3b52804d6be..470d300d2d9 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1191,7 +1191,7 @@ impl PathBuf {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    #[rustc_const_unstable(feature = "const_pathbuf_osstring_new", issue = "141520")]
+    #[rustc_const_stable(feature = "const_pathbuf_osstring_new", since = "CURRENT_RUSTC_VERSION")]
     pub const fn new() -> PathBuf {
         PathBuf { inner: OsString::new() }
     }
@@ -2105,6 +2105,38 @@ impl PartialEq for PathBuf {
     }
 }
 
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<str> for PathBuf {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        &*self == other
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<PathBuf> for str {
+    #[inline]
+    fn eq(&self, other: &PathBuf) -> bool {
+        other == self
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<String> for PathBuf {
+    #[inline]
+    fn eq(&self, other: &String) -> bool {
+        **self == **other
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<PathBuf> for String {
+    #[inline]
+    fn eq(&self, other: &PathBuf) -> bool {
+        other == self
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Hash for PathBuf {
     fn hash<H: Hasher>(&self, h: &mut H) {
@@ -3366,6 +3398,39 @@ impl PartialEq for Path {
     }
 }
 
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<str> for Path {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        let other: &OsStr = other.as_ref();
+        self == other
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<Path> for str {
+    #[inline]
+    fn eq(&self, other: &Path) -> bool {
+        other == self
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<String> for Path {
+    #[inline]
+    fn eq(&self, other: &String) -> bool {
+        self == &*other
+    }
+}
+
+#[stable(feature = "eq_str_for_path", since = "CURRENT_RUSTC_VERSION")]
+impl cmp::PartialEq<Path> for String {
+    #[inline]
+    fn eq(&self, other: &Path) -> bool {
+        other == self
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Hash for Path {
     fn hash<H: Hasher>(&self, h: &mut H) {
@@ -3612,19 +3677,13 @@ impl_cmp_os_str!(<'a> Cow<'a, Path>, OsString);
 
 #[stable(since = "1.7.0", feature = "strip_prefix")]
 impl fmt::Display for StripPrefixError {
-    #[allow(deprecated, deprecated_in_future)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.description().fmt(f)
+        "prefix not found".fmt(f)
     }
 }
 
 #[stable(since = "1.7.0", feature = "strip_prefix")]
-impl Error for StripPrefixError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "prefix not found"
-    }
-}
+impl Error for StripPrefixError {}
 
 #[unstable(feature = "normalize_lexically", issue = "134694")]
 impl fmt::Display for NormalizeError {
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index a40e29a772a..3231125f7a1 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -244,7 +244,11 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> {
     #[inline]
     #[stable(feature = "lazy_cell", since = "1.80.0")]
     pub fn force(this: &LazyLock<T, F>) -> &T {
-        this.once.call_once(|| {
+        this.once.call_once_force(|state| {
+            if state.is_poisoned() {
+                panic_poisoned();
+            }
+
             // SAFETY: `call_once` only runs this closure once, ever.
             let data = unsafe { &mut *this.data.get() };
             let f = unsafe { ManuallyDrop::take(&mut data.f) };
@@ -257,8 +261,7 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> {
         // * the closure was called and initialized `value`.
         // * the closure was called and panicked, so this point is never reached.
         // * the closure was not called, but a previous call initialized `value`.
-        // * the closure was not called because the Once is poisoned, so this point
-        //   is never reached.
+        // * the closure was not called because the Once is poisoned, which we handled above.
         // So `value` has definitely been initialized and will not be modified again.
         unsafe { &*(*this.data.get()).value }
     }
diff --git a/library/std/src/sync/mpmc/utils.rs b/library/std/src/sync/mpmc/utils.rs
index 0cbc61160f7..e3bcb149f64 100644
--- a/library/std/src/sync/mpmc/utils.rs
+++ b/library/std/src/sync/mpmc/utils.rs
@@ -23,14 +23,13 @@ use crate::ops::{Deref, DerefMut};
     any(target_arch = "x86_64", target_arch = "aarch64", target_arch = "powerpc64",),
     repr(align(128))
 )]
-// arm, mips, mips64, and riscv64 have 32-byte cache line size.
+// arm, mips and mips64 have 32-byte cache line size.
 //
 // Sources:
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_arm.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mipsle.go#L7
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_mips64x.go#L9
-// - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_riscv64.go#L7
 #[cfg_attr(
     any(
         target_arch = "arm",
@@ -38,7 +37,6 @@ use crate::ops::{Deref, DerefMut};
         target_arch = "mips32r6",
         target_arch = "mips64",
         target_arch = "mips64r6",
-        target_arch = "riscv64",
     ),
     repr(align(32))
 )]
@@ -47,11 +45,12 @@ use crate::ops::{Deref, DerefMut};
 // Sources:
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_s390x.go#L7
 #[cfg_attr(target_arch = "s390x", repr(align(256)))]
-// x86 and wasm have 64-byte cache line size.
+// x86, wasm and riscv have 64-byte cache line size.
 //
 // Sources:
 // - https://github.com/golang/go/blob/dda2991c2ea0c5914714469c4defc2562a907230/src/internal/cpu/cpu_x86.go#L9
 // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_wasm.go#L7
+// - https://github.com/golang/go/blob/5e31f78c8a4ed1b872ddc194f0cd1ae931b37d7e/src/internal/cpu/cpu_riscv64.go#L7
 //
 // All others are assumed to have 64-byte cache line size.
 #[cfg_attr(
@@ -64,7 +63,6 @@ use crate::ops::{Deref, DerefMut};
         target_arch = "mips32r6",
         target_arch = "mips64",
         target_arch = "mips64r6",
-        target_arch = "riscv64",
         target_arch = "s390x",
     )),
     repr(align(64))
diff --git a/library/std/src/sync/mpsc.rs b/library/std/src/sync/mpsc.rs
index 41d1dd3ce67..f91c26aa22c 100644
--- a/library/std/src/sync/mpsc.rs
+++ b/library/std/src/sync/mpsc.rs
@@ -697,14 +697,14 @@ impl<T> SyncSender<T> {
     /// let sync_sender2 = sync_sender.clone();
     ///
     /// // First thread owns sync_sender
-    /// thread::spawn(move || {
+    /// let handle1 = thread::spawn(move || {
     ///     sync_sender.send(1).unwrap();
     ///     sync_sender.send(2).unwrap();
     ///     // Thread blocked
     /// });
     ///
     /// // Second thread owns sync_sender2
-    /// thread::spawn(move || {
+    /// let handle2 = thread::spawn(move || {
     ///     // This will return an error and send
     ///     // no message if the buffer is full
     ///     let _ = sync_sender2.try_send(3);
@@ -722,6 +722,10 @@ impl<T> SyncSender<T> {
     ///     Ok(msg) => println!("message {msg} received"),
     ///     Err(_) => println!("the third message was never sent"),
     /// }
+    ///
+    /// // Wait for threads to complete
+    /// handle1.join().unwrap();
+    /// handle2.join().unwrap();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
@@ -1104,12 +1108,7 @@ impl<T> fmt::Display for SendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> error::Error for SendError<T> {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "sending on a closed channel"
-    }
-}
+impl<T> error::Error for SendError<T> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> fmt::Debug for TrySendError<T> {
@@ -1132,15 +1131,7 @@ impl<T> fmt::Display for TrySendError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> error::Error for TrySendError<T> {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        match *self {
-            TrySendError::Full(..) => "sending on a full channel",
-            TrySendError::Disconnected(..) => "sending on a closed channel",
-        }
-    }
-}
+impl<T> error::Error for TrySendError<T> {}
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
 impl<T> From<SendError<T>> for TrySendError<T> {
@@ -1164,12 +1155,7 @@ impl fmt::Display for RecvError {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl error::Error for RecvError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "receiving on a closed channel"
-    }
-}
+impl error::Error for RecvError {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for TryRecvError {
@@ -1182,15 +1168,7 @@ impl fmt::Display for TryRecvError {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl error::Error for TryRecvError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        match *self {
-            TryRecvError::Empty => "receiving on an empty channel",
-            TryRecvError::Disconnected => "receiving on a closed channel",
-        }
-    }
-}
+impl error::Error for TryRecvError {}
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
 impl From<RecvError> for TryRecvError {
@@ -1217,15 +1195,7 @@ impl fmt::Display for RecvTimeoutError {
 }
 
 #[stable(feature = "mpsc_recv_timeout_error", since = "1.15.0")]
-impl error::Error for RecvTimeoutError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        match *self {
-            RecvTimeoutError::Timeout => "timed out waiting on channel",
-            RecvTimeoutError::Disconnected => "channel is empty and sending half is closed",
-        }
-    }
-}
+impl error::Error for RecvTimeoutError {}
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
 impl From<RecvError> for RecvTimeoutError {
diff --git a/library/std/src/sync/nonpoison.rs b/library/std/src/sync/nonpoison.rs
index 2bbf226dc2c..b3ae376e70d 100644
--- a/library/std/src/sync/nonpoison.rs
+++ b/library/std/src/sync/nonpoison.rs
@@ -33,5 +33,10 @@ impl fmt::Display for WouldBlock {
 pub use self::mutex::MappedMutexGuard;
 #[unstable(feature = "nonpoison_mutex", issue = "134645")]
 pub use self::mutex::{Mutex, MutexGuard};
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+pub use self::rwlock::{MappedRwLockReadGuard, MappedRwLockWriteGuard};
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+pub use self::rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 mod mutex;
+mod rwlock;
diff --git a/library/std/src/sync/nonpoison/mutex.rs b/library/std/src/sync/nonpoison/mutex.rs
index b6861c78f00..fd1e671d7a3 100644
--- a/library/std/src/sync/nonpoison/mutex.rs
+++ b/library/std/src/sync/nonpoison/mutex.rs
@@ -100,7 +100,7 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
     lock: &'a Mutex<T>,
 }
 
-/// A [`MutexGuard`] is not `Send` to maximize platform portablity.
+/// A [`MutexGuard`] is not `Send` to maximize platform portability.
 ///
 /// On platforms that use POSIX threads (commonly referred to as pthreads) there is a requirement to
 /// release mutex locks on the same thread they were acquired.
diff --git a/library/std/src/sync/nonpoison/rwlock.rs b/library/std/src/sync/nonpoison/rwlock.rs
new file mode 100644
index 00000000000..eb0aef99cc1
--- /dev/null
+++ b/library/std/src/sync/nonpoison/rwlock.rs
@@ -0,0 +1,1081 @@
+use crate::cell::UnsafeCell;
+use crate::fmt;
+use crate::marker::PhantomData;
+use crate::mem::{self, ManuallyDrop, forget};
+use crate::ops::{Deref, DerefMut};
+use crate::ptr::NonNull;
+use crate::sync::nonpoison::{TryLockResult, WouldBlock};
+use crate::sys::sync as sys;
+
+/// A reader-writer lock that does not keep track of lock poisoning.
+///
+/// For more information about reader-writer locks, check out the documentation for the poisoning
+/// variant of this lock (which can be found at [`poison::RwLock`]).
+///
+/// [`poison::RwLock`]: crate::sync::poison::RwLock
+///
+/// # Examples
+///
+/// ```
+/// #![feature(nonpoison_rwlock)]
+///
+/// use std::sync::nonpoison::RwLock;
+///
+/// let lock = RwLock::new(5);
+///
+/// // many reader locks can be held at once
+/// {
+///     let r1 = lock.read();
+///     let r2 = lock.read();
+///     assert_eq!(*r1, 5);
+///     assert_eq!(*r2, 5);
+/// } // read locks are dropped at this point
+///
+/// // only one write lock may be held, however
+/// {
+///     let mut w = lock.write();
+///     *w += 1;
+///     assert_eq!(*w, 6);
+/// } // write lock is dropped here
+/// ```
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLock")]
+pub struct RwLock<T: ?Sized> {
+    /// The inner [`sys::RwLock`] that synchronizes thread access to the protected data.
+    inner: sys::RwLock,
+    /// The lock-protected data.
+    data: UnsafeCell<T>,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Guards
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+/// RAII structure used to release the shared read access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`read`] and [`try_read`] methods on
+/// [`RwLock`].
+///
+/// [`read`]: RwLock::read
+/// [`try_read`]: RwLock::try_read
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a RwLockReadGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Futures to not implement `Send`"]
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLockReadGuard")]
+pub struct RwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `RwLockReadGuard` instance only holds
+    /// immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
+    data: NonNull<T>,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
+
+/// RAII structure used to release the exclusive write access of a lock when
+/// dropped.
+///
+/// This structure is created by the [`write`] and [`try_write`] methods
+/// on [`RwLock`].
+///
+/// [`write`]: RwLock::write
+/// [`try_write`]: RwLock::try_write
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a RwLockWriteGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Future's to not implement `Send`"]
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+#[cfg_attr(not(test), rustc_diagnostic_item = "NonPoisonRwLockWriteGuard")]
+pub struct RwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A reference to the [`RwLock`] that we have write-locked.
+    lock: &'rwlock RwLock<T>,
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
+
+/// RAII structure used to release the shared read access of a lock when
+/// dropped, which can point to a subfield of the protected data.
+///
+/// This structure is created by the [`map`] and [`filter_map`] methods
+/// on [`RwLockReadGuard`].
+///
+/// [`map`]: RwLockReadGuard::map
+/// [`filter_map`]: RwLockReadGuard::filter_map
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a MappedRwLockReadGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Futures to not implement `Send`"]
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+pub struct MappedRwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockReadGuard` instance only
+    /// holds immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
+    data: NonNull<T>,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for MappedRwLockReadGuard<'_, T> {}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockReadGuard<'_, T> {}
+
+/// RAII structure used to release the exclusive write access of a lock when
+/// dropped, which can point to a subfield of the protected data.
+///
+/// This structure is created by the [`map`] and [`filter_map`] methods
+/// on [`RwLockWriteGuard`].
+///
+/// [`map`]: RwLockWriteGuard::map
+/// [`filter_map`]: RwLockWriteGuard::filter_map
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[must_not_suspend = "holding a MappedRwLockWriteGuard across suspend \
+                      points can cause deadlocks, delays, \
+                      and cause Future's to not implement `Send`"]
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+#[clippy::has_significant_drop]
+pub struct MappedRwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockWriteGuard` instance only
+    /// holds uniquneness until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations.
+    data: NonNull<T>,
+    /// `NonNull` is covariant over `T`, so we add a `PhantomData<&'rwlock mut T>` field here to
+    /// enforce the correct invariance over `T`.
+    _variance: PhantomData<&'rwlock mut T>,
+    /// A reference to the internal [`sys::RwLock`] that we have write-locked.
+    inner_lock: &'rwlock sys::RwLock,
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> !Send for MappedRwLockWriteGuard<'_, T> {}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockWriteGuard<'_, T> {}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Implementations
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+impl<T> RwLock<T> {
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(5);
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    #[inline]
+    pub const fn new(t: T) -> RwLock<T> {
+        RwLock { inner: sys::RwLock::new(), data: UnsafeCell::new(t) }
+    }
+
+    /// Returns the contained value by cloning it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.get_cloned(), 7);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn get_cloned(&self) -> T
+    where
+        T: Clone,
+    {
+        self.read().clone()
+    }
+
+    /// Sets the contained value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.get_cloned(), 7);
+    /// lock.set(11);
+    /// assert_eq!(lock.get_cloned(), 11);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn set(&self, value: T) {
+        if mem::needs_drop::<T>() {
+            // If the contained value has a non-trivial destructor, we
+            // call that destructor after the lock has been released.
+            drop(self.replace(value))
+        } else {
+            *self.write() = value;
+        }
+    }
+
+    /// Replaces the contained value with `value`, and returns the old contained value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(lock_value_accessors)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(7);
+    ///
+    /// assert_eq!(lock.replace(11), 7);
+    /// assert_eq!(lock.get_cloned(), 11);
+    /// ```
+    #[unstable(feature = "lock_value_accessors", issue = "133407")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn replace(&self, value: T) -> T {
+        let mut guard = self.write();
+        mem::replace(&mut *guard, value)
+    }
+}
+
+impl<T: ?Sized> RwLock<T> {
+    /// Locks this `RwLock` with shared read access, blocking the current thread
+    /// until it can be acquired.
+    ///
+    /// The calling thread will be blocked until there are no more writers which
+    /// hold the lock. There may be other readers currently inside the lock when
+    /// this method returns. This method does not provide any guarantees with
+    /// respect to the ordering of whether contentious readers or writers will
+    /// acquire the lock first.
+    ///
+    /// Returns an RAII guard which will release this thread's shared access
+    /// once it is dropped.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::Arc;
+    /// use std::sync::nonpoison::RwLock;
+    /// use std::thread;
+    ///
+    /// let lock = Arc::new(RwLock::new(1));
+    /// let c_lock = Arc::clone(&lock);
+    ///
+    /// let n = lock.read();
+    /// assert_eq!(*n, 1);
+    ///
+    /// thread::spawn(move || {
+    ///     let r = c_lock.read();
+    /// }).join().unwrap();
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn read(&self) -> RwLockReadGuard<'_, T> {
+        unsafe {
+            self.inner.read();
+            RwLockReadGuard::new(self)
+        }
+    }
+
+    /// Attempts to acquire this `RwLock` with shared read access.
+    ///
+    /// If the access could not be granted at this time, then `Err` is returned.
+    /// Otherwise, an RAII guard is returned which will release the shared access
+    /// when it is dropped.
+    ///
+    /// This function does not block.
+    ///
+    /// This function does not provide any guarantees with respect to the ordering
+    /// of whether contentious readers or writers will acquire the lock first.
+    ///
+    /// # Errors
+    ///
+    /// This function will return the [`WouldBlock`] error if the `RwLock` could
+    /// not be acquired because it was already locked exclusively.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// match lock.try_read() {
+    ///     Ok(n) => assert_eq!(*n, 1),
+    ///     Err(_) => unreachable!(),
+    /// };
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<'_, T>> {
+        unsafe {
+            if self.inner.try_read() { Ok(RwLockReadGuard::new(self)) } else { Err(WouldBlock) }
+        }
+    }
+
+    /// Locks this `RwLock` with exclusive write access, blocking the current
+    /// thread until it can be acquired.
+    ///
+    /// This function will not return while other writers or other readers
+    /// currently have access to the lock.
+    ///
+    /// Returns an RAII guard which will drop the write access of this `RwLock`
+    /// when dropped.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// let mut n = lock.write();
+    /// *n = 2;
+    ///
+    /// assert!(lock.try_read().is_err());
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn write(&self) -> RwLockWriteGuard<'_, T> {
+        unsafe {
+            self.inner.write();
+            RwLockWriteGuard::new(self)
+        }
+    }
+
+    /// Attempts to lock this `RwLock` with exclusive write access.
+    ///
+    /// If the lock could not be acquired at this time, then `Err` is returned.
+    /// Otherwise, an RAII guard is returned which will release the lock when
+    /// it is dropped.
+    ///
+    /// This function does not block.
+    ///
+    /// This function does not provide any guarantees with respect to the ordering
+    /// of whether contentious readers or writers will acquire the lock first.
+    ///
+    /// # Errors
+    ///
+    /// This function will return the [`WouldBlock`] error if the `RwLock` could
+    /// not be acquired because it was already locked.
+    ///
+    /// [`WouldBlock`]: WouldBlock
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(1);
+    ///
+    /// let n = lock.read();
+    /// assert_eq!(*n, 1);
+    ///
+    /// assert!(lock.try_write().is_err());
+    /// ```
+    #[inline]
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<'_, T>> {
+        unsafe {
+            if self.inner.try_write() { Ok(RwLockWriteGuard::new(self)) } else { Err(WouldBlock) }
+        }
+    }
+
+    /// Consumes this `RwLock`, returning the underlying data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let lock = RwLock::new(String::new());
+    /// {
+    ///     let mut s = lock.write();
+    ///     *s = "modified".to_owned();
+    /// }
+    /// assert_eq!(lock.into_inner(), "modified");
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn into_inner(self) -> T
+    where
+        T: Sized,
+    {
+        self.data.into_inner()
+    }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
+    /// take place -- the mutable borrow statically guarantees no new locks can be acquired
+    /// while this reference exists. Note that this method does not clear any previously abandoned
+    /// locks (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    ///
+    /// use std::sync::nonpoison::RwLock;
+    ///
+    /// let mut lock = RwLock::new(0);
+    /// *lock.get_mut() = 10;
+    /// assert_eq!(*lock.read(), 10);
+    /// ```
+    #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn get_mut(&mut self) -> &mut T {
+        self.data.get_mut()
+    }
+
+    /// Returns a raw pointer to the underlying data.
+    ///
+    /// The returned pointer is always non-null and properly aligned, but it is
+    /// the user's responsibility to ensure that any reads and writes through it
+    /// are properly synchronized to avoid data races, and that it is not read
+    /// or written through after the lock is dropped.
+    #[unstable(feature = "rwlock_data_ptr", issue = "140368")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn data_ptr(&self) -> *mut T {
+        self.data.get()
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_struct("RwLock");
+        match self.try_read() {
+            Ok(guard) => {
+                d.field("data", &&*guard);
+            }
+            Err(WouldBlock) => {
+                d.field("data", &format_args!("<locked>"));
+            }
+        }
+        d.finish_non_exhaustive()
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: Default> Default for RwLock<T> {
+    /// Creates a new `RwLock<T>`, with the `Default` value for T.
+    fn default() -> RwLock<T> {
+        RwLock::new(Default::default())
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T> From<T> for RwLock<T> {
+    /// Creates a new instance of an `RwLock<T>` which is unlocked.
+    /// This is equivalent to [`RwLock::new`].
+    fn from(t: T) -> Self {
+        RwLock::new(t)
+    }
+}
+
+impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockReadGuard<T>` from a `RwLock<T>`.
+    ///
+    /// # Safety
+    ///
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.read()`, `lock.inner.try_read()`, or `lock.inner.downgrade()` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> RwLockReadGuard<'rwlock, T> {
+        RwLockReadGuard {
+            data: unsafe { NonNull::new_unchecked(lock.data.get()) },
+            inner_lock: &lock.inner,
+        }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data, e.g.
+    /// an enum variant.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockReadGuard::map(...)`. A method would interfere with methods of
+    /// the same name on the contents of the `RwLockReadGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data. The
+    /// original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockReadGuard::filter_map(...)`. A method would interfere with methods
+    /// of the same name on the contents of the `RwLockReadGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
+    ///
+    /// # Safety
+    ///
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.write()`, `lock.inner.try_write()`, or `lock.inner.try_upgrade` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> RwLockWriteGuard<'rwlock, T> {
+        RwLockWriteGuard { lock }
+    }
+
+    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
+    ///
+    /// Since we have the `RwLockWriteGuard`, the [`RwLock`] must already be locked for writing, so
+    /// this method cannot fail.
+    ///
+    /// After downgrading, other readers will be allowed to read the protected data.
+    ///
+    /// # Examples
+    ///
+    /// `downgrade` takes ownership of the `RwLockWriteGuard` and returns a [`RwLockReadGuard`].
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = RwLock::new(0);
+    ///
+    /// let mut write_guard = rw.write();
+    /// *write_guard = 42;
+    ///
+    /// let read_guard = RwLockWriteGuard::downgrade(write_guard);
+    /// assert_eq!(42, *read_guard);
+    /// ```
+    ///
+    /// `downgrade` will _atomically_ change the state of the [`RwLock`] from exclusive mode into
+    /// shared mode. This means that it is impossible for another writing thread to get in between a
+    /// thread calling `downgrade` and any reads it performs after downgrading.
+    ///
+    /// ```
+    /// #![feature(nonpoison_rwlock)]
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::Arc;
+    /// use std::sync::nonpoison::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = Arc::new(RwLock::new(1));
+    ///
+    /// // Put the lock in write mode.
+    /// let mut main_write_guard = rw.write();
+    ///
+    /// let rw_clone = rw.clone();
+    /// let evil_handle = std::thread::spawn(move || {
+    ///     // This will not return until the main thread drops the `main_read_guard`.
+    ///     let mut evil_guard = rw_clone.write();
+    ///
+    ///     assert_eq!(*evil_guard, 2);
+    ///     *evil_guard = 3;
+    /// });
+    ///
+    /// *main_write_guard = 2;
+    ///
+    /// // Atomically downgrade the write guard into a read guard.
+    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    ///
+    /// // Since `downgrade` is atomic, the writer thread cannot have changed the protected data.
+    /// assert_eq!(*main_read_guard, 2, "`downgrade` was not atomic");
+    /// #
+    /// # drop(main_read_guard);
+    /// # evil_handle.join().unwrap();
+    /// #
+    /// # let final_check = rw.read();
+    /// # assert_eq!(*final_check, 3);
+    /// ```
+    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
+        let lock = s.lock;
+
+        // We don't want to call the destructor since that calls `write_unlock`.
+        forget(s);
+
+        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
+        // mode, satisfying the `downgrade` contract.
+        unsafe { lock.inner.downgrade() };
+
+        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
+        unsafe { RwLockReadGuard::new(lock) }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data, e.g.
+    /// an enum variant.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockWriteGuard::map(...)`. A method would interfere with methods of
+    /// the same name on the contents of the `RwLockWriteGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockWriteGuard { data, inner_lock: &orig.lock.inner, _variance: PhantomData }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data. The
+    /// original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `RwLockWriteGuard::filter_map(...)`. A method would interfere with methods
+    /// of the same name on the contents of the `RwLockWriteGuard` used through
+    /// `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&mut T) -> Option<&mut U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { &mut *orig.lock.data.get() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockWriteGuard {
+                    data,
+                    inner_lock: &orig.lock.inner,
+                    _variance: PhantomData,
+                })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> MappedRwLockReadGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for reading, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+impl<'rwlock, T: ?Sized> MappedRwLockWriteGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockWriteGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockWriteGuard { data, inner_lock: orig.inner_lock, _variance: PhantomData }
+    }
+
+    /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
+    ///
+    /// The `RwLock` is already locked for writing, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockWriteGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockWriteGuard`
+    /// used through `Deref`.
+    ///
+    /// # Panics
+    ///
+    /// If the closure panics, the guard will be dropped (unlocked).
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    // #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+    pub fn filter_map<U, F>(
+        mut orig: Self,
+        f: F,
+    ) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&mut T) -> Option<&mut U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_mut() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockWriteGuard {
+                    data,
+                    inner_lock: orig.inner_lock,
+                    _variance: PhantomData,
+                })
+            }
+            None => Err(orig),
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe {
+            self.lock.inner.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_mut() }
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+// #[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
diff --git a/library/std/src/sync/poison.rs b/library/std/src/sync/poison.rs
index 31889dcc10f..49a71b9ad10 100644
--- a/library/std/src/sync/poison.rs
+++ b/library/std/src/sync/poison.rs
@@ -263,12 +263,7 @@ impl<T> fmt::Display for PoisonError<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Error for PoisonError<T> {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "poisoned lock: another task failed inside"
-    }
-}
+impl<T> Error for PoisonError<T> {}
 
 impl<T> PoisonError<T> {
     /// Creates a `PoisonError`.
@@ -376,17 +371,6 @@ impl<T> fmt::Display for TryLockError<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Error for TryLockError<T> {
-    #[allow(deprecated, deprecated_in_future)]
-    fn description(&self) -> &str {
-        match *self {
-            #[cfg(panic = "unwind")]
-            TryLockError::Poisoned(ref p) => p.description(),
-            #[cfg(not(panic = "unwind"))]
-            TryLockError::Poisoned(ref p) => match p._never {},
-            TryLockError::WouldBlock => "try_lock failed because the operation would block",
-        }
-    }
-
     #[allow(deprecated)]
     fn cause(&self) -> Option<&dyn Error> {
         match *self {
diff --git a/library/std/src/sync/poison/mutex.rs b/library/std/src/sync/poison/mutex.rs
index 6205c4fa4ca..720c212c65c 100644
--- a/library/std/src/sync/poison/mutex.rs
+++ b/library/std/src/sync/poison/mutex.rs
@@ -279,7 +279,7 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
     poison: poison::Guard,
 }
 
-/// A [`MutexGuard`] is not `Send` to maximize platform portablity.
+/// A [`MutexGuard`] is not `Send` to maximize platform portability.
 ///
 /// On platforms that use POSIX threads (commonly referred to as pthreads) there is a requirement to
 /// release mutex locks on the same thread they were acquired.
diff --git a/library/std/src/sync/poison/rwlock.rs b/library/std/src/sync/poison/rwlock.rs
index 2c92602bc87..0a463f3f9c7 100644
--- a/library/std/src/sync/poison/rwlock.rs
+++ b/library/std/src/sync/poison/rwlock.rs
@@ -80,16 +80,24 @@ use crate::sys::sync as sys;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLock")]
 pub struct RwLock<T: ?Sized> {
+    /// The inner [`sys::RwLock`] that synchronizes thread access to the protected data.
     inner: sys::RwLock,
+    /// A flag denoting if this `RwLock` has been poisoned.
     poison: poison::Flag,
+    /// The lock-protected data.
     data: UnsafeCell<T>,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Guards
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 /// RAII structure used to release the shared read access of a lock when
 /// dropped.
 ///
@@ -105,13 +113,15 @@ unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 #[clippy::has_significant_drop]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockReadGuard")]
-pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
-    // `RwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
-    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
-    // is preferable over `const* T` to allow for niche optimization.
+pub struct RwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `RwLockReadGuard` instance only holds
+    /// immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -135,8 +145,10 @@ unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
 #[stable(feature = "rust1", since = "1.0.0")]
 #[clippy::has_significant_drop]
 #[cfg_attr(not(test), rustc_diagnostic_item = "RwLockWriteGuard")]
-pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
-    lock: &'a RwLock<T>,
+pub struct RwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A reference to the [`RwLock`] that we have write-locked.
+    lock: &'rwlock RwLock<T>,
+    /// The poison guard. See the [`poison`] module for more information.
     poison: poison::Guard,
 }
 
@@ -160,13 +172,15 @@ unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}
                       and cause Futures to not implement `Send`"]
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 #[clippy::has_significant_drop]
-pub struct MappedRwLockReadGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a T` to avoid `noalias` violations, because a
-    // `MappedRwLockReadGuard` argument doesn't hold immutability for its whole scope, only until it drops.
-    // `NonNull` is also covariant over `T`, just like we would have with `&T`. `NonNull`
-    // is preferable over `const* T` to allow for niche optimization.
+pub struct MappedRwLockReadGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockReadGuard` instance only
+    /// holds immutability until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations. `NonNull` is also
+    /// covariant over `T`, just like we would have with `&T`.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
+    /// A reference to the internal [`sys::RwLock`] that we have read-locked.
+    inner_lock: &'rwlock sys::RwLock,
 }
 
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
@@ -189,16 +203,21 @@ unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockReadGuard<'_, T> {}
                       and cause Future's to not implement `Send`"]
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 #[clippy::has_significant_drop]
-pub struct MappedRwLockWriteGuard<'a, T: ?Sized + 'a> {
-    // NB: we use a pointer instead of `&'a mut T` to avoid `noalias` violations, because a
-    // `MappedRwLockWriteGuard` argument doesn't hold uniqueness for its whole scope, only until it drops.
-    // `NonNull` is covariant over `T`, so we add a `PhantomData<&'a mut T>` field
-    // below for the correct variance over `T` (invariance).
+pub struct MappedRwLockWriteGuard<'rwlock, T: ?Sized + 'rwlock> {
+    /// A pointer to the data protected by the `RwLock`. Note that we use a pointer here instead of
+    /// `&'rwlock T` to avoid `noalias` violations, because a `MappedRwLockWriteGuard` instance only
+    /// holds uniquneness until it drops, not for its whole scope.
+    /// `NonNull` is preferable over `*const T` to allow for niche optimizations.
     data: NonNull<T>,
-    inner_lock: &'a sys::RwLock,
-    poison_flag: &'a poison::Flag,
-    poison: poison::Guard,
-    _variance: PhantomData<&'a mut T>,
+    /// `NonNull` is covariant over `T`, so we add a `PhantomData<&'rwlock mut T>` field here to
+    /// enforce the correct invariance over `T`.
+    _variance: PhantomData<&'rwlock mut T>,
+    /// A reference to the internal [`sys::RwLock`] that we have write-locked.
+    inner_lock: &'rwlock sys::RwLock,
+    /// A reference to the original `RwLock`'s poison state.
+    poison_flag: &'rwlock poison::Flag,
+    /// The poison guard. See the [`poison`] module for more information.
+    poison_guard: poison::Guard,
 }
 
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
@@ -207,6 +226,10 @@ impl<T: ?Sized> !Send for MappedRwLockWriteGuard<'_, T> {}
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 unsafe impl<T: ?Sized + Sync> Sync for MappedRwLockWriteGuard<'_, T> {}
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// Implementations
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 impl<T> RwLock<T> {
     /// Creates a new instance of an `RwLock<T>` which is unlocked.
     ///
@@ -611,8 +634,8 @@ impl<T: ?Sized> RwLock<T> {
     ///
     /// Since this call borrows the `RwLock` mutably, no actual locking needs to
     /// take place -- the mutable borrow statically guarantees no new locks can be acquired
-    /// while this reference exists. Note that this method does not clear any previously abandoned locks
-    /// (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
+    /// while this reference exists. Note that this method does not clear any previously abandoned
+    /// locks (e.g., via [`forget()`] on a [`RwLockReadGuard`] or [`RwLockWriteGuard`]).
     ///
     /// # Errors
     ///
@@ -700,177 +723,7 @@ impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
             inner_lock: &lock.inner,
         })
     }
-}
-
-impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
-    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
-    // SAFETY: if and only if `lock.inner.write()` (or `lock.inner.try_write()`) has been
-    // successfully called from the same thread before instantiating this object.
-    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
-        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
-    }
-}
-
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_guard_impls", since = "1.20.0")]
-impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "std_guard_impls", since = "1.20.0")]
-impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe { &*self.lock.data.get() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe { &mut *self.lock.data.get() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_ref() }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe { self.data.as_mut() }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
-    fn drop(&mut self) {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
-        unsafe {
-            self.inner_lock.read_unlock();
-        }
-    }
-}
-
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
-    fn drop(&mut self) {
-        self.lock.poison.done(&self.poison);
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
-        unsafe {
-            self.lock.inner.write_unlock();
-        }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
-    fn drop(&mut self) {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe {
-            self.inner_lock.read_unlock();
-        }
-    }
-}
-
-#[unstable(feature = "mapped_lock_guards", issue = "117108")]
-impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
-    fn drop(&mut self) {
-        self.poison_flag.done(&self.poison);
-        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        unsafe {
-            self.inner_lock.write_unlock();
-        }
-    }
-}
 
-impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data, e.g.
     /// an enum variant.
     ///
@@ -883,17 +736,18 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     ///
     /// # Panics
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
     where
         F: FnOnce(&T) -> &U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
@@ -912,17 +766,18 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     ///
     /// # Panics
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&T) -> Option<&U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { orig.data.as_ref() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -934,71 +789,95 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
     }
 }
 
-impl<'a, T: ?Sized> MappedRwLockReadGuard<'a, T> {
-    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
-    /// e.g. an enum variant.
-    ///
-    /// The `RwLock` is already locked for reading, so this cannot fail.
-    ///
-    /// This is an associated function that needs to be used as
-    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
-    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
-    /// used through `Deref`.
+impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
+    /// Creates a new instance of `RwLockWriteGuard<T>` from a `RwLock<T>`.
     ///
-    /// # Panics
+    /// # Safety
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
-    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'a, U>
-    where
-        F: FnOnce(&T) -> &U,
-        U: ?Sized,
-    {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
-        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
-        let orig = ManuallyDrop::new(orig);
-        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    /// This function is safe if and only if the same thread has successfully and safely called
+    /// `lock.inner.write()`, `lock.inner.try_write()`, or `lock.inner.try_upgrade` before
+    /// instantiating this object.
+    unsafe fn new(lock: &'rwlock RwLock<T>) -> LockResult<RwLockWriteGuard<'rwlock, T>> {
+        poison::map_result(lock.poison.guard(), |guard| RwLockWriteGuard { lock, poison: guard })
     }
 
-    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
-    /// The original guard is returned as an `Err(...)` if the closure returns
-    /// `None`.
+    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
     ///
-    /// The `RwLock` is already locked for reading, so this cannot fail.
+    /// Since we have the `RwLockWriteGuard`, the [`RwLock`] must already be locked for writing, so
+    /// this method cannot fail.
     ///
-    /// This is an associated function that needs to be used as
-    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
-    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
-    /// used through `Deref`.
+    /// After downgrading, other readers will be allowed to read the protected data.
     ///
-    /// # Panics
+    /// # Examples
     ///
-    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be poisoned.
-    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'a, U>, Self>
-    where
-        F: FnOnce(&T) -> Option<&U>,
-        U: ?Sized,
-    {
-        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
-        // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
-        match f(unsafe { orig.data.as_ref() }) {
-            Some(data) => {
-                let data = NonNull::from(data);
-                let orig = ManuallyDrop::new(orig);
-                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
-            }
-            None => Err(orig),
-        }
+    /// `downgrade` takes ownership of the `RwLockWriteGuard` and returns a [`RwLockReadGuard`].
+    ///
+    /// ```
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::{RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = RwLock::new(0);
+    ///
+    /// let mut write_guard = rw.write().unwrap();
+    /// *write_guard = 42;
+    ///
+    /// let read_guard = RwLockWriteGuard::downgrade(write_guard);
+    /// assert_eq!(42, *read_guard);
+    /// ```
+    ///
+    /// `downgrade` will _atomically_ change the state of the [`RwLock`] from exclusive mode into
+    /// shared mode. This means that it is impossible for another writing thread to get in between a
+    /// thread calling `downgrade` and any reads it performs after downgrading.
+    ///
+    /// ```
+    /// #![feature(rwlock_downgrade)]
+    ///
+    /// use std::sync::{Arc, RwLock, RwLockWriteGuard};
+    ///
+    /// let rw = Arc::new(RwLock::new(1));
+    ///
+    /// // Put the lock in write mode.
+    /// let mut main_write_guard = rw.write().unwrap();
+    ///
+    /// let rw_clone = rw.clone();
+    /// let evil_handle = std::thread::spawn(move || {
+    ///     // This will not return until the main thread drops the `main_read_guard`.
+    ///     let mut evil_guard = rw_clone.write().unwrap();
+    ///
+    ///     assert_eq!(*evil_guard, 2);
+    ///     *evil_guard = 3;
+    /// });
+    ///
+    /// *main_write_guard = 2;
+    ///
+    /// // Atomically downgrade the write guard into a read guard.
+    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    ///
+    /// // Since `downgrade` is atomic, the writer thread cannot have changed the protected data.
+    /// assert_eq!(*main_read_guard, 2, "`downgrade` was not atomic");
+    /// #
+    /// # drop(main_read_guard);
+    /// # evil_handle.join().unwrap();
+    /// #
+    /// # let final_check = rw.read().unwrap();
+    /// # assert_eq!(*final_check, 3);
+    /// ```
+    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
+    pub fn downgrade(s: Self) -> RwLockReadGuard<'rwlock, T> {
+        let lock = s.lock;
+
+        // We don't want to call the destructor since that calls `write_unlock`.
+        forget(s);
+
+        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
+        // mode, satisfying the `downgrade` contract.
+        unsafe { lock.inner.downgrade() };
+
+        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
+        unsafe { RwLockReadGuard::new(lock).unwrap_or_else(PoisonError::into_inner) }
     }
-}
 
-impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data, e.g.
     /// an enum variant.
     ///
@@ -1013,22 +892,22 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
     where
         F: FnOnce(&mut T) -> &mut U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { &mut *orig.lock.data.get() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockWriteGuard {
             data,
             inner_lock: &orig.lock.inner,
             poison_flag: &orig.lock.poison,
-            poison: orig.poison.clone(),
+            poison_guard: orig.poison.clone(),
             _variance: PhantomData,
         }
     }
@@ -1048,15 +927,15 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { &mut *orig.lock.data.get() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -1065,78 +944,82 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
                     data,
                     inner_lock: &orig.lock.inner,
                     poison_flag: &orig.lock.poison,
-                    poison: orig.poison.clone(),
+                    poison_guard: orig.poison.clone(),
                     _variance: PhantomData,
                 })
             }
             None => Err(orig),
         }
     }
+}
 
-    /// Downgrades a write-locked `RwLockWriteGuard` into a read-locked [`RwLockReadGuard`].
-    ///
-    /// This method will atomically change the state of the [`RwLock`] from exclusive mode into
-    /// shared mode. This means that it is impossible for a writing thread to get in between a
-    /// thread calling `downgrade` and the same thread reading whatever it wrote while it had the
-    /// [`RwLock`] in write mode.
-    ///
-    /// Note that since we have the `RwLockWriteGuard`, we know that the [`RwLock`] is already
-    /// locked for writing, so this method cannot fail.
-    ///
-    /// # Example
-    ///
-    /// ```
-    /// #![feature(rwlock_downgrade)]
-    /// use std::sync::{Arc, RwLock, RwLockWriteGuard};
-    ///
-    /// // The inner value starts as 0.
-    /// let rw = Arc::new(RwLock::new(0));
+impl<'rwlock, T: ?Sized> MappedRwLockReadGuard<'rwlock, T> {
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data,
+    /// e.g. an enum variant.
     ///
-    /// // Put the lock in write mode.
-    /// let mut main_write_guard = rw.write().unwrap();
+    /// The `RwLock` is already locked for reading, so this cannot fail.
     ///
-    /// let evil = rw.clone();
-    /// let handle = std::thread::spawn(move || {
-    ///     // This will not return until the main thread drops the `main_read_guard`.
-    ///     let mut evil_guard = evil.write().unwrap();
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
     ///
-    ///     assert_eq!(*evil_guard, 1);
-    ///     *evil_guard = 2;
-    /// });
+    /// # Panics
     ///
-    /// // After spawning the writer thread, set the inner value to 1.
-    /// *main_write_guard = 1;
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    pub fn map<U, F>(orig: Self, f: F) -> MappedRwLockReadGuard<'rwlock, U>
+    where
+        F: FnOnce(&T) -> &U,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        let data = NonNull::from(f(unsafe { orig.data.as_ref() }));
+        let orig = ManuallyDrop::new(orig);
+        MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock }
+    }
+
+    /// Makes a [`MappedRwLockReadGuard`] for a component of the borrowed data.
+    /// The original guard is returned as an `Err(...)` if the closure returns
+    /// `None`.
     ///
-    /// // Atomically downgrade the write guard into a read guard.
-    /// let main_read_guard = RwLockWriteGuard::downgrade(main_write_guard);
+    /// The `RwLock` is already locked for reading, so this cannot fail.
     ///
-    /// // Since `downgrade` is atomic, the writer thread cannot have set the inner value to 2.
-    /// assert_eq!(*main_read_guard, 1, "`downgrade` was not atomic");
+    /// This is an associated function that needs to be used as
+    /// `MappedRwLockReadGuard::filter_map(...)`. A method would interfere with
+    /// methods of the same name on the contents of the `MappedRwLockReadGuard`
+    /// used through `Deref`.
     ///
-    /// // Clean up everything now
-    /// drop(main_read_guard);
-    /// handle.join().unwrap();
+    /// # Panics
     ///
-    /// let final_check = rw.read().unwrap();
-    /// assert_eq!(*final_check, 2);
-    /// ```
-    #[unstable(feature = "rwlock_downgrade", issue = "128203")]
-    pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {
-        let lock = s.lock;
-
-        // We don't want to call the destructor since that calls `write_unlock`.
-        forget(s);
-
-        // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
-        // mode, satisfying the `downgrade` contract.
-        unsafe { lock.inner.downgrade() };
-
-        // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
-        unsafe { RwLockReadGuard::new(lock).unwrap_or_else(PoisonError::into_inner) }
+    /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will not be
+    /// poisoned.
+    #[unstable(feature = "mapped_lock_guards", issue = "117108")]
+    pub fn filter_map<U, F>(orig: Self, f: F) -> Result<MappedRwLockReadGuard<'rwlock, U>, Self>
+    where
+        F: FnOnce(&T) -> Option<&U>,
+        U: ?Sized,
+    {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
+        match f(unsafe { orig.data.as_ref() }) {
+            Some(data) => {
+                let data = NonNull::from(data);
+                let orig = ManuallyDrop::new(orig);
+                Ok(MappedRwLockReadGuard { data, inner_lock: &orig.inner_lock })
+            }
+            None => Err(orig),
+        }
     }
 }
 
-impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
+impl<'rwlock, T: ?Sized> MappedRwLockWriteGuard<'rwlock, T> {
     /// Makes a [`MappedRwLockWriteGuard`] for a component of the borrowed data,
     /// e.g. an enum variant.
     ///
@@ -1151,22 +1034,22 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'a, U>
+    pub fn map<U, F>(mut orig: Self, f: F) -> MappedRwLockWriteGuard<'rwlock, U>
     where
         F: FnOnce(&mut T) -> &mut U,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         let data = NonNull::from(f(unsafe { orig.data.as_mut() }));
         let orig = ManuallyDrop::new(orig);
         MappedRwLockWriteGuard {
             data,
             inner_lock: orig.inner_lock,
             poison_flag: orig.poison_flag,
-            poison: orig.poison.clone(),
+            poison_guard: orig.poison_guard.clone(),
             _variance: PhantomData,
         }
     }
@@ -1186,15 +1069,18 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
     ///
     /// If the closure panics, the guard will be dropped (unlocked) and the RwLock will be poisoned.
     #[unstable(feature = "mapped_lock_guards", issue = "117108")]
-    pub fn filter_map<U, F>(mut orig: Self, f: F) -> Result<MappedRwLockWriteGuard<'a, U>, Self>
+    pub fn filter_map<U, F>(
+        mut orig: Self,
+        f: F,
+    ) -> Result<MappedRwLockWriteGuard<'rwlock, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
         U: ?Sized,
     {
         // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
         // was created, and have been upheld throughout `map` and/or `filter_map`.
-        // The signature of the closure guarantees that it will not "leak" the lifetime of the reference
-        // passed to it. If the closure panics, the guard will be dropped.
+        // The signature of the closure guarantees that it will not "leak" the lifetime of the
+        // reference passed to it. If the closure panics, the guard will be dropped.
         match f(unsafe { orig.data.as_mut() }) {
             Some(data) => {
                 let data = NonNull::from(data);
@@ -1203,7 +1089,7 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
                     data,
                     inner_lock: orig.inner_lock,
                     poison_flag: orig.poison_flag,
-                    poison: orig.poison.clone(),
+                    poison_guard: orig.poison_guard.clone(),
                     _variance: PhantomData,
                 })
             }
@@ -1211,3 +1097,162 @@ impl<'a, T: ?Sized> MappedRwLockWriteGuard<'a, T> {
         }
     }
 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.poison.done(&self.poison);
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe {
+            self.lock.inner.write_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Drop for MappedRwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.read_unlock();
+        }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Drop for MappedRwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        self.poison_flag.done(&self.poison_guard);
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe {
+            self.inner_lock.write_unlock();
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Deref for RwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when created.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Deref for RwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when created.
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Deref for MappedRwLockReadGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockReadGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> Deref for MappedRwLockWriteGuard<'_, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_ref() }
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized> DerefMut for MappedRwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: the conditions of `RwLockWriteGuard::new` were satisfied when the original guard
+        // was created, and have been upheld throughout `map` and/or `filter_map`.
+        unsafe { self.data.as_mut() }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_guard_impls", since = "1.20.0")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[stable(feature = "std_guard_impls", since = "1.20.0")]
+impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockReadGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Debug> fmt::Debug for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+impl<T: ?Sized + fmt::Display> fmt::Display for MappedRwLockWriteGuard<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
diff --git a/library/std/src/sync/reentrant_lock.rs b/library/std/src/sync/reentrant_lock.rs
index 727252f03a2..4140718560c 100644
--- a/library/std/src/sync/reentrant_lock.rs
+++ b/library/std/src/sync/reentrant_lock.rs
@@ -1,5 +1,3 @@
-use cfg_if::cfg_if;
-
 use crate::cell::UnsafeCell;
 use crate::fmt;
 use crate::ops::Deref;
@@ -87,8 +85,8 @@ pub struct ReentrantLock<T: ?Sized> {
     data: T,
 }
 
-cfg_if!(
-    if #[cfg(target_has_atomic = "64")] {
+cfg_select!(
+    target_has_atomic = "64" => {
         use crate::sync::atomic::{Atomic, AtomicU64, Ordering::Relaxed};
 
         struct Tid(Atomic<u64>);
@@ -110,7 +108,8 @@ cfg_if!(
                 self.0.store(value, Relaxed);
             }
         }
-    } else {
+    }
+    _ => {
         /// Returns the address of a TLS variable. This is guaranteed to
         /// be unique across all currently alive threads.
         fn tls_addr() -> usize {
diff --git a/library/std/src/sys/alloc/mod.rs b/library/std/src/sys/alloc/mod.rs
index f3af1f7f599..6d4b09494a3 100644
--- a/library/std/src/sys/alloc/mod.rs
+++ b/library/std/src/sys/alloc/mod.rs
@@ -68,29 +68,37 @@ unsafe fn realloc_fallback(
     }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         target_family = "unix",
         target_os = "wasi",
         target_os = "teeos",
         target_os = "trusty",
-    ))] {
+    ) => {
         mod unix;
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
-    } else if #[cfg(target_family = "wasm")] {
+    }
+    target_family = "wasm" => {
         mod wasm;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
     }
 }
diff --git a/library/std/src/sys/alloc/unix.rs b/library/std/src/sys/alloc/unix.rs
index a7ac4117ec9..3d369b08abc 100644
--- a/library/std/src/sys/alloc/unix.rs
+++ b/library/std/src/sys/alloc/unix.rs
@@ -58,18 +58,16 @@ unsafe impl GlobalAlloc for System {
     }
 }
 
-cfg_if::cfg_if! {
+cfg_select! {
     // We use posix_memalign wherever possible, but some targets have very incomplete POSIX coverage
     // so we need a fallback for those.
-    if #[cfg(any(
-        target_os = "horizon",
-        target_os = "vita",
-    ))] {
+    any(target_os = "horizon", target_os = "vita") => {
         #[inline]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
             unsafe { libc::memalign(layout.align(), layout.size()) as *mut u8 }
         }
-    } else {
+    }
+    _ => {
         #[inline]
         #[cfg_attr(target_os = "vxworks", allow(unused_unsafe))]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
diff --git a/library/std/src/sys/alloc/wasm.rs b/library/std/src/sys/alloc/wasm.rs
index c8fab992a88..48e2fdd4ecc 100644
--- a/library/std/src/sys/alloc/wasm.rs
+++ b/library/std/src/sys/alloc/wasm.rs
@@ -16,12 +16,15 @@
 //! The crate itself provides a global allocator which on wasm has no
 //! synchronization as there are no threads!
 
-// FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
-#![allow(static_mut_refs)]
+use core::cell::SyncUnsafeCell;
 
 use crate::alloc::{GlobalAlloc, Layout, System};
 
-static mut DLMALLOC: dlmalloc::Dlmalloc = dlmalloc::Dlmalloc::new();
+struct SyncDlmalloc(dlmalloc::Dlmalloc);
+unsafe impl Sync for SyncDlmalloc {}
+
+static DLMALLOC: SyncUnsafeCell<SyncDlmalloc> =
+    SyncUnsafeCell::new(SyncDlmalloc(dlmalloc::Dlmalloc::new()));
 
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
 unsafe impl GlobalAlloc for System {
@@ -30,7 +33,7 @@ unsafe impl GlobalAlloc for System {
         // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
         // Calling malloc() is safe because preconditions on this function match the trait method preconditions.
         let _lock = lock::lock();
-        unsafe { DLMALLOC.malloc(layout.size(), layout.align()) }
+        unsafe { (*DLMALLOC.get()).0.malloc(layout.size(), layout.align()) }
     }
 
     #[inline]
@@ -38,7 +41,7 @@ unsafe impl GlobalAlloc for System {
         // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
         // Calling calloc() is safe because preconditions on this function match the trait method preconditions.
         let _lock = lock::lock();
-        unsafe { DLMALLOC.calloc(layout.size(), layout.align()) }
+        unsafe { (*DLMALLOC.get()).0.calloc(layout.size(), layout.align()) }
     }
 
     #[inline]
@@ -46,7 +49,7 @@ unsafe impl GlobalAlloc for System {
         // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
         // Calling free() is safe because preconditions on this function match the trait method preconditions.
         let _lock = lock::lock();
-        unsafe { DLMALLOC.free(ptr, layout.size(), layout.align()) }
+        unsafe { (*DLMALLOC.get()).0.free(ptr, layout.size(), layout.align()) }
     }
 
     #[inline]
@@ -54,7 +57,7 @@ unsafe impl GlobalAlloc for System {
         // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
         // Calling realloc() is safe because preconditions on this function match the trait method preconditions.
         let _lock = lock::lock();
-        unsafe { DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size) }
+        unsafe { (*DLMALLOC.get()).0.realloc(ptr, layout.size(), layout.align(), new_size) }
     }
 }
 
diff --git a/library/std/src/sys/anonymous_pipe/mod.rs b/library/std/src/sys/anonymous_pipe/mod.rs
index aa14c8b650d..b6f464161ee 100644
--- a/library/std/src/sys/anonymous_pipe/mod.rs
+++ b/library/std/src/sys/anonymous_pipe/mod.rs
@@ -1,13 +1,15 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-cfg_if::cfg_if! {
-    if #[cfg(unix)] {
+cfg_select! {
+    unix => {
         mod unix;
         pub use unix::{AnonPipe, pipe};
-    } else if #[cfg(windows)] {
+    }
+    windows => {
         mod windows;
         pub use windows::{AnonPipe, pipe};
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::{AnonPipe, pipe};
     }
diff --git a/library/std/src/sys/args/mod.rs b/library/std/src/sys/args/mod.rs
index 0011f55dc14..c9627322276 100644
--- a/library/std/src/sys/args/mod.rs
+++ b/library/std/src/sys/args/mod.rs
@@ -12,32 +12,39 @@
 ))]
 mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_family = "unix", not(any(target_os = "espidf", target_os = "vita"))),
         target_os = "hermit",
-    ))] {
+    ) => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(target_family = "windows")] {
+    }
+    target_family = "windows" => {
         mod windows;
         pub use windows::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::*;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::*;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
         pub use zkvm::*;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::*;
     }
diff --git a/library/std/src/sys/backtrace.rs b/library/std/src/sys/backtrace.rs
index 272d0fa4d1a..57682207e07 100644
--- a/library/std/src/sys/backtrace.rs
+++ b/library/std/src/sys/backtrace.rs
@@ -113,7 +113,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
                         res = bt_fmt.frame().symbol(frame, symbol);
                     }
                 });
-                #[cfg(target_os = "nto")]
+                #[cfg(all(target_os = "nto", any(target_env = "nto70", target_env = "nto71")))]
                 if libc::__my_thread_exit as *mut libc::c_void == frame.ip() {
                     if !hit && print {
                         use crate::backtrace_rs::SymbolName;
diff --git a/library/std/src/sys/cmath.rs b/library/std/src/sys/cmath.rs
index 299ce1a6ff0..1592218ead8 100644
--- a/library/std/src/sys/cmath.rs
+++ b/library/std/src/sys/cmath.rs
@@ -45,69 +45,70 @@ unsafe extern "C" {
     pub safe fn lgammaf128_r(n: f128, s: &mut i32) -> f128;
     pub safe fn erff128(n: f128) -> f128;
     pub safe fn erfcf128(n: f128) -> f128;
-
-    cfg_if::cfg_if! {
-    if #[cfg(not(all(target_os = "windows", target_env = "msvc", target_arch = "x86")))] {
-        pub safe fn acosf(n: f32) -> f32;
-        pub safe fn asinf(n: f32) -> f32;
-        pub safe fn atan2f(a: f32, b: f32) -> f32;
-        pub safe fn atanf(n: f32) -> f32;
-        pub safe fn coshf(n: f32) -> f32;
-        pub safe fn sinhf(n: f32) -> f32;
-        pub safe fn tanf(n: f32) -> f32;
-        pub safe fn tanhf(n: f32) -> f32;
-    }}
 }
 
-// On AIX, we don't have lgammaf_r only the f64 version, so we can
-// use the f64 version lgamma_r
-#[cfg(target_os = "aix")]
-pub fn lgammaf_r(n: f32, s: &mut i32) -> f32 {
-    lgamma_r(n.into(), s) as f32
-}
+cfg_select! {
+    all(target_os = "windows", target_env = "msvc", target_arch = "x86") => {
+        // On 32-bit x86 MSVC these functions aren't defined, so we just define shims
+        // which promote everything to f64, perform the calculation, and then demote
+        // back to f32. While not precisely correct should be "correct enough" for now.
+        #[inline]
+        pub fn acosf(n: f32) -> f32 {
+            f64::acos(n as f64) as f32
+        }
 
-// On 32-bit x86 MSVC these functions aren't defined, so we just define shims
-// which promote everything to f64, perform the calculation, and then demote
-// back to f32. While not precisely correct should be "correct enough" for now.
-cfg_if::cfg_if! {
-if #[cfg(all(target_os = "windows", target_env = "msvc", target_arch = "x86"))] {
-    #[inline]
-    pub fn acosf(n: f32) -> f32 {
-        f64::acos(n as f64) as f32
-    }
+        #[inline]
+        pub fn asinf(n: f32) -> f32 {
+            f64::asin(n as f64) as f32
+        }
 
-    #[inline]
-    pub fn asinf(n: f32) -> f32 {
-        f64::asin(n as f64) as f32
-    }
+        #[inline]
+        pub fn atan2f(n: f32, b: f32) -> f32 {
+            f64::atan2(n as f64, b as f64) as f32
+        }
 
-    #[inline]
-    pub fn atan2f(n: f32, b: f32) -> f32 {
-        f64::atan2(n as f64, b as f64) as f32
-    }
+        #[inline]
+        pub fn atanf(n: f32) -> f32 {
+            f64::atan(n as f64) as f32
+        }
 
-    #[inline]
-    pub fn atanf(n: f32) -> f32 {
-        f64::atan(n as f64) as f32
-    }
+        #[inline]
+        pub fn coshf(n: f32) -> f32 {
+            f64::cosh(n as f64) as f32
+        }
 
-    #[inline]
-    pub fn coshf(n: f32) -> f32 {
-        f64::cosh(n as f64) as f32
-    }
+        #[inline]
+        pub fn sinhf(n: f32) -> f32 {
+            f64::sinh(n as f64) as f32
+        }
 
-    #[inline]
-    pub fn sinhf(n: f32) -> f32 {
-        f64::sinh(n as f64) as f32
-    }
+        #[inline]
+        pub fn tanf(n: f32) -> f32 {
+            f64::tan(n as f64) as f32
+        }
 
-    #[inline]
-    pub fn tanf(n: f32) -> f32 {
-        f64::tan(n as f64) as f32
+        #[inline]
+        pub fn tanhf(n: f32) -> f32 {
+            f64::tanh(n as f64) as f32
+        }
     }
-
-    #[inline]
-    pub fn tanhf(n: f32) -> f32 {
-        f64::tanh(n as f64) as f32
+    _ => {
+        unsafe extern "C" {
+            pub safe fn acosf(n: f32) -> f32;
+            pub safe fn asinf(n: f32) -> f32;
+            pub safe fn atan2f(a: f32, b: f32) -> f32;
+            pub safe fn atanf(n: f32) -> f32;
+            pub safe fn coshf(n: f32) -> f32;
+            pub safe fn sinhf(n: f32) -> f32;
+            pub safe fn tanf(n: f32) -> f32;
+            pub safe fn tanhf(n: f32) -> f32;
+        }
     }
-}}
+}
+
+// On AIX, we don't have lgammaf_r only the f64 version, so we can
+// use the f64 version lgamma_r
+#[cfg(target_os = "aix")]
+pub fn lgammaf_r(n: f32, s: &mut i32) -> f32 {
+    lgamma_r(n.into(), s) as f32
+}
diff --git a/library/std/src/sys/env/mod.rs b/library/std/src/sys/env/mod.rs
index d81ff875c83..f211a9fc86b 100644
--- a/library/std/src/sys/env/mod.rs
+++ b/library/std/src/sys/env/mod.rs
@@ -13,35 +13,44 @@
 ))]
 mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_family = "unix")] {
+cfg_select! {
+    target_family = "unix" => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(target_family = "windows")] {
+    }
+    target_family = "windows" => {
         mod windows;
         pub use windows::*;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         pub use hermit::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use solid::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::*;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::*;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
         pub use zkvm::*;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::*;
     }
diff --git a/library/std/src/sys/env/wasi.rs b/library/std/src/sys/env/wasi.rs
index 3719f9db51e..1327cbc3263 100644
--- a/library/std/src/sys/env/wasi.rs
+++ b/library/std/src/sys/env/wasi.rs
@@ -7,8 +7,8 @@ use crate::os::wasi::prelude::*;
 use crate::sys::common::small_c_string::run_with_cstr;
 use crate::sys::pal::os::{cvt, libc};
 
-cfg_if::cfg_if! {
-    if #[cfg(target_feature = "atomics")] {
+cfg_select! {
+    target_feature = "atomics" => {
         // Access to the environment must be protected by a lock in multi-threaded scenarios.
         use crate::sync::{PoisonError, RwLock};
         static ENV_LOCK: RwLock<()> = RwLock::new(());
@@ -18,7 +18,8 @@ cfg_if::cfg_if! {
         pub fn env_write_lock() -> impl Drop {
             ENV_LOCK.write().unwrap_or_else(PoisonError::into_inner)
         }
-    } else {
+    }
+    _ => {
         // No need for a lock if we are single-threaded.
         pub fn env_read_lock() -> impl Drop {
             Box::new(())
diff --git a/library/std/src/sys/exit_guard.rs b/library/std/src/sys/exit_guard.rs
index bd70d178244..00b91842e9d 100644
--- a/library/std/src/sys/exit_guard.rs
+++ b/library/std/src/sys/exit_guard.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "linux")] {
+cfg_select! {
+    target_os = "linux" => {
         /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
         ///
         /// On glibc, `libc::exit` has been observed to not always be thread-safe.
@@ -56,7 +56,8 @@ cfg_if::cfg_if! {
                 }
             }
         }
-    } else {
+    }
+    _ => {
         /// Mitigation for <https://github.com/rust-lang/rust/issues/126600>
         ///
         /// Mitigation is ***NOT*** implemented on this platform, either because this platform
diff --git a/library/std/src/sys/fd/mod.rs b/library/std/src/sys/fd/mod.rs
index e0f5eab6951..7cb9dd1cba9 100644
--- a/library/std/src/sys/fd/mod.rs
+++ b/library/std/src/sys/fd/mod.rs
@@ -2,18 +2,22 @@
 
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-cfg_if::cfg_if! {
-    if #[cfg(target_family = "unix")] {
+cfg_select! {
+    target_family = "unix" => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         pub use hermit::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::*;
     }
+    _ => {}
 }
diff --git a/library/std/src/sys/fd/unix.rs b/library/std/src/sys/fd/unix.rs
index cdca73cdca1..a12f692e754 100644
--- a/library/std/src/sys/fd/unix.rs
+++ b/library/std/src/sys/fd/unix.rs
@@ -37,10 +37,10 @@ pub struct FileDesc(OwnedFd);
 //
 // On Apple targets however, apparently the 64-bit libc is either buggy or
 // intentionally showing odd behavior by rejecting any read with a size
-// larger than or equal to INT_MAX. To handle both of these the read
-// size is capped on both platforms.
+// larger than INT_MAX. To handle both of these the read size is capped on
+// both platforms.
 const READ_LIMIT: usize = if cfg!(target_vendor = "apple") {
-    libc::c_int::MAX as usize - 1
+    libc::c_int::MAX as usize
 } else {
     libc::ssize_t::MAX as usize
 };
diff --git a/library/std/src/sys/fs/mod.rs b/library/std/src/sys/fs/mod.rs
index d55e28074fe..0276bf6e64c 100644
--- a/library/std/src/sys/fs/mod.rs
+++ b/library/std/src/sys/fs/mod.rs
@@ -5,8 +5,8 @@ use crate::path::{Path, PathBuf};
 
 pub mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_family = "unix")] {
+cfg_select! {
+    target_family = "unix" => {
         mod unix;
         use unix as imp;
         pub use unix::{chown, fchown, lchown, mkfifo};
@@ -16,24 +16,30 @@ cfg_if::cfg_if! {
         #[cfg(any(target_os = "linux", target_os = "android"))]
         pub(crate) use unix::CachedFileMetadata;
         use crate::sys::common::small_c_string::run_path_with_cstr as with_native_path;
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
         use windows as imp;
         pub use windows::{symlink_inner, junction_point};
         use crate::sys::path::with_native_path;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         use hermit as imp;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         use solid as imp;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         use uefi as imp;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         use wasi as imp;
-    } else {
+    }
+    _ => {
         mod unsupported;
         use unsupported as imp;
     }
@@ -108,6 +114,30 @@ pub fn set_permissions(path: &Path, perm: FilePermissions) -> io::Result<()> {
     with_native_path(path, &|path| imp::set_perm(path, perm.clone()))
 }
 
+#[cfg(unix)]
+pub fn set_permissions_nofollow(path: &Path, perm: crate::fs::Permissions) -> io::Result<()> {
+    use crate::fs::OpenOptions;
+
+    let mut options = OpenOptions::new();
+
+    // ESP-IDF and Horizon do not support O_NOFOLLOW, so we skip setting it.
+    // Their filesystems do not have symbolic links, so no special handling is required.
+    #[cfg(not(any(target_os = "espidf", target_os = "horizon")))]
+    {
+        use crate::os::unix::fs::OpenOptionsExt;
+        options.custom_flags(libc::O_NOFOLLOW);
+    }
+
+    options.open(path)?.set_permissions(perm)
+}
+
+#[cfg(not(unix))]
+pub fn set_permissions_nofollow(_path: &Path, _perm: crate::fs::Permissions) -> io::Result<()> {
+    crate::unimplemented!(
+        "`set_permissions_nofollow` is currently only implemented on Unix platforms"
+    )
+}
+
 pub fn canonicalize(path: &Path) -> io::Result<PathBuf> {
     with_native_path(path, &imp::canonicalize)
 }
diff --git a/library/std/src/sys/fs/unix.rs b/library/std/src/sys/fs/unix.rs
index b310db2dac4..0d710a4b2a6 100644
--- a/library/std/src/sys/fs/unix.rs
+++ b/library/std/src/sys/fs/unix.rs
@@ -101,10 +101,11 @@ pub struct File(FileDesc);
 // https://github.com/rust-lang/rust/pull/67774
 macro_rules! cfg_has_statx {
     ({ $($then_tt:tt)* } else { $($else_tt:tt)* }) => {
-        cfg_if::cfg_if! {
-            if #[cfg(all(target_os = "linux", target_env = "gnu"))] {
+        cfg_select! {
+            all(target_os = "linux", target_env = "gnu") => {
                 $($then_tt)*
-            } else {
+            }
+            _ => {
                 $($else_tt)*
             }
         }
@@ -1263,6 +1264,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     ))]
     pub fn lock(&self) -> io::Result<()> {
@@ -1275,6 +1278,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     )))]
     pub fn lock(&self) -> io::Result<()> {
@@ -1286,6 +1291,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     ))]
     pub fn lock_shared(&self) -> io::Result<()> {
@@ -1298,6 +1305,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     )))]
     pub fn lock_shared(&self) -> io::Result<()> {
@@ -1309,6 +1318,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     ))]
     pub fn try_lock(&self) -> Result<(), TryLockError> {
@@ -1329,6 +1340,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     )))]
     pub fn try_lock(&self) -> Result<(), TryLockError> {
@@ -1343,6 +1356,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     ))]
     pub fn try_lock_shared(&self) -> Result<(), TryLockError> {
@@ -1363,6 +1378,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     )))]
     pub fn try_lock_shared(&self) -> Result<(), TryLockError> {
@@ -1377,6 +1394,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     ))]
     pub fn unlock(&self) -> io::Result<()> {
@@ -1389,6 +1408,8 @@ impl File {
         target_os = "fuchsia",
         target_os = "linux",
         target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "cygwin",
         target_vendor = "apple",
     )))]
     pub fn unlock(&self) -> io::Result<()> {
@@ -1505,8 +1526,8 @@ impl File {
             )),
             None => Ok(libc::timespec { tv_sec: 0, tv_nsec: libc::UTIME_OMIT as _ }),
         };
-        cfg_if::cfg_if! {
-            if #[cfg(any(target_os = "redox", target_os = "espidf", target_os = "horizon", target_os = "nuttx"))] {
+        cfg_select! {
+            any(target_os = "redox", target_os = "espidf", target_os = "horizon", target_os = "nuttx") => {
                 // Redox doesn't appear to support `UTIME_OMIT`.
                 // ESP-IDF and HorizonOS do not support `futimens` at all and the behavior for those OS is therefore
                 // the same as for Redox.
@@ -1515,7 +1536,8 @@ impl File {
                     io::ErrorKind::Unsupported,
                     "setting file times not supported",
                 ))
-            } else if #[cfg(target_vendor = "apple")] {
+            }
+            target_vendor = "apple" => {
                 let mut buf = [mem::MaybeUninit::<libc::timespec>::uninit(); 3];
                 let mut num_times = 0;
                 let mut attrlist: libc::attrlist = unsafe { mem::zeroed() };
@@ -1543,7 +1565,8 @@ impl File {
                     0
                 ) })?;
                 Ok(())
-            } else if #[cfg(target_os = "android")] {
+            }
+            target_os = "android" => {
                 let times = [to_timespec(times.accessed)?, to_timespec(times.modified)?];
                 // futimens requires Android API level 19
                 cvt(unsafe {
@@ -1559,7 +1582,8 @@ impl File {
                     }
                 })?;
                 Ok(())
-            } else {
+            }
+            _ => {
                 #[cfg(all(target_os = "linux", target_env = "gnu", target_pointer_width = "32", not(target_arch = "riscv32")))]
                 {
                     use crate::sys::{time::__timespec64, weak::weak};
@@ -1677,13 +1701,14 @@ impl fmt::Debug for File {
             let mut buf = vec![0; libc::PATH_MAX as usize];
             let n = unsafe { libc::fcntl(fd, libc::F_GETPATH, buf.as_ptr()) };
             if n == -1 {
-                cfg_if::cfg_if! {
-                    if #[cfg(target_os = "netbsd")] {
+                cfg_select! {
+                    target_os = "netbsd" => {
                         // fallback to procfs as last resort
                         let mut p = PathBuf::from("/proc/self/fd");
                         p.push(&fd.to_string());
                         return run_path_with_cstr(&p, &readlink).ok()
-                    } else {
+                    }
+                    _ => {
                         return None;
                     }
                 }
@@ -1884,15 +1909,16 @@ pub fn symlink(original: &CStr, link: &CStr) -> io::Result<()> {
 }
 
 pub fn link(original: &CStr, link: &CStr) -> io::Result<()> {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_os = "vxworks", target_os = "redox", target_os = "android", target_os = "espidf", target_os = "horizon", target_os = "vita", target_env = "nto70"))] {
+    cfg_select! {
+        any(target_os = "vxworks", target_os = "redox", target_os = "android", target_os = "espidf", target_os = "horizon", target_os = "vita", target_env = "nto70") => {
             // VxWorks, Redox and ESP-IDF lack `linkat`, so use `link` instead. POSIX leaves
             // it implementation-defined whether `link` follows symlinks, so rely on the
             // `symlink_hard_link` test in library/std/src/fs/tests.rs to check the behavior.
             // Android has `linkat` on newer versions, but we happen to know `link`
             // always has the correct behavior, so it's here as well.
             cvt(unsafe { libc::link(original.as_ptr(), link.as_ptr()) })?;
-        } else {
+        }
+        _ => {
             // Where we can, use `linkat` instead of `link`; see the comment above
             // this one for details on why.
             cvt(unsafe { libc::linkat(libc::AT_FDCWD, original.as_ptr(), libc::AT_FDCWD, link.as_ptr(), 0) })?;
diff --git a/library/std/src/sys/io/mod.rs b/library/std/src/sys/io/mod.rs
index ae75f4d97b4..fe8ec1dbb73 100644
--- a/library/std/src/sys/io/mod.rs
+++ b/library/std/src/sys/io/mod.rs
@@ -1,20 +1,24 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
 mod io_slice {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_family = "unix", target_os = "hermit", target_os = "solid_asp3", target_os = "trusty"))] {
+    cfg_select! {
+        any(target_family = "unix", target_os = "hermit", target_os = "solid_asp3", target_os = "trusty") => {
             mod iovec;
             pub use iovec::*;
-        } else if #[cfg(target_os = "windows")] {
+        }
+        target_os = "windows" => {
             mod windows;
             pub use windows::*;
-        } else if #[cfg(target_os = "wasi")] {
+        }
+        target_os = "wasi" => {
             mod wasi;
             pub use wasi::*;
-        } else if #[cfg(target_os = "uefi")] {
+        }
+        target_os = "uefi" => {
             mod uefi;
             pub use uefi::*;
-        } else {
+        }
+        _ => {
             mod unsupported;
             pub use unsupported::*;
         }
@@ -22,17 +26,20 @@ mod io_slice {
 }
 
 mod is_terminal {
-    cfg_if::cfg_if! {
-        if #[cfg(any(target_family = "unix", target_os = "wasi"))] {
+    cfg_select! {
+        any(target_family = "unix", target_os = "wasi") => {
             mod isatty;
             pub use isatty::*;
-        } else if #[cfg(target_os = "windows")] {
+        }
+        target_os = "windows" => {
             mod windows;
             pub use windows::*;
-        } else if #[cfg(target_os = "hermit")] {
+        }
+        target_os = "hermit" => {
             mod hermit;
             pub use hermit::*;
-        } else {
+        }
+        _ => {
             mod unsupported;
             pub use unsupported::*;
         }
diff --git a/library/std/src/sys/mod.rs b/library/std/src/sys/mod.rs
index 8ec0a0e3302..6324c1a232a 100644
--- a/library/std/src/sys/mod.rs
+++ b/library/std/src/sys/mod.rs
@@ -1,7 +1,7 @@
 #![allow(unsafe_op_in_unsafe_fn)]
 
 /// The configure builtins provides runtime support compiler-builtin features
-/// which require dynamic intialization to work as expected, e.g. aarch64
+/// which require dynamic initialization to work as expected, e.g. aarch64
 /// outline-atomics.
 mod configure_builtins;
 
diff --git a/library/std/src/sys/net/connection/sgx.rs b/library/std/src/sys/net/connection/sgx.rs
index 242df10bc32..2389fd1bcb6 100644
--- a/library/std/src/sys/net/connection/sgx.rs
+++ b/library/std/src/sys/net/connection/sgx.rs
@@ -452,12 +452,7 @@ pub struct NonIpSockAddr {
     host: String,
 }
 
-impl error::Error for NonIpSockAddr {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "Failed to convert address to SocketAddr"
-    }
-}
+impl error::Error for NonIpSockAddr {}
 
 impl fmt::Display for NonIpSockAddr {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/library/std/src/sys/net/connection/socket.rs b/library/std/src/sys/net/connection/socket.rs
index 7301bde6881..aa83ed65d4c 100644
--- a/library/std/src/sys/net/connection/socket.rs
+++ b/library/std/src/sys/net/connection/socket.rs
@@ -9,29 +9,34 @@ use crate::sys_common::{AsInner, FromInner};
 use crate::time::Duration;
 use crate::{cmp, fmt, mem, ptr};
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "hermit")] {
+cfg_select! {
+    target_os = "hermit" => {
         mod hermit;
         pub use hermit::*;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use solid::*;
-    } else if #[cfg(target_family = "unix")] {
+    }
+    target_family = "unix" => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(all(target_os = "wasi", target_env = "p2"))] {
+    }
+    all(target_os = "wasi", target_env = "p2") => {
         mod wasip2;
         pub use wasip2::*;
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
         pub use windows::*;
     }
+    _ => {}
 }
 
 use netc as c;
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         target_os = "dragonfly",
         target_os = "freebsd",
         target_os = "openbsd",
@@ -43,39 +48,44 @@ cfg_if::cfg_if! {
         target_os = "nto",
         target_os = "nuttx",
         target_vendor = "apple",
-    ))] {
+    ) => {
         use c::IPV6_JOIN_GROUP as IPV6_ADD_MEMBERSHIP;
         use c::IPV6_LEAVE_GROUP as IPV6_DROP_MEMBERSHIP;
-    } else {
+    }
+    _ => {
         use c::IPV6_ADD_MEMBERSHIP;
         use c::IPV6_DROP_MEMBERSHIP;
     }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         target_os = "linux", target_os = "android",
         target_os = "hurd",
         target_os = "dragonfly", target_os = "freebsd",
         target_os = "openbsd", target_os = "netbsd",
         target_os = "solaris", target_os = "illumos",
         target_os = "haiku", target_os = "nto",
-        target_os = "cygwin"))] {
+        target_os = "cygwin",
+    ) => {
         use libc::MSG_NOSIGNAL;
-    } else {
+    }
+    _ => {
         const MSG_NOSIGNAL: c_int = 0x0;
     }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         target_os = "dragonfly", target_os = "freebsd",
         target_os = "openbsd", target_os = "netbsd",
         target_os = "solaris", target_os = "illumos",
-        target_os = "nto"))] {
+        target_os = "nto",
+    ) => {
         use crate::ffi::c_uchar;
         type IpV4MultiCastType = c_uchar;
-    } else {
+    }
+    _ => {
         type IpV4MultiCastType = c_int;
     }
 }
@@ -523,17 +533,19 @@ impl TcpListener {
         let (addr, len) = socket_addr_to_c(addr);
         cvt(unsafe { c::bind(sock.as_raw(), addr.as_ptr(), len as _) })?;
 
-        cfg_if::cfg_if! {
-            if #[cfg(target_os = "horizon")] {
+        cfg_select! {
+            target_os = "horizon" => {
                 // The 3DS doesn't support a big connection backlog. Sometimes
                 // it allows up to about 37, but other times it doesn't even
                 // accept 32. There may be a global limitation causing this.
                 let backlog = 20;
-            } else if #[cfg(target_os = "haiku")] {
+            }
+            target_os = "haiku" => {
                 // Haiku does not support a queue length > 32
                 // https://github.com/haiku/haiku/blob/979a0bc487864675517fb2fab28f87dc8bf43041/headers/posix/sys/socket.h#L81
                 let backlog = 32;
-            } else {
+            }
+            _ => {
                 // The default for all other platforms
                 let backlog = 128;
             }
diff --git a/library/std/src/sys/net/connection/socket/unix.rs b/library/std/src/sys/net/connection/socket/unix.rs
index cc111f3521b..8b5970d1494 100644
--- a/library/std/src/sys/net/connection/socket/unix.rs
+++ b/library/std/src/sys/net/connection/socket/unix.rs
@@ -12,10 +12,11 @@ use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::time::{Duration, Instant};
 use crate::{cmp, mem};
 
-cfg_if::cfg_if! {
-    if #[cfg(target_vendor = "apple")] {
+cfg_select! {
+    target_vendor = "apple" => {
         use libc::SO_LINGER_SEC as SO_LINGER;
-    } else {
+    }
+    _ => {
         use libc::SO_LINGER;
     }
 }
@@ -72,8 +73,8 @@ impl Socket {
 
     pub fn new_raw(fam: c_int, ty: c_int) -> io::Result<Socket> {
         unsafe {
-            cfg_if::cfg_if! {
-                if #[cfg(any(
+            cfg_select! {
+                any(
                     target_os = "android",
                     target_os = "dragonfly",
                     target_os = "freebsd",
@@ -85,7 +86,7 @@ impl Socket {
                     target_os = "cygwin",
                     target_os = "nto",
                     target_os = "solaris",
-                ))] {
+                ) => {
                     // On platforms that support it we pass the SOCK_CLOEXEC
                     // flag to atomically create the socket and set it as
                     // CLOEXEC. On Linux this was added in 2.6.27.
@@ -98,7 +99,8 @@ impl Socket {
                     setsockopt(&socket, libc::SOL_SOCKET, libc::SO_NOSIGPIPE, 1)?;
 
                     Ok(socket)
-                } else {
+                }
+                _ => {
                     let fd = cvt(libc::socket(fam, ty, 0))?;
                     let fd = FileDesc::from_raw_fd(fd);
                     fd.set_cloexec()?;
@@ -120,8 +122,8 @@ impl Socket {
         unsafe {
             let mut fds = [0, 0];
 
-            cfg_if::cfg_if! {
-                if #[cfg(any(
+            cfg_select! {
+                any(
                     target_os = "android",
                     target_os = "dragonfly",
                     target_os = "freebsd",
@@ -132,11 +134,12 @@ impl Socket {
                     target_os = "openbsd",
                     target_os = "cygwin",
                     target_os = "nto",
-                ))] {
+                ) => {
                     // Like above, set cloexec atomically
                     cvt(libc::socketpair(fam, ty | libc::SOCK_CLOEXEC, 0, fds.as_mut_ptr()))?;
                     Ok((Socket(FileDesc::from_raw_fd(fds[0])), Socket(FileDesc::from_raw_fd(fds[1]))))
-                } else {
+                }
+                _ => {
                     cvt(libc::socketpair(fam, ty, 0, fds.as_mut_ptr()))?;
                     let a = FileDesc::from_raw_fd(fds[0]);
                     let b = FileDesc::from_raw_fd(fds[1]);
@@ -250,8 +253,8 @@ impl Socket {
         // atomically set the CLOEXEC flag is to use the `accept4` syscall on
         // platforms that support it. On Linux, this was added in 2.6.28,
         // glibc 2.10 and musl 0.9.5.
-        cfg_if::cfg_if! {
-            if #[cfg(any(
+        cfg_select! {
+            any(
                 target_os = "android",
                 target_os = "dragonfly",
                 target_os = "freebsd",
@@ -261,12 +264,13 @@ impl Socket {
                 target_os = "netbsd",
                 target_os = "openbsd",
                 target_os = "cygwin",
-            ))] {
+            ) => {
                 unsafe {
                     let fd = cvt_r(|| libc::accept4(self.as_raw_fd(), storage, len, libc::SOCK_CLOEXEC))?;
                     Ok(Socket(FileDesc::from_raw_fd(fd)))
                 }
-            } else {
+            }
+            _ => {
                 unsafe {
                     let fd = cvt_r(|| libc::accept(self.as_raw_fd(), storage, len))?;
                     let fd = FileDesc::from_raw_fd(fd);
diff --git a/library/std/src/sys/net/mod.rs b/library/std/src/sys/net/mod.rs
index 646679a1cc8..5df1fe138ab 100644
--- a/library/std/src/sys/net/mod.rs
+++ b/library/std/src/sys/net/mod.rs
@@ -1,36 +1,41 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_family = "unix", not(target_os = "l4re")),
         target_os = "windows",
         target_os = "hermit",
         all(target_os = "wasi", target_env = "p2"),
         target_os = "solid_asp3",
-    ))] {
+    ) => {
         mod connection {
             mod socket;
             pub use socket::*;
         }
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod connection {
             mod sgx;
             pub use sgx::*;
         }
-    } else if #[cfg(all(target_os = "wasi", target_env = "p1"))] {
+    }
+    all(target_os = "wasi", target_env = "p1") => {
         mod connection {
             mod wasip1;
             pub use wasip1::*;
         }
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod connection {
             mod xous;
             pub use xous::*;
         }
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod connection {
             mod uefi;
             pub use uefi::*;
         }
-    } else {
+    }
+    _ => {
         mod connection {
             mod unsupported;
             pub use unsupported::*;
diff --git a/library/std/src/sys/os_str/mod.rs b/library/std/src/sys/os_str/mod.rs
index 345e661586d..65c90d88049 100644
--- a/library/std/src/sys/os_str/mod.rs
+++ b/library/std/src/sys/os_str/mod.rs
@@ -1,13 +1,11 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
-        target_os = "windows",
-        target_os = "uefi",
-    ))] {
+cfg_select! {
+    any(target_os = "windows", target_os = "uefi") => {
         mod wtf8;
         pub use wtf8::{Buf, Slice};
-    } else {
+    }
+    _ => {
         mod bytes;
         pub use bytes::{Buf, Slice};
     }
diff --git a/library/std/src/sys/os_str/wtf8.rs b/library/std/src/sys/os_str/wtf8.rs
index bbc704ebf86..96da891874e 100644
--- a/library/std/src/sys/os_str/wtf8.rs
+++ b/library/std/src/sys/os_str/wtf8.rs
@@ -1,12 +1,12 @@
 //! The underlying OsString/OsStr implementation on Windows is a
 //! wrapper around the "WTF-8" encoding; see the `wtf8` module for more.
+use alloc::wtf8::{Wtf8, Wtf8Buf};
 use core::clone::CloneToUninit;
 
 use crate::borrow::Cow;
 use crate::collections::TryReserveError;
 use crate::rc::Rc;
 use crate::sync::Arc;
-use crate::sys_common::wtf8::{Wtf8, Wtf8Buf, check_utf8_boundary};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::{fmt, mem};
 
@@ -220,7 +220,9 @@ impl Buf {
     /// trailing surrogate half.
     #[inline]
     pub unsafe fn extend_from_slice_unchecked(&mut self, other: &[u8]) {
-        self.inner.extend_from_slice(other);
+        unsafe {
+            self.inner.extend_from_slice_unchecked(other);
+        }
     }
 }
 
@@ -238,7 +240,7 @@ impl Slice {
     #[track_caller]
     #[inline]
     pub fn check_public_boundary(&self, index: usize) {
-        check_utf8_boundary(&self.inner, index);
+        self.inner.check_utf8_boundary(index);
     }
 
     #[inline]
diff --git a/library/std/src/sys/pal/hermit/os.rs b/library/std/src/sys/pal/hermit/os.rs
index a998c3165e5..0fe713a503b 100644
--- a/library/std/src/sys/pal/hermit/os.rs
+++ b/library/std/src/sys/pal/hermit/os.rs
@@ -1,5 +1,4 @@
 use super::hermit_abi;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::path::{self, PathBuf};
@@ -52,12 +51,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on hermit yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/mod.rs b/library/std/src/sys/pal/mod.rs
index fbefc62ac88..9376f5249f1 100644
--- a/library/std/src/sys/pal/mod.rs
+++ b/library/std/src/sys/pal/mod.rs
@@ -24,60 +24,70 @@
 
 pub mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(unix)] {
+cfg_select! {
+    unix => {
         mod unix;
         pub use self::unix::*;
-    } else if #[cfg(windows)] {
+    }
+    windows => {
         mod windows;
         pub use self::windows::*;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use self::solid::*;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         pub use self::hermit::*;
-    } else if #[cfg(target_os = "trusty")] {
+    }
+    target_os = "trusty" => {
         mod trusty;
         pub use self::trusty::*;
-    } else if #[cfg(all(target_os = "wasi", target_env = "p2"))] {
+    }
+    all(target_os = "wasi", target_env = "p2") => {
         mod wasip2;
         pub use self::wasip2::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use self::wasi::*;
-    } else if #[cfg(target_family = "wasm")] {
+    }
+    target_family = "wasm" => {
         mod wasm;
         pub use self::wasm::*;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use self::xous::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use self::uefi::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use self::sgx::*;
-    } else if #[cfg(target_os = "teeos")] {
+    }
+    target_os = "teeos" => {
         mod teeos;
         pub use self::teeos::*;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
         pub use self::zkvm::*;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use self::unsupported::*;
     }
 }
 
-cfg_if::cfg_if! {
+pub const FULL_BACKTRACE_DEFAULT: bool = cfg_select! {
     // Fuchsia components default to full backtrace.
-    if #[cfg(target_os = "fuchsia")] {
-        pub const FULL_BACKTRACE_DEFAULT: bool = true;
-    } else {
-        pub const FULL_BACKTRACE_DEFAULT: bool = false;
-    }
-}
+    target_os = "fuchsia" => true,
+    _ => false,
+};
 
 #[cfg(not(target_os = "uefi"))]
 pub type RawOsError = i32;
diff --git a/library/std/src/sys/pal/sgx/mod.rs b/library/std/src/sys/pal/sgx/mod.rs
index 6e43a79ddec..4a297b6823f 100644
--- a/library/std/src/sys/pal/sgx/mod.rs
+++ b/library/std/src/sys/pal/sgx/mod.rs
@@ -59,8 +59,7 @@ pub fn sgx_ineffective<T>(v: T) -> crate::io::Result<T> {
 
 #[inline]
 pub fn is_interrupted(code: i32) -> bool {
-    use fortanix_sgx_abi::Error;
-    code == Error::Interrupted as _
+    code == fortanix_sgx_abi::Error::Interrupted as _
 }
 
 pub fn decode_error_kind(code: i32) -> ErrorKind {
diff --git a/library/std/src/sys/pal/sgx/os.rs b/library/std/src/sys/pal/sgx/os.rs
index 70f838679c9..28d79963ac8 100644
--- a/library/std/src/sys/pal/sgx/os.rs
+++ b/library/std/src/sys/pal/sgx/os.rs
@@ -1,11 +1,10 @@
 use fortanix_sgx_abi::{Error, RESULT_SUCCESS};
 
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::path::{self, PathBuf};
 use crate::sys::{decode_error_kind, sgx_ineffective, unsupported};
-use crate::{fmt, io, str};
+use crate::{fmt, io};
 
 pub fn errno() -> i32 {
     RESULT_SUCCESS
@@ -59,12 +58,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported in SGX yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/solid/os.rs b/library/std/src/sys/pal/solid/os.rs
index 8f5976b0592..cb6e2cbceae 100644
--- a/library/std/src/sys/pal/solid/os.rs
+++ b/library/std/src/sys/pal/solid/os.rs
@@ -1,5 +1,4 @@
 use super::{error, itron, unsupported};
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::path::{self, PathBuf};
 use crate::{fmt, io};
@@ -58,12 +57,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on this platform yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/teeos/os.rs b/library/std/src/sys/pal/teeos/os.rs
index 03f3c72b022..512b3e2885b 100644
--- a/library/std/src/sys/pal/teeos/os.rs
+++ b/library/std/src/sys/pal/teeos/os.rs
@@ -3,7 +3,6 @@
 use core::marker::PhantomData;
 
 use super::unsupported;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::path::PathBuf;
 use crate::{fmt, io, path};
@@ -62,12 +61,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on this platform yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/uefi/os.rs b/library/std/src/sys/pal/uefi/os.rs
index bfd4dc81cb4..aae6cb9e064 100644
--- a/library/std/src/sys/pal/uefi/os.rs
+++ b/library/std/src/sys/pal/uefi/os.rs
@@ -2,7 +2,6 @@ use r_efi::efi::Status;
 use r_efi::efi::protocols::{device_path, loaded_image_device_path};
 
 use super::{RawOsError, helpers, unsupported_err};
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::os::uefi;
@@ -122,7 +121,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     let protocol = helpers::image_handle_protocol::<device_path::Protocol>(
diff --git a/library/std/src/sys/pal/uefi/time.rs b/library/std/src/sys/pal/uefi/time.rs
index df5611b2ddd..36ce3f7ef96 100644
--- a/library/std/src/sys/pal/uefi/time.rs
+++ b/library/std/src/sys/pal/uefi/time.rs
@@ -188,7 +188,7 @@ pub(crate) mod system_time_internal {
         Duration::new(epoch, t.nanosecond)
     }
 
-    /// This algorithm is a modifed version of the one described in the post:
+    /// This algorithm is a modified version of the one described in the post:
     /// https://howardhinnant.github.io/date_algorithms.html#clive_from_days
     ///
     /// The changes are to use 1900-01-01-00:00:00 with timezone -1440 as anchor instead of UNIX
@@ -197,7 +197,7 @@ pub(crate) mod system_time_internal {
         // Check timzone validity
         assert!(timezone <= 1440 && timezone >= -1440);
 
-        // FIXME(#126043): use checked_sub_signed once stablized
+        // FIXME(#126043): use checked_sub_signed once stabilized
         let secs =
             dur.as_secs().checked_add_signed((-timezone as i64) * SECS_IN_MINUTE as i64).unwrap();
 
@@ -296,12 +296,9 @@ pub(crate) mod instant_internal {
     }
 
     pub fn platform_specific() -> Option<Instant> {
-        cfg_if::cfg_if! {
-            if #[cfg(any(target_arch = "x86_64", target_arch = "x86"))] {
-                timestamp_rdtsc().map(Instant)
-            } else {
-                None
-            }
+        cfg_select! {
+            any(target_arch = "x86_64", target_arch = "x86") => timestamp_rdtsc().map(Instant),
+            _ => None,
         }
     }
 
diff --git a/library/std/src/sys/pal/unix/futex.rs b/library/std/src/sys/pal/unix/futex.rs
index c23278bdf5e..265067d84d5 100644
--- a/library/std/src/sys/pal/unix/futex.rs
+++ b/library/std/src/sys/pal/unix/futex.rs
@@ -46,8 +46,8 @@ pub fn futex_wait(futex: &Atomic<u32>, expected: u32, timeout: Option<Duration>)
         }
 
         let r = unsafe {
-            cfg_if::cfg_if! {
-                if #[cfg(target_os = "freebsd")] {
+            cfg_select! {
+                target_os = "freebsd" => {
                     // FreeBSD doesn't have futex(), but it has
                     // _umtx_op(UMTX_OP_WAIT_UINT_PRIVATE), which is nearly
                     // identical. It supports absolute timeouts through a flag
@@ -66,7 +66,8 @@ pub fn futex_wait(futex: &Atomic<u32>, expected: u32, timeout: Option<Duration>)
                         crate::ptr::without_provenance_mut(umtx_timeout_size),
                         umtx_timeout_ptr as *mut _,
                     )
-                } else if #[cfg(any(target_os = "linux", target_os = "android"))] {
+                }
+                any(target_os = "linux", target_os = "android") => {
                     // Use FUTEX_WAIT_BITSET rather than FUTEX_WAIT to be able to give an
                     // absolute time rather than a relative time.
                     libc::syscall(
@@ -78,7 +79,8 @@ pub fn futex_wait(futex: &Atomic<u32>, expected: u32, timeout: Option<Duration>)
                         null::<u32>(), // This argument is unused for FUTEX_WAIT_BITSET.
                         !0u32,         // A full bitmask, to make it behave like a regular FUTEX_WAIT.
                     )
-                } else {
+                }
+                _ => {
                     compile_error!("unknown target_os");
                 }
             }
diff --git a/library/std/src/sys/pal/unix/mod.rs b/library/std/src/sys/pal/unix/mod.rs
index ba9e14b8009..aef7ab55088 100644
--- a/library/std/src/sys/pal/unix/mod.rs
+++ b/library/std/src/sys/pal/unix/mod.rs
@@ -59,6 +59,30 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     }
 
     unsafe fn sanitize_standard_fds() {
+        #[allow(dead_code, unused_variables, unused_mut)]
+        let mut opened_devnull = -1;
+        #[allow(dead_code, unused_variables, unused_mut)]
+        let mut open_devnull = || {
+            #[cfg(not(all(target_os = "linux", target_env = "gnu")))]
+            use libc::open;
+            #[cfg(all(target_os = "linux", target_env = "gnu"))]
+            use libc::open64 as open;
+
+            if opened_devnull != -1 {
+                if libc::dup(opened_devnull) != -1 {
+                    return;
+                }
+            }
+            opened_devnull = open(c"/dev/null".as_ptr(), libc::O_RDWR, 0);
+            if opened_devnull == -1 {
+                // If the stream is closed but we failed to reopen it, abort the
+                // process. Otherwise we wouldn't preserve the safety of
+                // operations on the corresponding Rust object Stdin, Stdout, or
+                // Stderr.
+                libc::abort();
+            }
+        };
+
         // fast path with a single syscall for systems with poll()
         #[cfg(not(any(
             miri,
@@ -74,11 +98,6 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
             target_vendor = "apple",
         )))]
         'poll: {
-            #[cfg(not(all(target_os = "linux", target_env = "gnu")))]
-            use libc::open as open64;
-            #[cfg(all(target_os = "linux", target_env = "gnu"))]
-            use libc::open64;
-
             use crate::sys::os::errno;
             let pfds: &mut [_] = &mut [
                 libc::pollfd { fd: 0, events: 0, revents: 0 },
@@ -106,13 +125,7 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
                 if pfd.revents & libc::POLLNVAL == 0 {
                     continue;
                 }
-                if open64(c"/dev/null".as_ptr(), libc::O_RDWR, 0) == -1 {
-                    // If the stream is closed but we failed to reopen it, abort the
-                    // process. Otherwise we wouldn't preserve the safety of
-                    // operations on the corresponding Rust object Stdin, Stdout, or
-                    // Stderr.
-                    libc::abort();
-                }
+                open_devnull();
             }
             return;
         }
@@ -129,21 +142,10 @@ pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
             target_os = "vita",
         )))]
         {
-            #[cfg(not(all(target_os = "linux", target_env = "gnu")))]
-            use libc::open as open64;
-            #[cfg(all(target_os = "linux", target_env = "gnu"))]
-            use libc::open64;
-
             use crate::sys::os::errno;
             for fd in 0..3 {
                 if libc::fcntl(fd, libc::F_GETFD) == -1 && errno() == libc::EBADF {
-                    if open64(c"/dev/null".as_ptr(), libc::O_RDWR, 0) == -1 {
-                        // If the stream is closed but we failed to reopen it, abort the
-                        // process. Otherwise we wouldn't preserve the safety of
-                        // operations on the corresponding Rust object Stdin, Stdout, or
-                        // Stderr.
-                        libc::abort();
-                    }
+                    open_devnull();
                 }
             }
         }
@@ -366,31 +368,36 @@ pub fn abort_internal() -> ! {
     unsafe { libc::abort() }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "android")] {
+cfg_select! {
+    target_os = "android" => {
         #[link(name = "dl", kind = "static", modifiers = "-bundle",
             cfg(target_feature = "crt-static"))]
         #[link(name = "dl", cfg(not(target_feature = "crt-static")))]
         #[link(name = "log", cfg(not(target_feature = "crt-static")))]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "freebsd")] {
+    }
+    target_os = "freebsd" => {
         #[link(name = "execinfo")]
         #[link(name = "pthread")]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "netbsd")] {
+    }
+    target_os = "netbsd" => {
         #[link(name = "pthread")]
         #[link(name = "rt")]
         unsafe extern "C" {}
-    } else if #[cfg(any(target_os = "dragonfly", target_os = "openbsd", target_os = "cygwin"))] {
+    }
+    any(target_os = "dragonfly", target_os = "openbsd", target_os = "cygwin") => {
         #[link(name = "pthread")]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "solaris")] {
+    }
+    target_os = "solaris" => {
         #[link(name = "socket")]
         #[link(name = "posix4")]
         #[link(name = "pthread")]
         #[link(name = "resolv")]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "illumos")] {
+    }
+    target_os = "illumos" => {
         #[link(name = "socket")]
         #[link(name = "posix4")]
         #[link(name = "pthread")]
@@ -399,24 +406,29 @@ cfg_if::cfg_if! {
         // Use libumem for the (malloc-compatible) allocator
         #[link(name = "umem")]
         unsafe extern "C" {}
-    } else if #[cfg(target_vendor = "apple")] {
+    }
+    target_vendor = "apple" => {
         // Link to `libSystem.dylib`.
         //
         // Don't get confused by the presence of `System.framework`,
         // it is a deprecated wrapper over the dynamic library.
         #[link(name = "System")]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "fuchsia")] {
+    }
+    target_os = "fuchsia" => {
         #[link(name = "zircon")]
         #[link(name = "fdio")]
         unsafe extern "C" {}
-    } else if #[cfg(all(target_os = "linux", target_env = "uclibc"))] {
+    }
+    all(target_os = "linux", target_env = "uclibc") => {
         #[link(name = "dl")]
         unsafe extern "C" {}
-    } else if #[cfg(target_os = "vita")] {
+    }
+    target_os = "vita" => {
         #[link(name = "pthread", kind = "static", modifiers = "-bundle")]
         unsafe extern "C" {}
     }
+    _ => {}
 }
 
 #[cfg(any(target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nuttx"))]
diff --git a/library/std/src/sys/pal/unix/os.rs b/library/std/src/sys/pal/unix/os.rs
index 0e68313cc3e..81275afa707 100644
--- a/library/std/src/sys/pal/unix/os.rs
+++ b/library/std/src/sys/pal/unix/os.rs
@@ -7,7 +7,6 @@ mod tests;
 
 use libc::{c_char, c_int, c_void};
 
-use crate::error::Error as StdError;
 use crate::ffi::{CStr, OsStr, OsString};
 use crate::os::unix::prelude::*;
 use crate::path::{self, PathBuf};
@@ -17,13 +16,10 @@ use crate::{fmt, io, iter, mem, ptr, slice, str};
 
 const TMPBUF_SZ: usize = 128;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "redox")] {
-        const PATH_SEPARATOR: u8 = b';';
-    } else {
-        const PATH_SEPARATOR: u8 = b':';
-    }
-}
+const PATH_SEPARATOR: u8 = cfg_select! {
+    target_os = "redox" => b';',
+    _ => b':',
+};
 
 unsafe extern "C" {
     #[cfg(not(any(target_os = "dragonfly", target_os = "vxworks", target_os = "rtems")))]
@@ -251,12 +247,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "failed to join paths"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 #[cfg(target_os = "aix")]
 pub fn current_exe() -> io::Result<PathBuf> {
@@ -620,14 +611,10 @@ fn darwin_temp_dir() -> PathBuf {
 
 pub fn temp_dir() -> PathBuf {
     crate::env::var_os("TMPDIR").map(PathBuf::from).unwrap_or_else(|| {
-        cfg_if::cfg_if! {
-            if #[cfg(all(target_vendor = "apple", not(miri)))] {
-                darwin_temp_dir()
-            } else if #[cfg(target_os = "android")] {
-                PathBuf::from("/data/local/tmp")
-            } else {
-                PathBuf::from("/tmp")
-            }
+        cfg_select! {
+            all(target_vendor = "apple", not(miri)) => darwin_temp_dir(),
+            target_os = "android" => PathBuf::from("/data/local/tmp"),
+            _ => PathBuf::from("/tmp"),
         }
     })
 }
diff --git a/library/std/src/sys/pal/unix/pipe.rs b/library/std/src/sys/pal/unix/pipe.rs
index 55510153dc8..6b0cd14da4f 100644
--- a/library/std/src/sys/pal/unix/pipe.rs
+++ b/library/std/src/sys/pal/unix/pipe.rs
@@ -18,8 +18,8 @@ pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
     // The only known way right now to create atomically set the CLOEXEC flag is
     // to use the `pipe2` syscall. This was added to Linux in 2.6.27, glibc 2.9
     // and musl 0.9.3, and some other targets also have it.
-    cfg_if::cfg_if! {
-        if #[cfg(any(
+    cfg_select! {
+        any(
             target_os = "dragonfly",
             target_os = "freebsd",
             target_os = "hurd",
@@ -29,12 +29,13 @@ pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
             target_os = "openbsd",
             target_os = "cygwin",
             target_os = "redox"
-        ))] {
+        ) => {
             unsafe {
                 cvt(libc::pipe2(fds.as_mut_ptr(), libc::O_CLOEXEC))?;
                 Ok((AnonPipe(FileDesc::from_raw_fd(fds[0])), AnonPipe(FileDesc::from_raw_fd(fds[1]))))
             }
-        } else {
+        }
+        _ => {
             unsafe {
                 cvt(libc::pipe(fds.as_mut_ptr()))?;
 
diff --git a/library/std/src/sys/pal/unix/thread.rs b/library/std/src/sys/pal/unix/thread.rs
index 36e53e7cadc..3389b8c0c8a 100644
--- a/library/std/src/sys/pal/unix/thread.rs
+++ b/library/std/src/sys/pal/unix/thread.rs
@@ -77,7 +77,18 @@ impl Thread {
                     let page_size = os::page_size();
                     let stack_size =
                         (stack_size + page_size - 1) & (-(page_size as isize - 1) as usize - 1);
-                    assert_eq!(libc::pthread_attr_setstacksize(attr.as_mut_ptr(), stack_size), 0);
+
+                    // Some libc implementations, e.g. musl, place an upper bound
+                    // on the stack size, in which case we can only gracefully return
+                    // an error here.
+                    if libc::pthread_attr_setstacksize(attr.as_mut_ptr(), stack_size) != 0 {
+                        assert_eq!(libc::pthread_attr_destroy(attr.as_mut_ptr()), 0);
+                        drop(Box::from_raw(data));
+                        return Err(io::const_error!(
+                            io::ErrorKind::InvalidInput,
+                            "invalid stack size"
+                        ));
+                    }
                 }
             };
         }
@@ -140,12 +151,13 @@ impl Thread {
     ))]
     pub fn set_name(name: &CStr) {
         unsafe {
-            cfg_if::cfg_if! {
-                if #[cfg(any(target_os = "linux", target_os = "cygwin"))] {
+            cfg_select! {
+                any(target_os = "linux", target_os = "cygwin") => {
                     // Linux and Cygwin limits the allowed length of the name.
                     const TASK_COMM_LEN: usize = 16;
                     let name = truncate_cstr::<{ TASK_COMM_LEN }>(name);
-                } else {
+                }
+                _ => {
                     // FreeBSD, DragonFly BSD and NuttX do not enforce length limits.
                 }
             };
@@ -404,9 +416,9 @@ pub(crate) fn current_os_id() -> Option<u64> {
     //
     // The OS thread ID is used rather than `pthread_self` so as to match what will be displayed
     // for process inspection (debuggers, trace, `top`, etc.).
-    cfg_if::cfg_if! {
+    cfg_select! {
         // Most platforms have a function returning a `pid_t` or int, which is an `i32`.
-        if #[cfg(any(target_os = "android", target_os = "linux"))] {
+        any(target_os = "android", target_os = "linux") => {
             use crate::sys::weak::syscall;
 
             // `libc::gettid` is only available on glibc 2.30+, but the syscall is available
@@ -416,28 +428,34 @@ pub(crate) fn current_os_id() -> Option<u64> {
             // SAFETY: FFI call with no preconditions.
             let id: libc::pid_t = unsafe { gettid() };
             Some(id as u64)
-        } else if #[cfg(target_os = "nto")] {
+        }
+        target_os = "nto" => {
             // SAFETY: FFI call with no preconditions.
             let id: libc::pid_t = unsafe { libc::gettid() };
             Some(id as u64)
-        } else if #[cfg(target_os = "openbsd")] {
+        }
+        target_os = "openbsd" => {
             // SAFETY: FFI call with no preconditions.
             let id: libc::pid_t = unsafe { libc::getthrid() };
             Some(id as u64)
-        } else if #[cfg(target_os = "freebsd")] {
+        }
+        target_os = "freebsd" => {
             // SAFETY: FFI call with no preconditions.
             let id: libc::c_int = unsafe { libc::pthread_getthreadid_np() };
             Some(id as u64)
-        } else if #[cfg(target_os = "netbsd")] {
+        }
+        target_os = "netbsd" => {
             // SAFETY: FFI call with no preconditions.
             let id: libc::lwpid_t = unsafe { libc::_lwp_self() };
             Some(id as u64)
-        } else if #[cfg(any(target_os = "illumos", target_os = "solaris"))] {
+        }
+        any(target_os = "illumos", target_os = "solaris") => {
             // On Illumos and Solaris, the `pthread_t` is the same as the OS thread ID.
             // SAFETY: FFI call with no preconditions.
             let id: libc::pthread_t = unsafe { libc::pthread_self() };
             Some(id as u64)
-        } else if #[cfg(target_vendor = "apple")] {
+        }
+        target_vendor = "apple" => {
             // Apple allows querying arbitrary thread IDs, `thread=NULL` queries the current thread.
             let mut id = 0u64;
             // SAFETY: `thread_id` is a valid pointer, no other preconditions.
@@ -447,10 +465,9 @@ pub(crate) fn current_os_id() -> Option<u64> {
             } else {
                 None
             }
-        } else {
-            // Other platforms don't have an OS thread ID or don't have a way to access it.
-            None
         }
+        // Other platforms don't have an OS thread ID or don't have a way to access it.
+        _ => None,
     }
 }
 
@@ -472,8 +489,8 @@ fn truncate_cstr<const MAX_WITH_NUL: usize>(cstr: &CStr) -> [libc::c_char; MAX_W
 }
 
 pub fn available_parallelism() -> io::Result<NonZero<usize>> {
-    cfg_if::cfg_if! {
-        if #[cfg(any(
+    cfg_select! {
+        any(
             target_os = "android",
             target_os = "emscripten",
             target_os = "fuchsia",
@@ -482,7 +499,7 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
             target_os = "aix",
             target_vendor = "apple",
             target_os = "cygwin",
-        ))] {
+        ) => {
             #[allow(unused_assignments)]
             #[allow(unused_mut)]
             let mut quota = usize::MAX;
@@ -516,12 +533,13 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
                     Ok(unsafe { NonZero::new_unchecked(count) })
                 }
             }
-        } else if #[cfg(any(
-                   target_os = "freebsd",
-                   target_os = "dragonfly",
-                   target_os = "openbsd",
-                   target_os = "netbsd",
-               ))] {
+        }
+        any(
+           target_os = "freebsd",
+           target_os = "dragonfly",
+           target_os = "openbsd",
+           target_os = "netbsd",
+        ) => {
             use crate::ptr;
 
             #[cfg(target_os = "freebsd")]
@@ -596,7 +614,8 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
             }
 
             Ok(unsafe { NonZero::new_unchecked(cpus as usize) })
-        } else if #[cfg(target_os = "nto")] {
+        }
+        target_os = "nto" => {
             unsafe {
                 use libc::_syspage_ptr;
                 if _syspage_ptr.is_null() {
@@ -607,13 +626,15 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
                         .ok_or(io::Error::UNKNOWN_THREAD_COUNT)
                 }
             }
-        } else if #[cfg(any(target_os = "solaris", target_os = "illumos"))] {
+        }
+        any(target_os = "solaris", target_os = "illumos") => {
             let mut cpus = 0u32;
             if unsafe { libc::pset_info(libc::PS_MYID, core::ptr::null_mut(), &mut cpus, core::ptr::null_mut()) } != 0 {
                 return Err(io::Error::UNKNOWN_THREAD_COUNT);
             }
             Ok(unsafe { NonZero::new_unchecked(cpus as usize) })
-        } else if #[cfg(target_os = "haiku")] {
+        }
+        target_os = "haiku" => {
             // system_info cpu_count field gets the static data set at boot time with `smp_set_num_cpus`
             // `get_system_info` calls then `smp_get_num_cpus`
             unsafe {
@@ -626,7 +647,8 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
 
                 Ok(NonZero::new_unchecked(sinfo.cpu_count as usize))
             }
-        } else if #[cfg(target_os = "vxworks")] {
+        }
+        target_os = "vxworks" => {
             // Note: there is also `vxCpuConfiguredGet`, closer to _SC_NPROCESSORS_CONF
             // expectations than the actual cores availability.
             unsafe extern "C" {
@@ -638,7 +660,8 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
                 let set = vxCpuEnabledGet();
                 Ok(NonZero::new_unchecked(set.count_ones() as usize))
             }
-        } else {
+        }
+        _ => {
             // FIXME: implement on Redox, l4re
             Err(io::const_error!(io::ErrorKind::Unsupported, "getting the number of hardware threads is not supported on the target platform"))
         }
diff --git a/library/std/src/sys/pal/unsupported/os.rs b/library/std/src/sys/pal/unsupported/os.rs
index a8ef97ecf67..13d2a2044f4 100644
--- a/library/std/src/sys/pal/unsupported/os.rs
+++ b/library/std/src/sys/pal/unsupported/os.rs
@@ -1,5 +1,4 @@
 use super::unsupported;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::path::{self, PathBuf};
@@ -51,12 +50,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on this platform yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/wasi/os.rs b/library/std/src/sys/pal/wasi/os.rs
index 672cf70d1a5..151ba254ec4 100644
--- a/library/std/src/sys/pal/wasi/os.rs
+++ b/library/std/src/sys/pal/wasi/os.rs
@@ -1,6 +1,5 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-use crate::error::Error as StdError;
 use crate::ffi::{CStr, OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::os::wasi::prelude::*;
@@ -105,12 +104,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on wasm yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/wasi/thread.rs b/library/std/src/sys/pal/wasi/thread.rs
index 4755e2ef5da..e062b49bd7a 100644
--- a/library/std/src/sys/pal/wasi/thread.rs
+++ b/library/std/src/sys/pal/wasi/thread.rs
@@ -5,8 +5,8 @@ use crate::num::NonZero;
 use crate::time::{Duration, Instant};
 use crate::{io, mem};
 
-cfg_if::cfg_if! {
-    if #[cfg(target_feature = "atomics")] {
+cfg_select! {
+    target_feature = "atomics" => {
         use crate::cmp;
         use crate::ptr;
         use crate::sys::os;
@@ -62,7 +62,8 @@ cfg_if::cfg_if! {
                 debug_assert_eq!(ret, 0);
             }
         }
-    } else {
+    }
+    _ => {
         pub struct Thread(!);
     }
 }
@@ -71,8 +72,8 @@ pub const DEFAULT_MIN_STACK_SIZE: usize = 1024 * 1024;
 
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
-    cfg_if::cfg_if! {
-        if #[cfg(target_feature = "atomics")] {
+    cfg_select! {
+        target_feature = "atomics" => {
             pub unsafe fn new(stack: usize, _name: Option<&str>, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
                 let p = Box::into_raw(Box::new(p));
                 let mut native: libc::pthread_t = unsafe { mem::zeroed() };
@@ -119,7 +120,8 @@ impl Thread {
                     ptr::null_mut()
                 }
             }
-        } else {
+        }
+        _ => {
             pub unsafe fn new(_stack: usize, _name: Option<&str>, _p: Box<dyn FnOnce()>) -> io::Result<Thread> {
                 crate::sys::unsupported()
             }
@@ -180,14 +182,15 @@ impl Thread {
     }
 
     pub fn join(self) {
-        cfg_if::cfg_if! {
-            if #[cfg(target_feature = "atomics")] {
+        cfg_select! {
+            target_feature = "atomics" => {
                 let id = mem::ManuallyDrop::new(self).id;
                 let ret = unsafe { libc::pthread_join(id, ptr::null_mut()) };
                 if ret != 0 {
                     rtabort!("failed to join thread: {}", io::Error::from_raw_os_error(ret));
                 }
-            } else {
+            }
+            _ => {
                 self.0
             }
         }
@@ -199,14 +202,13 @@ pub(crate) fn current_os_id() -> Option<u64> {
 }
 
 pub fn available_parallelism() -> io::Result<NonZero<usize>> {
-    cfg_if::cfg_if! {
-        if #[cfg(target_feature = "atomics")] {
+    cfg_select! {
+        target_feature = "atomics" => {
             match unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) } {
                 -1 => Err(io::Error::last_os_error()),
                 cpus => NonZero::new(cpus as usize).ok_or(io::Error::UNKNOWN_THREAD_COUNT),
             }
-        } else {
-            crate::sys::unsupported()
         }
+        _ => crate::sys::unsupported(),
     }
 }
diff --git a/library/std/src/sys/pal/wasm/mod.rs b/library/std/src/sys/pal/wasm/mod.rs
index 37cb46a8f6b..346c9ff88c9 100644
--- a/library/std/src/sys/pal/wasm/mod.rs
+++ b/library/std/src/sys/pal/wasm/mod.rs
@@ -23,13 +23,14 @@ pub mod pipe;
 #[path = "../unsupported/time.rs"]
 pub mod time;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_feature = "atomics")] {
+cfg_select! {
+    target_feature = "atomics" => {
         #[path = "atomics/futex.rs"]
         pub mod futex;
         #[path = "atomics/thread.rs"]
         pub mod thread;
-    } else {
+    }
+    _ => {
         #[path = "../unsupported/thread.rs"]
         pub mod thread;
     }
diff --git a/library/std/src/sys/pal/windows/c.rs b/library/std/src/sys/pal/windows/c.rs
index edac5262a4e..25c1a82cc42 100644
--- a/library/std/src/sys/pal/windows/c.rs
+++ b/library/std/src/sys/pal/windows/c.rs
@@ -95,11 +95,8 @@ pub struct MOUNT_POINT_REPARSE_BUFFER {
 }
 
 // Desktop specific functions & types
-cfg_if::cfg_if! {
-if #[cfg(not(target_vendor = "uwp"))] {
-    pub const EXCEPTION_CONTINUE_SEARCH: i32 = 0;
-}
-}
+#[cfg(not(target_vendor = "uwp"))]
+pub const EXCEPTION_CONTINUE_SEARCH: i32 = 0;
 
 // Use raw-dylib to import ProcessPrng as we can't rely on there being an import library.
 #[cfg(not(target_vendor = "win7"))]
@@ -230,12 +227,13 @@ compat_fn_with_fallback! {
     }
 }
 
-cfg_if::cfg_if! {
-    if #[cfg(target_vendor = "uwp")] {
+cfg_select! {
+    target_vendor = "uwp" => {
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn NtCreateFile(filehandle : *mut HANDLE, desiredaccess : FILE_ACCESS_RIGHTS, objectattributes : *const OBJECT_ATTRIBUTES, iostatusblock : *mut IO_STATUS_BLOCK, allocationsize : *const i64, fileattributes : FILE_FLAGS_AND_ATTRIBUTES, shareaccess : FILE_SHARE_MODE, createdisposition : NTCREATEFILE_CREATE_DISPOSITION, createoptions : NTCREATEFILE_CREATE_OPTIONS, eabuffer : *const core::ffi::c_void, ealength : u32) -> NTSTATUS);
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn NtOpenFile(filehandle : *mut HANDLE, desiredaccess : u32, objectattributes : *const OBJECT_ATTRIBUTES, iostatusblock : *mut IO_STATUS_BLOCK, shareaccess : u32, openoptions : u32) -> NTSTATUS);
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn NtReadFile(filehandle : HANDLE, event : HANDLE, apcroutine : PIO_APC_ROUTINE, apccontext : *const core::ffi::c_void, iostatusblock : *mut IO_STATUS_BLOCK, buffer : *mut core::ffi::c_void, length : u32, byteoffset : *const i64, key : *const u32) -> NTSTATUS);
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn NtWriteFile(filehandle : HANDLE, event : HANDLE, apcroutine : PIO_APC_ROUTINE, apccontext : *const core::ffi::c_void, iostatusblock : *mut IO_STATUS_BLOCK, buffer : *const core::ffi::c_void, length : u32, byteoffset : *const i64, key : *const u32) -> NTSTATUS);
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn RtlNtStatusToDosError(status : NTSTATUS) -> u32);
     }
+    _ => {}
 }
diff --git a/library/std/src/sys/pal/windows/c/bindings.txt b/library/std/src/sys/pal/windows/c/bindings.txt
index c8e4dca4781..abc1c19827f 100644
--- a/library/std/src/sys/pal/windows/c/bindings.txt
+++ b/library/std/src/sys/pal/windows/c/bindings.txt
@@ -2158,6 +2158,7 @@ GetCurrentDirectoryW
 GetCurrentProcess
 GetCurrentProcessId
 GetCurrentThread
+GetCurrentThreadId
 GetEnvironmentStringsW
 GetEnvironmentVariableW
 GetExitCodeProcess
@@ -2185,7 +2186,6 @@ GetSystemInfo
 GetSystemTimeAsFileTime
 GetSystemTimePreciseAsFileTime
 GetTempPathW
-GetThreadId
 GetUserProfileDirectoryW
 GetWindowsDirectoryW
 HANDLE
diff --git a/library/std/src/sys/pal/windows/c/windows_sys.rs b/library/std/src/sys/pal/windows/c/windows_sys.rs
index 45a273d241a..989a1246650 100644
--- a/library/std/src/sys/pal/windows/c/windows_sys.rs
+++ b/library/std/src/sys/pal/windows/c/windows_sys.rs
@@ -38,6 +38,7 @@ windows_targets::link!("kernel32.dll" "system" fn GetCurrentDirectoryW(nbufferle
 windows_targets::link!("kernel32.dll" "system" fn GetCurrentProcess() -> HANDLE);
 windows_targets::link!("kernel32.dll" "system" fn GetCurrentProcessId() -> u32);
 windows_targets::link!("kernel32.dll" "system" fn GetCurrentThread() -> HANDLE);
+windows_targets::link!("kernel32.dll" "system" fn GetCurrentThreadId() -> u32);
 windows_targets::link!("kernel32.dll" "system" fn GetEnvironmentStringsW() -> PWSTR);
 windows_targets::link!("kernel32.dll" "system" fn GetEnvironmentVariableW(lpname : PCWSTR, lpbuffer : PWSTR, nsize : u32) -> u32);
 windows_targets::link!("kernel32.dll" "system" fn GetExitCodeProcess(hprocess : HANDLE, lpexitcode : *mut u32) -> BOOL);
@@ -61,7 +62,6 @@ windows_targets::link!("kernel32.dll" "system" fn GetSystemInfo(lpsysteminfo : *
 windows_targets::link!("kernel32.dll" "system" fn GetSystemTimeAsFileTime(lpsystemtimeasfiletime : *mut FILETIME));
 windows_targets::link!("kernel32.dll" "system" fn GetSystemTimePreciseAsFileTime(lpsystemtimeasfiletime : *mut FILETIME));
 windows_targets::link!("kernel32.dll" "system" fn GetTempPathW(nbufferlength : u32, lpbuffer : PWSTR) -> u32);
-windows_targets::link!("kernel32.dll" "system" fn GetThreadId(thread : HANDLE) -> u32);
 windows_targets::link!("userenv.dll" "system" fn GetUserProfileDirectoryW(htoken : HANDLE, lpprofiledir : PWSTR, lpcchsize : *mut u32) -> BOOL);
 windows_targets::link!("kernel32.dll" "system" fn GetWindowsDirectoryW(lpbuffer : PWSTR, usize : u32) -> u32);
 windows_targets::link!("kernel32.dll" "system" fn InitOnceBeginInitialize(lpinitonce : *mut INIT_ONCE, dwflags : u32, fpending : *mut BOOL, lpcontext : *mut *mut core::ffi::c_void) -> BOOL);
diff --git a/library/std/src/sys/pal/windows/mod.rs b/library/std/src/sys/pal/windows/mod.rs
index 8f54e2376eb..10ad4541bed 100644
--- a/library/std/src/sys/pal/windows/mod.rs
+++ b/library/std/src/sys/pal/windows/mod.rs
@@ -22,10 +22,11 @@ pub mod os;
 pub mod pipe;
 pub mod thread;
 pub mod time;
-cfg_if::cfg_if! {
-    if #[cfg(not(target_vendor = "uwp"))] {
+cfg_select! {
+    not(target_vendor = "uwp") => {
         pub mod stack_overflow;
-    } else {
+    }
+    _ => {
         pub mod stack_overflow_uwp;
         pub use self::stack_overflow_uwp as stack_overflow;
     }
@@ -337,14 +338,17 @@ pub fn dur2timeout(dur: Duration) -> u32 {
 #[cfg(not(miri))] // inline assembly does not work in Miri
 pub fn abort_internal() -> ! {
     unsafe {
-        cfg_if::cfg_if! {
-            if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
+        cfg_select! {
+            any(target_arch = "x86", target_arch = "x86_64") => {
                 core::arch::asm!("int $$0x29", in("ecx") c::FAST_FAIL_FATAL_APP_EXIT, options(noreturn, nostack));
-            } else if #[cfg(all(target_arch = "arm", target_feature = "thumb-mode"))] {
+            }
+            all(target_arch = "arm", target_feature = "thumb-mode") => {
                 core::arch::asm!(".inst 0xDEFB", in("r0") c::FAST_FAIL_FATAL_APP_EXIT, options(noreturn, nostack));
-            } else if #[cfg(any(target_arch = "aarch64", target_arch = "arm64ec"))] {
+            }
+            any(target_arch = "aarch64", target_arch = "arm64ec") => {
                 core::arch::asm!("brk 0xF003", in("x0") c::FAST_FAIL_FATAL_APP_EXIT, options(noreturn, nostack));
-            } else {
+            }
+            _ => {
                 core::intrinsics::abort();
             }
         }
diff --git a/library/std/src/sys/pal/windows/os.rs b/library/std/src/sys/pal/windows/os.rs
index f331282d2d7..1b3c80c079b 100644
--- a/library/std/src/sys/pal/windows/os.rs
+++ b/library/std/src/sys/pal/windows/os.rs
@@ -8,7 +8,6 @@ mod tests;
 use super::api;
 #[cfg(not(target_vendor = "uwp"))]
 use super::api::WinError;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::os::windows::ffi::EncodeWide;
 use crate::os::windows::prelude::*;
@@ -162,12 +161,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "failed to join paths"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     super::fill_utf16_buf(
diff --git a/library/std/src/sys/pal/windows/thread.rs b/library/std/src/sys/pal/windows/thread.rs
index c708da5af12..b0e38220a2d 100644
--- a/library/std/src/sys/pal/windows/thread.rs
+++ b/library/std/src/sys/pal/windows/thread.rs
@@ -129,7 +129,7 @@ impl Thread {
 
 pub(crate) fn current_os_id() -> Option<u64> {
     // SAFETY: FFI call with no preconditions.
-    let id: u32 = unsafe { c::GetThreadId(c::GetCurrentThread()) };
+    let id: u32 = unsafe { c::GetCurrentThreadId() };
 
     // A return value of 0 indicates failed lookup.
     if id == 0 { None } else { Some(id.into()) }
diff --git a/library/std/src/sys/pal/xous/os.rs b/library/std/src/sys/pal/xous/os.rs
index d612a27d2bd..d9b8418e6c3 100644
--- a/library/std/src/sys/pal/xous/os.rs
+++ b/library/std/src/sys/pal/xous/os.rs
@@ -1,5 +1,4 @@
 use super::unsupported;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::os::xous::ffi::Error as XousError;
@@ -110,12 +109,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on this platform yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/pal/zkvm/os.rs b/library/std/src/sys/pal/zkvm/os.rs
index a8ef97ecf67..13d2a2044f4 100644
--- a/library/std/src/sys/pal/zkvm/os.rs
+++ b/library/std/src/sys/pal/zkvm/os.rs
@@ -1,5 +1,4 @@
 use super::unsupported;
-use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
 use crate::path::{self, PathBuf};
@@ -51,12 +50,7 @@ impl fmt::Display for JoinPathsError {
     }
 }
 
-impl StdError for JoinPathsError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "not supported on this platform yet"
-    }
-}
+impl crate::error::Error for JoinPathsError {}
 
 pub fn current_exe() -> io::Result<PathBuf> {
     unsupported()
diff --git a/library/std/src/sys/path/mod.rs b/library/std/src/sys/path/mod.rs
index a4ff4338cf5..254683bc83f 100644
--- a/library/std/src/sys/path/mod.rs
+++ b/library/std/src/sys/path/mod.rs
@@ -1,22 +1,27 @@
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "windows")] {
+cfg_select! {
+    target_os = "windows" => {
         mod windows;
         mod windows_prefix;
         pub use windows::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod unsupported_backslash;
         pub use unsupported_backslash::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::*;
-    } else if #[cfg(target_os = "cygwin")] {
+    }
+    target_os = "cygwin" => {
         mod cygwin;
         mod windows_prefix;
         pub use cygwin::*;
-    } else {
+    }
+    _ => {
         mod unix;
         pub use unix::*;
     }
diff --git a/library/std/src/sys/personality/gcc.rs b/library/std/src/sys/personality/gcc.rs
index 75e793f18b8..019d5629d6d 100644
--- a/library/std/src/sys/personality/gcc.rs
+++ b/library/std/src/sys/personality/gcc.rs
@@ -93,12 +93,12 @@ const UNWIND_DATA_REG: (i32, i32) = (4, 5); // a0, a1
 // https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/libsupc++/eh_personality.cc
 // https://github.com/gcc-mirror/gcc/blob/trunk/libgcc/unwind-c.c
 
-cfg_if::cfg_if! {
-    if #[cfg(all(
+cfg_select! {
+    all(
         target_arch = "arm",
         not(target_vendor = "apple"),
         not(target_os = "netbsd"),
-    ))] {
+    ) => {
         /// personality fn called by [ARM EHABI][armeabi-eh]
         ///
         /// 32-bit ARM on iOS/tvOS/watchOS does not use ARM EHABI, it uses
@@ -202,7 +202,8 @@ cfg_if::cfg_if! {
                 }
             }
         }
-    } else {
+    }
+    _ => {
         /// Default personality routine, which is used directly on most targets
         /// and indirectly on Windows x86_64 and AArch64 via SEH.
         unsafe extern "C" fn rust_eh_personality_impl(
@@ -247,11 +248,11 @@ cfg_if::cfg_if! {
             }
         }
 
-        cfg_if::cfg_if! {
-            if #[cfg(any(
-                    all(windows, any(target_arch = "aarch64", target_arch = "x86_64"), target_env = "gnu"),
-                    target_os = "cygwin",
-                ))] {
+        cfg_select! {
+            any(
+                all(windows, any(target_arch = "aarch64", target_arch = "x86_64"), target_env = "gnu"),
+                target_os = "cygwin",
+            ) => {
                 /// personality fn called by [Windows Structured Exception Handling][windows-eh]
                 ///
                 /// On x86_64 and AArch64 MinGW targets, the unwinding mechanism is SEH,
@@ -279,7 +280,8 @@ cfg_if::cfg_if! {
                         )
                     }
                 }
-            } else {
+            }
+            _ => {
                 /// personality fn called by [Itanium C++ ABI Exception Handling][itanium-eh]
                 ///
                 /// The personality routine for most non-Windows targets. This will be called by
diff --git a/library/std/src/sys/personality/mod.rs b/library/std/src/sys/personality/mod.rs
index 2e1d2e53a29..158e44e1764 100644
--- a/library/std/src/sys/personality/mod.rs
+++ b/library/std/src/sys/personality/mod.rs
@@ -13,10 +13,11 @@
 mod dwarf;
 
 #[cfg(not(any(test, doctest)))]
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "emscripten")] {
+cfg_select! {
+    target_os = "emscripten" => {
         mod emcc;
-    } else if #[cfg(any(target_env = "msvc", target_family = "wasm"))] {
+    }
+    any(target_env = "msvc", target_family = "wasm") => {
         // This is required by the compiler to exist (e.g., it's a lang item),
         // but it's never actually called by the compiler because
         // __CxxFrameHandler3 (msvc) / __gxx_wasm_personality_v0 (wasm) is the
@@ -26,16 +27,18 @@ cfg_if::cfg_if! {
         fn rust_eh_personality() {
             core::intrinsics::abort()
         }
-    } else if #[cfg(any(
+    }
+    any(
         all(target_family = "windows", target_env = "gnu"),
         target_os = "psp",
         target_os = "xous",
         target_os = "solid_asp3",
         all(target_family = "unix", not(target_os = "espidf"), not(target_os = "l4re"), not(target_os = "nuttx")),
         all(target_vendor = "fortanix", target_env = "sgx"),
-    ))] {
+    ) => {
         mod gcc;
-    } else {
+    }
+    _ => {
         // Targets that don't support unwinding.
         // - os=none ("bare metal" targets)
         // - os=uefi
diff --git a/library/std/src/sys/process/mod.rs b/library/std/src/sys/process/mod.rs
index 91c7005a328..9ef5496e57a 100644
--- a/library/std/src/sys/process/mod.rs
+++ b/library/std/src/sys/process/mod.rs
@@ -1,14 +1,17 @@
-cfg_if::cfg_if! {
-    if #[cfg(target_family = "unix")] {
+cfg_select! {
+    target_family = "unix" => {
         mod unix;
         use unix as imp;
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
         use windows as imp;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         use uefi as imp;
-    } else {
+    }
+    _ => {
         mod unsupported;
         use unsupported as imp;
     }
diff --git a/library/std/src/sys/process/unix/common.rs b/library/std/src/sys/process/unix/common.rs
index 6219be60caf..ea45b08e90a 100644
--- a/library/std/src/sys/process/unix/common.rs
+++ b/library/std/src/sys/process/unix/common.rs
@@ -20,12 +20,14 @@ use crate::{fmt, io};
 
 mod cstring_array;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "fuchsia")] {
+cfg_select! {
+    target_os = "fuchsia" => {
         // fuchsia doesn't have /dev/null
-    } else if #[cfg(target_os = "vxworks")] {
+    }
+    target_os = "vxworks" => {
         const DEV_NULL: &CStr = c"/null";
-    } else {
+    }
+    _ => {
         const DEV_NULL: &CStr = c"/dev/null";
     }
 }
@@ -35,8 +37,8 @@ cfg_if::cfg_if! {
 // to support older Android version (independent of libc version).
 // The following implementations are based on
 // https://github.com/aosp-mirror/platform_bionic/blob/ad8dcd6023294b646e5a8288c0ed431b0845da49/libc/include/android/legacy_signal_inlines.h
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "android")] {
+cfg_select! {
+    target_os = "android" => {
         #[allow(dead_code)]
         pub unsafe fn sigemptyset(set: *mut libc::sigset_t) -> libc::c_int {
             set.write_bytes(0u8, 1);
@@ -69,7 +71,8 @@ cfg_if::cfg_if! {
             raw[bit / LONG_BIT] |= 1 << (bit % LONG_BIT);
             return 0;
         }
-    } else {
+    }
+    _ => {
         #[allow(unused_imports)]
         pub use libc::{sigemptyset, sigaddset};
     }
diff --git a/library/std/src/sys/process/unix/mod.rs b/library/std/src/sys/process/unix/mod.rs
index ee8fd8b2ca3..b4cf060fba9 100644
--- a/library/std/src/sys/process/unix/mod.rs
+++ b/library/std/src/sys/process/unix/mod.rs
@@ -1,18 +1,21 @@
 #[cfg_attr(any(target_os = "espidf", target_os = "horizon", target_os = "nuttx"), allow(unused))]
 mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "fuchsia")] {
+cfg_select! {
+    target_os = "fuchsia" => {
         mod fuchsia;
         use fuchsia as imp;
-    } else if #[cfg(target_os = "vxworks")] {
+    }
+    target_os = "vxworks" => {
         mod vxworks;
         use vxworks as imp;
-    } else if #[cfg(any(target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nuttx"))] {
+    }
+    any(target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nuttx") => {
         mod unsupported;
         use unsupported as imp;
         pub use unsupported::output;
-    } else {
+    }
+    _ => {
         mod unix;
         use unix as imp;
     }
diff --git a/library/std/src/sys/process/unix/unix.rs b/library/std/src/sys/process/unix/unix.rs
index 5d13d6da185..11d48878727 100644
--- a/library/std/src/sys/process/unix/unix.rs
+++ b/library/std/src/sys/process/unix/unix.rs
@@ -18,8 +18,8 @@ use crate::sys::cvt;
 use crate::sys::pal::linux::pidfd::PidFd;
 use crate::{fmt, mem, sys};
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "nto")] {
+cfg_select! {
+    target_os = "nto" => {
         use crate::thread;
         use libc::{c_char, posix_spawn_file_actions_t, posix_spawnattr_t};
         use crate::time::Duration;
@@ -43,6 +43,7 @@ cfg_if::cfg_if! {
         // Maximum duration of sleeping before giving up and returning an error
         const MAX_FORKSPAWN_SLEEP: Duration = Duration::from_millis(1000);
     }
+    _ => {}
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -465,8 +466,8 @@ impl Command {
             return Ok(None);
         }
 
-        cfg_if::cfg_if! {
-            if #[cfg(target_os = "linux")] {
+        cfg_select! {
+            target_os = "linux" => {
                 use crate::sys::weak::weak;
 
                 weak!(
@@ -526,7 +527,8 @@ impl Command {
                     }
                     core::assert_matches::debug_assert_matches!(support, SPAWN | NO);
                 }
-            } else {
+            }
+            _ => {
                 if self.get_create_pidfd() {
                     unreachable!("only implemented on linux")
                 }
@@ -746,10 +748,11 @@ impl Command {
             }
 
             if self.get_setsid() {
-                cfg_if::cfg_if! {
-                    if #[cfg(all(target_os = "linux", target_env = "gnu"))] {
+                cfg_select! {
+                    all(target_os = "linux", target_env = "gnu") => {
                         flags |= libc::POSIX_SPAWN_SETSID;
-                    } else {
+                    }
+                    _ => {
                         return Ok(None);
                     }
                 }
diff --git a/library/std/src/sys/random/mod.rs b/library/std/src/sys/random/mod.rs
index fc85797dcc2..a7fbae86099 100644
--- a/library/std/src/sys/random/mod.rs
+++ b/library/std/src/sys/random/mod.rs
@@ -1,16 +1,19 @@
-cfg_if::cfg_if! {
+cfg_select! {
     // Tier 1
-    if #[cfg(any(target_os = "linux", target_os = "android"))] {
+    any(target_os = "linux", target_os = "android") => {
         mod linux;
         pub use linux::{fill_bytes, hashmap_random_keys};
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
         pub use windows::fill_bytes;
-    } else if #[cfg(target_vendor = "apple")] {
+    }
+    target_vendor = "apple" => {
         mod apple;
         pub use apple::fill_bytes;
     // Others, in alphabetical ordering.
-    } else if #[cfg(any(
+    }
+    any(
         target_os = "dragonfly",
         target_os = "freebsd",
         target_os = "haiku",
@@ -21,69 +24,86 @@ cfg_if::cfg_if! {
         target_os = "solaris",
         target_os = "vita",
         target_os = "nuttx",
-    ))] {
+    ) => {
         mod arc4random;
         pub use arc4random::fill_bytes;
-    } else if #[cfg(target_os = "emscripten")] {
+    }
+    target_os = "emscripten" => {
         mod getentropy;
         pub use getentropy::fill_bytes;
-    } else if #[cfg(target_os = "espidf")] {
+    }
+    target_os = "espidf" => {
         mod espidf;
         pub use espidf::fill_bytes;
-    } else if #[cfg(target_os = "fuchsia")] {
+    }
+    target_os = "fuchsia" => {
         mod fuchsia;
         pub use fuchsia::fill_bytes;
-    } else if #[cfg(target_os = "hermit")] {
+    }
+    target_os = "hermit" => {
         mod hermit;
         pub use hermit::fill_bytes;
-    } else if #[cfg(any(target_os = "horizon", target_os = "cygwin"))] {
+    }
+    any(target_os = "horizon", target_os = "cygwin") => {
         // FIXME(horizon): add arc4random_buf to shim-3ds
         mod getrandom;
         pub use getrandom::fill_bytes;
-    } else if #[cfg(any(
+    }
+    any(
         target_os = "aix",
         target_os = "hurd",
         target_os = "l4re",
         target_os = "nto",
-    ))] {
+    ) => {
         mod unix_legacy;
         pub use unix_legacy::fill_bytes;
-    } else if #[cfg(target_os = "redox")] {
+    }
+    target_os = "redox" => {
         mod redox;
         pub use redox::fill_bytes;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::fill_bytes;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use solid::fill_bytes;
-    } else if #[cfg(target_os = "teeos")] {
+    }
+    target_os = "teeos" => {
         mod teeos;
         pub use teeos::fill_bytes;
-    } else if #[cfg(target_os = "trusty")] {
+    }
+    target_os = "trusty" => {
         mod trusty;
         pub use trusty::fill_bytes;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::fill_bytes;
-    } else if #[cfg(target_os = "vxworks")] {
+    }
+    target_os = "vxworks" => {
         mod vxworks;
         pub use vxworks::fill_bytes;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::fill_bytes;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
         pub use zkvm::fill_bytes;
-    } else if #[cfg(any(
+    }
+    any(
         all(target_family = "wasm", target_os = "unknown"),
         target_os = "xous",
-    ))] {
+    ) => {
         // FIXME: finally remove std support for wasm32-unknown-unknown
         // FIXME: add random data generation to xous
         mod unsupported;
         pub use unsupported::{fill_bytes, hashmap_random_keys};
     }
+    _ => {}
 }
 
 #[cfg(not(any(
diff --git a/library/std/src/sys/random/uefi.rs b/library/std/src/sys/random/uefi.rs
index 4a71d32fffe..697933f197b 100644
--- a/library/std/src/sys/random/uefi.rs
+++ b/library/std/src/sys/random/uefi.rs
@@ -55,12 +55,13 @@ mod rng_protocol {
 /// Port from [getrandom](https://github.com/rust-random/getrandom/blob/master/src/backends/rdrand.rs)
 #[cfg(any(target_arch = "x86_64", target_arch = "x86"))]
 mod rdrand {
-    cfg_if::cfg_if! {
-        if #[cfg(target_arch = "x86_64")] {
+    cfg_select! {
+        target_arch = "x86_64" => {
             use crate::arch::x86_64 as arch;
             use arch::_rdrand64_step as rdrand_step;
             type Word = u64;
-        } else if #[cfg(target_arch = "x86")] {
+        }
+        target_arch = "x86" => {
             use crate::arch::x86 as arch;
             use arch::_rdrand32_step as rdrand_step;
             type Word = u32;
diff --git a/library/std/src/sys/stdio/mod.rs b/library/std/src/sys/stdio/mod.rs
index 336d4c8527d..314f226f07b 100644
--- a/library/std/src/sys/stdio/mod.rs
+++ b/library/std/src/sys/stdio/mod.rs
@@ -1,40 +1,47 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
-        target_family = "unix",
-        target_os = "hermit"
-    ))] {
+cfg_select! {
+    any(target_family = "unix", target_os = "hermit") => {
         mod unix;
         pub use unix::*;
-    } else if #[cfg(target_os = "windows")] {
+    }
+    target_os = "windows" => {
         mod windows;
         pub use windows::*;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::*;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use solid::*;
-    } else if #[cfg(target_os = "teeos")] {
+    }
+    target_os = "teeos" => {
         mod teeos;
         pub use teeos::*;
-    } else if #[cfg(target_os = "trusty")] {
+    }
+    target_os = "trusty" => {
         mod trusty;
         pub use trusty::*;
-    } else if #[cfg(target_os = "uefi")] {
+    }
+    target_os = "uefi" => {
         mod uefi;
         pub use uefi::*;
-    } else if #[cfg(target_os = "wasi")] {
+    }
+    target_os = "wasi" => {
         mod wasi;
         pub use wasi::*;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::*;
-    } else if #[cfg(target_os = "zkvm")] {
+    }
+    target_os = "zkvm" => {
         mod zkvm;
         pub use zkvm::*;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::*;
     }
diff --git a/library/std/src/sys/sync/condvar/mod.rs b/library/std/src/sys/sync/condvar/mod.rs
index d0c998a5597..cb67d273759 100644
--- a/library/std/src/sys/sync/condvar/mod.rs
+++ b/library/std/src/sys/sync/condvar/mod.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor="win7")),
         target_os = "linux",
         target_os = "android",
@@ -9,28 +9,34 @@ cfg_if::cfg_if! {
         target_os = "fuchsia",
         all(target_family = "wasm", target_feature = "atomics"),
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::Condvar;
-    } else if #[cfg(any(
+    }
+    any(
         target_family = "unix",
         target_os = "teeos",
-    ))] {
+    ) => {
         mod pthread;
         pub use pthread::Condvar;
-    } else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+    }
+    all(target_os = "windows", target_vendor = "win7") => {
         mod windows7;
         pub use windows7::Condvar;
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::Condvar;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod itron;
         pub use itron::Condvar;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::Condvar;
-    } else {
+    }
+    _ => {
         mod no_threads;
         pub use no_threads::Condvar;
     }
diff --git a/library/std/src/sys/sync/mutex/mod.rs b/library/std/src/sys/sync/mutex/mod.rs
index 360df3fc4b5..c885b0eabae 100644
--- a/library/std/src/sys/sync/mutex/mod.rs
+++ b/library/std/src/sys/sync/mutex/mod.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
@@ -8,31 +8,38 @@ cfg_if::cfg_if! {
         target_os = "dragonfly",
         all(target_family = "wasm", target_feature = "atomics"),
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::Mutex;
-    } else if #[cfg(target_os = "fuchsia")] {
+    }
+    target_os = "fuchsia" => {
         mod fuchsia;
         pub use fuchsia::Mutex;
-    } else if #[cfg(any(
+    }
+    any(
         target_family = "unix",
         target_os = "teeos",
-    ))] {
+    ) => {
         mod pthread;
         pub use pthread::Mutex;
-    } else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+    }
+    all(target_os = "windows", target_vendor = "win7") => {
         mod windows7;
         pub use windows7::{Mutex, raw};
-    } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+    }
+    all(target_vendor = "fortanix", target_env = "sgx") => {
         mod sgx;
         pub use sgx::Mutex;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod itron;
         pub use itron::Mutex;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::Mutex;
-    } else {
+    }
+    _ => {
         mod no_threads;
         pub use no_threads::Mutex;
     }
diff --git a/library/std/src/sys/sync/once/mod.rs b/library/std/src/sys/sync/once/mod.rs
index 0e38937b121..8adeb1f259d 100644
--- a/library/std/src/sys/sync/once/mod.rs
+++ b/library/std/src/sys/sync/once/mod.rs
@@ -7,8 +7,8 @@
 // This also gives us the opportunity to optimize the implementation a bit which
 // should help the fast path on call sites.
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor="win7")),
         target_os = "linux",
         target_os = "android",
@@ -18,19 +18,21 @@ cfg_if::cfg_if! {
         target_os = "dragonfly",
         target_os = "fuchsia",
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::{Once, OnceState};
-    } else if #[cfg(any(
+    }
+    any(
         windows,
         target_family = "unix",
         all(target_vendor = "fortanix", target_env = "sgx"),
         target_os = "solid_asp3",
         target_os = "xous",
-    ))] {
+    ) => {
         mod queue;
         pub use queue::{Once, OnceState};
-    } else {
+    }
+    _ => {
         mod no_threads;
         pub use no_threads::{Once, OnceState};
     }
diff --git a/library/std/src/sys/sync/rwlock/mod.rs b/library/std/src/sys/sync/rwlock/mod.rs
index 70ba6bf38ef..82f1dd18dee 100644
--- a/library/std/src/sys/sync/rwlock/mod.rs
+++ b/library/std/src/sys/sync/rwlock/mod.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
@@ -9,24 +9,28 @@ cfg_if::cfg_if! {
         target_os = "fuchsia",
         all(target_family = "wasm", target_feature = "atomics"),
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::RwLock;
-    } else if #[cfg(any(
+    }
+    any(
         target_family = "unix",
         all(target_os = "windows", target_vendor = "win7"),
         all(target_vendor = "fortanix", target_env = "sgx"),
         target_os = "xous",
-    ))] {
+    ) => {
         mod queue;
         pub use queue::RwLock;
-    } else if #[cfg(target_os = "solid_asp3")] {
+    }
+    target_os = "solid_asp3" => {
         mod solid;
         pub use solid::RwLock;
-    } else if #[cfg(target_os = "teeos")] {
+    }
+    target_os = "teeos" => {
         mod teeos;
         pub use teeos::RwLock;
-    } else {
+    }
+    _ => {
         mod no_threads;
         pub use no_threads::RwLock;
     }
diff --git a/library/std/src/sys/sync/thread_parking/mod.rs b/library/std/src/sys/sync/thread_parking/mod.rs
index f4d8fa0a58c..b9fb27b4eef 100644
--- a/library/std/src/sys/sync/thread_parking/mod.rs
+++ b/library/std/src/sys/sync/thread_parking/mod.rs
@@ -1,5 +1,5 @@
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
@@ -9,30 +9,36 @@ cfg_if::cfg_if! {
         target_os = "dragonfly",
         target_os = "fuchsia",
         target_os = "hermit",
-    ))] {
+    ) => {
         mod futex;
         pub use futex::Parker;
-    } else if #[cfg(any(
+    }
+    any(
         target_os = "netbsd",
         all(target_vendor = "fortanix", target_env = "sgx"),
         target_os = "solid_asp3",
-    ))] {
+    ) => {
         mod id;
         pub use id::Parker;
-    } else if #[cfg(target_vendor = "win7")] {
+    }
+    target_vendor = "win7" => {
         mod windows7;
         pub use windows7::Parker;
-    } else if #[cfg(all(target_vendor = "apple", not(miri)))] {
+    }
+    all(target_vendor = "apple", not(miri)) => {
         // Doesn't work in Miri, see <https://github.com/rust-lang/miri/issues/2589>.
         mod darwin;
         pub use darwin::Parker;
-    } else if #[cfg(target_os = "xous")] {
+    }
+    target_os = "xous" => {
         mod xous;
         pub use xous::Parker;
-    } else if #[cfg(target_family = "unix")] {
+    }
+    target_family = "unix" => {
         mod pthread;
         pub use pthread::Parker;
-    } else {
+    }
+    _ => {
         mod unsupported;
         pub use unsupported::Parker;
     }
diff --git a/library/std/src/sys/thread_local/mod.rs b/library/std/src/sys/thread_local/mod.rs
index 9fafac3aa5b..cff74857c47 100644
--- a/library/std/src/sys/thread_local/mod.rs
+++ b/library/std/src/sys/thread_local/mod.rs
@@ -23,21 +23,23 @@
     issue = "none"
 )]
 
-cfg_if::cfg_if! {
-    if #[cfg(any(
+cfg_select! {
+    any(
         all(target_family = "wasm", not(target_feature = "atomics")),
         target_os = "uefi",
         target_os = "zkvm",
         target_os = "trusty",
-    ))] {
+    ) => {
         mod no_threads;
         pub use no_threads::{EagerStorage, LazyStorage, thread_local_inner};
         pub(crate) use no_threads::{LocalPointer, local_pointer};
-    } else if #[cfg(target_thread_local)] {
+    }
+    target_thread_local => {
         mod native;
         pub use native::{EagerStorage, LazyStorage, thread_local_inner};
         pub(crate) use native::{LocalPointer, local_pointer};
-    } else {
+    }
+    _ => {
         mod os;
         pub use os::{Storage, thread_local_inner};
         pub(crate) use os::{LocalPointer, local_pointer};
@@ -53,8 +55,8 @@ cfg_if::cfg_if! {
 /// single callback that runs all of the destructors in the list.
 #[cfg(all(target_thread_local, not(all(target_family = "wasm", not(target_feature = "atomics")))))]
 pub(crate) mod destructors {
-    cfg_if::cfg_if! {
-        if #[cfg(any(
+    cfg_select! {
+        any(
             target_os = "linux",
             target_os = "android",
             target_os = "fuchsia",
@@ -62,12 +64,13 @@ pub(crate) mod destructors {
             target_os = "hurd",
             target_os = "netbsd",
             target_os = "dragonfly"
-        ))] {
+        ) => {
             mod linux_like;
             mod list;
             pub(super) use linux_like::register;
             pub(super) use list::run;
-        } else {
+        }
+        _ => {
             mod list;
             pub(super) use list::register;
             pub(crate) use list::run;
@@ -79,21 +82,23 @@ pub(crate) mod destructors {
 /// and the [runtime cleanup](crate::rt::thread_cleanup) function. Calling `enable`
 /// should ensure that these functions are called at the right times.
 pub(crate) mod guard {
-    cfg_if::cfg_if! {
-        if #[cfg(all(target_thread_local, target_vendor = "apple"))] {
+    cfg_select! {
+        all(target_thread_local, target_vendor = "apple") => {
             mod apple;
             pub(crate) use apple::enable;
-        } else if #[cfg(target_os = "windows")] {
+        }
+        target_os = "windows" => {
             mod windows;
             pub(crate) use windows::enable;
-        } else if #[cfg(any(
+        }
+        any(
             all(target_family = "wasm", not(
                 all(target_os = "wasi", target_env = "p1", target_feature = "atomics")
             )),
             target_os = "uefi",
             target_os = "zkvm",
             target_os = "trusty",
-        ))] {
+        ) => {
             pub(crate) fn enable() {
                 // FIXME: Right now there is no concept of "thread exit" on
                 // wasm, but this is likely going to show up at some point in
@@ -107,17 +112,20 @@ pub(crate) mod guard {
                 #[allow(unused)]
                 use crate::rt::thread_cleanup;
             }
-        } else if #[cfg(any(
+        }
+        any(
             target_os = "hermit",
             target_os = "xous",
-        ))] {
+        ) => {
             // `std` is the only runtime, so it just calls the destructor functions
             // itself when the time comes.
             pub(crate) fn enable() {}
-        } else if #[cfg(target_os = "solid_asp3")] {
+        }
+        target_os = "solid_asp3" => {
             mod solid;
             pub(crate) use solid::enable;
-        } else {
+        }
+        _ => {
             mod key;
             pub(crate) use key::enable;
         }
@@ -131,8 +139,8 @@ pub(crate) mod guard {
 /// reference an entry in a thread-local table. This then associates each key
 /// with a pointer which we can get and set to store our data.
 pub(crate) mod key {
-    cfg_if::cfg_if! {
-        if #[cfg(any(
+    cfg_select! {
+        any(
             all(
                 not(target_vendor = "apple"),
                 not(target_family = "wasm"),
@@ -141,7 +149,7 @@ pub(crate) mod key {
             all(not(target_thread_local), target_vendor = "apple"),
             target_os = "teeos",
             all(target_os = "wasi", target_env = "p1", target_feature = "atomics"),
-        ))] {
+        ) => {
             mod racy;
             mod unix;
             #[cfg(test)]
@@ -151,12 +159,14 @@ pub(crate) mod key {
             #[cfg(any(not(target_thread_local), test))]
             pub(super) use unix::get;
             use unix::{create, destroy};
-        } else if #[cfg(all(not(target_thread_local), target_os = "windows"))] {
+        }
+        all(not(target_thread_local), target_os = "windows") => {
             #[cfg(test)]
             mod tests;
             mod windows;
             pub(super) use windows::{Key, LazyKey, get, run_dtors, set};
-        } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
+        }
+        all(target_vendor = "fortanix", target_env = "sgx") => {
             mod racy;
             mod sgx;
             #[cfg(test)]
@@ -164,7 +174,8 @@ pub(crate) mod key {
             pub(super) use racy::LazyKey;
             pub(super) use sgx::{Key, get, set};
             use sgx::{create, destroy};
-        } else if #[cfg(target_os = "xous")] {
+        }
+        target_os = "xous" => {
             mod racy;
             #[cfg(test)]
             mod tests;
@@ -174,6 +185,7 @@ pub(crate) mod key {
             pub(super) use xous::{Key, get, set};
             use xous::{create, destroy};
         }
+        _ => {}
     }
 }
 
diff --git a/library/std/src/sys_common/mod.rs b/library/std/src/sys_common/mod.rs
index cce88d936b7..ec45c723e0d 100644
--- a/library/std/src/sys_common/mod.rs
+++ b/library/std/src/sys_common/mod.rs
@@ -21,7 +21,6 @@
 mod tests;
 
 pub mod wstr;
-pub mod wtf8;
 
 // common error constructors
 
@@ -51,17 +50,16 @@ pub trait FromInner<Inner> {
     fn from_inner(inner: Inner) -> Self;
 }
 
-// Computes (value*numer)/denom without overflow, as long as both
-// (numer*denom) and the overall result fit into i64 (which is the case
-// for our time conversions).
+// Computes (value*numerator)/denom without overflow, as long as both (numerator*denom) and the
+// overall result fit into i64 (which is the case for our time conversions).
 #[allow(dead_code)] // not used on all platforms
-pub fn mul_div_u64(value: u64, numer: u64, denom: u64) -> u64 {
+pub fn mul_div_u64(value: u64, numerator: u64, denom: u64) -> u64 {
     let q = value / denom;
     let r = value % denom;
     // Decompose value as (value/denom*denom + value%denom),
-    // substitute into (value*numer)/denom and simplify.
-    // r < denom, so (denom*numer) is the upper bound of (r*numer)
-    q * numer + r * numer / denom
+    // substitute into (value*numerator)/denom and simplify.
+    // r < denom, so (denom*numerator) is the upper bound of (r*numerator)
+    q * numerator + r * numerator / denom
 }
 
 pub fn ignore_notfound<T>(result: crate::io::Result<T>) -> crate::io::Result<()> {
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
deleted file mode 100644
index 50bde88b5a4..00000000000
--- a/library/std/src/sys_common/wtf8.rs
+++ /dev/null
@@ -1,1083 +0,0 @@
-//! Implementation of [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
-//!
-//! This library uses Rust’s type system to maintain
-//! [well-formedness](https://simonsapin.github.io/wtf-8/#well-formed),
-//! like the `String` and `&str` types do for UTF-8.
-//!
-//! Since [WTF-8 must not be used
-//! for interchange](https://simonsapin.github.io/wtf-8/#intended-audience),
-//! this library deliberately does not provide access to the underlying bytes
-//! of WTF-8 strings,
-//! nor can it decode WTF-8 from arbitrary bytes.
-//! WTF-8 strings can be obtained from UTF-8, UTF-16, or code points.
-
-// this module is imported from @SimonSapin's repo and has tons of dead code on
-// unix (it's mostly used on windows), so don't worry about dead code here.
-#![allow(dead_code)]
-
-#[cfg(test)]
-mod tests;
-
-use core::char::{MAX_LEN_UTF8, MAX_LEN_UTF16, encode_utf8_raw, encode_utf16_raw};
-use core::clone::CloneToUninit;
-use core::str::next_code_point;
-
-use crate::borrow::Cow;
-use crate::collections::TryReserveError;
-use crate::hash::{Hash, Hasher};
-use crate::iter::FusedIterator;
-use crate::rc::Rc;
-use crate::sync::Arc;
-use crate::sys_common::AsInner;
-use crate::{fmt, mem, ops, slice, str};
-
-const UTF8_REPLACEMENT_CHARACTER: &str = "\u{FFFD}";
-
-/// A Unicode code point: from U+0000 to U+10FFFF.
-///
-/// Compares with the `char` type,
-/// which represents a Unicode scalar value:
-/// a code point that is not a surrogate (U+D800 to U+DFFF).
-#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy)]
-pub struct CodePoint {
-    value: u32,
-}
-
-/// Format the code point as `U+` followed by four to six hexadecimal digits.
-/// Example: `U+1F4A9`
-impl fmt::Debug for CodePoint {
-    #[inline]
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(formatter, "U+{:04X}", self.value)
-    }
-}
-
-impl CodePoint {
-    /// Unsafely creates a new `CodePoint` without checking the value.
-    ///
-    /// Only use when `value` is known to be less than or equal to 0x10FFFF.
-    #[inline]
-    pub unsafe fn from_u32_unchecked(value: u32) -> CodePoint {
-        CodePoint { value }
-    }
-
-    /// Creates a new `CodePoint` if the value is a valid code point.
-    ///
-    /// Returns `None` if `value` is above 0x10FFFF.
-    #[inline]
-    pub fn from_u32(value: u32) -> Option<CodePoint> {
-        match value {
-            0..=0x10FFFF => Some(CodePoint { value }),
-            _ => None,
-        }
-    }
-
-    /// Creates a new `CodePoint` from a `char`.
-    ///
-    /// Since all Unicode scalar values are code points, this always succeeds.
-    #[inline]
-    pub fn from_char(value: char) -> CodePoint {
-        CodePoint { value: value as u32 }
-    }
-
-    /// Returns the numeric value of the code point.
-    #[inline]
-    pub fn to_u32(&self) -> u32 {
-        self.value
-    }
-
-    /// Returns the numeric value of the code point if it is a leading surrogate.
-    #[inline]
-    pub fn to_lead_surrogate(&self) -> Option<u16> {
-        match self.value {
-            lead @ 0xD800..=0xDBFF => Some(lead as u16),
-            _ => None,
-        }
-    }
-
-    /// Returns the numeric value of the code point if it is a trailing surrogate.
-    #[inline]
-    pub fn to_trail_surrogate(&self) -> Option<u16> {
-        match self.value {
-            trail @ 0xDC00..=0xDFFF => Some(trail as u16),
-            _ => None,
-        }
-    }
-
-    /// Optionally returns a Unicode scalar value for the code point.
-    ///
-    /// Returns `None` if the code point is a surrogate (from U+D800 to U+DFFF).
-    #[inline]
-    pub fn to_char(&self) -> Option<char> {
-        match self.value {
-            0xD800..=0xDFFF => None,
-            _ => Some(unsafe { char::from_u32_unchecked(self.value) }),
-        }
-    }
-
-    /// Returns a Unicode scalar value for the code point.
-    ///
-    /// Returns `'\u{FFFD}'` (the replacement character “�”)
-    /// if the code point is a surrogate (from U+D800 to U+DFFF).
-    #[inline]
-    pub fn to_char_lossy(&self) -> char {
-        self.to_char().unwrap_or('\u{FFFD}')
-    }
-}
-
-/// An owned, growable string of well-formed WTF-8 data.
-///
-/// Similar to `String`, but can additionally contain surrogate code points
-/// if they’re not in a surrogate pair.
-#[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
-pub struct Wtf8Buf {
-    bytes: Vec<u8>,
-
-    /// Do we know that `bytes` holds a valid UTF-8 encoding? We can easily
-    /// know this if we're constructed from a `String` or `&str`.
-    ///
-    /// It is possible for `bytes` to have valid UTF-8 without this being
-    /// set, such as when we're concatenating `&Wtf8`'s and surrogates become
-    /// paired, as we don't bother to rescan the entire string.
-    is_known_utf8: bool,
-}
-
-impl ops::Deref for Wtf8Buf {
-    type Target = Wtf8;
-
-    fn deref(&self) -> &Wtf8 {
-        self.as_slice()
-    }
-}
-
-impl ops::DerefMut for Wtf8Buf {
-    fn deref_mut(&mut self) -> &mut Wtf8 {
-        self.as_mut_slice()
-    }
-}
-
-/// Formats the string in double quotes, with characters escaped according to
-/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
-/// where each `x` is a hexadecimal digit.
-///
-/// For example, the code units [U+0061, U+D800, U+000A] are formatted as
-/// `"a\u{D800}\n"`.
-impl fmt::Debug for Wtf8Buf {
-    #[inline]
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&**self, formatter)
-    }
-}
-
-/// Formats the string with unpaired surrogates substituted with the replacement
-/// character, U+FFFD.
-impl fmt::Display for Wtf8Buf {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if let Some(s) = self.as_known_utf8() {
-            fmt::Display::fmt(s, formatter)
-        } else {
-            fmt::Display::fmt(&**self, formatter)
-        }
-    }
-}
-
-impl Wtf8Buf {
-    /// Creates a new, empty WTF-8 string.
-    #[inline]
-    pub fn new() -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::new(), is_known_utf8: true }
-    }
-
-    /// Creates a new, empty WTF-8 string with pre-allocated capacity for `capacity` bytes.
-    #[inline]
-    pub fn with_capacity(capacity: usize) -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::with_capacity(capacity), is_known_utf8: true }
-    }
-
-    /// Creates a WTF-8 string from a WTF-8 byte vec.
-    ///
-    /// Since the byte vec is not checked for valid WTF-8, this function is
-    /// marked unsafe.
-    #[inline]
-    pub unsafe fn from_bytes_unchecked(value: Vec<u8>) -> Wtf8Buf {
-        Wtf8Buf { bytes: value, is_known_utf8: false }
-    }
-
-    /// Creates a WTF-8 string from a UTF-8 `String`.
-    ///
-    /// This takes ownership of the `String` and does not copy.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub const fn from_string(string: String) -> Wtf8Buf {
-        Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
-    }
-
-    /// Creates a WTF-8 string from a UTF-8 `&str` slice.
-    ///
-    /// This copies the content of the slice.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub fn from_str(s: &str) -> Wtf8Buf {
-        Wtf8Buf { bytes: s.as_bytes().to_vec(), is_known_utf8: true }
-    }
-
-    pub fn clear(&mut self) {
-        self.bytes.clear();
-        self.is_known_utf8 = true;
-    }
-
-    /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
-    ///
-    /// This is lossless: calling `.encode_wide()` on the resulting string
-    /// will always return the original code units.
-    pub fn from_wide(v: &[u16]) -> Wtf8Buf {
-        let mut string = Wtf8Buf::with_capacity(v.len());
-        for item in char::decode_utf16(v.iter().cloned()) {
-            match item {
-                Ok(ch) => string.push_char(ch),
-                Err(surrogate) => {
-                    let surrogate = surrogate.unpaired_surrogate();
-                    // Surrogates are known to be in the code point range.
-                    let code_point = unsafe { CodePoint::from_u32_unchecked(surrogate as u32) };
-                    // The string will now contain an unpaired surrogate.
-                    string.is_known_utf8 = false;
-                    // Skip the WTF-8 concatenation check,
-                    // surrogate pairs are already decoded by decode_utf16
-                    string.push_code_point_unchecked(code_point);
-                }
-            }
-        }
-        string
-    }
-
-    /// Appends the given `char` to the end of this string.
-    /// This does **not** include the WTF-8 concatenation check or `is_known_utf8` check.
-    /// Copied from String::push.
-    fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
-        let mut bytes = [0; MAX_LEN_UTF8];
-        let bytes = encode_utf8_raw(code_point.value, &mut bytes);
-        self.bytes.extend_from_slice(bytes)
-    }
-
-    #[inline]
-    pub fn as_slice(&self) -> &Wtf8 {
-        unsafe { Wtf8::from_bytes_unchecked(&self.bytes) }
-    }
-
-    #[inline]
-    pub fn as_mut_slice(&mut self) -> &mut Wtf8 {
-        // Safety: `Wtf8` doesn't expose any way to mutate the bytes that would
-        // cause them to change from well-formed UTF-8 to ill-formed UTF-8,
-        // which would break the assumptions of the `is_known_utf8` field.
-        unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
-    }
-
-    /// Converts the string to UTF-8 without validation, if it was created from
-    /// valid UTF-8.
-    #[inline]
-    fn as_known_utf8(&self) -> Option<&str> {
-        if self.is_known_utf8 {
-            // SAFETY: The buffer is known to be valid UTF-8.
-            Some(unsafe { str::from_utf8_unchecked(self.as_bytes()) })
-        } else {
-            None
-        }
-    }
-
-    /// Reserves capacity for at least `additional` more bytes to be inserted
-    /// in the given `Wtf8Buf`.
-    /// The collection may reserve more space to avoid frequent reallocations.
-    ///
-    /// # Panics
-    ///
-    /// Panics if the new capacity exceeds `isize::MAX` bytes.
-    #[inline]
-    pub fn reserve(&mut self, additional: usize) {
-        self.bytes.reserve(additional)
-    }
-
-    /// Tries to reserve capacity for at least `additional` more bytes to be
-    /// inserted in the given `Wtf8Buf`. The `Wtf8Buf` may reserve more space to
-    /// avoid frequent reallocations. After calling `try_reserve`, capacity will
-    /// be greater than or equal to `self.len() + additional`. Does nothing if
-    /// capacity is already sufficient. This method preserves the contents even
-    /// if an error occurs.
-    ///
-    /// # Errors
-    ///
-    /// If the capacity overflows, or the allocator reports a failure, then an error
-    /// is returned.
-    #[inline]
-    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        self.bytes.try_reserve(additional)
-    }
-
-    #[inline]
-    pub fn reserve_exact(&mut self, additional: usize) {
-        self.bytes.reserve_exact(additional)
-    }
-
-    /// Tries to reserve the minimum capacity for exactly `additional` more
-    /// bytes to be inserted in the given `Wtf8Buf`. 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 `Wtf8Buf` 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`]: Wtf8Buf::try_reserve
-    ///
-    /// # Errors
-    ///
-    /// If the capacity overflows, or the allocator reports a failure, then an error
-    /// is returned.
-    #[inline]
-    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
-        self.bytes.try_reserve_exact(additional)
-    }
-
-    #[inline]
-    pub fn shrink_to_fit(&mut self) {
-        self.bytes.shrink_to_fit()
-    }
-
-    #[inline]
-    pub fn shrink_to(&mut self, min_capacity: usize) {
-        self.bytes.shrink_to(min_capacity)
-    }
-
-    #[inline]
-    pub fn leak<'a>(self) -> &'a mut Wtf8 {
-        unsafe { Wtf8::from_mut_bytes_unchecked(self.bytes.leak()) }
-    }
-
-    /// Returns the number of bytes that this string buffer can hold without reallocating.
-    #[inline]
-    pub fn capacity(&self) -> usize {
-        self.bytes.capacity()
-    }
-
-    /// Append a UTF-8 slice at the end of the string.
-    #[inline]
-    pub fn push_str(&mut self, other: &str) {
-        self.bytes.extend_from_slice(other.as_bytes())
-    }
-
-    /// Append a WTF-8 slice at the end of the string.
-    ///
-    /// This replaces newly paired surrogates at the boundary
-    /// with a supplementary code point,
-    /// like concatenating ill-formed UTF-16 strings effectively would.
-    #[inline]
-    pub fn push_wtf8(&mut self, other: &Wtf8) {
-        match ((&*self).final_lead_surrogate(), other.initial_trail_surrogate()) {
-            // Replace newly paired surrogates by a supplementary code point.
-            (Some(lead), Some(trail)) => {
-                let len_without_lead_surrogate = self.len() - 3;
-                self.bytes.truncate(len_without_lead_surrogate);
-                let other_without_trail_surrogate = &other.bytes[3..];
-                // 4 bytes for the supplementary code point
-                self.bytes.reserve(4 + other_without_trail_surrogate.len());
-                self.push_char(decode_surrogate_pair(lead, trail));
-                self.bytes.extend_from_slice(other_without_trail_surrogate);
-            }
-            _ => {
-                // If we'll be pushing a string containing a surrogate, we may
-                // no longer have UTF-8.
-                if self.is_known_utf8 && other.next_surrogate(0).is_some() {
-                    self.is_known_utf8 = false;
-                }
-
-                self.bytes.extend_from_slice(&other.bytes);
-            }
-        }
-    }
-
-    /// Append a Unicode scalar value at the end of the string.
-    #[inline]
-    pub fn push_char(&mut self, c: char) {
-        self.push_code_point_unchecked(CodePoint::from_char(c))
-    }
-
-    /// Append a code point at the end of the string.
-    ///
-    /// This replaces newly paired surrogates at the boundary
-    /// with a supplementary code point,
-    /// like concatenating ill-formed UTF-16 strings effectively would.
-    #[inline]
-    pub fn push(&mut self, code_point: CodePoint) {
-        if let Some(trail) = code_point.to_trail_surrogate() {
-            if let Some(lead) = (&*self).final_lead_surrogate() {
-                let len_without_lead_surrogate = self.len() - 3;
-                self.bytes.truncate(len_without_lead_surrogate);
-                self.push_char(decode_surrogate_pair(lead, trail));
-                return;
-            }
-
-            // We're pushing a trailing surrogate.
-            self.is_known_utf8 = false;
-        } else if code_point.to_lead_surrogate().is_some() {
-            // We're pushing a leading surrogate.
-            self.is_known_utf8 = false;
-        }
-
-        // No newly paired surrogates at the boundary.
-        self.push_code_point_unchecked(code_point)
-    }
-
-    /// Shortens a string to the specified length.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `new_len` > current length,
-    /// or if `new_len` is not a code point boundary.
-    #[inline]
-    pub fn truncate(&mut self, new_len: usize) {
-        assert!(is_code_point_boundary(self, new_len));
-        self.bytes.truncate(new_len)
-    }
-
-    /// Consumes the WTF-8 string and tries to convert it to a vec of bytes.
-    #[inline]
-    pub fn into_bytes(self) -> Vec<u8> {
-        self.bytes
-    }
-
-    /// Consumes the WTF-8 string and tries to convert it to UTF-8.
-    ///
-    /// This does not copy the data.
-    ///
-    /// If the contents are not well-formed UTF-8
-    /// (that is, if the string contains surrogates),
-    /// the original WTF-8 string is returned instead.
-    pub fn into_string(self) -> Result<String, Wtf8Buf> {
-        if self.is_known_utf8 || self.next_surrogate(0).is_none() {
-            Ok(unsafe { String::from_utf8_unchecked(self.bytes) })
-        } else {
-            Err(self)
-        }
-    }
-
-    /// Consumes the WTF-8 string and converts it lossily to UTF-8.
-    ///
-    /// This does not copy the data (but may overwrite parts of it in place).
-    ///
-    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
-    pub fn into_string_lossy(mut self) -> String {
-        if !self.is_known_utf8 {
-            let mut pos = 0;
-            while let Some((surrogate_pos, _)) = self.next_surrogate(pos) {
-                pos = surrogate_pos + 3;
-                // Surrogates and the replacement character are all 3 bytes, so
-                // they can substituted in-place.
-                self.bytes[surrogate_pos..pos]
-                    .copy_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-            }
-        }
-        unsafe { String::from_utf8_unchecked(self.bytes) }
-    }
-
-    /// Converts this `Wtf8Buf` into a boxed `Wtf8`.
-    #[inline]
-    pub fn into_box(self) -> Box<Wtf8> {
-        // SAFETY: relies on `Wtf8` being `repr(transparent)`.
-        unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
-    }
-
-    /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
-    pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
-        let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
-        Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
-    }
-
-    /// Provides plumbing to core `Vec::extend_from_slice`.
-    /// More well behaving alternative to allowing outer types
-    /// full mutable access to the core `Vec`.
-    #[inline]
-    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
-        self.bytes.extend_from_slice(other);
-        self.is_known_utf8 = false;
-    }
-}
-
-/// Creates a new WTF-8 string from an iterator of code points.
-///
-/// This replaces surrogate code point pairs with supplementary code points,
-/// like concatenating ill-formed UTF-16 strings effectively would.
-impl FromIterator<CodePoint> for Wtf8Buf {
-    fn from_iter<T: IntoIterator<Item = CodePoint>>(iter: T) -> Wtf8Buf {
-        let mut string = Wtf8Buf::new();
-        string.extend(iter);
-        string
-    }
-}
-
-/// Append code points from an iterator to the string.
-///
-/// This replaces surrogate code point pairs with supplementary code points,
-/// like concatenating ill-formed UTF-16 strings effectively would.
-impl Extend<CodePoint> for Wtf8Buf {
-    fn extend<T: IntoIterator<Item = CodePoint>>(&mut self, iter: T) {
-        let iterator = iter.into_iter();
-        let (low, _high) = iterator.size_hint();
-        // Lower bound of one byte per code point (ASCII only)
-        self.bytes.reserve(low);
-        iterator.for_each(move |code_point| self.push(code_point));
-    }
-
-    #[inline]
-    fn extend_one(&mut self, code_point: CodePoint) {
-        self.push(code_point);
-    }
-
-    #[inline]
-    fn extend_reserve(&mut self, additional: usize) {
-        // Lower bound of one byte per code point (ASCII only)
-        self.bytes.reserve(additional);
-    }
-}
-
-/// A borrowed slice of well-formed WTF-8 data.
-///
-/// Similar to `&str`, but can additionally contain surrogate code points
-/// if they’re not in a surrogate pair.
-#[derive(Eq, Ord, PartialEq, PartialOrd)]
-#[repr(transparent)]
-pub struct Wtf8 {
-    bytes: [u8],
-}
-
-impl AsInner<[u8]> for Wtf8 {
-    #[inline]
-    fn as_inner(&self) -> &[u8] {
-        &self.bytes
-    }
-}
-
-/// Formats the string in double quotes, with characters escaped according to
-/// [`char::escape_debug`] and unpaired surrogates represented as `\u{xxxx}`,
-/// where each `x` is a hexadecimal digit.
-impl fmt::Debug for Wtf8 {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fn write_str_escaped(f: &mut fmt::Formatter<'_>, s: &str) -> fmt::Result {
-            use crate::fmt::Write;
-            for c in s.chars().flat_map(|c| c.escape_debug()) {
-                f.write_char(c)?
-            }
-            Ok(())
-        }
-
-        formatter.write_str("\"")?;
-        let mut pos = 0;
-        while let Some((surrogate_pos, surrogate)) = self.next_surrogate(pos) {
-            write_str_escaped(formatter, unsafe {
-                str::from_utf8_unchecked(&self.bytes[pos..surrogate_pos])
-            })?;
-            write!(formatter, "\\u{{{:x}}}", surrogate)?;
-            pos = surrogate_pos + 3;
-        }
-        write_str_escaped(formatter, unsafe { str::from_utf8_unchecked(&self.bytes[pos..]) })?;
-        formatter.write_str("\"")
-    }
-}
-
-/// Formats the string with unpaired surrogates substituted with the replacement
-/// character, U+FFFD.
-impl fmt::Display for Wtf8 {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let wtf8_bytes = &self.bytes;
-        let mut pos = 0;
-        loop {
-            match self.next_surrogate(pos) {
-                Some((surrogate_pos, _)) => {
-                    formatter.write_str(unsafe {
-                        str::from_utf8_unchecked(&wtf8_bytes[pos..surrogate_pos])
-                    })?;
-                    formatter.write_str(UTF8_REPLACEMENT_CHARACTER)?;
-                    pos = surrogate_pos + 3;
-                }
-                None => {
-                    let s = unsafe { str::from_utf8_unchecked(&wtf8_bytes[pos..]) };
-                    if pos == 0 { return s.fmt(formatter) } else { return formatter.write_str(s) }
-                }
-            }
-        }
-    }
-}
-
-impl Wtf8 {
-    /// Creates a WTF-8 slice from a UTF-8 `&str` slice.
-    ///
-    /// Since WTF-8 is a superset of UTF-8, this always succeeds.
-    #[inline]
-    pub fn from_str(value: &str) -> &Wtf8 {
-        unsafe { Wtf8::from_bytes_unchecked(value.as_bytes()) }
-    }
-
-    /// Creates a WTF-8 slice from a WTF-8 byte slice.
-    ///
-    /// Since the byte slice is not checked for valid WTF-8, this functions is
-    /// marked unsafe.
-    #[inline]
-    pub unsafe fn from_bytes_unchecked(value: &[u8]) -> &Wtf8 {
-        // SAFETY: start with &[u8], end with fancy &[u8]
-        unsafe { &*(value as *const [u8] as *const Wtf8) }
-    }
-
-    /// Creates a mutable WTF-8 slice from a mutable WTF-8 byte slice.
-    ///
-    /// Since the byte slice is not checked for valid WTF-8, this functions is
-    /// marked unsafe.
-    #[inline]
-    unsafe fn from_mut_bytes_unchecked(value: &mut [u8]) -> &mut Wtf8 {
-        // SAFETY: start with &mut [u8], end with fancy &mut [u8]
-        unsafe { &mut *(value as *mut [u8] as *mut Wtf8) }
-    }
-
-    /// Returns the length, in WTF-8 bytes.
-    #[inline]
-    pub fn len(&self) -> usize {
-        self.bytes.len()
-    }
-
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        self.bytes.is_empty()
-    }
-
-    /// Returns the code point at `position` if it is in the ASCII range,
-    /// or `b'\xFF'` otherwise.
-    ///
-    /// # Panics
-    ///
-    /// Panics if `position` is beyond the end of the string.
-    #[inline]
-    pub fn ascii_byte_at(&self, position: usize) -> u8 {
-        match self.bytes[position] {
-            ascii_byte @ 0x00..=0x7F => ascii_byte,
-            _ => 0xFF,
-        }
-    }
-
-    /// Returns an iterator for the string’s code points.
-    #[inline]
-    pub fn code_points(&self) -> Wtf8CodePoints<'_> {
-        Wtf8CodePoints { bytes: self.bytes.iter() }
-    }
-
-    /// Access raw bytes of WTF-8 data
-    #[inline]
-    pub fn as_bytes(&self) -> &[u8] {
-        &self.bytes
-    }
-
-    /// Tries to convert the string to UTF-8 and return a `&str` slice.
-    ///
-    /// Returns `None` if the string contains surrogates.
-    ///
-    /// This does not copy the data.
-    #[inline]
-    pub fn as_str(&self) -> Result<&str, str::Utf8Error> {
-        str::from_utf8(&self.bytes)
-    }
-
-    /// Creates an owned `Wtf8Buf` from a borrowed `Wtf8`.
-    pub fn to_owned(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_vec(), is_known_utf8: false }
-    }
-
-    /// Lossily converts the string to UTF-8.
-    /// Returns a UTF-8 `&str` slice if the contents are well-formed in UTF-8.
-    ///
-    /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”).
-    ///
-    /// This only copies the data if necessary (if it contains any surrogate).
-    pub fn to_string_lossy(&self) -> Cow<'_, str> {
-        let Some((surrogate_pos, _)) = self.next_surrogate(0) else {
-            return Cow::Borrowed(unsafe { str::from_utf8_unchecked(&self.bytes) });
-        };
-        let wtf8_bytes = &self.bytes;
-        let mut utf8_bytes = Vec::with_capacity(self.len());
-        utf8_bytes.extend_from_slice(&wtf8_bytes[..surrogate_pos]);
-        utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-        let mut pos = surrogate_pos + 3;
-        loop {
-            match self.next_surrogate(pos) {
-                Some((surrogate_pos, _)) => {
-                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..surrogate_pos]);
-                    utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
-                    pos = surrogate_pos + 3;
-                }
-                None => {
-                    utf8_bytes.extend_from_slice(&wtf8_bytes[pos..]);
-                    return Cow::Owned(unsafe { String::from_utf8_unchecked(utf8_bytes) });
-                }
-            }
-        }
-    }
-
-    /// Converts the WTF-8 string to potentially ill-formed UTF-16
-    /// and return an iterator of 16-bit code units.
-    ///
-    /// This is lossless:
-    /// calling `Wtf8Buf::from_ill_formed_utf16` on the resulting code units
-    /// would always return the original WTF-8 string.
-    #[inline]
-    pub fn encode_wide(&self) -> EncodeWide<'_> {
-        EncodeWide { code_points: self.code_points(), extra: 0 }
-    }
-
-    #[inline]
-    fn next_surrogate(&self, mut pos: usize) -> Option<(usize, u16)> {
-        let mut iter = self.bytes[pos..].iter();
-        loop {
-            let b = *iter.next()?;
-            if b < 0x80 {
-                pos += 1;
-            } else if b < 0xE0 {
-                iter.next();
-                pos += 2;
-            } else if b == 0xED {
-                match (iter.next(), iter.next()) {
-                    (Some(&b2), Some(&b3)) if b2 >= 0xA0 => {
-                        return Some((pos, decode_surrogate(b2, b3)));
-                    }
-                    _ => pos += 3,
-                }
-            } else if b < 0xF0 {
-                iter.next();
-                iter.next();
-                pos += 3;
-            } else {
-                iter.next();
-                iter.next();
-                iter.next();
-                pos += 4;
-            }
-        }
-    }
-
-    #[inline]
-    fn final_lead_surrogate(&self) -> Option<u16> {
-        match self.bytes {
-            [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
-            _ => None,
-        }
-    }
-
-    #[inline]
-    fn initial_trail_surrogate(&self) -> Option<u16> {
-        match self.bytes {
-            [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(b2, b3)),
-            _ => None,
-        }
-    }
-
-    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
-        buf.is_known_utf8 = false;
-        self.bytes.clone_into(&mut buf.bytes);
-    }
-
-    /// Boxes this `Wtf8`.
-    #[inline]
-    pub fn into_box(&self) -> Box<Wtf8> {
-        let boxed: Box<[u8]> = self.bytes.into();
-        unsafe { mem::transmute(boxed) }
-    }
-
-    /// Creates a boxed, empty `Wtf8`.
-    pub fn empty_box() -> Box<Wtf8> {
-        let boxed: Box<[u8]> = Default::default();
-        unsafe { mem::transmute(boxed) }
-    }
-
-    #[inline]
-    pub fn into_arc(&self) -> Arc<Wtf8> {
-        let arc: Arc<[u8]> = Arc::from(&self.bytes);
-        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Wtf8) }
-    }
-
-    #[inline]
-    pub fn into_rc(&self) -> Rc<Wtf8> {
-        let rc: Rc<[u8]> = Rc::from(&self.bytes);
-        unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Wtf8) }
-    }
-
-    #[inline]
-    pub fn make_ascii_lowercase(&mut self) {
-        self.bytes.make_ascii_lowercase()
-    }
-
-    #[inline]
-    pub fn make_ascii_uppercase(&mut self) {
-        self.bytes.make_ascii_uppercase()
-    }
-
-    #[inline]
-    pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase(), is_known_utf8: false }
-    }
-
-    #[inline]
-    pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase(), is_known_utf8: false }
-    }
-
-    #[inline]
-    pub fn is_ascii(&self) -> bool {
-        self.bytes.is_ascii()
-    }
-
-    #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
-        self.bytes.eq_ignore_ascii_case(&other.bytes)
-    }
-}
-
-/// Returns a slice of the given string for the byte range \[`begin`..`end`).
-///
-/// # Panics
-///
-/// Panics when `begin` and `end` do not point to code point boundaries,
-/// or point beyond the end of the string.
-impl ops::Index<ops::Range<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::Range<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if range.start <= range.end
-            && is_code_point_boundary(self, range.start)
-            && is_code_point_boundary(self, range.end)
-        {
-            unsafe { slice_unchecked(self, range.start, range.end) }
-        } else {
-            slice_error_fail(self, range.start, range.end)
-        }
-    }
-}
-
-/// Returns a slice of the given string from byte `begin` to its end.
-///
-/// # Panics
-///
-/// Panics when `begin` is not at a code point boundary,
-/// or is beyond the end of the string.
-impl ops::Index<ops::RangeFrom<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::RangeFrom<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if is_code_point_boundary(self, range.start) {
-            unsafe { slice_unchecked(self, range.start, self.len()) }
-        } else {
-            slice_error_fail(self, range.start, self.len())
-        }
-    }
-}
-
-/// Returns a slice of the given string from its beginning to byte `end`.
-///
-/// # Panics
-///
-/// Panics when `end` is not at a code point boundary,
-/// or is beyond the end of the string.
-impl ops::Index<ops::RangeTo<usize>> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, range: ops::RangeTo<usize>) -> &Wtf8 {
-        // is_code_point_boundary checks that the index is in [0, .len()]
-        if is_code_point_boundary(self, range.end) {
-            unsafe { slice_unchecked(self, 0, range.end) }
-        } else {
-            slice_error_fail(self, 0, range.end)
-        }
-    }
-}
-
-impl ops::Index<ops::RangeFull> for Wtf8 {
-    type Output = Wtf8;
-
-    #[inline]
-    fn index(&self, _range: ops::RangeFull) -> &Wtf8 {
-        self
-    }
-}
-
-#[inline]
-fn decode_surrogate(second_byte: u8, third_byte: u8) -> u16 {
-    // The first byte is assumed to be 0xED
-    0xD800 | (second_byte as u16 & 0x3F) << 6 | third_byte as u16 & 0x3F
-}
-
-#[inline]
-fn decode_surrogate_pair(lead: u16, trail: u16) -> char {
-    let code_point = 0x10000 + ((((lead - 0xD800) as u32) << 10) | (trail - 0xDC00) as u32);
-    unsafe { char::from_u32_unchecked(code_point) }
-}
-
-/// Copied from str::is_char_boundary
-#[inline]
-pub fn is_code_point_boundary(slice: &Wtf8, index: usize) -> bool {
-    if index == 0 {
-        return true;
-    }
-    match slice.bytes.get(index) {
-        None => index == slice.len(),
-        Some(&b) => (b as i8) >= -0x40,
-    }
-}
-
-/// Verify that `index` is at the edge of either a valid UTF-8 codepoint
-/// (i.e. a codepoint that's not a surrogate) or of the whole string.
-///
-/// These are the cases currently permitted by `OsStr::slice_encoded_bytes`.
-/// Splitting between surrogates is valid as far as WTF-8 is concerned, but
-/// we do not permit it in the public API because WTF-8 is considered an
-/// implementation detail.
-#[track_caller]
-#[inline]
-pub fn check_utf8_boundary(slice: &Wtf8, index: usize) {
-    if index == 0 {
-        return;
-    }
-    match slice.bytes.get(index) {
-        Some(0xED) => (), // Might be a surrogate
-        Some(&b) if (b as i8) >= -0x40 => return,
-        Some(_) => panic!("byte index {index} is not a codepoint boundary"),
-        None if index == slice.len() => return,
-        None => panic!("byte index {index} is out of bounds"),
-    }
-    if slice.bytes[index + 1] >= 0xA0 {
-        // There's a surrogate after index. Now check before index.
-        if index >= 3 && slice.bytes[index - 3] == 0xED && slice.bytes[index - 2] >= 0xA0 {
-            panic!("byte index {index} lies between surrogate codepoints");
-        }
-    }
-}
-
-/// Copied from core::str::raw::slice_unchecked
-#[inline]
-pub unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
-    // SAFETY: memory layout of a &[u8] and &Wtf8 are the same
-    unsafe {
-        let len = end - begin;
-        let start = s.as_bytes().as_ptr().add(begin);
-        Wtf8::from_bytes_unchecked(slice::from_raw_parts(start, len))
-    }
-}
-
-/// Copied from core::str::raw::slice_error_fail
-#[inline(never)]
-pub fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
-    assert!(begin <= end);
-    panic!("index {begin} and/or {end} in `{s:?}` do not lie on character boundary");
-}
-
-/// Iterator for the code points of a WTF-8 string.
-///
-/// Created with the method `.code_points()`.
-#[derive(Clone)]
-pub struct Wtf8CodePoints<'a> {
-    bytes: slice::Iter<'a, u8>,
-}
-
-impl Iterator for Wtf8CodePoints<'_> {
-    type Item = CodePoint;
-
-    #[inline]
-    fn next(&mut self) -> Option<CodePoint> {
-        // SAFETY: `self.bytes` has been created from a WTF-8 string
-        unsafe { next_code_point(&mut self.bytes).map(|c| CodePoint { value: c }) }
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = self.bytes.len();
-        (len.saturating_add(3) / 4, Some(len))
-    }
-}
-
-/// Generates a wide character sequence for potentially ill-formed UTF-16.
-#[stable(feature = "rust1", since = "1.0.0")]
-#[derive(Clone)]
-pub struct EncodeWide<'a> {
-    code_points: Wtf8CodePoints<'a>,
-    extra: u16,
-}
-
-// Copied from libunicode/u_str.rs
-#[stable(feature = "rust1", since = "1.0.0")]
-impl Iterator for EncodeWide<'_> {
-    type Item = u16;
-
-    #[inline]
-    fn next(&mut self) -> Option<u16> {
-        if self.extra != 0 {
-            let tmp = self.extra;
-            self.extra = 0;
-            return Some(tmp);
-        }
-
-        let mut buf = [0; MAX_LEN_UTF16];
-        self.code_points.next().map(|code_point| {
-            let n = encode_utf16_raw(code_point.value, &mut buf).len();
-            if n == 2 {
-                self.extra = buf[1];
-            }
-            buf[0]
-        })
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        let (low, high) = self.code_points.size_hint();
-        let ext = (self.extra != 0) as usize;
-        // every code point gets either one u16 or two u16,
-        // so this iterator is between 1 or 2 times as
-        // long as the underlying iterator.
-        (low + ext, high.and_then(|n| n.checked_mul(2)).and_then(|n| n.checked_add(ext)))
-    }
-}
-
-#[stable(feature = "encode_wide_fused_iterator", since = "1.62.0")]
-impl FusedIterator for EncodeWide<'_> {}
-
-impl Hash for CodePoint {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        self.value.hash(state)
-    }
-}
-
-impl Hash for Wtf8Buf {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write(&self.bytes);
-        0xfeu8.hash(state)
-    }
-}
-
-impl Hash for Wtf8 {
-    #[inline]
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        state.write(&self.bytes);
-        0xfeu8.hash(state)
-    }
-}
-
-#[unstable(feature = "clone_to_uninit", issue = "126799")]
-unsafe impl CloneToUninit for Wtf8 {
-    #[inline]
-    #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
-        // SAFETY: we're just a transparent wrapper around [u8]
-        unsafe { self.bytes.clone_to_uninit(dst) }
-    }
-}
diff --git a/library/std/src/sys_common/wtf8/tests.rs b/library/std/src/sys_common/wtf8/tests.rs
deleted file mode 100644
index b57c99a8452..00000000000
--- a/library/std/src/sys_common/wtf8/tests.rs
+++ /dev/null
@@ -1,751 +0,0 @@
-use super::*;
-
-#[test]
-fn code_point_from_u32() {
-    assert!(CodePoint::from_u32(0).is_some());
-    assert!(CodePoint::from_u32(0xD800).is_some());
-    assert!(CodePoint::from_u32(0x10FFFF).is_some());
-    assert!(CodePoint::from_u32(0x110000).is_none());
-}
-
-#[test]
-fn code_point_to_u32() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    assert_eq!(c(0).to_u32(), 0);
-    assert_eq!(c(0xD800).to_u32(), 0xD800);
-    assert_eq!(c(0x10FFFF).to_u32(), 0x10FFFF);
-}
-
-#[test]
-fn code_point_to_lead_surrogate() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    assert_eq!(c(0).to_lead_surrogate(), None);
-    assert_eq!(c(0xE9).to_lead_surrogate(), None);
-    assert_eq!(c(0xD800).to_lead_surrogate(), Some(0xD800));
-    assert_eq!(c(0xDBFF).to_lead_surrogate(), Some(0xDBFF));
-    assert_eq!(c(0xDC00).to_lead_surrogate(), None);
-    assert_eq!(c(0xDFFF).to_lead_surrogate(), None);
-    assert_eq!(c(0x1F4A9).to_lead_surrogate(), None);
-    assert_eq!(c(0x10FFFF).to_lead_surrogate(), None);
-}
-
-#[test]
-fn code_point_to_trail_surrogate() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    assert_eq!(c(0).to_trail_surrogate(), None);
-    assert_eq!(c(0xE9).to_trail_surrogate(), None);
-    assert_eq!(c(0xD800).to_trail_surrogate(), None);
-    assert_eq!(c(0xDBFF).to_trail_surrogate(), None);
-    assert_eq!(c(0xDC00).to_trail_surrogate(), Some(0xDC00));
-    assert_eq!(c(0xDFFF).to_trail_surrogate(), Some(0xDFFF));
-    assert_eq!(c(0x1F4A9).to_trail_surrogate(), None);
-    assert_eq!(c(0x10FFFF).to_trail_surrogate(), None);
-}
-
-#[test]
-fn code_point_from_char() {
-    assert_eq!(CodePoint::from_char('a').to_u32(), 0x61);
-    assert_eq!(CodePoint::from_char('💩').to_u32(), 0x1F4A9);
-}
-
-#[test]
-fn code_point_to_string() {
-    assert_eq!(format!("{:?}", CodePoint::from_char('a')), "U+0061");
-    assert_eq!(format!("{:?}", CodePoint::from_char('💩')), "U+1F4A9");
-}
-
-#[test]
-fn code_point_to_char() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    assert_eq!(c(0x61).to_char(), Some('a'));
-    assert_eq!(c(0x1F4A9).to_char(), Some('💩'));
-    assert_eq!(c(0xD800).to_char(), None);
-}
-
-#[test]
-fn code_point_to_char_lossy() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    assert_eq!(c(0x61).to_char_lossy(), 'a');
-    assert_eq!(c(0x1F4A9).to_char_lossy(), '💩');
-    assert_eq!(c(0xD800).to_char_lossy(), '\u{FFFD}');
-}
-
-#[test]
-fn wtf8buf_new() {
-    assert_eq!(Wtf8Buf::new().bytes, b"");
-}
-
-#[test]
-fn wtf8buf_from_str() {
-    assert_eq!(Wtf8Buf::from_str("").bytes, b"");
-    assert_eq!(Wtf8Buf::from_str("aé 💩").bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-}
-
-#[test]
-fn wtf8buf_from_string() {
-    assert_eq!(Wtf8Buf::from_string(String::from("")).bytes, b"");
-    assert_eq!(Wtf8Buf::from_string(String::from("aé 💩")).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-}
-
-#[test]
-fn wtf8buf_from_wide() {
-    let buf = Wtf8Buf::from_wide(&[]);
-    assert_eq!(buf.bytes, b"");
-    assert!(buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xDCA9]);
-    assert_eq!(buf.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-    assert!(buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]);
-    assert_eq!(buf.bytes, b"a\xC3\xA9 \xED\xA0\xBD\xF0\x9F\x92\xA9");
-    assert!(!buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0xD800]);
-    assert_eq!(buf.bytes, b"\xED\xA0\x80");
-    assert!(!buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0xDBFF]);
-    assert_eq!(buf.bytes, b"\xED\xAF\xBF");
-    assert!(!buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0xDC00]);
-    assert_eq!(buf.bytes, b"\xED\xB0\x80");
-    assert!(!buf.is_known_utf8);
-
-    let buf = Wtf8Buf::from_wide(&[0xDFFF]);
-    assert_eq!(buf.bytes, b"\xED\xBF\xBF");
-    assert!(!buf.is_known_utf8);
-}
-
-#[test]
-fn wtf8buf_push_str() {
-    let mut string = Wtf8Buf::new();
-    assert_eq!(string.bytes, b"");
-    assert!(string.is_known_utf8);
-
-    string.push_str("aé 💩");
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-    assert!(string.is_known_utf8);
-}
-
-#[test]
-fn wtf8buf_push_char() {
-    let mut string = Wtf8Buf::from_str("aé ");
-    assert_eq!(string.bytes, b"a\xC3\xA9 ");
-    assert!(string.is_known_utf8);
-
-    string.push_char('💩');
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-    assert!(string.is_known_utf8);
-}
-
-#[test]
-fn wtf8buf_push() {
-    let mut string = Wtf8Buf::from_str("aé ");
-    assert_eq!(string.bytes, b"a\xC3\xA9 ");
-    assert!(string.is_known_utf8);
-
-    string.push(CodePoint::from_char('💩'));
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-    assert!(string.is_known_utf8);
-
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xD83D)); // lead
-    assert!(!string.is_known_utf8);
-    string.push(c(0xDCA9)); // trail
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9"); // Magic!
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xD83D)); // lead
-    assert!(!string.is_known_utf8);
-    string.push(c(0x20)); // not surrogate
-    string.push(c(0xDCA9)); // trail
-    assert_eq!(string.bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xD800)); // lead
-    assert!(!string.is_known_utf8);
-    string.push(c(0xDBFF)); // lead
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xD800)); // lead
-    assert!(!string.is_known_utf8);
-    string.push(c(0xE000)); // not surrogate
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xEE\x80\x80");
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xD7FF)); // not surrogate
-    assert!(string.is_known_utf8);
-    string.push(c(0xDC00)); // trail
-    assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0x61)); // not surrogate, < 3 bytes
-    assert!(string.is_known_utf8);
-    string.push(c(0xDC00)); // trail
-    assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\x61\xED\xB0\x80");
-
-    let mut string = Wtf8Buf::new();
-    string.push(c(0xDC00)); // trail
-    assert!(!string.is_known_utf8);
-    assert_eq!(string.bytes, b"\xED\xB0\x80");
-}
-
-#[test]
-fn wtf8buf_push_wtf8() {
-    let mut string = Wtf8Buf::from_str("aé");
-    assert_eq!(string.bytes, b"a\xC3\xA9");
-    string.push_wtf8(Wtf8::from_str(" 💩"));
-    assert_eq!(string.bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-    assert!(string.is_known_utf8);
-
-    fn w(v: &[u8]) -> &Wtf8 {
-        unsafe { Wtf8::from_bytes_unchecked(v) }
-    }
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\xA0\xBD")); // lead
-    string.push_wtf8(w(b"\xED\xB2\xA9")); // trail
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9"); // Magic!
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\xA0\xBD")); // lead
-    string.push_wtf8(w(b" ")); // not surrogate
-    string.push_wtf8(w(b"\xED\xB2\xA9")); // trail
-    assert_eq!(string.bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-    assert!(!string.is_known_utf8);
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\xA0\x80")); // lead
-    string.push_wtf8(w(b"\xED\xAF\xBF")); // lead
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-    assert!(!string.is_known_utf8);
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\xA0\x80")); // lead
-    string.push_wtf8(w(b"\xEE\x80\x80")); // not surrogate
-    assert_eq!(string.bytes, b"\xED\xA0\x80\xEE\x80\x80");
-    assert!(!string.is_known_utf8);
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\x9F\xBF")); // not surrogate
-    string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-    assert!(!string.is_known_utf8);
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"a")); // not surrogate, < 3 bytes
-    string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\x61\xED\xB0\x80");
-    assert!(!string.is_known_utf8);
-
-    let mut string = Wtf8Buf::new();
-    string.push_wtf8(w(b"\xED\xB0\x80")); // trail
-    assert_eq!(string.bytes, b"\xED\xB0\x80");
-    assert!(!string.is_known_utf8);
-}
-
-#[test]
-fn wtf8buf_truncate() {
-    let mut string = Wtf8Buf::from_str("aé");
-    assert!(string.is_known_utf8);
-
-    string.truncate(3);
-    assert_eq!(string.bytes, b"a\xC3\xA9");
-    assert!(string.is_known_utf8);
-
-    string.truncate(1);
-    assert_eq!(string.bytes, b"a");
-    assert!(string.is_known_utf8);
-
-    string.truncate(0);
-    assert_eq!(string.bytes, b"");
-    assert!(string.is_known_utf8);
-}
-
-#[test]
-fn wtf8buf_truncate_around_non_bmp() {
-    let mut string = Wtf8Buf::from_str("💩");
-    assert!(string.is_known_utf8);
-
-    string.truncate(4);
-    assert_eq!(string.bytes, b"\xF0\x9F\x92\xA9");
-    assert!(string.is_known_utf8);
-
-    string.truncate(0);
-    assert_eq!(string.bytes, b"");
-    assert!(string.is_known_utf8);
-}
-
-#[test]
-#[should_panic]
-fn wtf8buf_truncate_fail_code_point_boundary() {
-    let mut string = Wtf8Buf::from_str("aé");
-    string.truncate(2);
-}
-
-#[test]
-#[should_panic]
-fn wtf8buf_truncate_fail_longer() {
-    let mut string = Wtf8Buf::from_str("aé");
-    string.truncate(4);
-}
-
-#[test]
-#[should_panic]
-fn wtf8buf_truncate_splitting_non_bmp3() {
-    let mut string = Wtf8Buf::from_str("💩");
-    assert!(string.is_known_utf8);
-    string.truncate(3);
-}
-
-#[test]
-#[should_panic]
-fn wtf8buf_truncate_splitting_non_bmp2() {
-    let mut string = Wtf8Buf::from_str("💩");
-    assert!(string.is_known_utf8);
-    string.truncate(2);
-}
-
-#[test]
-#[should_panic]
-fn wtf8buf_truncate_splitting_non_bmp1() {
-    let mut string = Wtf8Buf::from_str("💩");
-    assert!(string.is_known_utf8);
-    string.truncate(1);
-}
-
-#[test]
-fn wtf8buf_into_string() {
-    let mut string = Wtf8Buf::from_str("aé 💩");
-    assert!(string.is_known_utf8);
-    assert_eq!(string.clone().into_string(), Ok(String::from("aé 💩")));
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert!(!string.is_known_utf8);
-    assert_eq!(string.clone().into_string(), Err(string));
-}
-
-#[test]
-fn wtf8buf_into_string_lossy() {
-    let mut string = Wtf8Buf::from_str("aé 💩");
-    assert_eq!(string.clone().into_string_lossy(), String::from("aé 💩"));
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert_eq!(string.clone().into_string_lossy(), String::from("aé 💩�"));
-}
-
-#[test]
-fn wtf8buf_from_iterator() {
-    fn f(values: &[u32]) -> Wtf8Buf {
-        values.iter().map(|&c| CodePoint::from_u32(c).unwrap()).collect::<Wtf8Buf>()
-    }
-    assert_eq!(
-        f(&[0x61, 0xE9, 0x20, 0x1F4A9]),
-        Wtf8Buf { bytes: b"a\xC3\xA9 \xF0\x9F\x92\xA9".to_vec(), is_known_utf8: true }
-    );
-
-    assert_eq!(f(&[0xD83D, 0xDCA9]).bytes, b"\xF0\x9F\x92\xA9"); // Magic!
-    assert_eq!(
-        f(&[0xD83D, 0x20, 0xDCA9]),
-        Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        f(&[0xD800, 0xDBFF]),
-        Wtf8Buf { bytes: b"\xED\xA0\x80\xED\xAF\xBF".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        f(&[0xD800, 0xE000]),
-        Wtf8Buf { bytes: b"\xED\xA0\x80\xEE\x80\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        f(&[0xD7FF, 0xDC00]),
-        Wtf8Buf { bytes: b"\xED\x9F\xBF\xED\xB0\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        f(&[0x61, 0xDC00]),
-        Wtf8Buf { bytes: b"\x61\xED\xB0\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(f(&[0xDC00]), Wtf8Buf { bytes: b"\xED\xB0\x80".to_vec(), is_known_utf8: false });
-}
-
-#[test]
-fn wtf8buf_extend() {
-    fn e(initial: &[u32], extended: &[u32]) -> Wtf8Buf {
-        fn c(value: &u32) -> CodePoint {
-            CodePoint::from_u32(*value).unwrap()
-        }
-        let mut string = initial.iter().map(c).collect::<Wtf8Buf>();
-        string.extend(extended.iter().map(c));
-        string
-    }
-
-    assert_eq!(
-        e(&[0x61, 0xE9], &[0x20, 0x1F4A9]),
-        Wtf8Buf { bytes: b"a\xC3\xA9 \xF0\x9F\x92\xA9".to_vec(), is_known_utf8: true }
-    );
-
-    assert_eq!(e(&[0xD83D], &[0xDCA9]).bytes, b"\xF0\x9F\x92\xA9"); // Magic!
-    assert_eq!(
-        e(&[0xD83D, 0x20], &[0xDCA9]),
-        Wtf8Buf { bytes: b"\xED\xA0\xBD \xED\xB2\xA9".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        e(&[0xD800], &[0xDBFF]),
-        Wtf8Buf { bytes: b"\xED\xA0\x80\xED\xAF\xBF".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        e(&[0xD800], &[0xE000]),
-        Wtf8Buf { bytes: b"\xED\xA0\x80\xEE\x80\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        e(&[0xD7FF], &[0xDC00]),
-        Wtf8Buf { bytes: b"\xED\x9F\xBF\xED\xB0\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        e(&[0x61], &[0xDC00]),
-        Wtf8Buf { bytes: b"\x61\xED\xB0\x80".to_vec(), is_known_utf8: false }
-    );
-    assert_eq!(
-        e(&[], &[0xDC00]),
-        Wtf8Buf { bytes: b"\xED\xB0\x80".to_vec(), is_known_utf8: false }
-    );
-}
-
-#[test]
-fn wtf8buf_show() {
-    let mut string = Wtf8Buf::from_str("a\té \u{7f}💩\r");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert_eq!(format!("{string:?}"), "\"a\\té \\u{7f}\u{1f4a9}\\r\\u{d800}\"");
-}
-
-#[test]
-fn wtf8buf_as_slice() {
-    assert_eq!(Wtf8Buf::from_str("aé").as_slice(), Wtf8::from_str("aé"));
-}
-
-#[test]
-fn wtf8buf_show_str() {
-    let text = "a\té 💩\r";
-    let string = Wtf8Buf::from_str(text);
-    assert_eq!(format!("{text:?}"), format!("{string:?}"));
-}
-
-#[test]
-fn wtf8_from_str() {
-    assert_eq!(&Wtf8::from_str("").bytes, b"");
-    assert_eq!(&Wtf8::from_str("aé 💩").bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
-}
-
-#[test]
-fn wtf8_len() {
-    assert_eq!(Wtf8::from_str("").len(), 0);
-    assert_eq!(Wtf8::from_str("aé 💩").len(), 8);
-}
-
-#[test]
-fn wtf8_slice() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[1..4].bytes, b"\xC3\xA9 ");
-}
-
-#[test]
-#[should_panic]
-fn wtf8_slice_not_code_point_boundary() {
-    let _ = &Wtf8::from_str("aé 💩")[2..4];
-}
-
-#[test]
-fn wtf8_slice_from() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[1..].bytes, b"\xC3\xA9 \xF0\x9F\x92\xA9");
-}
-
-#[test]
-#[should_panic]
-fn wtf8_slice_from_not_code_point_boundary() {
-    let _ = &Wtf8::from_str("aé 💩")[2..];
-}
-
-#[test]
-fn wtf8_slice_to() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[..4].bytes, b"a\xC3\xA9 ");
-}
-
-#[test]
-#[should_panic]
-fn wtf8_slice_to_not_code_point_boundary() {
-    let _ = &Wtf8::from_str("aé 💩")[5..];
-}
-
-#[test]
-fn wtf8_ascii_byte_at() {
-    let slice = Wtf8::from_str("aé 💩");
-    assert_eq!(slice.ascii_byte_at(0), b'a');
-    assert_eq!(slice.ascii_byte_at(1), b'\xFF');
-    assert_eq!(slice.ascii_byte_at(2), b'\xFF');
-    assert_eq!(slice.ascii_byte_at(3), b' ');
-    assert_eq!(slice.ascii_byte_at(4), b'\xFF');
-}
-
-#[test]
-fn wtf8_code_points() {
-    fn c(value: u32) -> CodePoint {
-        CodePoint::from_u32(value).unwrap()
-    }
-    fn cp(string: &Wtf8Buf) -> Vec<Option<char>> {
-        string.code_points().map(|c| c.to_char()).collect::<Vec<_>>()
-    }
-    let mut string = Wtf8Buf::from_str("é ");
-    assert_eq!(cp(&string), [Some('é'), Some(' ')]);
-    string.push(c(0xD83D));
-    assert_eq!(cp(&string), [Some('é'), Some(' '), None]);
-    string.push(c(0xDCA9));
-    assert_eq!(cp(&string), [Some('é'), Some(' '), Some('💩')]);
-}
-
-#[test]
-fn wtf8_as_str() {
-    assert_eq!(Wtf8::from_str("").as_str(), Ok(""));
-    assert_eq!(Wtf8::from_str("aé 💩").as_str(), Ok("aé 💩"));
-    let mut string = Wtf8Buf::new();
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert!(string.as_str().is_err());
-}
-
-#[test]
-fn wtf8_to_string_lossy() {
-    assert_eq!(Wtf8::from_str("").to_string_lossy(), Cow::Borrowed(""));
-    assert_eq!(Wtf8::from_str("aé 💩").to_string_lossy(), Cow::Borrowed("aé 💩"));
-    let mut string = Wtf8Buf::from_str("aé 💩");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    let expected: Cow<'_, str> = Cow::Owned(String::from("aé 💩�"));
-    assert_eq!(string.to_string_lossy(), expected);
-}
-
-#[test]
-fn wtf8_display() {
-    fn d(b: &[u8]) -> String {
-        (&unsafe { Wtf8::from_bytes_unchecked(b) }).to_string()
-    }
-
-    assert_eq!("", d("".as_bytes()));
-    assert_eq!("aé 💩", d("aé 💩".as_bytes()));
-
-    let mut string = Wtf8Buf::from_str("aé 💩");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    assert_eq!("aé 💩�", d(string.as_inner()));
-}
-
-#[test]
-fn wtf8_encode_wide() {
-    let mut string = Wtf8Buf::from_str("aé ");
-    string.push(CodePoint::from_u32(0xD83D).unwrap());
-    string.push_char('💩');
-    assert_eq!(
-        string.encode_wide().collect::<Vec<_>>(),
-        vec![0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]
-    );
-}
-
-#[test]
-fn wtf8_encode_wide_size_hint() {
-    let string = Wtf8Buf::from_str("\u{12345}");
-    let mut iter = string.encode_wide();
-    assert_eq!((1, Some(8)), iter.size_hint());
-    iter.next().unwrap();
-    assert_eq!((1, Some(1)), iter.size_hint());
-    iter.next().unwrap();
-    assert_eq!((0, Some(0)), iter.size_hint());
-    assert!(iter.next().is_none());
-}
-
-#[test]
-fn wtf8_clone_into() {
-    let mut string = Wtf8Buf::new();
-    Wtf8::from_str("green").clone_into(&mut string);
-    assert_eq!(string.bytes, b"green");
-
-    let mut string = Wtf8Buf::from_str("green");
-    Wtf8::from_str("").clone_into(&mut string);
-    assert_eq!(string.bytes, b"");
-
-    let mut string = Wtf8Buf::from_str("red");
-    Wtf8::from_str("green").clone_into(&mut string);
-    assert_eq!(string.bytes, b"green");
-
-    let mut string = Wtf8Buf::from_str("green");
-    Wtf8::from_str("red").clone_into(&mut string);
-    assert_eq!(string.bytes, b"red");
-
-    let mut string = Wtf8Buf::from_str("green");
-    assert!(string.is_known_utf8);
-    unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").clone_into(&mut string) };
-    assert_eq!(string.bytes, b"\xED\xA0\x80");
-    assert!(!string.is_known_utf8);
-}
-
-#[test]
-fn wtf8_to_ascii_lowercase() {
-    let lowercase = Wtf8::from_str("").to_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"");
-
-    let lowercase = Wtf8::from_str("GrEeN gRaPeS! 🍇").to_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
-
-    let lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_ascii_lowercase() };
-    assert_eq!(lowercase.bytes, b"\xED\xA0\x80");
-    assert!(!lowercase.is_known_utf8);
-}
-
-#[test]
-fn wtf8_to_ascii_uppercase() {
-    let uppercase = Wtf8::from_str("").to_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"");
-
-    let uppercase = Wtf8::from_str("GrEeN gRaPeS! 🍇").to_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"GREEN GRAPES! \xf0\x9f\x8d\x87");
-
-    let uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_ascii_uppercase() };
-    assert_eq!(uppercase.bytes, b"\xED\xA0\x80");
-    assert!(!uppercase.is_known_utf8);
-}
-
-#[test]
-fn wtf8_make_ascii_lowercase() {
-    let mut lowercase = Wtf8Buf::from_str("");
-    lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"");
-
-    let mut lowercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
-    lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
-
-    let mut lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    lowercase.make_ascii_lowercase();
-    assert_eq!(lowercase.bytes, b"\xED\xA0\x80");
-    assert!(!lowercase.is_known_utf8);
-}
-
-#[test]
-fn wtf8_make_ascii_uppercase() {
-    let mut uppercase = Wtf8Buf::from_str("");
-    uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"");
-
-    let mut uppercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
-    uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"GREEN GRAPES! \xf0\x9f\x8d\x87");
-
-    let mut uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    uppercase.make_ascii_uppercase();
-    assert_eq!(uppercase.bytes, b"\xED\xA0\x80");
-    assert!(!uppercase.is_known_utf8);
-}
-
-#[test]
-fn wtf8_to_owned() {
-    let string = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    assert_eq!(string.bytes, b"\xED\xA0\x80");
-    assert!(!string.is_known_utf8);
-}
-
-#[test]
-fn wtf8_valid_utf8_boundaries() {
-    let mut string = Wtf8Buf::from_str("aé 💩");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 0);
-    check_utf8_boundary(&string, 1);
-    check_utf8_boundary(&string, 3);
-    check_utf8_boundary(&string, 4);
-    check_utf8_boundary(&string, 8);
-    check_utf8_boundary(&string, 14);
-    assert_eq!(string.len(), 14);
-
-    string.push_char('a');
-    check_utf8_boundary(&string, 14);
-    check_utf8_boundary(&string, 15);
-
-    let mut string = Wtf8Buf::from_str("a");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 1);
-
-    let mut string = Wtf8Buf::from_str("\u{D7FF}");
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 3);
-
-    let mut string = Wtf8Buf::new();
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    string.push_char('\u{D7FF}');
-    check_utf8_boundary(&string, 3);
-}
-
-#[test]
-#[should_panic(expected = "byte index 4 is out of bounds")]
-fn wtf8_utf8_boundary_out_of_bounds() {
-    let string = Wtf8::from_str("aé");
-    check_utf8_boundary(&string, 4);
-}
-
-#[test]
-#[should_panic(expected = "byte index 1 is not a codepoint boundary")]
-fn wtf8_utf8_boundary_inside_codepoint() {
-    let string = Wtf8::from_str("é");
-    check_utf8_boundary(&string, 1);
-}
-
-#[test]
-#[should_panic(expected = "byte index 1 is not a codepoint boundary")]
-fn wtf8_utf8_boundary_inside_surrogate() {
-    let mut string = Wtf8Buf::new();
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 1);
-}
-
-#[test]
-#[should_panic(expected = "byte index 3 lies between surrogate codepoints")]
-fn wtf8_utf8_boundary_between_surrogates() {
-    let mut string = Wtf8Buf::new();
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    string.push(CodePoint::from_u32(0xD800).unwrap());
-    check_utf8_boundary(&string, 3);
-}
-
-#[test]
-fn wobbled_wtf8_plus_bytes_isnt_utf8() {
-    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    assert!(!string.is_known_utf8);
-    string.extend_from_slice(b"some utf-8");
-    assert!(!string.is_known_utf8);
-}
-
-#[test]
-fn wobbled_wtf8_plus_str_isnt_utf8() {
-    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
-    assert!(!string.is_known_utf8);
-    string.push_str("some utf-8");
-    assert!(!string.is_known_utf8);
-}
-
-#[test]
-fn unwobbly_wtf8_plus_utf8_is_utf8() {
-    let mut string: Wtf8Buf = Wtf8Buf::from_str("hello world");
-    assert!(string.is_known_utf8);
-    string.push_str("some utf-8");
-    assert!(string.is_known_utf8);
-}
diff --git a/library/std/src/thread/current.rs b/library/std/src/thread/current.rs
index 5c879903526..7da1621da45 100644
--- a/library/std/src/thread/current.rs
+++ b/library/std/src/thread/current.rs
@@ -18,8 +18,8 @@ local_pointer! {
 pub(super) mod id {
     use super::*;
 
-    cfg_if::cfg_if! {
-        if #[cfg(target_thread_local)] {
+    cfg_select! {
+        target_thread_local => {
             use crate::cell::Cell;
 
             #[thread_local]
@@ -34,7 +34,8 @@ pub(super) mod id {
             pub(super) fn set(id: ThreadId) {
                 ID.set(Some(id))
             }
-        } else if #[cfg(target_pointer_width = "16")] {
+        }
+        target_pointer_width = "16" => {
             local_pointer! {
                 static ID0;
                 static ID16;
@@ -59,7 +60,8 @@ pub(super) mod id {
                 ID32.set(ptr::without_provenance_mut((val >> 32) as usize));
                 ID48.set(ptr::without_provenance_mut((val >> 48) as usize));
             }
-        } else if #[cfg(target_pointer_width = "32")] {
+        }
+        target_pointer_width = "32" => {
             local_pointer! {
                 static ID0;
                 static ID32;
@@ -78,7 +80,8 @@ pub(super) mod id {
                 ID0.set(ptr::without_provenance_mut(val as usize));
                 ID32.set(ptr::without_provenance_mut((val >> 32) as usize));
             }
-        } else {
+        }
+        _ => {
             local_pointer! {
                 static ID;
             }
diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs
index 0ad014ccd3e..797feeb2bbb 100644
--- a/library/std/src/thread/local.rs
+++ b/library/std/src/thread/local.rs
@@ -8,8 +8,8 @@ use crate::fmt;
 
 /// A thread local storage (TLS) key which owns its contents.
 ///
-/// This key uses the fastest possible implementation available to it for the
-/// target platform. It is instantiated with the [`thread_local!`] macro and the
+/// This key uses the fastest implementation available on the target platform.
+/// It is instantiated with the [`thread_local!`] macro and the
 /// primary method is the [`with`] method, though there are helpers to make
 /// working with [`Cell`] types easier.
 ///
@@ -24,10 +24,10 @@ use crate::fmt;
 /// [`with`]) within a thread, and values that implement [`Drop`] get
 /// destructed when a thread exits. Some platform-specific caveats apply, which
 /// are explained below.
-/// Note that, should the destructor panics, the whole process will be [aborted].
+/// Note that if the destructor panics, the whole process will be [aborted].
 ///
 /// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
-/// `LocalKey` in this way may cause panics, aborts or infinite recursion on
+/// `LocalKey` in this way may cause panics, aborts, or infinite recursion on
 /// the first call to `with`.
 ///
 /// [aborted]: crate::process::abort
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 292323d0118..b6059c28cec 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -1212,8 +1212,8 @@ impl ThreadId {
             panic!("failed to generate unique thread ID: bitspace exhausted")
         }
 
-        cfg_if::cfg_if! {
-            if #[cfg(target_has_atomic = "64")] {
+        cfg_select! {
+            target_has_atomic = "64" => {
                 use crate::sync::atomic::{Atomic, AtomicU64};
 
                 static COUNTER: Atomic<u64> = AtomicU64::new(0);
@@ -1229,7 +1229,8 @@ impl ThreadId {
                         Err(id) => last = id,
                     }
                 }
-            } else {
+            }
+            _ => {
                 use crate::sync::{Mutex, PoisonError};
 
                 static COUNTER: Mutex<u64> = Mutex::new(0);
@@ -1318,8 +1319,8 @@ use thread_name_string::ThreadNameString;
 /// Note however that this also means that the name reported in pre-main functions
 /// will be incorrect, but that's just something we have to live with.
 pub(crate) mod main_thread {
-    cfg_if::cfg_if! {
-        if #[cfg(target_has_atomic = "64")] {
+    cfg_select! {
+        target_has_atomic = "64" => {
             use super::ThreadId;
             use crate::sync::atomic::{Atomic, AtomicU64};
             use crate::sync::atomic::Ordering::Relaxed;
@@ -1335,7 +1336,8 @@ pub(crate) mod main_thread {
             pub(crate) unsafe fn set(id: ThreadId) {
                 MAIN.store(id.as_u64().get(), Relaxed)
             }
-        } else {
+        }
+        _ => {
             use super::ThreadId;
             use crate::mem::MaybeUninit;
             use crate::sync::atomic::{Atomic, AtomicBool};
diff --git a/library/std/src/time.rs b/library/std/src/time.rs
index 07bb41f1496..84fbb4c2fe4 100644
--- a/library/std/src/time.rs
+++ b/library/std/src/time.rs
@@ -717,12 +717,7 @@ impl SystemTimeError {
 }
 
 #[stable(feature = "time2", since = "1.8.0")]
-impl Error for SystemTimeError {
-    #[allow(deprecated)]
-    fn description(&self) -> &str {
-        "other time was not earlier than self"
-    }
-}
+impl Error for SystemTimeError {}
 
 #[stable(feature = "time2", since = "1.8.0")]
 impl fmt::Display for SystemTimeError {