Adding debian version 1.65.0+dfsg1-2.debian/1.65.0+dfsg1-2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

144 files changed, 4003 insertions, 2021 deletions

diff --git a/library/std/src/alloc.rs b/library/std/src/alloc.rs
index a05e0db3a..61c1ff578 100644
--- a/library/std/src/alloc.rs
+++ b/library/std/src/alloc.rs
@@ -68,7 +68,10 @@ pub use alloc_crate::alloc::*;
 /// The default memory allocator provided by the operating system.
 ///
 /// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
-/// plus related functions.
+/// plus related functions. However, it is not valid to mix use of the backing
+/// system allocator with `System`, as this implementation may include extra
+/// work, such as to serve alignment requests greater than the alignment
+/// provided directly by the backing system allocator.
 ///
 /// This type implements the `GlobalAlloc` trait and Rust programs by default
 /// work as if they had this definition:
diff --git a/library/std/src/backtrace.rs b/library/std/src/backtrace.rs
index 05e9b2eb6..5cf6ec817 100644
--- a/library/std/src/backtrace.rs
+++ b/library/std/src/backtrace.rs
@@ -9,12 +9,6 @@
 //! implementing `std::error::Error`) to get a causal chain of where an error
 //! was generated.
 //!
-//! > **Note**: this module is unstable and is designed in [RFC 2504], and you
-//! > can learn more about its status in the [tracking issue].
-//!
-//! [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
-//! [tracking issue]: https://github.com/rust-lang/rust/issues/53487
-//!
 //! ## Accuracy
 //!
 //! Backtraces are attempted to be as accurate as possible, but no guarantees
@@ -64,7 +58,7 @@
 //! `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` at runtime might not actually change
 //! how backtraces are captured.
 
-#![unstable(feature = "backtrace", issue = "53487")]
+#![stable(feature = "backtrace", since = "1.65.0")]
 
 #[cfg(test)]
 mod tests;
@@ -110,6 +104,7 @@ use crate::vec::Vec;
 /// previous point in time. In some instances the `Backtrace` type may
 /// internally be empty due to configuration. For more information see
 /// `Backtrace::capture`.
+#[stable(feature = "backtrace", since = "1.65.0")]
 #[must_use]
 pub struct Backtrace {
     inner: Inner,
@@ -117,17 +112,21 @@ pub struct Backtrace {
 
 /// The current status of a backtrace, indicating whether it was captured or
 /// whether it is empty for some other reason.
+#[stable(feature = "backtrace", since = "1.65.0")]
 #[non_exhaustive]
 #[derive(Debug, PartialEq, Eq)]
 pub enum BacktraceStatus {
     /// Capturing a backtrace is not supported, likely because it's not
     /// implemented for the current platform.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     Unsupported,
     /// Capturing a backtrace has been disabled through either the
     /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     Disabled,
     /// A backtrace has been captured and the `Backtrace` should print
     /// reasonable information when rendered.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     Captured,
 }
 
@@ -174,6 +173,7 @@ enum BytesOrWide {
     Wide(Vec<u16>),
 }
 
+#[stable(feature = "backtrace", since = "1.65.0")]
 impl fmt::Debug for Backtrace {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let capture = match &self.inner {
@@ -200,6 +200,7 @@ impl fmt::Debug for Backtrace {
     }
 }
 
+#[unstable(feature = "backtrace_frames", issue = "79676")]
 impl fmt::Debug for BacktraceFrame {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut dbg = fmt.debug_list();
@@ -288,6 +289,7 @@ impl Backtrace {
     ///
     /// To forcibly capture a backtrace regardless of environment variables, use
     /// the `Backtrace::force_capture` function.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     #[inline(never)] // want to make sure there's a frame here to remove
     pub fn capture() -> Backtrace {
         if !Backtrace::enabled() {
@@ -306,6 +308,7 @@ impl Backtrace {
     /// Note that capturing a backtrace can be an expensive operation on some
     /// platforms, so this should be used with caution in performance-sensitive
     /// parts of code.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     #[inline(never)] // want to make sure there's a frame here to remove
     pub fn force_capture() -> Backtrace {
         Backtrace::create(Backtrace::force_capture as usize)
@@ -313,6 +316,8 @@ impl Backtrace {
 
     /// Forcibly captures a disabled backtrace, regardless of environment
     /// variable configuration.
+    #[stable(feature = "backtrace", since = "1.65.0")]
+    #[rustc_const_stable(feature = "backtrace", since = "1.65.0")]
     pub const fn disabled() -> Backtrace {
         Backtrace { inner: Inner::Disabled }
     }
@@ -356,6 +361,7 @@ impl Backtrace {
     /// Returns the status of this backtrace, indicating whether this backtrace
     /// request was unsupported, disabled, or a stack trace was actually
     /// captured.
+    #[stable(feature = "backtrace", since = "1.65.0")]
     #[must_use]
     pub fn status(&self) -> BacktraceStatus {
         match self.inner {
@@ -375,6 +381,7 @@ impl<'a> Backtrace {
     }
 }
 
+#[stable(feature = "backtrace", since = "1.65.0")]
 impl fmt::Display for Backtrace {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let capture = match &self.inner {
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index db811343f..9845d1faf 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -9,6 +9,8 @@ use crate::borrow::Borrow;
 use crate::cell::Cell;
 use crate::collections::TryReserveError;
 use crate::collections::TryReserveErrorKind;
+#[cfg(not(bootstrap))]
+use crate::error::Error;
 use crate::fmt::{self, Debug};
 #[allow(deprecated)]
 use crate::hash::{BuildHasher, Hash, Hasher, SipHasher13};
@@ -2158,6 +2160,15 @@ impl<'a, K: Debug, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[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"
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> {
     type Item = (&'a K, &'a V);
diff --git a/library/std/src/collections/hash/map/tests.rs b/library/std/src/collections/hash/map/tests.rs
index 7ebc41588..cb3032719 100644
--- a/library/std/src/collections/hash/map/tests.rs
+++ b/library/std/src/collections/hash/map/tests.rs
@@ -268,10 +268,13 @@ fn test_lots_of_insertions() {
 
     // Try this a few times to make sure we never screw up the hashmap's
     // internal state.
-    for _ in 0..10 {
+    let loops = if cfg!(miri) { 2 } else { 10 };
+    for _ in 0..loops {
         assert!(m.is_empty());
 
-        for i in 1..1001 {
+        let count = if cfg!(miri) { 101 } else { 1001 };
+
+        for i in 1..count {
             assert!(m.insert(i, i).is_none());
 
             for j in 1..=i {
@@ -279,42 +282,42 @@ fn test_lots_of_insertions() {
                 assert_eq!(r, Some(&j));
             }
 
-            for j in i + 1..1001 {
+            for j in i + 1..count {
                 let r = m.get(&j);
                 assert_eq!(r, None);
             }
         }
 
-        for i in 1001..2001 {
+        for i in count..(2 * count) {
             assert!(!m.contains_key(&i));
         }
 
         // remove forwards
-        for i in 1..1001 {
+        for i in 1..count {
             assert!(m.remove(&i).is_some());
 
             for j in 1..=i {
                 assert!(!m.contains_key(&j));
             }
 
-            for j in i + 1..1001 {
+            for j in i + 1..count {
                 assert!(m.contains_key(&j));
             }
         }
 
-        for i in 1..1001 {
+        for i in 1..count {
             assert!(!m.contains_key(&i));
         }
 
-        for i in 1..1001 {
+        for i in 1..count {
             assert!(m.insert(i, i).is_none());
         }
 
         // remove backwards
-        for i in (1..1001).rev() {
+        for i in (1..count).rev() {
             assert!(m.remove(&i).is_some());
 
-            for j in i..1001 {
+            for j in i..count {
                 assert!(!m.contains_key(&j));
             }
 
@@ -817,6 +820,7 @@ fn test_retain() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // Miri does not support signalling OOM
 #[cfg_attr(target_os = "android", ignore)] // Android used in CI has a broken dlmalloc
 fn test_try_reserve() {
     let mut empty_bytes: HashMap<u8, u8> = HashMap::new();
diff --git a/library/std/src/collections/hash/set.rs b/library/std/src/collections/hash/set.rs
index abff82788..5b6a415fa 100644
--- a/library/std/src/collections/hash/set.rs
+++ b/library/std/src/collections/hash/set.rs
@@ -239,7 +239,7 @@ impl<T, S> HashSet<T, S> {
     ///
     /// If the returned iterator is dropped before being fully consumed, it
     /// drops the remaining elements. The returned iterator keeps a mutable
-    /// borrow on the vector to optimize its implementation.
+    /// borrow on the set to optimize its implementation.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/error.rs b/library/std/src/error.rs
index 722df119d..e45059595 100644
--- a/library/std/src/error.rs
+++ b/library/std/src/error.rs
@@ -1,175 +1,51 @@
-//! Interfaces for working with Errors.
-//!
-//! # Error Handling In Rust
-//!
-//! The Rust language provides two complementary systems for constructing /
-//! representing, reporting, propagating, reacting to, and discarding errors.
-//! These responsibilities are collectively known as "error handling." The
-//! components of the first system, the panic runtime and interfaces, are most
-//! commonly used to represent bugs that have been detected in your program. The
-//! components of the second system, `Result`, the error traits, and user
-//! defined types, are used to represent anticipated runtime failure modes of
-//! your program.
-//!
-//! ## The Panic Interfaces
-//!
-//! The following are the primary interfaces of the panic system and the
-//! responsibilities they cover:
-//!
-//! * [`panic!`] and [`panic_any`] (Constructing, Propagated automatically)
-//! * [`PanicInfo`] (Reporting)
-//! * [`set_hook`], [`take_hook`], and [`#[panic_handler]`][panic-handler] (Reporting)
-//! * [`catch_unwind`] and [`resume_unwind`] (Discarding, Propagating)
-//!
-//! The following are the primary interfaces of the error system and the
-//! responsibilities they cover:
-//!
-//! * [`Result`] (Propagating, Reacting)
-//! * The [`Error`] trait (Reporting)
-//! * User defined types (Constructing / Representing)
-//! * [`match`] and [`downcast`] (Reacting)
-//! * The question mark operator ([`?`]) (Propagating)
-//! * The partially stable [`Try`] traits (Propagating, Constructing)
-//! * [`Termination`] (Reporting)
-//!
-//! ## Converting Errors into Panics
-//!
-//! The panic and error systems are not entirely distinct. Often times errors
-//! that are anticipated runtime failures in an API might instead represent bugs
-//! to a caller. For these situations the standard library provides APIs for
-//! constructing panics with an `Error` as it's source.
-//!
-//! * [`Result::unwrap`]
-//! * [`Result::expect`]
-//!
-//! These functions are equivalent, they either return the inner value if the
-//! `Result` is `Ok` or panic if the `Result` is `Err` printing the inner error
-//! as the source. The only difference between them is that with `expect` you
-//! provide a panic error message to be printed alongside the source, whereas
-//! `unwrap` has a default message indicating only that you unwraped an `Err`.
-//!
-//! Of the two, `expect` is generally preferred since its `msg` field allows you
-//! to convey your intent and assumptions which makes tracking down the source
-//! of a panic easier. `unwrap` on the other hand can still be a good fit in
-//! situations where you can trivially show that a piece of code will never
-//! panic, such as `"127.0.0.1".parse::<std::net::IpAddr>().unwrap()` or early
-//! prototyping.
-//!
-//! # Common Message Styles
-//!
-//! There are two common styles for how people word `expect` messages. Using
-//! the message to present information to users encountering a panic
-//! ("expect as error message") or using the message to present information
-//! to developers debugging the panic ("expect as precondition").
-//!
-//! In the former case the expect message is used to describe the error that
-//! has occurred which is considered a bug. Consider the following example:
-//!
-//! ```should_panic
-//! // Read environment variable, panic if it is not present
-//! let path = std::env::var("IMPORTANT_PATH").unwrap();
-//! ```
-//!
-//! In the "expect as error message" style we would use expect to describe
-//! that the environment variable was not set when it should have been:
-//!
-//! ```should_panic
-//! let path = std::env::var("IMPORTANT_PATH")
-//!     .expect("env variable `IMPORTANT_PATH` is not set");
-//! ```
-//!
-//! In the "expect as precondition" style, we would instead describe the
-//! reason we _expect_ the `Result` should be `Ok`. With this style we would
-//! prefer to write:
-//!
-//! ```should_panic
-//! let path = std::env::var("IMPORTANT_PATH")
-//!     .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
-//! ```
-//!
-//! The "expect as error message" style does not work as well with the
-//! default output of the std panic hooks, and often ends up repeating
-//! information that is already communicated by the source error being
-//! unwrapped:
-//!
-//! ```text
-//! thread 'main' panicked at 'env variable `IMPORTANT_PATH` is not set: NotPresent', src/main.rs:4:6
-//! ```
-//!
-//! In this example we end up mentioning that an env variable is not set,
-//! followed by our source message that says the env is not present, the
-//! only additional information we're communicating is the name of the
-//! environment variable being checked.
-//!
-//! The "expect as precondition" style instead focuses on source code
-//! readability, making it easier to understand what must have gone wrong in
-//! situations where panics are being used to represent bugs exclusively.
-//! Also, by framing our expect in terms of what "SHOULD" have happened to
-//! prevent the source error, we end up introducing new information that is
-//! independent from our source error.
-//!
-//! ```text
-//! thread 'main' panicked at 'env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`: NotPresent', src/main.rs:4:6
-//! ```
-//!
-//! In this example we are communicating not only the name of the
-//! environment variable that should have been set, but also an explanation
-//! for why it should have been set, and we let the source error display as
-//! a clear contradiction to our expectation.
-//!
-//! **Hint**: If you're having trouble remembering how to phrase
-//! expect-as-precondition style error messages remember to focus on the word
-//! "should" as in "env variable should be set by blah" or "the given binary
-//! should be available and executable by the current user".
-//!
-//! [`panic_any`]: crate::panic::panic_any
-//! [`PanicInfo`]: crate::panic::PanicInfo
-//! [`catch_unwind`]: crate::panic::catch_unwind
-//! [`resume_unwind`]: crate::panic::resume_unwind
-//! [`downcast`]: crate::error::Error
-//! [`Termination`]: crate::process::Termination
-//! [`Try`]: crate::ops::Try
-//! [panic hook]: crate::panic::set_hook
-//! [`set_hook`]: crate::panic::set_hook
-//! [`take_hook`]: crate::panic::take_hook
-//! [panic-handler]: <https://doc.rust-lang.org/nomicon/panic-handler.html>
-//! [`match`]: ../../std/keyword.match.html
-//! [`?`]: ../../std/result/index.html#the-question-mark-operator-
-
+#![doc = include_str!("../../core/src/error.md")]
 #![stable(feature = "rust1", since = "1.0.0")]
 
-// A note about crates and the facade:
-//
-// Originally, the `Error` trait was defined in libcore, and the impls
-// were scattered about. However, coherence objected to this
-// arrangement, because to create the blanket impls for `Box` required
-// knowing that `&str: !Error`, and we have no means to deal with that
-// sort of conflict just now. Therefore, for the time being, we have
-// moved the `Error` trait into libstd. As we evolve a sol'n to the
-// coherence challenge (e.g., specialization, neg impls, etc) we can
-// reconsider what crate these items belong in.
-
 #[cfg(test)]
 mod tests;
 
+#[cfg(bootstrap)]
 use core::array;
+#[cfg(bootstrap)]
 use core::convert::Infallible;
 
+#[cfg(bootstrap)]
 use crate::alloc::{AllocError, LayoutError};
-use crate::any::{Demand, Provider, TypeId};
+#[cfg(bootstrap)]
+use crate::any::Demand;
+#[cfg(bootstrap)]
+use crate::any::{Provider, TypeId};
 use crate::backtrace::Backtrace;
+#[cfg(bootstrap)]
 use crate::borrow::Cow;
+#[cfg(bootstrap)]
 use crate::cell;
+#[cfg(bootstrap)]
 use crate::char;
-use crate::fmt::{self, Debug, Display, Write};
+#[cfg(bootstrap)]
+use crate::fmt::Debug;
+#[cfg(bootstrap)]
+use crate::fmt::Display;
+use crate::fmt::{self, Write};
+#[cfg(bootstrap)]
 use crate::io;
+#[cfg(bootstrap)]
 use crate::mem::transmute;
+#[cfg(bootstrap)]
 use crate::num;
+#[cfg(bootstrap)]
 use crate::str;
+#[cfg(bootstrap)]
 use crate::string;
+#[cfg(bootstrap)]
 use crate::sync::Arc;
+#[cfg(bootstrap)]
 use crate::time;
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::error::Error;
+
 /// `Error` is a trait representing the basic expectations for error values,
 /// i.e., values of type `E` in [`Result<T, E>`].
 ///
@@ -182,14 +58,15 @@ use crate::time;
 /// assert_eq!(err.to_string(), "invalid digit found in string");
 /// ```
 ///
-/// Errors may provide cause chain information. [`Error::source()`] is generally
+/// Errors may provide cause information. [`Error::source()`] is generally
 /// used when errors cross "abstraction boundaries". If one module must report
 /// an error that is caused by an error from a lower-level module, it can allow
 /// accessing that error via [`Error::source()`]. This makes it possible for the
 /// high-level module to provide its own errors while also revealing some of the
-/// implementation for debugging via `source` chains.
+/// implementation for debugging.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
+#[cfg(bootstrap)]
 pub trait Error: Debug + Display {
     /// The lower-level source of this error, if any.
     ///
@@ -333,8 +210,8 @@ pub trait Error: Debug + Display {
     /// }
     ///
     /// impl std::error::Error for Error {
-    ///     fn provide<'a>(&'a self, req: &mut Demand<'a>) {
-    ///         req
+    ///     fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+    ///         demand
     ///             .provide_ref::<MyBacktrace>(&self.backtrace)
     ///             .provide_ref::<dyn std::error::Error + 'static>(&self.source);
     ///     }
@@ -352,13 +229,14 @@ pub trait Error: Debug + Display {
     /// ```
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
     #[allow(unused_variables)]
-    fn provide<'a>(&'a self, req: &mut Demand<'a>) {}
+    fn provide<'a>(&'a self, demand: &mut Demand<'a>) {}
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "error_generic_member_access", issue = "99301")]
 impl<'b> Provider for dyn Error + 'b {
-    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
-        self.provide(req)
+    fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+        self.provide(demand)
     }
 }
 
@@ -370,6 +248,7 @@ mod private {
     pub struct Internal;
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
     /// Converts a type of [`Error`] into a box of dyn [`Error`].
@@ -402,6 +281,7 @@ impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
     /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
@@ -440,6 +320,7 @@ impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync +
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<String> for Box<dyn Error + Send + Sync> {
     /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
@@ -483,6 +364,7 @@ impl From<String> for Box<dyn Error + Send + Sync> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "string_box_error", since = "1.6.0")]
 impl From<String> for Box<dyn Error> {
     /// Converts a [`String`] into a box of dyn [`Error`].
@@ -504,6 +386,7 @@ impl From<String> for Box<dyn Error> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
     /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
@@ -527,6 +410,7 @@ impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "string_box_error", since = "1.6.0")]
 impl From<&str> for Box<dyn Error> {
     /// Converts a [`str`] into a box of dyn [`Error`].
@@ -548,6 +432,7 @@ impl From<&str> for Box<dyn Error> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "cow_box_error", since = "1.22.0")]
 impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
     /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
@@ -569,6 +454,7 @@ impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "cow_box_error", since = "1.22.0")]
 impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
     /// Converts a [`Cow`] into a box of dyn [`Error`].
@@ -589,9 +475,11 @@ impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
     }
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {}
 
+#[cfg(bootstrap)]
 #[unstable(
     feature = "allocator_api",
     reason = "the precise API and guarantees it provides may be tweaked.",
@@ -599,9 +487,11 @@ impl Error for ! {}
 )]
 impl Error for AllocError {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 impl Error for LayoutError {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for str::ParseBoolError {
     #[allow(deprecated)]
@@ -610,6 +500,7 @@ impl Error for str::ParseBoolError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for str::Utf8Error {
     #[allow(deprecated)]
@@ -618,6 +509,7 @@ impl Error for str::Utf8Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseIntError {
     #[allow(deprecated)]
@@ -626,6 +518,7 @@ impl Error for num::ParseIntError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for num::TryFromIntError {
     #[allow(deprecated)]
@@ -634,6 +527,7 @@ impl Error for num::TryFromIntError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for array::TryFromSliceError {
     #[allow(deprecated)]
@@ -642,6 +536,7 @@ impl Error for array::TryFromSliceError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for num::ParseFloatError {
     #[allow(deprecated)]
@@ -650,6 +545,7 @@ impl Error for num::ParseFloatError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for string::FromUtf8Error {
     #[allow(deprecated)]
@@ -658,6 +554,7 @@ impl Error for string::FromUtf8Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for string::FromUtf16Error {
     #[allow(deprecated)]
@@ -666,6 +563,7 @@ impl Error for string::FromUtf16Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "str_parse_error2", since = "1.8.0")]
 impl Error for Infallible {
     fn description(&self) -> &str {
@@ -673,6 +571,7 @@ impl Error for Infallible {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "decode_utf16", since = "1.9.0")]
 impl Error for char::DecodeUtf16Error {
     #[allow(deprecated)]
@@ -681,9 +580,11 @@ impl Error for char::DecodeUtf16Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "u8_from_char", since = "1.59.0")]
 impl Error for char::TryFromCharError {}
 
+#[cfg(bootstrap)]
 #[unstable(feature = "map_try_insert", issue = "82766")]
 impl<'a, K: Debug + Ord, V: Debug> Error
     for crate::collections::btree_map::OccupiedError<'a, K, V>
@@ -694,6 +595,7 @@ impl<'a, K: Debug + Ord, V: Debug> Error
     }
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "map_try_insert", issue = "82766")]
 impl<'a, K: Debug, V: Debug> Error for crate::collections::hash_map::OccupiedError<'a, K, V> {
     #[allow(deprecated)]
@@ -702,6 +604,7 @@ impl<'a, K: Debug, V: Debug> Error for crate::collections::hash_map::OccupiedErr
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "box_error", since = "1.8.0")]
 impl<T: Error> Error for Box<T> {
     #[allow(deprecated, deprecated_in_future)]
@@ -719,6 +622,7 @@ impl<T: Error> Error for Box<T> {
     }
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "thin_box", issue = "92791")]
 impl<T: ?Sized + crate::error::Error> crate::error::Error for crate::boxed::ThinBox<T> {
     fn source(&self) -> Option<&(dyn crate::error::Error + 'static)> {
@@ -727,6 +631,7 @@ impl<T: ?Sized + crate::error::Error> crate::error::Error for crate::boxed::Thin
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "error_by_ref", since = "1.51.0")]
 impl<'a, T: Error + ?Sized> Error for &'a T {
     #[allow(deprecated, deprecated_in_future)]
@@ -743,11 +648,12 @@ impl<'a, T: Error + ?Sized> Error for &'a T {
         Error::source(&**self)
     }
 
-    fn provide<'b>(&'b self, req: &mut Demand<'b>) {
-        Error::provide(&**self, req);
+    fn provide<'b>(&'b self, demand: &mut Demand<'b>) {
+        Error::provide(&**self, demand);
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "arc_error", since = "1.52.0")]
 impl<T: Error + ?Sized> Error for Arc<T> {
     #[allow(deprecated, deprecated_in_future)]
@@ -764,11 +670,12 @@ impl<T: Error + ?Sized> Error for Arc<T> {
         Error::source(&**self)
     }
 
-    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
-        Error::provide(&**self, req);
+    fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+        Error::provide(&**self, demand);
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "fmt_error", since = "1.11.0")]
 impl Error for fmt::Error {
     #[allow(deprecated)]
@@ -777,6 +684,7 @@ impl Error for fmt::Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Error for cell::BorrowError {
     #[allow(deprecated)]
@@ -785,6 +693,7 @@ impl Error for cell::BorrowError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Error for cell::BorrowMutError {
     #[allow(deprecated)]
@@ -793,6 +702,7 @@ impl Error for cell::BorrowMutError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for char::CharTryFromError {
     #[allow(deprecated)]
@@ -801,6 +711,7 @@ impl Error for char::CharTryFromError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "char_from_str", since = "1.20.0")]
 impl Error for char::ParseCharError {
     #[allow(deprecated)]
@@ -809,12 +720,15 @@ impl Error for char::ParseCharError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "try_reserve", since = "1.57.0")]
 impl Error for alloc::collections::TryReserveError {}
 
+#[cfg(bootstrap)]
 #[unstable(feature = "duration_checked_float", issue = "83400")]
 impl Error for time::FromFloatSecsError {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Error for alloc::ffi::NulError {
     #[allow(deprecated)]
@@ -823,6 +737,7 @@ impl Error for alloc::ffi::NulError {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<alloc::ffi::NulError> for io::Error {
     /// Converts a [`alloc::ffi::NulError`] into a [`io::Error`].
@@ -831,6 +746,7 @@ impl From<alloc::ffi::NulError> for io::Error {
     }
 }
 
+#[cfg(bootstrap)]
 #[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
 impl Error for core::ffi::FromBytesWithNulError {
     #[allow(deprecated)]
@@ -839,12 +755,15 @@ impl Error for core::ffi::FromBytesWithNulError {
     }
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
 impl Error for core::ffi::FromBytesUntilNulError {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "cstring_from_vec_with_nul", since = "1.58.0")]
 impl Error for alloc::ffi::FromVecWithNulError {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "cstring_into", since = "1.7.0")]
 impl Error for alloc::ffi::IntoStringError {
     #[allow(deprecated)]
@@ -857,6 +776,7 @@ impl Error for alloc::ffi::IntoStringError {
     }
 }
 
+#[cfg(bootstrap)]
 impl<'a> dyn Error + 'a {
     /// Request a reference of type `T` as context about this error.
     #[unstable(feature = "error_generic_member_access", issue = "99301")]
@@ -872,6 +792,7 @@ impl<'a> dyn Error + 'a {
 }
 
 // Copied from `any.rs`.
+#[cfg(bootstrap)]
 impl dyn Error + 'static {
     /// Returns `true` if the inner type is the same as `T`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -912,6 +833,7 @@ impl dyn Error + 'static {
     }
 }
 
+#[cfg(bootstrap)]
 impl dyn Error + 'static + Send {
     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -947,6 +869,7 @@ impl dyn Error + 'static + Send {
     }
 }
 
+#[cfg(bootstrap)]
 impl dyn Error + 'static + Send + Sync {
     /// Forwards to the method defined on the type `dyn Error`.
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -982,6 +905,7 @@ impl dyn Error + 'static + Send + Sync {
     }
 }
 
+#[cfg(bootstrap)]
 impl dyn Error {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -1041,7 +965,7 @@ impl dyn Error {
     /// // let err : Box<Error> = b.into(); // or
     /// let err = &b as &(dyn Error);
     ///
-    /// let mut iter = err.chain();
+    /// let mut iter = err.sources();
     ///
     /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
     /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
@@ -1050,8 +974,19 @@ impl dyn Error {
     /// ```
     #[unstable(feature = "error_iter", issue = "58520")]
     #[inline]
-    pub fn chain(&self) -> Chain<'_> {
-        Chain { current: Some(self) }
+    pub fn sources(&self) -> Sources<'_> {
+        // You may think this method would be better in the Error trait, and you'd be right.
+        // Unfortunately that doesn't work, not because of the object safety rules but because we
+        // save a reference to self in Sources below as a trait object. If this method was
+        // declared in Error, then self would have the type &T where T is some concrete type which
+        // implements Error. We would need to coerce self to have type &dyn Error, but that requires
+        // that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
+        // since that would forbid Error trait objects, and we can't put that bound on the method
+        // because that means the method can't be called on trait objects (we'd also need the
+        // 'static bound, but that isn't allowed because methods with bounds on Self other than
+        // Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.
+
+        Sources { current: Some(self) }
     }
 }
 
@@ -1061,12 +996,14 @@ impl dyn Error {
 /// its sources, use `skip(1)`.
 #[unstable(feature = "error_iter", issue = "58520")]
 #[derive(Clone, Debug)]
-pub struct Chain<'a> {
+#[cfg(bootstrap)]
+pub struct Sources<'a> {
     current: Option<&'a (dyn Error + 'static)>,
 }
 
+#[cfg(bootstrap)]
 #[unstable(feature = "error_iter", issue = "58520")]
-impl<'a> Iterator for Chain<'a> {
+impl<'a> Iterator for Sources<'a> {
     type Item = &'a (dyn Error + 'static);
 
     fn next(&mut self) -> Option<Self::Item> {
@@ -1076,6 +1013,7 @@ impl<'a> Iterator for Chain<'a> {
     }
 }
 
+#[cfg(bootstrap)]
 impl dyn Error + Send {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -1089,6 +1027,7 @@ impl dyn Error + Send {
     }
 }
 
+#[cfg(bootstrap)]
 impl dyn Error + Send + Sync {
     #[inline]
     #[stable(feature = "error_downcast", since = "1.3.0")]
@@ -1104,8 +1043,8 @@ impl dyn Error + Send + Sync {
 
 /// An error reporter that prints an error and its sources.
 ///
-/// Report also exposes configuration options for formatting the error chain, either entirely on a
-/// single line, or in multi-line format with each cause in the error chain on a new line.
+/// Report also exposes configuration options for formatting the error sources, either entirely on a
+/// single line, or in multi-line format with each source on a new line.
 ///
 /// `Report` only requires that the wrapped error implement `Error`. It doesn't require that the
 /// wrapped error be `Send`, `Sync`, or `'static`.
@@ -1246,7 +1185,7 @@ impl dyn Error + Send + Sync {
 /// #     Err(SuperError { source: SuperErrorSideKick })
 /// # }
 ///
-/// fn main() -> Result<(), Report> {
+/// fn main() -> Result<(), Report<SuperError>> {
 ///     get_super_error()?;
 ///     Ok(())
 /// }
@@ -1293,7 +1232,7 @@ impl dyn Error + Send + Sync {
 /// #     Err(SuperError { source: SuperErrorSideKick })
 /// # }
 ///
-/// fn main() -> Result<(), Report> {
+/// fn main() -> Result<(), Report<SuperError>> {
 ///     get_super_error()
 ///         .map_err(Report::from)
 ///         .map_err(|r| r.pretty(true).show_backtrace(true))?;
@@ -1450,11 +1389,10 @@ impl<E> Report<E> {
     ///
     /// **Note**: Report will search for the first `Backtrace` it can find starting from the
     /// outermost error. In this example it will display the backtrace from the second error in the
-    /// chain, `SuperErrorSideKick`.
+    /// sources, `SuperErrorSideKick`.
     ///
     /// ```rust
     /// #![feature(error_reporter)]
-    /// #![feature(backtrace)]
     /// #![feature(provide_any)]
     /// #![feature(error_generic_member_access)]
     /// # use std::error::Error;
@@ -1489,9 +1427,8 @@ impl<E> Report<E> {
     /// }
     ///
     /// impl Error for SuperErrorSideKick {
-    ///     fn provide<'a>(&'a self, req: &mut Demand<'a>) {
-    ///         req
-    ///             .provide_ref::<Backtrace>(&self.backtrace);
+    ///     fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+    ///         demand.provide_ref::<Backtrace>(&self.backtrace);
     ///     }
     /// }
     ///
@@ -1548,7 +1485,7 @@ where
         let backtrace = backtrace.or_else(|| {
             self.error
                 .source()
-                .map(|source| source.chain().find_map(|source| source.request_ref()))
+                .map(|source| source.sources().find_map(|source| source.request_ref()))
                 .flatten()
         });
         backtrace
@@ -1559,7 +1496,7 @@ where
     fn fmt_singleline(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}", self.error)?;
 
-        let sources = self.error.source().into_iter().flat_map(<dyn Error>::chain);
+        let sources = self.error.source().into_iter().flat_map(<dyn Error>::sources);
 
         for cause in sources {
             write!(f, ": {cause}")?;
@@ -1580,73 +1517,7 @@ where
 
             let multiple = cause.source().is_some();
 
-            for (ind, error) in cause.chain().enumerate() {
-                writeln!(f)?;
-                let mut indented = Indented { inner: f };
-                if multiple {
-                    write!(indented, "{ind: >4}: {error}")?;
-                } else {
-                    write!(indented, "      {error}")?;
-                }
-            }
-        }
-
-        if self.show_backtrace {
-            let backtrace = self.backtrace();
-
-            if let Some(backtrace) = backtrace {
-                let backtrace = backtrace.to_string();
-
-                f.write_str("\n\nStack backtrace:\n")?;
-                f.write_str(backtrace.trim_end())?;
-            }
-        }
-
-        Ok(())
-    }
-}
-
-impl Report<Box<dyn Error>> {
-    fn backtrace(&self) -> Option<&Backtrace> {
-        // have to grab the backtrace on the first error directly since that error may not be
-        // 'static
-        let backtrace = self.error.request_ref();
-        let backtrace = backtrace.or_else(|| {
-            self.error
-                .source()
-                .map(|source| source.chain().find_map(|source| source.request_ref()))
-                .flatten()
-        });
-        backtrace
-    }
-
-    /// Format the report as a single line.
-    #[unstable(feature = "error_reporter", issue = "90172")]
-    fn fmt_singleline(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.error)?;
-
-        let sources = self.error.source().into_iter().flat_map(<dyn Error>::chain);
-
-        for cause in sources {
-            write!(f, ": {cause}")?;
-        }
-
-        Ok(())
-    }
-
-    /// Format the report as multiple lines, with each error cause on its own line.
-    #[unstable(feature = "error_reporter", issue = "90172")]
-    fn fmt_multiline(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let error = &self.error;
-
-        write!(f, "{error}")?;
-
-        if let Some(cause) = error.source() {
-            write!(f, "\n\nCaused by:")?;
-
-            let multiple = cause.source().is_some();
-
-            for (ind, error) in cause.chain().enumerate() {
+            for (ind, error) in cause.sources().enumerate() {
                 writeln!(f)?;
                 let mut indented = Indented { inner: f };
                 if multiple {
@@ -1683,17 +1554,6 @@ where
 }
 
 #[unstable(feature = "error_reporter", issue = "90172")]
-impl<'a, E> From<E> for Report<Box<dyn Error + 'a>>
-where
-    E: Error + 'a,
-{
-    fn from(error: E) -> Self {
-        let error = box error;
-        Report { error, show_backtrace: false, pretty: false }
-    }
-}
-
-#[unstable(feature = "error_reporter", issue = "90172")]
 impl<E> fmt::Display for Report<E>
 where
     E: Error,
@@ -1703,13 +1563,6 @@ where
     }
 }
 
-#[unstable(feature = "error_reporter", issue = "90172")]
-impl fmt::Display for Report<Box<dyn Error>> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if self.pretty { self.fmt_multiline(f) } else { self.fmt_singleline(f) }
-    }
-}
-
 // This type intentionally outputs the same format for `Display` and `Debug`for
 // situations where you unwrap a `Report` or return it from main.
 #[unstable(feature = "error_reporter", issue = "90172")]
diff --git a/library/std/src/f32.rs b/library/std/src/f32.rs
index 933b52b4d..3dd5b1250 100644
--- a/library/std/src/f32.rs
+++ b/library/std/src/f32.rs
@@ -1,4 +1,4 @@
-//! Constants specific to the `f32` single-precision floating point type.
+//! Constants for the `f32` single-precision floating point type.
 //!
 //! *[See also the `f32` primitive type](primitive@f32).*
 //!
diff --git a/library/std/src/f32/tests.rs b/library/std/src/f32/tests.rs
index 69fa203ff..4ec16c84a 100644
--- a/library/std/src/f32/tests.rs
+++ b/library/std/src/f32/tests.rs
@@ -299,6 +299,84 @@ fn test_is_sign_negative() {
     assert!((-f32::NAN).is_sign_negative());
 }
 
+#[allow(unused_macros)]
+macro_rules! assert_f32_biteq {
+    ($left : expr, $right : expr) => {
+        let l: &f32 = &$left;
+        let r: &f32 = &$right;
+        let lb = l.to_bits();
+        let rb = r.to_bits();
+        assert_eq!(lb, rb, "float {} ({:#x}) is not equal to {} ({:#x})", *l, lb, *r, rb);
+    };
+}
+
+// Ignore test on x87 floating point, these platforms do not guarantee NaN
+// payloads are preserved and flush denormals to zero, failing the tests.
+#[cfg(not(target_arch = "x86"))]
+#[test]
+fn test_next_up() {
+    let tiny = f32::from_bits(1);
+    let tiny_up = f32::from_bits(2);
+    let max_down = f32::from_bits(0x7f7f_fffe);
+    let largest_subnormal = f32::from_bits(0x007f_ffff);
+    let smallest_normal = f32::from_bits(0x0080_0000);
+    assert_f32_biteq!(f32::NEG_INFINITY.next_up(), f32::MIN);
+    assert_f32_biteq!(f32::MIN.next_up(), -max_down);
+    assert_f32_biteq!((-1.0 - f32::EPSILON).next_up(), -1.0);
+    assert_f32_biteq!((-smallest_normal).next_up(), -largest_subnormal);
+    assert_f32_biteq!((-tiny_up).next_up(), -tiny);
+    assert_f32_biteq!((-tiny).next_up(), -0.0f32);
+    assert_f32_biteq!((-0.0f32).next_up(), tiny);
+    assert_f32_biteq!(0.0f32.next_up(), tiny);
+    assert_f32_biteq!(tiny.next_up(), tiny_up);
+    assert_f32_biteq!(largest_subnormal.next_up(), smallest_normal);
+    assert_f32_biteq!(1.0f32.next_up(), 1.0 + f32::EPSILON);
+    assert_f32_biteq!(f32::MAX.next_up(), f32::INFINITY);
+    assert_f32_biteq!(f32::INFINITY.next_up(), f32::INFINITY);
+
+    // Check that NaNs roundtrip.
+    let nan0 = f32::NAN;
+    let nan1 = f32::from_bits(f32::NAN.to_bits() ^ 0x002a_aaaa);
+    let nan2 = f32::from_bits(f32::NAN.to_bits() ^ 0x0055_5555);
+    assert_f32_biteq!(nan0.next_up(), nan0);
+    assert_f32_biteq!(nan1.next_up(), nan1);
+    assert_f32_biteq!(nan2.next_up(), nan2);
+}
+
+// Ignore test on x87 floating point, these platforms do not guarantee NaN
+// payloads are preserved and flush denormals to zero, failing the tests.
+#[cfg(not(target_arch = "x86"))]
+#[test]
+fn test_next_down() {
+    let tiny = f32::from_bits(1);
+    let tiny_up = f32::from_bits(2);
+    let max_down = f32::from_bits(0x7f7f_fffe);
+    let largest_subnormal = f32::from_bits(0x007f_ffff);
+    let smallest_normal = f32::from_bits(0x0080_0000);
+    assert_f32_biteq!(f32::NEG_INFINITY.next_down(), f32::NEG_INFINITY);
+    assert_f32_biteq!(f32::MIN.next_down(), f32::NEG_INFINITY);
+    assert_f32_biteq!((-max_down).next_down(), f32::MIN);
+    assert_f32_biteq!((-1.0f32).next_down(), -1.0 - f32::EPSILON);
+    assert_f32_biteq!((-largest_subnormal).next_down(), -smallest_normal);
+    assert_f32_biteq!((-tiny).next_down(), -tiny_up);
+    assert_f32_biteq!((-0.0f32).next_down(), -tiny);
+    assert_f32_biteq!((0.0f32).next_down(), -tiny);
+    assert_f32_biteq!(tiny.next_down(), 0.0f32);
+    assert_f32_biteq!(tiny_up.next_down(), tiny);
+    assert_f32_biteq!(smallest_normal.next_down(), largest_subnormal);
+    assert_f32_biteq!((1.0 + f32::EPSILON).next_down(), 1.0f32);
+    assert_f32_biteq!(f32::MAX.next_down(), max_down);
+    assert_f32_biteq!(f32::INFINITY.next_down(), f32::MAX);
+
+    // Check that NaNs roundtrip.
+    let nan0 = f32::NAN;
+    let nan1 = f32::from_bits(f32::NAN.to_bits() ^ 0x002a_aaaa);
+    let nan2 = f32::from_bits(f32::NAN.to_bits() ^ 0x0055_5555);
+    assert_f32_biteq!(nan0.next_down(), nan0);
+    assert_f32_biteq!(nan1.next_down(), nan1);
+    assert_f32_biteq!(nan2.next_down(), nan2);
+}
+
 #[test]
 fn test_mul_add() {
     let nan: f32 = f32::NAN;
diff --git a/library/std/src/f64.rs b/library/std/src/f64.rs
index a9aa84f70..31351a879 100644
--- a/library/std/src/f64.rs
+++ b/library/std/src/f64.rs
@@ -1,4 +1,4 @@
-//! Constants specific to the `f64` double-precision floating point type.
+//! Constants for the `f64` double-precision floating point type.
 //!
 //! *[See also the `f64` primitive type](primitive@f64).*
 //!
diff --git a/library/std/src/f64/tests.rs b/library/std/src/f64/tests.rs
index 5c163cfe9..12baa68f4 100644
--- a/library/std/src/f64/tests.rs
+++ b/library/std/src/f64/tests.rs
@@ -289,6 +289,82 @@ fn test_is_sign_negative() {
     assert!((-f64::NAN).is_sign_negative());
 }
 
+#[allow(unused_macros)]
+macro_rules! assert_f64_biteq {
+    ($left : expr, $right : expr) => {
+        let l: &f64 = &$left;
+        let r: &f64 = &$right;
+        let lb = l.to_bits();
+        let rb = r.to_bits();
+        assert_eq!(lb, rb, "float {} ({:#x}) is not equal to {} ({:#x})", *l, lb, *r, rb);
+    };
+}
+
+// Ignore test on x87 floating point, these platforms do not guarantee NaN
+// payloads are preserved and flush denormals to zero, failing the tests.
+#[cfg(not(target_arch = "x86"))]
+#[test]
+fn test_next_up() {
+    let tiny = f64::from_bits(1);
+    let tiny_up = f64::from_bits(2);
+    let max_down = f64::from_bits(0x7fef_ffff_ffff_fffe);
+    let largest_subnormal = f64::from_bits(0x000f_ffff_ffff_ffff);
+    let smallest_normal = f64::from_bits(0x0010_0000_0000_0000);
+    assert_f64_biteq!(f64::NEG_INFINITY.next_up(), f64::MIN);
+    assert_f64_biteq!(f64::MIN.next_up(), -max_down);
+    assert_f64_biteq!((-1.0 - f64::EPSILON).next_up(), -1.0);
+    assert_f64_biteq!((-smallest_normal).next_up(), -largest_subnormal);
+    assert_f64_biteq!((-tiny_up).next_up(), -tiny);
+    assert_f64_biteq!((-tiny).next_up(), -0.0f64);
+    assert_f64_biteq!((-0.0f64).next_up(), tiny);
+    assert_f64_biteq!(0.0f64.next_up(), tiny);
+    assert_f64_biteq!(tiny.next_up(), tiny_up);
+    assert_f64_biteq!(largest_subnormal.next_up(), smallest_normal);
+    assert_f64_biteq!(1.0f64.next_up(), 1.0 + f64::EPSILON);
+    assert_f64_biteq!(f64::MAX.next_up(), f64::INFINITY);
+    assert_f64_biteq!(f64::INFINITY.next_up(), f64::INFINITY);
+
+    let nan0 = f64::NAN;
+    let nan1 = f64::from_bits(f64::NAN.to_bits() ^ 0x000a_aaaa_aaaa_aaaa);
+    let nan2 = f64::from_bits(f64::NAN.to_bits() ^ 0x0005_5555_5555_5555);
+    assert_f64_biteq!(nan0.next_up(), nan0);
+    assert_f64_biteq!(nan1.next_up(), nan1);
+    assert_f64_biteq!(nan2.next_up(), nan2);
+}
+
+// Ignore test on x87 floating point, these platforms do not guarantee NaN
+// payloads are preserved and flush denormals to zero, failing the tests.
+#[cfg(not(target_arch = "x86"))]
+#[test]
+fn test_next_down() {
+    let tiny = f64::from_bits(1);
+    let tiny_up = f64::from_bits(2);
+    let max_down = f64::from_bits(0x7fef_ffff_ffff_fffe);
+    let largest_subnormal = f64::from_bits(0x000f_ffff_ffff_ffff);
+    let smallest_normal = f64::from_bits(0x0010_0000_0000_0000);
+    assert_f64_biteq!(f64::NEG_INFINITY.next_down(), f64::NEG_INFINITY);
+    assert_f64_biteq!(f64::MIN.next_down(), f64::NEG_INFINITY);
+    assert_f64_biteq!((-max_down).next_down(), f64::MIN);
+    assert_f64_biteq!((-1.0f64).next_down(), -1.0 - f64::EPSILON);
+    assert_f64_biteq!((-largest_subnormal).next_down(), -smallest_normal);
+    assert_f64_biteq!((-tiny).next_down(), -tiny_up);
+    assert_f64_biteq!((-0.0f64).next_down(), -tiny);
+    assert_f64_biteq!((0.0f64).next_down(), -tiny);
+    assert_f64_biteq!(tiny.next_down(), 0.0f64);
+    assert_f64_biteq!(tiny_up.next_down(), tiny);
+    assert_f64_biteq!(smallest_normal.next_down(), largest_subnormal);
+    assert_f64_biteq!((1.0 + f64::EPSILON).next_down(), 1.0f64);
+    assert_f64_biteq!(f64::MAX.next_down(), max_down);
+    assert_f64_biteq!(f64::INFINITY.next_down(), f64::MAX);
+
+    let nan0 = f64::NAN;
+    let nan1 = f64::from_bits(f64::NAN.to_bits() ^ 0x000a_aaaa_aaaa_aaaa);
+    let nan2 = f64::from_bits(f64::NAN.to_bits() ^ 0x0005_5555_5555_5555);
+    assert_f64_biteq!(nan0.next_down(), nan0);
+    assert_f64_biteq!(nan1.next_down(), nan1);
+    assert_f64_biteq!(nan2.next_down(), nan2);
+}
+
 #[test]
 fn test_mul_add() {
     let nan: f64 = f64::NAN;
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index a0a5c003d..80ed34157 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -290,7 +290,8 @@ impl OsString {
     /// in the given `OsString`. The string may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
     /// greater than or equal to `self.len() + additional` if it returns `Ok(())`.
-    /// Does nothing if capacity is already sufficient.
+    /// Does nothing if capacity is already sufficient. This method preserves
+    /// the contents even if an error occurs.
     ///
     /// See the main `OsString` documentation information about encoding and capacity units.
     ///
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index c8e131b6e..c6c78dc39 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -13,13 +13,13 @@ mod tests;
 
 use crate::ffi::OsString;
 use crate::fmt;
-use crate::io::{self, IoSlice, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, Write};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
 use crate::path::{Path, PathBuf};
 use crate::sys::fs as fs_imp;
 use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 use crate::time::SystemTime;
 
-/// A reference to an open file on the filesystem.
+/// An object providing access to an open file on the filesystem.
 ///
 /// An instance of a `File` can be read and/or written depending on what options
 /// it was opened with. Files also implement [`Seek`] to alter the logical cursor
@@ -377,6 +377,35 @@ impl File {
         OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref())
     }
 
+    /// Creates a new file in read-write mode; error if the file exists.
+    ///
+    /// This function will create a file if it does not exist, or return an error if it does. This
+    /// way, if the call succeeds, the file returned is guaranteed to be new.
+    ///
+    /// This option is useful because it is atomic. Otherwise between checking whether a file
+    /// exists and creating a new one, the file may have been created by another process (a TOCTOU
+    /// race condition / attack).
+    ///
+    /// This can also be written using
+    /// `File::options().read(true).write(true).create_new(true).open(...)`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(file_create_new)]
+    ///
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut f = File::create_new("foo.txt")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "file_create_new", issue = "none")]
+    pub fn create_new<P: AsRef<Path>>(path: P) -> io::Result<File> {
+        OpenOptions::new().read(true).write(true).create_new(true).open(path.as_ref())
+    }
+
     /// Returns a new OpenOptions object.
     ///
     /// This function returns a new OpenOptions object that you can use to
@@ -703,8 +732,8 @@ impl Read for File {
         self.inner.read_vectored(bufs)
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        self.inner.read_buf(buf)
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.inner.read_buf(cursor)
     }
 
     #[inline]
@@ -755,8 +784,8 @@ impl Read for &File {
         self.inner.read(buf)
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        self.inner.read_buf(buf)
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.inner.read_buf(cursor)
     }
 
     fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
diff --git a/library/std/src/io/buffered/bufreader.rs b/library/std/src/io/buffered/bufreader.rs
index f7fbaa9c2..4f339a18a 100644
--- a/library/std/src/io/buffered/bufreader.rs
+++ b/library/std/src/io/buffered/bufreader.rs
@@ -2,7 +2,7 @@ mod buffer;
 
 use crate::fmt;
 use crate::io::{
-    self, BufRead, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, SizeHint, DEFAULT_BUF_SIZE,
+    self, BorrowedCursor, BufRead, IoSliceMut, Read, Seek, SeekFrom, SizeHint, DEFAULT_BUF_SIZE,
 };
 use buffer::Buffer;
 
@@ -224,6 +224,14 @@ impl<R> BufReader<R> {
     }
 }
 
+// This is only used by a test which asserts that the initialization-tracking is correct.
+#[cfg(test)]
+impl<R> BufReader<R> {
+    pub fn initialized(&self) -> usize {
+        self.buf.initialized()
+    }
+}
+
 impl<R: Seek> BufReader<R> {
     /// Seeks relative to the current position. If the new position lies within the buffer,
     /// the buffer will not be flushed, allowing for more efficient seeks.
@@ -266,21 +274,21 @@ impl<R: Read> Read for BufReader<R> {
         Ok(nread)
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
         // If we don't have any buffered data and we're doing a massive read
         // (larger than our internal buffer), bypass our internal buffer
         // entirely.
-        if self.buf.pos() == self.buf.filled() && buf.remaining() >= self.capacity() {
+        if self.buf.pos() == self.buf.filled() && cursor.capacity() >= self.capacity() {
             self.discard_buffer();
-            return self.inner.read_buf(buf);
+            return self.inner.read_buf(cursor);
         }
 
-        let prev = buf.filled_len();
+        let prev = cursor.written();
 
         let mut rem = self.fill_buf()?;
-        rem.read_buf(buf)?;
+        rem.read_buf(cursor.reborrow())?;
 
-        self.consume(buf.filled_len() - prev); //slice impl of read_buf known to never unfill buf
+        self.consume(cursor.written() - prev); //slice impl of read_buf known to never unfill buf
 
         Ok(())
     }
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 8ae01f3b0..e9e29d60c 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -9,7 +9,7 @@
 /// that user code which wants to do reads from a `BufReader` via `buffer` + `consume` can do so
 /// without encountering any runtime bounds checks.
 use crate::cmp;
-use crate::io::{self, Read, ReadBuf};
+use crate::io::{self, BorrowedBuf, Read};
 use crate::mem::MaybeUninit;
 
 pub struct Buffer {
@@ -20,13 +20,19 @@ pub struct Buffer {
     // Each call to `fill_buf` sets `filled` to indicate how many bytes at the start of `buf` are
     // initialized with bytes from a read.
     filled: usize,
+    // This is the max number of bytes returned across all `fill_buf` calls. We track this so that we
+    // can accurately tell `read_buf` how many bytes of buf are initialized, to bypass as much of its
+    // defensive initialization as possible. Note that while this often the same as `filled`, it
+    // doesn't need to be. Calls to `fill_buf` are not required to actually fill the buffer, and
+    // omitting this is a huge perf regression for `Read` impls that do not.
+    initialized: usize,
 }
 
 impl Buffer {
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
         let buf = Box::new_uninit_slice(capacity);
-        Self { buf, pos: 0, filled: 0 }
+        Self { buf, pos: 0, filled: 0, initialized: 0 }
     }
 
     #[inline]
@@ -51,6 +57,12 @@ impl Buffer {
         self.pos
     }
 
+    // This is only used by a test which asserts that the initialization-tracking is correct.
+    #[cfg(test)]
+    pub fn initialized(&self) -> usize {
+        self.initialized
+    }
+
     #[inline]
     pub fn discard_buffer(&mut self) {
         self.pos = 0;
@@ -93,12 +105,17 @@ impl Buffer {
         if self.pos >= self.filled {
             debug_assert!(self.pos == self.filled);
 
-            let mut readbuf = ReadBuf::uninit(&mut self.buf);
+            let mut buf = BorrowedBuf::from(&mut *self.buf);
+            // SAFETY: `self.filled` bytes will always have been initialized.
+            unsafe {
+                buf.set_init(self.initialized);
+            }
 
-            reader.read_buf(&mut readbuf)?;
+            reader.read_buf(buf.unfilled())?;
 
-            self.filled = readbuf.filled_len();
             self.pos = 0;
+            self.filled = buf.len();
+            self.initialized = buf.init_len();
         }
         Ok(self.buffer())
     }
diff --git a/library/std/src/io/buffered/tests.rs b/library/std/src/io/buffered/tests.rs
index fe45b1326..f4e688eb9 100644
--- a/library/std/src/io/buffered/tests.rs
+++ b/library/std/src/io/buffered/tests.rs
@@ -1,5 +1,7 @@
 use crate::io::prelude::*;
-use crate::io::{self, BufReader, BufWriter, ErrorKind, IoSlice, LineWriter, ReadBuf, SeekFrom};
+use crate::io::{
+    self, BorrowedBuf, BufReader, BufWriter, ErrorKind, IoSlice, LineWriter, SeekFrom,
+};
 use crate::mem::MaybeUninit;
 use crate::panic;
 use crate::sync::atomic::{AtomicUsize, Ordering};
@@ -61,48 +63,48 @@ fn test_buffered_reader_read_buf() {
     let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
     let mut reader = BufReader::with_capacity(2, inner);
 
-    let mut buf = [MaybeUninit::uninit(); 3];
-    let mut buf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 3];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
     assert_eq!(buf.filled(), [5, 6, 7]);
     assert_eq!(reader.buffer(), []);
 
-    let mut buf = [MaybeUninit::uninit(); 2];
-    let mut buf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 2];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
     assert_eq!(buf.filled(), [0, 1]);
     assert_eq!(reader.buffer(), []);
 
-    let mut buf = [MaybeUninit::uninit(); 1];
-    let mut buf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
     assert_eq!(buf.filled(), [2]);
     assert_eq!(reader.buffer(), [3]);
 
-    let mut buf = [MaybeUninit::uninit(); 3];
-    let mut buf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 3];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
     assert_eq!(buf.filled(), [3]);
     assert_eq!(reader.buffer(), []);
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
     assert_eq!(buf.filled(), [3, 4]);
     assert_eq!(reader.buffer(), []);
 
     buf.clear();
 
-    reader.read_buf(&mut buf).unwrap();
+    reader.read_buf(buf.unfilled()).unwrap();
 
-    assert_eq!(buf.filled_len(), 0);
+    assert!(buf.filled().is_empty());
 }
 
 #[test]
@@ -1037,3 +1039,27 @@ fn single_formatted_write() {
     writeln!(&mut writer, "{}, {}!", "hello", "world").unwrap();
     assert_eq!(writer.get_ref().events, [RecordedEvent::Write("hello, world!\n".to_string())]);
 }
+
+#[test]
+fn bufreader_full_initialize() {
+    struct OneByteReader;
+    impl Read for OneByteReader {
+        fn read(&mut self, buf: &mut [u8]) -> crate::io::Result<usize> {
+            if buf.len() > 0 {
+                buf[0] = 0;
+                Ok(1)
+            } else {
+                Ok(0)
+            }
+        }
+    }
+    let mut reader = BufReader::new(OneByteReader);
+    // Nothing is initialized yet.
+    assert_eq!(reader.initialized(), 0);
+
+    let buf = reader.fill_buf().unwrap();
+    // We read one byte...
+    assert_eq!(buf.len(), 1);
+    // But we initialized the whole buffer!
+    assert_eq!(reader.initialized(), reader.capacity());
+}
diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
index 1a10245e4..38b98afff 100644
--- a/library/std/src/io/copy.rs
+++ b/library/std/src/io/copy.rs
@@ -1,4 +1,4 @@
-use super::{BufWriter, ErrorKind, Read, ReadBuf, Result, Write, DEFAULT_BUF_SIZE};
+use super::{BorrowedBuf, BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
 use crate::mem::MaybeUninit;
 
 /// Copies the entire contents of a reader into a writer.
@@ -97,37 +97,39 @@ impl<I: Write> BufferedCopySpec for BufWriter<I> {
 
         loop {
             let buf = writer.buffer_mut();
-            let mut read_buf = ReadBuf::uninit(buf.spare_capacity_mut());
+            let mut read_buf: BorrowedBuf<'_> = buf.spare_capacity_mut().into();
 
-            // SAFETY: init is either 0 or the initialized_len of the previous iteration
             unsafe {
-                read_buf.assume_init(init);
+                // SAFETY: init is either 0 or the init_len from the previous iteration.
+                read_buf.set_init(init);
             }
 
             if read_buf.capacity() >= DEFAULT_BUF_SIZE {
-                match reader.read_buf(&mut read_buf) {
+                let mut cursor = read_buf.unfilled();
+                match reader.read_buf(cursor.reborrow()) {
                     Ok(()) => {
-                        let bytes_read = read_buf.filled_len();
+                        let bytes_read = cursor.written();
 
                         if bytes_read == 0 {
                             return Ok(len);
                         }
 
-                        init = read_buf.initialized_len() - bytes_read;
+                        init = read_buf.init_len() - bytes_read;
+                        len += bytes_read as u64;
 
-                        // SAFETY: ReadBuf guarantees all of its filled bytes are init
+                        // SAFETY: BorrowedBuf guarantees all of its filled bytes are init
                         unsafe { buf.set_len(buf.len() + bytes_read) };
-                        len += bytes_read as u64;
+
                         // Read again if the buffer still has enough capacity, as BufWriter itself would do
                         // This will occur if the reader returns short reads
-                        continue;
                     }
-                    Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                    Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                     Err(e) => return Err(e),
                 }
+            } else {
+                writer.flush_buf()?;
+                init = 0;
             }
-
-            writer.flush_buf()?;
         }
     }
 }
@@ -136,13 +138,13 @@ fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
     reader: &mut R,
     writer: &mut W,
 ) -> Result<u64> {
-    let mut buf = [MaybeUninit::uninit(); DEFAULT_BUF_SIZE];
-    let mut buf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); DEFAULT_BUF_SIZE];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
     let mut len = 0;
 
     loop {
-        match reader.read_buf(&mut buf) {
+        match reader.read_buf(buf.unfilled()) {
             Ok(()) => {}
             Err(e) if e.kind() == ErrorKind::Interrupted => continue,
             Err(e) => return Err(e),
diff --git a/library/std/src/io/cursor.rs b/library/std/src/io/cursor.rs
index f3fbfc447..d98ab021c 100644
--- a/library/std/src/io/cursor.rs
+++ b/library/std/src/io/cursor.rs
@@ -5,7 +5,7 @@ use crate::io::prelude::*;
 
 use crate::alloc::Allocator;
 use crate::cmp;
-use crate::io::{self, ErrorKind, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
 
 /// A `Cursor` wraps an in-memory buffer and provides it with a
 /// [`Seek`] implementation.
@@ -323,12 +323,12 @@ where
         Ok(n)
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        let prev_filled = buf.filled_len();
+    fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        let prev_written = cursor.written();
 
-        Read::read_buf(&mut self.fill_buf()?, buf)?;
+        Read::read_buf(&mut self.fill_buf()?, cursor.reborrow())?;
 
-        self.pos += (buf.filled_len() - prev_filled) as u64;
+        self.pos += (cursor.written() - prev_written) as u64;
 
         Ok(())
     }
diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
index ff7fdcae1..29b09fcc5 100644
--- a/library/std/src/io/error.rs
+++ b/library/std/src/io/error.rs
@@ -76,6 +76,15 @@ impl fmt::Debug for Error {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<alloc::ffi::NulError> for Error {
+    /// Converts a [`alloc::ffi::NulError`] into a [`Error`].
+    fn from(_: alloc::ffi::NulError) -> Error {
+        const_io_error!(ErrorKind::InvalidInput, "data provided contains a nul byte")
+    }
+}
+
 // Only derive debug in tests, to make sure it
 // doesn't accidentally get printed.
 #[cfg_attr(test, derive(Debug))]
@@ -564,6 +573,8 @@ impl Error {
     /// println!("last OS error: {os_error:?}");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[doc(alias = "GetLastError")]
+    #[doc(alias = "errno")]
     #[must_use]
     #[inline]
     pub fn last_os_error() -> Error {
diff --git a/library/std/src/io/error/repr_bitpacked.rs b/library/std/src/io/error/repr_bitpacked.rs
index 292bf4826..781ae03ad 100644
--- a/library/std/src/io/error/repr_bitpacked.rs
+++ b/library/std/src/io/error/repr_bitpacked.rs
@@ -269,10 +269,10 @@ where
         }
         TAG_SIMPLE_MESSAGE => ErrorData::SimpleMessage(&*ptr.cast::<SimpleMessage>().as_ptr()),
         TAG_CUSTOM => {
-            // It would be correct for us to use `ptr::sub` here (see the
+            // It would be correct for us to use `ptr::byte_sub` here (see the
             // comment above the `wrapping_add` call in `new_custom` for why),
             // but it isn't clear that it makes a difference, so we don't.
-            let custom = ptr.as_ptr().cast::<u8>().wrapping_sub(TAG_CUSTOM).cast::<Custom>();
+            let custom = ptr.as_ptr().wrapping_byte_sub(TAG_CUSTOM).cast::<Custom>();
             ErrorData::Custom(make_custom(custom))
         }
         _ => {
diff --git a/library/std/src/io/impls.rs b/library/std/src/io/impls.rs
index 950725473..e5048dcc8 100644
--- a/library/std/src/io/impls.rs
+++ b/library/std/src/io/impls.rs
@@ -6,7 +6,7 @@ use crate::cmp;
 use crate::collections::VecDeque;
 use crate::fmt;
 use crate::io::{
-    self, BufRead, ErrorKind, IoSlice, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, Write,
+    self, BorrowedCursor, BufRead, ErrorKind, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write,
 };
 use crate::mem;
 
@@ -21,8 +21,8 @@ impl<R: Read + ?Sized> Read for &mut R {
     }
 
     #[inline]
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        (**self).read_buf(buf)
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (**self).read_buf(cursor)
     }
 
     #[inline]
@@ -125,8 +125,8 @@ impl<R: Read + ?Sized> Read for Box<R> {
     }
 
     #[inline]
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        (**self).read_buf(buf)
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (**self).read_buf(cursor)
     }
 
     #[inline]
@@ -249,11 +249,11 @@ impl Read for &[u8] {
     }
 
     #[inline]
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        let amt = cmp::min(buf.remaining(), self.len());
+    fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        let amt = cmp::min(cursor.capacity(), self.len());
         let (a, b) = self.split_at(amt);
 
-        buf.append(a);
+        cursor.append(a);
 
         *self = b;
         Ok(())
@@ -427,10 +427,10 @@ impl<A: Allocator> Read for VecDeque<u8, A> {
     }
 
     #[inline]
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
         let (ref mut front, _) = self.as_slices();
-        let n = cmp::min(buf.remaining(), front.len());
-        Read::read_buf(front, buf)?;
+        let n = cmp::min(cursor.capacity(), front.len());
+        Read::read_buf(front, cursor)?;
         self.drain(..n);
         Ok(())
     }
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 96addbd1a..eeace2c43 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -278,7 +278,7 @@ pub use self::{
 };
 
 #[unstable(feature = "read_buf", issue = "78485")]
-pub use self::readbuf::ReadBuf;
+pub use self::readbuf::{BorrowedBuf, BorrowedCursor};
 pub(crate) use error::const_io_error;
 
 mod buffered;
@@ -362,29 +362,30 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>
             buf.reserve(32); // buf is full, need more space
         }
 
-        let mut read_buf = ReadBuf::uninit(buf.spare_capacity_mut());
+        let mut read_buf: BorrowedBuf<'_> = buf.spare_capacity_mut().into();
 
         // SAFETY: These bytes were initialized but not filled in the previous loop
         unsafe {
-            read_buf.assume_init(initialized);
+            read_buf.set_init(initialized);
         }
 
-        match r.read_buf(&mut read_buf) {
+        let mut cursor = read_buf.unfilled();
+        match r.read_buf(cursor.reborrow()) {
             Ok(()) => {}
             Err(e) if e.kind() == ErrorKind::Interrupted => continue,
             Err(e) => return Err(e),
         }
 
-        if read_buf.filled_len() == 0 {
+        if cursor.written() == 0 {
             return Ok(buf.len() - start_len);
         }
 
         // store how much was initialized but not filled
-        initialized = read_buf.initialized_len() - read_buf.filled_len();
-        let new_len = read_buf.filled_len() + buf.len();
+        initialized = cursor.init_ref().len();
 
-        // SAFETY: ReadBuf's invariants mean this much memory is init
+        // SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
         unsafe {
+            let new_len = read_buf.filled().len() + buf.len();
             buf.set_len(new_len);
         }
 
@@ -461,12 +462,15 @@ pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [
     }
 }
 
-pub(crate) fn default_read_buf<F>(read: F, buf: &mut ReadBuf<'_>) -> Result<()>
+pub(crate) fn default_read_buf<F>(read: F, mut cursor: BorrowedCursor<'_>) -> Result<()>
 where
     F: FnOnce(&mut [u8]) -> Result<usize>,
 {
-    let n = read(buf.initialize_unfilled())?;
-    buf.add_filled(n);
+    let n = read(cursor.ensure_init().init_mut())?;
+    unsafe {
+        // SAFETY: we initialised using `ensure_init` so there is no uninit data to advance to.
+        cursor.advance(n);
+    }
     Ok(())
 }
 
@@ -803,30 +807,30 @@ pub trait Read {
 
     /// Pull some bytes from this source into the specified buffer.
     ///
-    /// This is equivalent to the [`read`](Read::read) method, except that it is passed a [`ReadBuf`] rather than `[u8]` to allow use
+    /// This is equivalent to the [`read`](Read::read) method, except that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
     /// with uninitialized buffers. The new data will be appended to any existing contents of `buf`.
     ///
     /// The default implementation delegates to `read`.
     #[unstable(feature = "read_buf", issue = "78485")]
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
+    fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {
         default_read_buf(|b| self.read(b), buf)
     }
 
-    /// Read the exact number of bytes required to fill `buf`.
+    /// Read the exact number of bytes required to fill `cursor`.
     ///
-    /// This is equivalent to the [`read_exact`](Read::read_exact) method, except that it is passed a [`ReadBuf`] rather than `[u8]` to
+    /// This is equivalent to the [`read_exact`](Read::read_exact) method, except that it is passed a [`BorrowedCursor`] rather than `[u8]` to
     /// allow use with uninitialized buffers.
     #[unstable(feature = "read_buf", issue = "78485")]
-    fn read_buf_exact(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
-        while buf.remaining() > 0 {
-            let prev_filled = buf.filled().len();
-            match self.read_buf(buf) {
+    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> Result<()> {
+        while cursor.capacity() > 0 {
+            let prev_written = cursor.written();
+            match self.read_buf(cursor.reborrow()) {
                 Ok(()) => {}
                 Err(e) if e.kind() == ErrorKind::Interrupted => continue,
                 Err(e) => return Err(e),
             }
 
-            if buf.filled().len() == prev_filled {
+            if cursor.written() == prev_written {
                 return Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill buffer"));
             }
         }
@@ -883,6 +887,10 @@ pub trait Read {
     /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
     /// otherwise. EOF is mapped to returning [`None`] from this iterator.
     ///
+    /// The default implementation calls `read` for each byte,
+    /// which can be very inefficient for data that's not in memory,
+    /// such as [`File`]. Consider using a [`BufReader`] in such cases.
+    ///
     /// # Examples
     ///
     /// [`File`]s implement `Read`:
@@ -895,10 +903,11 @@ pub trait Read {
     /// ```no_run
     /// use std::io;
     /// use std::io::prelude::*;
+    /// use std::io::BufReader;
     /// use std::fs::File;
     ///
     /// fn main() -> io::Result<()> {
-    ///     let f = File::open("foo.txt")?;
+    ///     let f = BufReader::new(File::open("foo.txt")?);
     ///
     ///     for byte in f.bytes() {
     ///         println!("{}", byte.unwrap());
@@ -1028,8 +1037,6 @@ pub trait Read {
 /// # Examples
 ///
 /// ```no_run
-/// #![feature(io_read_to_string)]
-///
 /// # use std::io;
 /// fn main() -> io::Result<()> {
 ///     let stdin = io::read_to_string(io::stdin())?;
@@ -1038,7 +1045,7 @@ pub trait Read {
 ///     Ok(())
 /// }
 /// ```
-#[unstable(feature = "io_read_to_string", issue = "80218")]
+#[stable(feature = "io_read_to_string", since = "1.65.0")]
 pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
     let mut buf = String::new();
     reader.read_to_string(&mut buf)?;
@@ -2582,50 +2589,48 @@ impl<T: Read> Read for Take<T> {
         Ok(n)
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
+    fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
         // Don't call into inner reader at all at EOF because it may still block
         if self.limit == 0 {
             return Ok(());
         }
 
-        let prev_filled = buf.filled_len();
-
-        if self.limit <= buf.remaining() as u64 {
+        if self.limit <= buf.capacity() as u64 {
             // if we just use an as cast to convert, limit may wrap around on a 32 bit target
             let limit = cmp::min(self.limit, usize::MAX as u64) as usize;
 
-            let extra_init = cmp::min(limit as usize, buf.initialized_len() - buf.filled_len());
+            let extra_init = cmp::min(limit as usize, buf.init_ref().len());
 
             // SAFETY: no uninit data is written to ibuf
-            let ibuf = unsafe { &mut buf.unfilled_mut()[..limit] };
+            let ibuf = unsafe { &mut buf.as_mut()[..limit] };
 
-            let mut sliced_buf = ReadBuf::uninit(ibuf);
+            let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();
 
             // SAFETY: extra_init bytes of ibuf are known to be initialized
             unsafe {
-                sliced_buf.assume_init(extra_init);
+                sliced_buf.set_init(extra_init);
             }
 
-            self.inner.read_buf(&mut sliced_buf)?;
+            let mut cursor = sliced_buf.unfilled();
+            self.inner.read_buf(cursor.reborrow())?;
 
-            let new_init = sliced_buf.initialized_len();
-            let filled = sliced_buf.filled_len();
+            let new_init = cursor.init_ref().len();
+            let filled = sliced_buf.len();
 
-            // sliced_buf / ibuf must drop here
+            // cursor / sliced_buf / ibuf must drop here
 
-            // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
             unsafe {
-                buf.assume_init(new_init);
+                // SAFETY: filled bytes have been filled and therefore initialized
+                buf.advance(filled);
+                // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
+                buf.set_init(new_init);
             }
 
-            buf.add_filled(filled);
-
             self.limit -= filled as u64;
         } else {
-            self.inner.read_buf(buf)?;
-
-            //inner may unfill
-            self.limit -= buf.filled_len().saturating_sub(prev_filled) as u64;
+            let written = buf.written();
+            self.inner.read_buf(buf.reborrow())?;
+            self.limit -= (buf.written() - written) as u64;
         }
 
         Ok(())
diff --git a/library/std/src/io/readbuf.rs b/library/std/src/io/readbuf.rs
index 78d1113f8..b1a84095f 100644
--- a/library/std/src/io/readbuf.rs
+++ b/library/std/src/io/readbuf.rs
@@ -5,9 +5,10 @@ mod tests;
 
 use crate::cmp;
 use crate::fmt::{self, Debug, Formatter};
-use crate::mem::MaybeUninit;
+use crate::io::{Result, Write};
+use crate::mem::{self, MaybeUninit};
 
-/// A wrapper around a byte buffer that is incrementally filled and initialized.
+/// A borrowed byte buffer which is incrementally filled and initialized.
 ///
 /// This type is a sort of "double cursor". It tracks three regions in the buffer: a region at the beginning of the
 /// buffer that has been logically filled with data, a region that has been initialized at some point but not yet
@@ -20,230 +21,286 @@ use crate::mem::MaybeUninit;
 /// [ filled |         unfilled         ]
 /// [    initialized    | uninitialized ]
 /// ```
-pub struct ReadBuf<'a> {
-    buf: &'a mut [MaybeUninit<u8>],
+///
+/// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique reference
+/// (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear` or `set_init`), but cannot be
+/// directly written. To write into the buffer, use `unfilled` to create a `BorrowedCursor`. The cursor
+/// has write-only access to the unfilled portion of the buffer (you can think of it as a
+/// write-only iterator).
+///
+/// The lifetime `'data` is a bound on the lifetime of the underlying data.
+pub struct BorrowedBuf<'data> {
+    /// The buffer's underlying data.
+    buf: &'data mut [MaybeUninit<u8>],
+    /// The length of `self.buf` which is known to be filled.
     filled: usize,
-    initialized: usize,
+    /// The length of `self.buf` which is known to be initialized.
+    init: usize,
 }
 
-impl Debug for ReadBuf<'_> {
+impl Debug for BorrowedBuf<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        f.debug_struct("ReadBuf")
-            .field("init", &self.initialized())
+        f.debug_struct("BorrowedBuf")
+            .field("init", &self.init)
             .field("filled", &self.filled)
             .field("capacity", &self.capacity())
             .finish()
     }
 }
 
-impl<'a> ReadBuf<'a> {
-    /// Creates a new `ReadBuf` from a fully initialized buffer.
+/// Create a new `BorrowedBuf` from a fully initialized slice.
+impl<'data> From<&'data mut [u8]> for BorrowedBuf<'data> {
     #[inline]
-    pub fn new(buf: &'a mut [u8]) -> ReadBuf<'a> {
-        let len = buf.len();
+    fn from(slice: &'data mut [u8]) -> BorrowedBuf<'data> {
+        let len = slice.len();
 
-        ReadBuf {
-            //SAFETY: initialized data never becoming uninitialized is an invariant of ReadBuf
-            buf: unsafe { (buf as *mut [u8]).as_uninit_slice_mut().unwrap() },
+        BorrowedBuf {
+            // SAFETY: initialized data never becoming uninitialized is an invariant of BorrowedBuf
+            buf: unsafe { (slice as *mut [u8]).as_uninit_slice_mut().unwrap() },
             filled: 0,
-            initialized: len,
+            init: len,
         }
     }
+}
 
-    /// Creates a new `ReadBuf` from a fully uninitialized buffer.
-    ///
-    /// Use `assume_init` if part of the buffer is known to be already initialized.
+/// Create a new `BorrowedBuf` from an uninitialized buffer.
+///
+/// Use `set_init` if part of the buffer is known to be already initialized.
+impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
     #[inline]
-    pub fn uninit(buf: &'a mut [MaybeUninit<u8>]) -> ReadBuf<'a> {
-        ReadBuf { buf, filled: 0, initialized: 0 }
+    fn from(buf: &'data mut [MaybeUninit<u8>]) -> BorrowedBuf<'data> {
+        BorrowedBuf { buf, filled: 0, init: 0 }
     }
+}
 
+impl<'data> BorrowedBuf<'data> {
     /// Returns the total capacity of the buffer.
     #[inline]
     pub fn capacity(&self) -> usize {
         self.buf.len()
     }
 
+    /// Returns the length of the filled part of the buffer.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.filled
+    }
+
+    /// Returns the length of the initialized part of the buffer.
+    #[inline]
+    pub fn init_len(&self) -> usize {
+        self.init
+    }
+
     /// Returns a shared reference to the filled portion of the buffer.
     #[inline]
     pub fn filled(&self) -> &[u8] {
-        //SAFETY: We only slice the filled part of the buffer, which is always valid
+        // SAFETY: We only slice the filled part of the buffer, which is always valid
         unsafe { MaybeUninit::slice_assume_init_ref(&self.buf[0..self.filled]) }
     }
 
-    /// Returns a mutable reference to the filled portion of the buffer.
+    /// Returns a cursor over the unfilled part of the buffer.
     #[inline]
-    pub fn filled_mut(&mut self) -> &mut [u8] {
-        //SAFETY: We only slice the filled part of the buffer, which is always valid
-        unsafe { MaybeUninit::slice_assume_init_mut(&mut self.buf[0..self.filled]) }
+    pub fn unfilled<'this>(&'this mut self) -> BorrowedCursor<'this> {
+        BorrowedCursor {
+            start: self.filled,
+            // SAFETY: we never assign into `BorrowedCursor::buf`, so treating its
+            // lifetime covariantly is safe.
+            buf: unsafe {
+                mem::transmute::<&'this mut BorrowedBuf<'data>, &'this mut BorrowedBuf<'this>>(self)
+            },
+        }
     }
 
-    /// Returns a shared reference to the initialized portion of the buffer.
+    /// Clears the buffer, resetting the filled region to empty.
     ///
-    /// This includes the filled portion.
+    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
     #[inline]
-    pub fn initialized(&self) -> &[u8] {
-        //SAFETY: We only slice the initialized part of the buffer, which is always valid
-        unsafe { MaybeUninit::slice_assume_init_ref(&self.buf[0..self.initialized]) }
+    pub fn clear(&mut self) -> &mut Self {
+        self.filled = 0;
+        self
     }
 
-    /// Returns a mutable reference to the initialized portion of the buffer.
+    /// Asserts that the first `n` bytes of the buffer are initialized.
     ///
-    /// This includes the filled portion.
-    #[inline]
-    pub fn initialized_mut(&mut self) -> &mut [u8] {
-        //SAFETY: We only slice the initialized part of the buffer, which is always valid
-        unsafe { MaybeUninit::slice_assume_init_mut(&mut self.buf[0..self.initialized]) }
-    }
-
-    /// Returns a mutable reference to the unfilled part of the buffer without ensuring that it has been fully
-    /// initialized.
+    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
+    /// bytes than are already known to be initialized.
     ///
     /// # Safety
     ///
-    /// The caller must not de-initialize portions of the buffer that have already been initialized.
+    /// The caller must ensure that the first `n` unfilled bytes of the buffer have already been initialized.
     #[inline]
-    pub unsafe fn unfilled_mut(&mut self) -> &mut [MaybeUninit<u8>] {
-        &mut self.buf[self.filled..]
+    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
+        self.init = cmp::max(self.init, n);
+        self
     }
+}
 
-    /// Returns a mutable reference to the uninitialized part of the buffer.
+/// A writeable view of the unfilled portion of a [`BorrowedBuf`](BorrowedBuf).
+///
+/// Provides access to the initialized and uninitialized parts of the underlying `BorrowedBuf`.
+/// Data can be written directly to the cursor by using [`append`](BorrowedCursor::append) or
+/// indirectly by getting a slice of part or all of the cursor and writing into the slice. In the
+/// indirect case, the caller must call [`advance`](BorrowedCursor::advance) after writing to inform
+/// the cursor how many bytes have been written.
+///
+/// Once data is written to the cursor, it becomes part of the filled portion of the underlying
+/// `BorrowedBuf` and can no longer be accessed or re-written by the cursor. I.e., the cursor tracks
+/// the unfilled part of the underlying `BorrowedBuf`.
+///
+/// The lifetime `'a` is a bound on the lifetime of the underlying buffer (which means it is a bound
+/// on the data in that buffer by transitivity).
+#[derive(Debug)]
+pub struct BorrowedCursor<'a> {
+    /// The underlying buffer.
+    // Safety invariant: we treat the type of buf as covariant in the lifetime of `BorrowedBuf` when
+    // we create a `BorrowedCursor`. This is only safe if we never replace `buf` by assigning into
+    // it, so don't do that!
+    buf: &'a mut BorrowedBuf<'a>,
+    /// The length of the filled portion of the underlying buffer at the time of the cursor's
+    /// creation.
+    start: usize,
+}
+
+impl<'a> BorrowedCursor<'a> {
+    /// Reborrow this cursor by cloning it with a smaller lifetime.
     ///
-    /// It is safe to uninitialize any of these bytes.
+    /// Since a cursor maintains unique access to its underlying buffer, the borrowed cursor is
+    /// not accessible while the new cursor exists.
     #[inline]
-    pub fn uninitialized_mut(&mut self) -> &mut [MaybeUninit<u8>] {
-        &mut self.buf[self.initialized..]
+    pub fn reborrow<'this>(&'this mut self) -> BorrowedCursor<'this> {
+        BorrowedCursor {
+            // SAFETY: we never assign into `BorrowedCursor::buf`, so treating its
+            // lifetime covariantly is safe.
+            buf: unsafe {
+                mem::transmute::<&'this mut BorrowedBuf<'a>, &'this mut BorrowedBuf<'this>>(
+                    self.buf,
+                )
+            },
+            start: self.start,
+        }
     }
 
-    /// Returns a mutable reference to the unfilled part of the buffer, ensuring it is fully initialized.
-    ///
-    /// Since `ReadBuf` tracks the region of the buffer that has been initialized, this is effectively "free" after
-    /// the first use.
+    /// Returns the available space in the cursor.
     #[inline]
-    pub fn initialize_unfilled(&mut self) -> &mut [u8] {
-        // should optimize out the assertion
-        self.initialize_unfilled_to(self.remaining())
+    pub fn capacity(&self) -> usize {
+        self.buf.capacity() - self.buf.filled
     }
 
-    /// Returns a mutable reference to the first `n` bytes of the unfilled part of the buffer, ensuring it is
-    /// fully initialized.
-    ///
-    /// # Panics
+    /// Returns the number of bytes written to this cursor since it was created from a `BorrowedBuf`.
     ///
-    /// Panics if `self.remaining()` is less than `n`.
+    /// Note that if this cursor is a reborrowed clone of another, then the count returned is the
+    /// count written via either cursor, not the count since the cursor was reborrowed.
     #[inline]
-    pub fn initialize_unfilled_to(&mut self, n: usize) -> &mut [u8] {
-        assert!(self.remaining() >= n);
-
-        let extra_init = self.initialized - self.filled;
-        // If we don't have enough initialized, do zeroing
-        if n > extra_init {
-            let uninit = n - extra_init;
-            let unfilled = &mut self.uninitialized_mut()[0..uninit];
-
-            for byte in unfilled.iter_mut() {
-                byte.write(0);
-            }
-
-            // SAFETY: we just initialized uninit bytes, and the previous bytes were already init
-            unsafe {
-                self.assume_init(n);
-            }
-        }
-
-        let filled = self.filled;
+    pub fn written(&self) -> usize {
+        self.buf.filled - self.start
+    }
 
-        &mut self.initialized_mut()[filled..filled + n]
+    /// Returns a shared reference to the initialized portion of the cursor.
+    #[inline]
+    pub fn init_ref(&self) -> &[u8] {
+        // SAFETY: We only slice the initialized part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_ref(&self.buf.buf[self.buf.filled..self.buf.init]) }
     }
 
-    /// Returns the number of bytes at the end of the slice that have not yet been filled.
+    /// Returns a mutable reference to the initialized portion of the cursor.
     #[inline]
-    pub fn remaining(&self) -> usize {
-        self.capacity() - self.filled
+    pub fn init_mut(&mut self) -> &mut [u8] {
+        // SAFETY: We only slice the initialized part of the buffer, which is always valid
+        unsafe {
+            MaybeUninit::slice_assume_init_mut(&mut self.buf.buf[self.buf.filled..self.buf.init])
+        }
     }
 
-    /// Clears the buffer, resetting the filled region to empty.
+    /// Returns a mutable reference to the uninitialized part of the cursor.
     ///
-    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
+    /// It is safe to uninitialize any of these bytes.
     #[inline]
-    pub fn clear(&mut self) -> &mut Self {
-        self.set_filled(0) // The assertion in `set_filled` is optimized out
+    pub fn uninit_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        &mut self.buf.buf[self.buf.init..]
     }
 
-    /// Increases the size of the filled region of the buffer.
-    ///
-    /// The number of initialized bytes is not changed.
+    /// Returns a mutable reference to the whole cursor.
     ///
-    /// # Panics
+    /// # Safety
     ///
-    /// Panics if the filled region of the buffer would become larger than the initialized region.
+    /// The caller must not uninitialize any bytes in the initialized portion of the cursor.
     #[inline]
-    pub fn add_filled(&mut self, n: usize) -> &mut Self {
-        self.set_filled(self.filled + n)
+    pub unsafe fn as_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        &mut self.buf.buf[self.buf.filled..]
     }
 
-    /// Sets the size of the filled region of the buffer.
+    /// Advance the cursor by asserting that `n` bytes have been filled.
     ///
-    /// The number of initialized bytes is not changed.
+    /// After advancing, the `n` bytes are no longer accessible via the cursor and can only be
+    /// accessed via the underlying buffer. I.e., the buffer's filled portion grows by `n` elements
+    /// and its unfilled portion (and the capacity of this cursor) shrinks by `n` elements.
     ///
-    /// Note that this can be used to *shrink* the filled region of the buffer in addition to growing it (for
-    /// example, by a `Read` implementation that compresses data in-place).
-    ///
-    /// # Panics
+    /// # Safety
     ///
-    /// Panics if the filled region of the buffer would become larger than the initialized region.
+    /// The caller must ensure that the first `n` bytes of the cursor have been properly
+    /// initialised.
+    #[inline]
+    pub unsafe fn advance(&mut self, n: usize) -> &mut Self {
+        self.buf.filled += n;
+        self.buf.init = cmp::max(self.buf.init, self.buf.filled);
+        self
+    }
+
+    /// Initializes all bytes in the cursor.
     #[inline]
-    pub fn set_filled(&mut self, n: usize) -> &mut Self {
-        assert!(n <= self.initialized);
+    pub fn ensure_init(&mut self) -> &mut Self {
+        for byte in self.uninit_mut() {
+            byte.write(0);
+        }
+        self.buf.init = self.buf.capacity();
 
-        self.filled = n;
         self
     }
 
-    /// Asserts that the first `n` unfilled bytes of the buffer are initialized.
+    /// Asserts that the first `n` unfilled bytes of the cursor are initialized.
     ///
-    /// `ReadBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
-    /// bytes than are already known to be initialized.
+    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when
+    /// called with fewer bytes than are already known to be initialized.
     ///
     /// # Safety
     ///
-    /// The caller must ensure that the first `n` unfilled bytes of the buffer have already been initialized.
+    /// The caller must ensure that the first `n` bytes of the buffer have already been initialized.
     #[inline]
-    pub unsafe fn assume_init(&mut self, n: usize) -> &mut Self {
-        self.initialized = cmp::max(self.initialized, self.filled + n);
+    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
+        self.buf.init = cmp::max(self.buf.init, self.buf.filled + n);
         self
     }
 
-    /// Appends data to the buffer, advancing the written position and possibly also the initialized position.
+    /// Appends data to the cursor, advancing position within its buffer.
     ///
     /// # Panics
     ///
-    /// Panics if `self.remaining()` is less than `buf.len()`.
+    /// Panics if `self.capacity()` is less than `buf.len()`.
     #[inline]
     pub fn append(&mut self, buf: &[u8]) {
-        assert!(self.remaining() >= buf.len());
+        assert!(self.capacity() >= buf.len());
 
         // SAFETY: we do not de-initialize any of the elements of the slice
         unsafe {
-            MaybeUninit::write_slice(&mut self.unfilled_mut()[..buf.len()], buf);
+            MaybeUninit::write_slice(&mut self.as_mut()[..buf.len()], buf);
         }
 
         // SAFETY: We just added the entire contents of buf to the filled section.
         unsafe {
-            self.assume_init(buf.len());
+            self.set_init(buf.len());
         }
-        self.add_filled(buf.len());
+        self.buf.filled += buf.len();
     }
+}
 
-    /// Returns the amount of bytes that have been filled.
-    #[inline]
-    pub fn filled_len(&self) -> usize {
-        self.filled
+impl<'a> Write for BorrowedCursor<'a> {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.append(buf);
+        Ok(buf.len())
     }
 
-    /// Returns the amount of bytes that have been initialized.
-    #[inline]
-    pub fn initialized_len(&self) -> usize {
-        self.initialized
+    fn flush(&mut self) -> Result<()> {
+        Ok(())
     }
 }
diff --git a/library/std/src/io/readbuf/tests.rs b/library/std/src/io/readbuf/tests.rs
index 3b7a5a56d..cc1b423f2 100644
--- a/library/std/src/io/readbuf/tests.rs
+++ b/library/std/src/io/readbuf/tests.rs
@@ -1,181 +1,175 @@
-use super::ReadBuf;
+use super::BorrowedBuf;
 use crate::mem::MaybeUninit;
 
-/// Test that ReadBuf has the correct numbers when created with new
+/// Test that BorrowedBuf has the correct numbers when created with new
 #[test]
 fn new() {
-    let mut buf = [0; 16];
-    let rbuf = ReadBuf::new(&mut buf);
+    let buf: &mut [_] = &mut [0; 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    assert_eq!(rbuf.filled_len(), 0);
-    assert_eq!(rbuf.initialized_len(), 16);
+    assert_eq!(rbuf.filled().len(), 0);
+    assert_eq!(rbuf.init_len(), 16);
     assert_eq!(rbuf.capacity(), 16);
-    assert_eq!(rbuf.remaining(), 16);
+    assert_eq!(rbuf.unfilled().capacity(), 16);
 }
 
-/// Test that ReadBuf has the correct numbers when created with uninit
+/// Test that BorrowedBuf has the correct numbers when created with uninit
 #[test]
 fn uninit() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let rbuf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    assert_eq!(rbuf.filled_len(), 0);
-    assert_eq!(rbuf.initialized_len(), 0);
+    assert_eq!(rbuf.filled().len(), 0);
+    assert_eq!(rbuf.init_len(), 0);
     assert_eq!(rbuf.capacity(), 16);
-    assert_eq!(rbuf.remaining(), 16);
+    assert_eq!(rbuf.unfilled().capacity(), 16);
 }
 
 #[test]
 fn initialize_unfilled() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.initialize_unfilled();
+    rbuf.unfilled().ensure_init();
 
-    assert_eq!(rbuf.initialized_len(), 16);
+    assert_eq!(rbuf.init_len(), 16);
 }
 
 #[test]
-fn initialize_unfilled_to() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
+fn addvance_filled() {
+    let buf: &mut [_] = &mut [0; 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.initialize_unfilled_to(8);
-
-    assert_eq!(rbuf.initialized_len(), 8);
-
-    rbuf.initialize_unfilled_to(4);
-
-    assert_eq!(rbuf.initialized_len(), 8);
-
-    rbuf.set_filled(8);
-
-    rbuf.initialize_unfilled_to(6);
-
-    assert_eq!(rbuf.initialized_len(), 14);
-
-    rbuf.initialize_unfilled_to(8);
-
-    assert_eq!(rbuf.initialized_len(), 16);
-}
-
-#[test]
-fn add_filled() {
-    let mut buf = [0; 16];
-    let mut rbuf = ReadBuf::new(&mut buf);
-
-    rbuf.add_filled(1);
-
-    assert_eq!(rbuf.filled_len(), 1);
-    assert_eq!(rbuf.remaining(), 15);
-}
-
-#[test]
-#[should_panic]
-fn add_filled_panic() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
-
-    rbuf.add_filled(1);
-}
-
-#[test]
-fn set_filled() {
-    let mut buf = [0; 16];
-    let mut rbuf = ReadBuf::new(&mut buf);
-
-    rbuf.set_filled(16);
-
-    assert_eq!(rbuf.filled_len(), 16);
-    assert_eq!(rbuf.remaining(), 0);
-
-    rbuf.set_filled(6);
-
-    assert_eq!(rbuf.filled_len(), 6);
-    assert_eq!(rbuf.remaining(), 10);
-}
-
-#[test]
-#[should_panic]
-fn set_filled_panic() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
+    unsafe {
+        rbuf.unfilled().advance(1);
+    }
 
-    rbuf.set_filled(16);
+    assert_eq!(rbuf.filled().len(), 1);
+    assert_eq!(rbuf.unfilled().capacity(), 15);
 }
 
 #[test]
 fn clear() {
-    let mut buf = [255; 16];
-    let mut rbuf = ReadBuf::new(&mut buf);
+    let buf: &mut [_] = &mut [255; 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.set_filled(16);
+    unsafe {
+        rbuf.unfilled().advance(16);
+    }
 
-    assert_eq!(rbuf.filled_len(), 16);
-    assert_eq!(rbuf.remaining(), 0);
+    assert_eq!(rbuf.filled().len(), 16);
+    assert_eq!(rbuf.unfilled().capacity(), 0);
 
     rbuf.clear();
 
-    assert_eq!(rbuf.filled_len(), 0);
-    assert_eq!(rbuf.remaining(), 16);
+    assert_eq!(rbuf.filled().len(), 0);
+    assert_eq!(rbuf.unfilled().capacity(), 16);
 
-    assert_eq!(rbuf.initialized(), [255; 16]);
+    assert_eq!(rbuf.unfilled().init_ref(), [255; 16]);
 }
 
 #[test]
-fn assume_init() {
-    let mut buf = [MaybeUninit::uninit(); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
+fn set_init() {
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
     unsafe {
-        rbuf.assume_init(8);
+        rbuf.set_init(8);
     }
 
-    assert_eq!(rbuf.initialized_len(), 8);
+    assert_eq!(rbuf.init_len(), 8);
 
-    rbuf.add_filled(4);
+    unsafe {
+        rbuf.unfilled().advance(4);
+    }
 
     unsafe {
-        rbuf.assume_init(2);
+        rbuf.set_init(2);
     }
 
-    assert_eq!(rbuf.initialized_len(), 8);
+    assert_eq!(rbuf.init_len(), 8);
 
     unsafe {
-        rbuf.assume_init(8);
+        rbuf.set_init(8);
     }
 
-    assert_eq!(rbuf.initialized_len(), 12);
+    assert_eq!(rbuf.init_len(), 8);
 }
 
 #[test]
 fn append() {
-    let mut buf = [MaybeUninit::new(255); 16];
-    let mut rbuf = ReadBuf::uninit(&mut buf);
+    let buf: &mut [_] = &mut [MaybeUninit::new(255); 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
 
-    rbuf.append(&[0; 8]);
+    rbuf.unfilled().append(&[0; 8]);
 
-    assert_eq!(rbuf.initialized_len(), 8);
-    assert_eq!(rbuf.filled_len(), 8);
+    assert_eq!(rbuf.init_len(), 8);
+    assert_eq!(rbuf.filled().len(), 8);
     assert_eq!(rbuf.filled(), [0; 8]);
 
     rbuf.clear();
 
-    rbuf.append(&[1; 16]);
+    rbuf.unfilled().append(&[1; 16]);
 
-    assert_eq!(rbuf.initialized_len(), 16);
-    assert_eq!(rbuf.filled_len(), 16);
+    assert_eq!(rbuf.init_len(), 16);
+    assert_eq!(rbuf.filled().len(), 16);
     assert_eq!(rbuf.filled(), [1; 16]);
 }
 
 #[test]
-fn filled_mut() {
-    let mut buf = [0; 16];
-    let mut rbuf = ReadBuf::new(&mut buf);
+fn reborrow_written() {
+    let buf: &mut [_] = &mut [MaybeUninit::new(0); 32];
+    let mut buf: BorrowedBuf<'_> = buf.into();
+
+    let mut cursor = buf.unfilled();
+    cursor.append(&[1; 16]);
+
+    let mut cursor2 = cursor.reborrow();
+    cursor2.append(&[2; 16]);
+
+    assert_eq!(cursor2.written(), 32);
+    assert_eq!(cursor.written(), 32);
+
+    assert_eq!(buf.unfilled().written(), 0);
+    assert_eq!(buf.init_len(), 32);
+    assert_eq!(buf.filled().len(), 32);
+    let filled = buf.filled();
+    assert_eq!(&filled[..16], [1; 16]);
+    assert_eq!(&filled[16..], [2; 16]);
+}
+
+#[test]
+fn cursor_set_init() {
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 16];
+    let mut rbuf: BorrowedBuf<'_> = buf.into();
+
+    unsafe {
+        rbuf.unfilled().set_init(8);
+    }
 
-    rbuf.add_filled(8);
+    assert_eq!(rbuf.init_len(), 8);
+    assert_eq!(rbuf.unfilled().init_ref().len(), 8);
+    assert_eq!(rbuf.unfilled().init_mut().len(), 8);
+    assert_eq!(rbuf.unfilled().uninit_mut().len(), 8);
+    assert_eq!(unsafe { rbuf.unfilled().as_mut() }.len(), 16);
+
+    unsafe {
+        rbuf.unfilled().advance(4);
+    }
 
-    let filled = rbuf.filled().to_vec();
+    unsafe {
+        rbuf.unfilled().set_init(2);
+    }
+
+    assert_eq!(rbuf.init_len(), 8);
+
+    unsafe {
+        rbuf.unfilled().set_init(8);
+    }
 
-    assert_eq!(&*filled, &*rbuf.filled_mut());
+    assert_eq!(rbuf.init_len(), 12);
+    assert_eq!(rbuf.unfilled().init_ref().len(), 8);
+    assert_eq!(rbuf.unfilled().init_mut().len(), 8);
+    assert_eq!(rbuf.unfilled().uninit_mut().len(), 4);
+    assert_eq!(unsafe { rbuf.unfilled().as_mut() }.len(), 12);
 }
diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
index 4d3736f79..2dc12a18a 100644
--- a/library/std/src/io/stdio.rs
+++ b/library/std/src/io/stdio.rs
@@ -8,7 +8,6 @@ use crate::io::prelude::*;
 use crate::cell::{Cell, RefCell};
 use crate::fmt;
 use crate::io::{self, BufReader, IoSlice, IoSliceMut, LineWriter, Lines};
-use crate::pin::Pin;
 use crate::sync::atomic::{AtomicBool, Ordering};
 use crate::sync::{Arc, Mutex, MutexGuard, OnceLock};
 use crate::sys::stdio;
@@ -526,7 +525,7 @@ pub struct Stdout {
     // FIXME: this should be LineWriter or BufWriter depending on the state of
     //        stdout (tty or not). Note that if this is not line buffered it
     //        should also flush-on-panic or some form of flush-on-abort.
-    inner: Pin<&'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>>,
+    inner: &'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>,
 }
 
 /// A locked reference to the [`Stdout`] handle.
@@ -603,22 +602,27 @@ static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLo
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdout() -> Stdout {
     Stdout {
-        inner: Pin::static_ref(&STDOUT).get_or_init_pin(
-            || unsafe { ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw()))) },
-            |mutex| unsafe { mutex.init() },
-        ),
+        inner: STDOUT
+            .get_or_init(|| ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw())))),
     }
 }
 
+// Flush the data and disable buffering during shutdown
+// by replacing the line writer by one with zero
+// buffering capacity.
 pub fn cleanup() {
-    if let Some(instance) = STDOUT.get() {
-        // Flush the data and disable buffering during shutdown
-        // by replacing the line writer by one with zero
-        // buffering capacity.
+    let mut initialized = false;
+    let stdout = STDOUT.get_or_init(|| {
+        initialized = true;
+        ReentrantMutex::new(RefCell::new(LineWriter::with_capacity(0, stdout_raw())))
+    });
+
+    if !initialized {
+        // The buffer was previously initialized, overwrite it here.
         // We use try_lock() instead of lock(), because someone
         // might have leaked a StdoutLock, which would
         // otherwise cause a deadlock here.
-        if let Some(lock) = Pin::static_ref(instance).try_lock() {
+        if let Some(lock) = stdout.try_lock() {
             *lock.borrow_mut() = LineWriter::with_capacity(0, stdout_raw());
         }
     }
@@ -761,7 +765,7 @@ impl fmt::Debug for StdoutLock<'_> {
 /// standard library or via raw Windows API calls, will fail.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: Pin<&'static ReentrantMutex<RefCell<StderrRaw>>>,
+    inner: &'static ReentrantMutex<RefCell<StderrRaw>>,
 }
 
 /// A locked reference to the [`Stderr`] handle.
@@ -834,16 +838,12 @@ pub struct StderrLock<'a> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
     // Note that unlike `stdout()` we don't use `at_exit` here to register a
-    // destructor. Stderr is not buffered , so there's no need to run a
+    // destructor. Stderr is not buffered, so there's no need to run a
     // destructor for flushing the buffer
-    static INSTANCE: OnceLock<ReentrantMutex<RefCell<StderrRaw>>> = OnceLock::new();
+    static INSTANCE: ReentrantMutex<RefCell<StderrRaw>> =
+        ReentrantMutex::new(RefCell::new(stderr_raw()));
 
-    Stderr {
-        inner: Pin::static_ref(&INSTANCE).get_or_init_pin(
-            || unsafe { ReentrantMutex::new(RefCell::new(stderr_raw())) },
-            |mutex| unsafe { mutex.init() },
-        ),
-    }
+    Stderr { inner: &INSTANCE }
 }
 
 impl Stderr {
@@ -986,12 +986,15 @@ pub fn set_output_capture(sink: Option<LocalStream>) -> Option<LocalStream> {
 /// otherwise. `label` identifies the stream in a panic message.
 ///
 /// This function is used to print error messages, so it takes extra
-/// care to avoid causing a panic when `local_s` is unusable.
-/// For instance, if the TLS key for the local stream is
-/// already destroyed, or if the local stream is locked by another
-/// thread, it will just fall back to the global stream.
+/// care to avoid causing a panic when `OUTPUT_CAPTURE` is unusable.
+/// For instance, if the TLS key for output capturing is already destroyed, or
+/// if the local stream is in use by another thread, it will just fall back to
+/// the global stream.
 ///
 /// However, if the actual I/O causes an error, this function does panic.
+///
+/// Writing to non-blocking stdout/stderr can cause an error, which will lead
+/// this function to panic.
 fn print_to<T>(args: fmt::Arguments<'_>, global_s: fn() -> T, label: &str)
 where
     T: Write,
diff --git a/library/std/src/io/tests.rs b/library/std/src/io/tests.rs
index f357f33ec..f4a886d88 100644
--- a/library/std/src/io/tests.rs
+++ b/library/std/src/io/tests.rs
@@ -1,4 +1,4 @@
-use super::{repeat, Cursor, ReadBuf, SeekFrom};
+use super::{repeat, BorrowedBuf, Cursor, SeekFrom};
 use crate::cmp::{self, min};
 use crate::io::{self, IoSlice, IoSliceMut};
 use crate::io::{BufRead, BufReader, Read, Seek, Write};
@@ -94,7 +94,7 @@ fn read_to_end() {
     assert_eq!(c.read_to_end(&mut v).unwrap(), 1);
     assert_eq!(v, b"1");
 
-    let cap = 1024 * 1024;
+    let cap = if cfg!(miri) { 1024 } else { 1024 * 1024 };
     let data = (0..cap).map(|i| (i / 3) as u8).collect::<Vec<_>>();
     let mut v = Vec::new();
     let (a, b) = data.split_at(data.len() / 2);
@@ -159,24 +159,24 @@ fn read_exact_slice() {
 
 #[test]
 fn read_buf_exact() {
-    let mut buf = [0; 4];
-    let mut buf = ReadBuf::new(&mut buf);
+    let buf: &mut [_] = &mut [0; 4];
+    let mut buf: BorrowedBuf<'_> = buf.into();
 
     let mut c = Cursor::new(&b""[..]);
-    assert_eq!(c.read_buf_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+    assert_eq!(c.read_buf_exact(buf.unfilled()).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
 
     let mut c = Cursor::new(&b"123456789"[..]);
-    c.read_buf_exact(&mut buf).unwrap();
+    c.read_buf_exact(buf.unfilled()).unwrap();
     assert_eq!(buf.filled(), b"1234");
 
     buf.clear();
 
-    c.read_buf_exact(&mut buf).unwrap();
+    c.read_buf_exact(buf.unfilled()).unwrap();
     assert_eq!(buf.filled(), b"5678");
 
     buf.clear();
 
-    assert_eq!(c.read_buf_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+    assert_eq!(c.read_buf_exact(buf.unfilled()).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
 }
 
 #[test]
@@ -309,6 +309,7 @@ fn chain_zero_length_read_is_not_eof() {
 
 #[bench]
 #[cfg_attr(target_os = "emscripten", ignore)]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_read_to_end(b: &mut test::Bencher) {
     b.iter(|| {
         let mut lr = repeat(1).take(10000000);
@@ -614,10 +615,10 @@ fn bench_take_read(b: &mut test::Bencher) {
 #[bench]
 fn bench_take_read_buf(b: &mut test::Bencher) {
     b.iter(|| {
-        let mut buf = [MaybeUninit::uninit(); 64];
+        let buf: &mut [_] = &mut [MaybeUninit::uninit(); 64];
 
-        let mut rbuf = ReadBuf::uninit(&mut buf);
+        let mut buf: BorrowedBuf<'_> = buf.into();
 
-        [255; 128].take(64).read_buf(&mut rbuf).unwrap();
+        [255; 128].take(64).read_buf(buf.unfilled()).unwrap();
     });
 }
diff --git a/library/std/src/io/util.rs b/library/std/src/io/util.rs
index c1300cd67..f076ee092 100644
--- a/library/std/src/io/util.rs
+++ b/library/std/src/io/util.rs
@@ -5,7 +5,7 @@ mod tests;
 
 use crate::fmt;
 use crate::io::{
-    self, BufRead, IoSlice, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, SizeHint, Write,
+    self, BorrowedCursor, BufRead, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write,
 };
 
 /// A reader which is always at EOF.
@@ -47,7 +47,7 @@ impl Read for Empty {
     }
 
     #[inline]
-    fn read_buf(&mut self, _buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    fn read_buf(&mut self, _cursor: BorrowedCursor<'_>) -> io::Result<()> {
         Ok(())
     }
 }
@@ -130,21 +130,19 @@ impl Read for Repeat {
         Ok(buf.len())
     }
 
-    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> io::Result<()> {
         // SAFETY: No uninit bytes are being written
-        for slot in unsafe { buf.unfilled_mut() } {
+        for slot in unsafe { buf.as_mut() } {
             slot.write(self.byte);
         }
 
-        let remaining = buf.remaining();
+        let remaining = buf.capacity();
 
         // SAFETY: the entire unfilled portion of buf has been initialized
         unsafe {
-            buf.assume_init(remaining);
+            buf.advance(remaining);
         }
 
-        buf.add_filled(remaining);
-
         Ok(())
     }
 
diff --git a/library/std/src/io/util/tests.rs b/library/std/src/io/util/tests.rs
index 08972a59a..ce5e2c9da 100644
--- a/library/std/src/io/util/tests.rs
+++ b/library/std/src/io/util/tests.rs
@@ -1,7 +1,7 @@
 use crate::cmp::{max, min};
 use crate::io::prelude::*;
 use crate::io::{
-    copy, empty, repeat, sink, BufWriter, Empty, ReadBuf, Repeat, Result, SeekFrom, Sink,
+    copy, empty, repeat, sink, BorrowedBuf, BufWriter, Empty, Repeat, Result, SeekFrom, Sink,
     DEFAULT_BUF_SIZE,
 };
 
@@ -79,29 +79,29 @@ fn empty_reads() {
     assert_eq!(e.read(&mut [0; 1024]).unwrap(), 0);
     assert_eq!(e.by_ref().read(&mut [0; 1024]).unwrap(), 0);
 
-    let mut buf = [];
-    let mut buf = ReadBuf::uninit(&mut buf);
-    e.read_buf(&mut buf).unwrap();
-    assert_eq!(buf.filled_len(), 0);
-    assert_eq!(buf.initialized_len(), 0);
-
-    let mut buf = [MaybeUninit::uninit()];
-    let mut buf = ReadBuf::uninit(&mut buf);
-    e.read_buf(&mut buf).unwrap();
-    assert_eq!(buf.filled_len(), 0);
-    assert_eq!(buf.initialized_len(), 0);
-
-    let mut buf = [MaybeUninit::uninit(); 1024];
-    let mut buf = ReadBuf::uninit(&mut buf);
-    e.read_buf(&mut buf).unwrap();
-    assert_eq!(buf.filled_len(), 0);
-    assert_eq!(buf.initialized_len(), 0);
-
-    let mut buf = [MaybeUninit::uninit(); 1024];
-    let mut buf = ReadBuf::uninit(&mut buf);
-    e.by_ref().read_buf(&mut buf).unwrap();
-    assert_eq!(buf.filled_len(), 0);
-    assert_eq!(buf.initialized_len(), 0);
+    let buf: &mut [MaybeUninit<_>] = &mut [];
+    let mut buf: BorrowedBuf<'_> = buf.into();
+    e.read_buf(buf.unfilled()).unwrap();
+    assert_eq!(buf.len(), 0);
+    assert_eq!(buf.init_len(), 0);
+
+    let buf: &mut [_] = &mut [MaybeUninit::uninit()];
+    let mut buf: BorrowedBuf<'_> = buf.into();
+    e.read_buf(buf.unfilled()).unwrap();
+    assert_eq!(buf.len(), 0);
+    assert_eq!(buf.init_len(), 0);
+
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
+    let mut buf: BorrowedBuf<'_> = buf.into();
+    e.read_buf(buf.unfilled()).unwrap();
+    assert_eq!(buf.len(), 0);
+    assert_eq!(buf.init_len(), 0);
+
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); 1024];
+    let mut buf: BorrowedBuf<'_> = buf.into();
+    e.by_ref().read_buf(buf.unfilled()).unwrap();
+    assert_eq!(buf.len(), 0);
+    assert_eq!(buf.init_len(), 0);
 }
 
 #[test]
diff --git a/library/std/src/keyword_docs.rs b/library/std/src/keyword_docs.rs
index 7157b5af0..a4b0522b0 100644
--- a/library/std/src/keyword_docs.rs
+++ b/library/std/src/keyword_docs.rs
@@ -1921,7 +1921,7 @@ mod type_keyword {}
 /// and [proposal]s exist to use `unsafe {}` blocks inside such functions when
 /// making `unsafe` operations.
 ///
-/// See the [Rustnomicon] and the [Reference] for more informations.
+/// See the [Rustnomicon] and the [Reference] for more information.
 ///
 /// # Examples
 ///
@@ -2113,7 +2113,7 @@ mod use_keyword {}
 /// Add constraints that must be upheld to use an item.
 ///
 /// `where` allows specifying constraints on lifetime and generic parameters.
-/// The [RFC] introducing `where` contains detailed informations about the
+/// The [RFC] introducing `where` contains detailed information about the
 /// keyword.
 ///
 /// # Examples
@@ -2355,7 +2355,7 @@ mod dyn_keyword {}
 /// println!("f = {f} and i = {i}");
 /// ```
 ///
-/// See the [Reference][union] for more informations on `union`s.
+/// See the [Reference][union] for more information on `union`s.
 ///
 /// [`struct`]: keyword.struct.html
 /// [union]: ../reference/items/unions.html
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 20d25a608..bc4f1b27c 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -187,6 +187,7 @@
 //! [rust-discord]: https://discord.gg/rust-lang
 //! [array]: prim@array
 //! [slice]: prim@slice
+
 #![cfg_attr(not(feature = "restricted-std"), stable(feature = "rust1", since = "1.0.0"))]
 #![cfg_attr(feature = "restricted-std", unstable(feature = "restricted_std", issue = "none"))]
 #![doc(
@@ -201,25 +202,35 @@
     no_global_oom_handling,
     not(no_global_oom_handling)
 ))]
+// To run libstd tests without x.py without ending up with two copies of libstd, Miri needs to be
+// able to "empty" this crate. See <https://github.com/rust-lang/miri-test-libstd/issues/4>.
+// rustc itself never sets the feature, so this line has no affect there.
+#![cfg(any(not(feature = "miri-test-libstd"), test, doctest))]
+// miri-test-libstd also prefers to make std use the sysroot versions of the dependencies.
+#![cfg_attr(feature = "miri-test-libstd", feature(rustc_private))]
 // Don't link to std. We are std.
 #![no_std]
+// Tell the compiler to link to either panic_abort or panic_unwind
+#![needs_panic_runtime]
+//
+// Lints:
 #![warn(deprecated_in_future)]
 #![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
 #![allow(explicit_outlives_requirements)]
 #![allow(unused_lifetimes)]
-// Tell the compiler to link to either panic_abort or panic_unwind
-#![needs_panic_runtime]
+#![deny(rustc::existing_doc_keyword)]
 // Ensure that std can be linked against panic_abort despite compiled with `-C panic=unwind`
-#![cfg_attr(not(bootstrap), deny(ffi_unwind_calls))]
+#![deny(ffi_unwind_calls)]
 // std may use features in a platform-specific way
 #![allow(unused_features)]
+//
+// Features:
 #![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count, rt))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
 )]
-#![deny(rustc::existing_doc_keyword)]
 //
 // Language features:
 #![feature(alloc_error_handler)]
@@ -241,11 +252,12 @@
 #![feature(dropck_eyepatch)]
 #![feature(exhaustive_patterns)]
 #![feature(intra_doc_pointers)]
-#![feature(label_break_value)]
+#![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(linkage)]
+#![feature(link_cfg)]
 #![feature(min_specialization)]
 #![feature(must_not_suspend)]
 #![feature(needs_panic_runtime)]
@@ -258,6 +270,7 @@
 #![feature(staged_api)]
 #![feature(thread_local)]
 #![feature(try_blocks)]
+#![feature(utf8_chunks)]
 //
 // Library features (core):
 #![feature(array_error_internals)]
@@ -269,10 +282,14 @@
 #![feature(cstr_internals)]
 #![feature(duration_checked_float)]
 #![feature(duration_constants)]
+#![cfg_attr(not(bootstrap), feature(error_generic_member_access))]
+#![cfg_attr(not(bootstrap), feature(error_in_core))]
+#![cfg_attr(not(bootstrap), feature(error_iter))]
 #![feature(exact_size_is_empty)]
 #![feature(exclusive_wrapper)]
 #![feature(extend_one)]
 #![feature(float_minimum_maximum)]
+#![feature(float_next_up_down)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
@@ -284,6 +301,8 @@
 #![feature(panic_can_unwind)]
 #![feature(panic_info_message)]
 #![feature(panic_internals)]
+#![feature(pointer_byte_offsets)]
+#![feature(pointer_is_aligned)]
 #![feature(portable_simd)]
 #![feature(prelude_2024)]
 #![feature(provide_any)]
@@ -294,6 +313,8 @@
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(strict_provenance)]
+#![feature(maybe_uninit_uninit_array)]
+#![feature(const_maybe_uninit_uninit_array)]
 //
 // Library features (alloc):
 #![feature(alloc_layout_extra)]
@@ -576,6 +597,7 @@ pub mod alloc;
 
 // Private support modules
 mod panicking;
+mod personality;
 
 #[path = "../../backtrace/src/lib.rs"]
 #[allow(dead_code, unused_attributes)]
diff --git a/library/std/src/macros.rs b/library/std/src/macros.rs
index 0cb21ef53..6e4ba1404 100644
--- a/library/std/src/macros.rs
+++ b/library/std/src/macros.rs
@@ -27,17 +27,31 @@ macro_rules! panic {
 /// necessary to use [`io::stdout().flush()`][flush] to ensure the output is emitted
 /// immediately.
 ///
+/// The `print!` macro will lock the standard output on each call. If you call
+/// `print!` within a hot loop, this behavior may be the bottleneck of the loop.
+/// To avoid this, lock stdout with [`io::stdout().lock()`][lock]:
+/// ```
+/// use std::io::{stdout, Write};
+///
+/// let mut lock = stdout().lock();
+/// write!(lock, "hello world").unwrap();
+/// ```
+///
 /// Use `print!` only for the primary output of your program. Use
 /// [`eprint!`] instead to print error and progress messages.
 ///
 /// [flush]: crate::io::Write::flush
 /// [`println!`]: crate::println
 /// [`eprint!`]: crate::eprint
+/// [lock]: crate::io::Stdout
 ///
 /// # Panics
 ///
 /// Panics if writing to `io::stdout()` fails.
 ///
+/// Writing to non-blocking stdout can cause an error, which will lead
+/// this macro to panic.
+///
 /// # Examples
 ///
 /// ```
@@ -75,16 +89,30 @@ macro_rules! print {
 /// This macro uses the same syntax as [`format!`], but writes to the standard output instead.
 /// See [`std::fmt`] for more information.
 ///
+/// The `println!` macro will lock the standard output on each call. If you call
+/// `println!` within a hot loop, this behavior may be the bottleneck of the loop.
+/// To avoid this, lock stdout with [`io::stdout().lock()`][lock]:
+/// ```
+/// use std::io::{stdout, Write};
+///
+/// let mut lock = stdout().lock();
+/// writeln!(lock, "hello world").unwrap();
+/// ```
+///
 /// Use `println!` only for the primary output of your program. Use
 /// [`eprintln!`] instead to print error and progress messages.
 ///
 /// [`std::fmt`]: crate::fmt
 /// [`eprintln!`]: crate::eprintln
+/// [lock]: crate::io::Stdout
 ///
 /// # Panics
 ///
 /// Panics if writing to [`io::stdout`] fails.
 ///
+/// Writing to non-blocking stdout can cause an error, which will lead
+/// this macro to panic.
+///
 /// [`io::stdout`]: crate::io::stdout
 ///
 /// # Examples
@@ -93,6 +121,8 @@ macro_rules! print {
 /// println!(); // prints just a newline
 /// println!("hello there!");
 /// println!("format {} arguments", "some");
+/// let local_variable = "some";
+/// println!("format {local_variable} arguments");
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -123,6 +153,9 @@ macro_rules! println {
 ///
 /// Panics if writing to `io::stderr` fails.
 ///
+/// Writing to non-blocking stdout can cause an error, which will lead
+/// this macro to panic.
+///
 /// # Examples
 ///
 /// ```
@@ -155,6 +188,9 @@ macro_rules! eprint {
 ///
 /// Panics if writing to `io::stderr` fails.
 ///
+/// Writing to non-blocking stdout can cause an error, which will lead
+/// this macro to panic.
+///
 /// # Examples
 ///
 /// ```
diff --git a/library/std/src/net/display_buffer.rs b/library/std/src/net/display_buffer.rs
new file mode 100644
index 000000000..7aadf06e9
--- /dev/null
+++ b/library/std/src/net/display_buffer.rs
@@ -0,0 +1,40 @@
+use crate::fmt;
+use crate::mem::MaybeUninit;
+use crate::str;
+
+/// Used for slow path in `Display` implementations when alignment is required.
+pub struct DisplayBuffer<const SIZE: usize> {
+    buf: [MaybeUninit<u8>; SIZE],
+    len: usize,
+}
+
+impl<const SIZE: usize> DisplayBuffer<SIZE> {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { buf: MaybeUninit::uninit_array(), len: 0 }
+    }
+
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        // SAFETY: `buf` is only written to by the `fmt::Write::write_str` implementation
+        // which writes a valid UTF-8 string to `buf` and correctly sets `len`.
+        unsafe {
+            let s = MaybeUninit::slice_assume_init_ref(&self.buf[..self.len]);
+            str::from_utf8_unchecked(s)
+        }
+    }
+}
+
+impl<const SIZE: usize> fmt::Write for DisplayBuffer<SIZE> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let bytes = s.as_bytes();
+
+        if let Some(buf) = self.buf.get_mut(self.len..(self.len + bytes.len())) {
+            MaybeUninit::write_slice(buf, bytes);
+            self.len += bytes.len();
+            Ok(())
+        } else {
+            Err(fmt::Error)
+        }
+    }
+}
diff --git a/library/std/src/net/ip.rs b/library/std/src/net/ip_addr.rs
index 41ca9ba84..4f14fc280 100644
--- a/library/std/src/net/ip.rs
+++ b/library/std/src/net/ip_addr.rs
@@ -3,12 +3,13 @@
 mod tests;
 
 use crate::cmp::Ordering;
-use crate::fmt::{self, Write as FmtWrite};
-use crate::io::Write as IoWrite;
+use crate::fmt::{self, Write};
 use crate::mem::transmute;
 use crate::sys::net::netc as c;
 use crate::sys_common::{FromInner, IntoInner};
 
+use super::display_buffer::DisplayBuffer;
+
 /// An IP address, either IPv4 or IPv6.
 ///
 /// This enum can contain either an [`Ipv4Addr`] or an [`Ipv6Addr`], see their
@@ -28,6 +29,7 @@ use crate::sys_common::{FromInner, IntoInner};
 /// assert_eq!(localhost_v4.is_ipv6(), false);
 /// assert_eq!(localhost_v4.is_ipv4(), true);
 /// ```
+#[cfg_attr(not(test), rustc_diagnostic_item = "IpAddr")]
 #[stable(feature = "ip_addr", since = "1.7.0")]
 #[derive(Copy, Clone, Eq, PartialEq, Hash, PartialOrd, Ord)]
 pub enum IpAddr {
@@ -71,6 +73,7 @@ pub enum IpAddr {
 /// assert!("0xcb.0x0.0x71.0x00".parse::<Ipv4Addr>().is_err()); // all octets are in hex
 /// ```
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "Ipv4Addr")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Ipv4Addr {
     octets: [u8; 4],
@@ -153,6 +156,7 @@ pub struct Ipv4Addr {
 /// assert_eq!(localhost.is_loopback(), true);
 /// ```
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "Ipv6Addr")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Ipv6Addr {
     octets: [u8; 16],
@@ -618,25 +622,31 @@ impl Ipv4Addr {
         matches!(self.octets(), [169, 254, ..])
     }
 
-    /// Returns [`true`] if the address appears to be globally routable.
-    /// See [iana-ipv4-special-registry][ipv4-sr].
+    /// Returns [`true`] if the address appears to be globally reachable
+    /// as specified by the [IANA IPv4 Special-Purpose Address Registry].
+    /// Whether or not an address is practically reachable will depend on your network configuration.
+    ///
+    /// Most IPv4 addresses are globally reachable;
+    /// unless they are specifically defined as *not* globally reachable.
     ///
-    /// The following return [`false`]:
+    /// Non-exhaustive list of notable addresses that are not globally reachable:
     ///
-    /// - private addresses (see [`Ipv4Addr::is_private()`])
-    /// - the loopback address (see [`Ipv4Addr::is_loopback()`])
-    /// - the link-local address (see [`Ipv4Addr::is_link_local()`])
-    /// - the broadcast address (see [`Ipv4Addr::is_broadcast()`])
-    /// - addresses used for documentation (see [`Ipv4Addr::is_documentation()`])
-    /// - the unspecified address (see [`Ipv4Addr::is_unspecified()`]), and the whole
-    ///   `0.0.0.0/8` block
-    /// - addresses reserved for future protocols, except
-    /// `192.0.0.9/32` and `192.0.0.10/32` which are globally routable
-    /// - addresses reserved for future use (see [`Ipv4Addr::is_reserved()`]
-    /// - addresses reserved for networking devices benchmarking (see
-    /// [`Ipv4Addr::is_benchmarking()`])
+    /// - The [unspecified address] ([`is_unspecified`](Ipv4Addr::is_unspecified))
+    /// - Addresses reserved for private use ([`is_private`](Ipv4Addr::is_private))
+    /// - Addresses in the shared address space ([`is_shared`](Ipv4Addr::is_shared))
+    /// - Loopback addresses ([`is_loopback`](Ipv4Addr::is_loopback))
+    /// - Link-local addresses ([`is_link_local`](Ipv4Addr::is_link_local))
+    /// - Addresses reserved for documentation ([`is_documentation`](Ipv4Addr::is_documentation))
+    /// - Addresses reserved for benchmarking ([`is_benchmarking`](Ipv4Addr::is_benchmarking))
+    /// - Reserved addresses ([`is_reserved`](Ipv4Addr::is_reserved))
+    /// - The [broadcast address] ([`is_broadcast`](Ipv4Addr::is_broadcast))
     ///
-    /// [ipv4-sr]: https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
+    /// For the complete overview of which addresses are globally reachable, see the table at the [IANA IPv4 Special-Purpose Address Registry].
+    ///
+    /// [IANA IPv4 Special-Purpose Address Registry]: https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
+    /// [unspecified address]: Ipv4Addr::UNSPECIFIED
+    /// [broadcast address]: Ipv4Addr::BROADCAST
+
     ///
     /// # Examples
     ///
@@ -645,71 +655,61 @@ impl Ipv4Addr {
     ///
     /// use std::net::Ipv4Addr;
     ///
-    /// // private addresses are not global
+    /// // Most IPv4 addresses are globally reachable:
+    /// assert_eq!(Ipv4Addr::new(80, 9, 12, 3).is_global(), true);
+    ///
+    /// // However some addresses have been assigned a special meaning
+    /// // that makes them not globally reachable. Some examples are:
+    ///
+    /// // The unspecified address (`0.0.0.0`)
+    /// assert_eq!(Ipv4Addr::UNSPECIFIED.is_global(), false);
+    ///
+    /// // Addresses reserved for private use (`10.0.0.0/8`, `172.16.0.0/12`, 192.168.0.0/16)
     /// assert_eq!(Ipv4Addr::new(10, 254, 0, 0).is_global(), false);
     /// assert_eq!(Ipv4Addr::new(192, 168, 10, 65).is_global(), false);
     /// assert_eq!(Ipv4Addr::new(172, 16, 10, 65).is_global(), false);
     ///
-    /// // the 0.0.0.0/8 block is not global
-    /// assert_eq!(Ipv4Addr::new(0, 1, 2, 3).is_global(), false);
-    /// // in particular, the unspecified address is not global
-    /// assert_eq!(Ipv4Addr::new(0, 0, 0, 0).is_global(), false);
+    /// // Addresses in the shared address space (`100.64.0.0/10`)
+    /// assert_eq!(Ipv4Addr::new(100, 100, 0, 0).is_global(), false);
     ///
-    /// // the loopback address is not global
-    /// assert_eq!(Ipv4Addr::new(127, 0, 0, 1).is_global(), false);
+    /// // The loopback addresses (`127.0.0.0/8`)
+    /// assert_eq!(Ipv4Addr::LOCALHOST.is_global(), false);
     ///
-    /// // link local addresses are not global
+    /// // Link-local addresses (`169.254.0.0/16`)
     /// assert_eq!(Ipv4Addr::new(169, 254, 45, 1).is_global(), false);
     ///
-    /// // the broadcast address is not global
-    /// assert_eq!(Ipv4Addr::new(255, 255, 255, 255).is_global(), false);
-    ///
-    /// // the address space designated for documentation is not global
+    /// // Addresses reserved for documentation (`192.0.2.0/24`, `198.51.100.0/24`, `203.0.113.0/24`)
     /// assert_eq!(Ipv4Addr::new(192, 0, 2, 255).is_global(), false);
     /// assert_eq!(Ipv4Addr::new(198, 51, 100, 65).is_global(), false);
     /// assert_eq!(Ipv4Addr::new(203, 0, 113, 6).is_global(), false);
     ///
-    /// // shared addresses are not global
-    /// assert_eq!(Ipv4Addr::new(100, 100, 0, 0).is_global(), false);
-    ///
-    /// // addresses reserved for protocol assignment are not global
-    /// assert_eq!(Ipv4Addr::new(192, 0, 0, 0).is_global(), false);
-    /// assert_eq!(Ipv4Addr::new(192, 0, 0, 255).is_global(), false);
+    /// // Addresses reserved for benchmarking (`198.18.0.0/15`)
+    /// assert_eq!(Ipv4Addr::new(198, 18, 0, 0).is_global(), false);
     ///
-    /// // addresses reserved for future use are not global
+    /// // Reserved addresses (`240.0.0.0/4`)
     /// assert_eq!(Ipv4Addr::new(250, 10, 20, 30).is_global(), false);
     ///
-    /// // addresses reserved for network devices benchmarking are not global
-    /// assert_eq!(Ipv4Addr::new(198, 18, 0, 0).is_global(), false);
+    /// // The broadcast address (`255.255.255.255`)
+    /// assert_eq!(Ipv4Addr::BROADCAST.is_global(), false);
     ///
-    /// // All the other addresses are global
-    /// assert_eq!(Ipv4Addr::new(1, 1, 1, 1).is_global(), true);
-    /// assert_eq!(Ipv4Addr::new(80, 9, 12, 3).is_global(), true);
+    /// // For a complete overview see the IANA IPv4 Special-Purpose Address Registry.
     /// ```
     #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
     #[unstable(feature = "ip", issue = "27709")]
     #[must_use]
     #[inline]
     pub const fn is_global(&self) -> bool {
-        // check if this address is 192.0.0.9 or 192.0.0.10. These addresses are the only two
-        // globally routable addresses in the 192.0.0.0/24 range.
-        if u32::from_be_bytes(self.octets()) == 0xc0000009
-            || u32::from_be_bytes(self.octets()) == 0xc000000a
-        {
-            return true;
-        }
-        !self.is_private()
-            && !self.is_loopback()
-            && !self.is_link_local()
-            && !self.is_broadcast()
-            && !self.is_documentation()
-            && !self.is_shared()
+        !(self.octets()[0] == 0 // "This network"
+            || self.is_private()
+            || self.is_shared()
+            || self.is_loopback()
+            || self.is_link_local()
             // addresses reserved for future protocols (`192.0.0.0/24`)
-            && !(self.octets()[0] == 192 && self.octets()[1] == 0 && self.octets()[2] == 0)
-            && !self.is_reserved()
-            && !self.is_benchmarking()
-            // Make sure the address is not in 0.0.0.0/8
-            && self.octets()[0] != 0
+            ||(self.octets()[0] == 192 && self.octets()[1] == 0 && self.octets()[2] == 0)
+            || self.is_documentation()
+            || self.is_benchmarking()
+            || self.is_reserved()
+            || self.is_broadcast())
     }
 
     /// Returns [`true`] if this address is part of the Shared Address Space defined in
@@ -991,21 +991,19 @@ impl From<Ipv6Addr> for IpAddr {
 impl fmt::Display for Ipv4Addr {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let octets = self.octets();
-        // Fast Path: if there's no alignment stuff, write directly to the buffer
+
+        // If there are no alignment requirements, write the IP address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if fmt.precision().is_none() && fmt.width().is_none() {
             write!(fmt, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3])
         } else {
-            const IPV4_BUF_LEN: usize = 15; // Long enough for the longest possible IPv4 address
-            let mut buf = [0u8; IPV4_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
+            const LONGEST_IPV4_ADDR: &str = "255.255.255.255";
 
-            // Note: The call to write should never fail, hence the unwrap
-            write!(buf_slice, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3]).unwrap();
-            let len = IPV4_BUF_LEN - buf_slice.len();
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV4_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv4 address, so this should never fail.
+            write!(buf, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3]).unwrap();
 
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            fmt.pad(buf)
+            fmt.pad(buf.as_str())
         }
     }
 }
@@ -1300,13 +1298,33 @@ impl Ipv6Addr {
         u128::from_be_bytes(self.octets()) == u128::from_be_bytes(Ipv6Addr::LOCALHOST.octets())
     }
 
-    /// Returns [`true`] if the address appears to be globally routable.
+    /// Returns [`true`] if the address appears to be globally reachable
+    /// as specified by the [IANA IPv6 Special-Purpose Address Registry].
+    /// Whether or not an address is practically reachable will depend on your network configuration.
     ///
-    /// The following return [`false`]:
+    /// Most IPv6 addresses are globally reachable;
+    /// unless they are specifically defined as *not* globally reachable.
     ///
-    /// - the loopback address
-    /// - link-local and unique local unicast addresses
-    /// - interface-, link-, realm-, admin- and site-local multicast addresses
+    /// Non-exhaustive list of notable addresses that are not globally reachable:
+    /// - The [unspecified address] ([`is_unspecified`](Ipv6Addr::is_unspecified))
+    /// - The [loopback address] ([`is_loopback`](Ipv6Addr::is_loopback))
+    /// - IPv4-mapped addresses
+    /// - Addresses reserved for benchmarking
+    /// - Addresses reserved for documentation ([`is_documentation`](Ipv6Addr::is_documentation))
+    /// - Unique local addresses ([`is_unique_local`](Ipv6Addr::is_unique_local))
+    /// - Unicast addresses with link-local scope ([`is_unicast_link_local`](Ipv6Addr::is_unicast_link_local))
+    ///
+    /// For the complete overview of which addresses are globally reachable, see the table at the [IANA IPv6 Special-Purpose Address Registry].
+    ///
+    /// Note that an address having global scope is not the same as being globally reachable,
+    /// and there is no direct relation between the two concepts: There exist addresses with global scope
+    /// that are not globally reachable (for example unique local addresses),
+    /// and addresses that are globally reachable without having global scope
+    /// (multicast addresses with non-global scope).
+    ///
+    /// [IANA IPv6 Special-Purpose Address Registry]: https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml
+    /// [unspecified address]: Ipv6Addr::UNSPECIFIED
+    /// [loopback address]: Ipv6Addr::LOCALHOST
     ///
     /// # Examples
     ///
@@ -1315,20 +1333,65 @@ impl Ipv6Addr {
     ///
     /// use std::net::Ipv6Addr;
     ///
-    /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff).is_global(), true);
-    /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 0x1).is_global(), false);
-    /// assert_eq!(Ipv6Addr::new(0, 0, 0x1c9, 0, 0, 0xafc8, 0, 0x1).is_global(), true);
+    /// // Most IPv6 addresses are globally reachable:
+    /// assert_eq!(Ipv6Addr::new(0x26, 0, 0x1c9, 0, 0, 0xafc8, 0x10, 0x1).is_global(), true);
+    ///
+    /// // However some addresses have been assigned a special meaning
+    /// // that makes them not globally reachable. Some examples are:
+    ///
+    /// // The unspecified address (`::`)
+    /// assert_eq!(Ipv6Addr::UNSPECIFIED.is_global(), false);
+    ///
+    /// // The loopback address (`::1`)
+    /// assert_eq!(Ipv6Addr::LOCALHOST.is_global(), false);
+    ///
+    /// // IPv4-mapped addresses (`::ffff:0:0/96`)
+    /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff).is_global(), false);
+    ///
+    /// // Addresses reserved for benchmarking (`2001:2::/48`)
+    /// assert_eq!(Ipv6Addr::new(0x2001, 2, 0, 0, 0, 0, 0, 1,).is_global(), false);
+    ///
+    /// // Addresses reserved for documentation (`2001:db8::/32`)
+    /// assert_eq!(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 1).is_global(), false);
+    ///
+    /// // Unique local addresses (`fc00::/7`)
+    /// assert_eq!(Ipv6Addr::new(0xfc02, 0, 0, 0, 0, 0, 0, 1).is_global(), false);
+    ///
+    /// // Unicast addresses with link-local scope (`fe80::/10`)
+    /// assert_eq!(Ipv6Addr::new(0xfe81, 0, 0, 0, 0, 0, 0, 1).is_global(), false);
+    ///
+    /// // For a complete overview see the IANA IPv6 Special-Purpose Address Registry.
     /// ```
     #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
     #[unstable(feature = "ip", issue = "27709")]
     #[must_use]
     #[inline]
     pub const fn is_global(&self) -> bool {
-        match self.multicast_scope() {
-            Some(Ipv6MulticastScope::Global) => true,
-            None => self.is_unicast_global(),
-            _ => false,
-        }
+        !(self.is_unspecified()
+            || self.is_loopback()
+            // IPv4-mapped Address (`::ffff:0:0/96`)
+            || matches!(self.segments(), [0, 0, 0, 0, 0, 0xffff, _, _])
+            // IPv4-IPv6 Translat. (`64:ff9b:1::/48`)
+            || matches!(self.segments(), [0x64, 0xff9b, 1, _, _, _, _, _])
+            // Discard-Only Address Block (`100::/64`)
+            || matches!(self.segments(), [0x100, 0, 0, 0, _, _, _, _])
+            // IETF Protocol Assignments (`2001::/23`)
+            || (matches!(self.segments(), [0x2001, b, _, _, _, _, _, _] if b < 0x200)
+                && !(
+                    // Port Control Protocol Anycast (`2001:1::1`)
+                    u128::from_be_bytes(self.octets()) == 0x2001_0001_0000_0000_0000_0000_0000_0001
+                    // Traversal Using Relays around NAT Anycast (`2001:1::2`)
+                    || u128::from_be_bytes(self.octets()) == 0x2001_0001_0000_0000_0000_0000_0000_0002
+                    // AMT (`2001:3::/32`)
+                    || matches!(self.segments(), [0x2001, 3, _, _, _, _, _, _])
+                    // AS112-v6 (`2001:4:112::/48`)
+                    || matches!(self.segments(), [0x2001, 4, 0x112, _, _, _, _, _])
+                    // ORCHIDv2 (`2001:20::/28`)
+                    || matches!(self.segments(), [0x2001, b, _, _, _, _, _, _] if b >= 0x20 && b <= 0x2F)
+                ))
+            || self.is_documentation()
+            || self.is_unique_local()
+            || self.is_unicast_link_local())
     }
 
     /// Returns [`true`] if this is a unique local address (`fc00::/7`).
@@ -1525,6 +1588,7 @@ impl Ipv6Addr {
             && !self.is_unique_local()
             && !self.is_unspecified()
             && !self.is_documentation()
+            && !self.is_benchmarking()
     }
 
     /// Returns the address's multicast scope if the address is multicast.
@@ -1708,8 +1772,8 @@ impl Ipv6Addr {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for Ipv6Addr {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // If there are no alignment requirements, write out the IP address to
-        // f. Otherwise, write it to a local buffer, then use f.pad.
+        // If there are no alignment requirements, write the IP address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             let segments = self.segments();
 
@@ -1780,22 +1844,13 @@ impl fmt::Display for Ipv6Addr {
                 }
             }
         } else {
-            // Slow path: write the address to a local buffer, then use f.pad.
-            // Defined recursively by using the fast path to write to the
-            // buffer.
-
-            // This is the largest possible size of an IPv6 address
-            const IPV6_BUF_LEN: usize = (4 * 8) + 7;
-            let mut buf = [0u8; IPV6_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
-
-            // Note: This call to write should never fail, so unwrap is okay.
-            write!(buf_slice, "{}", self).unwrap();
-            let len = IPV6_BUF_LEN - buf_slice.len();
-
-            // This is safe because we know exactly what can be in this buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            const LONGEST_IPV6_ADDR: &str = "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff";
+
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV6_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv6 address, so this should never fail.
+            write!(buf, "{}", self).unwrap();
+
+            f.pad(buf.as_str())
         }
     }
 }
diff --git a/library/std/src/net/ip/tests.rs b/library/std/src/net/ip_addr/tests.rs
index c29509331..7c3430b2b 100644
--- a/library/std/src/net/ip/tests.rs
+++ b/library/std/src/net/ip_addr/tests.rs
@@ -321,15 +321,15 @@ fn ip_properties() {
     check!("fe80:ffff::");
     check!("febf:ffff::");
     check!("fec0::", global);
-    check!("ff01::", multicast);
-    check!("ff02::", multicast);
-    check!("ff03::", multicast);
-    check!("ff04::", multicast);
-    check!("ff05::", multicast);
-    check!("ff08::", multicast);
+    check!("ff01::", global | multicast);
+    check!("ff02::", global | multicast);
+    check!("ff03::", global | multicast);
+    check!("ff04::", global | multicast);
+    check!("ff05::", global | multicast);
+    check!("ff08::", global | multicast);
     check!("ff0e::", global | multicast);
     check!("2001:db8:85a3::8a2e:370:7334", doc);
-    check!("2001:2::ac32:23ff:21", global | benchmarking);
+    check!("2001:2::ac32:23ff:21", benchmarking);
     check!("102:304:506:708:90a:b0c:d0e:f10", global);
 }
 
@@ -609,6 +609,60 @@ fn ipv6_properties() {
 
     check!("1::", &[0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], global | unicast_global);
 
+    check!(
+        "::ffff:127.0.0.1",
+        &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 0x7f, 0, 0, 1],
+        unicast_global
+    );
+
+    check!(
+        "64:ff9b:1::",
+        &[0, 0x64, 0xff, 0x9b, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        unicast_global
+    );
+
+    check!("100::", &[0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], unicast_global);
+
+    check!("2001::", &[0x20, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], unicast_global);
+
+    check!(
+        "2001:1::1",
+        &[0x20, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1],
+        global | unicast_global
+    );
+
+    check!(
+        "2001:1::2",
+        &[0x20, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2],
+        global | unicast_global
+    );
+
+    check!(
+        "2001:3::",
+        &[0x20, 1, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        global | unicast_global
+    );
+
+    check!(
+        "2001:4:112::",
+        &[0x20, 1, 0, 4, 1, 0x12, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        global | unicast_global
+    );
+
+    check!(
+        "2001:20::",
+        &[0x20, 1, 0, 0x20, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        global | unicast_global
+    );
+
+    check!("2001:30::", &[0x20, 1, 0, 0x30, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], unicast_global);
+
+    check!(
+        "2001:200::",
+        &[0x20, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        global | unicast_global
+    );
+
     check!("fc00::", &[0xfc, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], unique_local);
 
     check!(
@@ -666,21 +720,37 @@ fn ipv6_properties() {
     check!(
         "ff01::",
         &[0xff, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
-        multicast_interface_local
+        multicast_interface_local | global
     );
 
-    check!("ff02::", &[0xff, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], multicast_link_local);
+    check!(
+        "ff02::",
+        &[0xff, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        multicast_link_local | global
+    );
 
-    check!("ff03::", &[0xff, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], multicast_realm_local);
+    check!(
+        "ff03::",
+        &[0xff, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        multicast_realm_local | global
+    );
 
-    check!("ff04::", &[0xff, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], multicast_admin_local);
+    check!(
+        "ff04::",
+        &[0xff, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        multicast_admin_local | global
+    );
 
-    check!("ff05::", &[0xff, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], multicast_site_local);
+    check!(
+        "ff05::",
+        &[0xff, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+        multicast_site_local | global
+    );
 
     check!(
         "ff08::",
         &[0xff, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
-        multicast_organization_local
+        multicast_organization_local | global
     );
 
     check!(
@@ -698,7 +768,7 @@ fn ipv6_properties() {
     check!(
         "2001:2::ac32:23ff:21",
         &[0x20, 1, 0, 2, 0, 0, 0, 0, 0, 0, 0xac, 0x32, 0x23, 0xff, 0, 0x21],
-        global | unicast_global | benchmarking
+        benchmarking
     );
 
     check!(
diff --git a/library/std/src/net/mod.rs b/library/std/src/net/mod.rs
index e7a40bdaf..01e3db9de 100644
--- a/library/std/src/net/mod.rs
+++ b/library/std/src/net/mod.rs
@@ -24,11 +24,11 @@
 use crate::io::{self, ErrorKind};
 
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use self::addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
-#[stable(feature = "rust1", since = "1.0.0")]
-pub use self::ip::{IpAddr, Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
+pub use self::ip_addr::{IpAddr, Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::parser::AddrParseError;
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::socket_addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
 #[unstable(feature = "tcplistener_into_incoming", issue = "88339")]
 pub use self::tcp::IntoIncoming;
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -36,12 +36,13 @@ pub use self::tcp::{Incoming, TcpListener, TcpStream};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::udp::UdpSocket;
 
-mod addr;
-mod ip;
+mod display_buffer;
+mod ip_addr;
 mod parser;
+mod socket_addr;
 mod tcp;
 #[cfg(test)]
-mod test;
+pub(crate) mod test;
 mod udp;
 
 /// Possible values which can be passed to the [`TcpStream::shutdown`] method.
diff --git a/library/std/src/net/parser.rs b/library/std/src/net/parser.rs
index 069b66099..a38031c48 100644
--- a/library/std/src/net/parser.rs
+++ b/library/std/src/net/parser.rs
@@ -39,8 +39,8 @@ struct Parser<'a> {
 }
 
 impl<'a> Parser<'a> {
-    fn new(input: &'a str) -> Parser<'a> {
-        Parser { state: input.as_bytes() }
+    fn new(input: &'a [u8]) -> Parser<'a> {
+        Parser { state: input }
     }
 
     /// Run a parser, and restore the pre-parse state if it fails.
@@ -273,32 +273,106 @@ impl<'a> Parser<'a> {
     }
 }
 
+impl IpAddr {
+    /// Parse an IP address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
+    ///
+    /// let localhost_v4 = IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1));
+    /// let localhost_v6 = IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1));
+    ///
+    /// assert_eq!(IpAddr::parse_ascii(b"127.0.0.1"), Ok(localhost_v4));
+    /// assert_eq!(IpAddr::parse_ascii(b"::1"), Ok(localhost_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+    }
+}
+
 #[stable(feature = "ip_addr", since = "1.7.0")]
 impl FromStr for IpAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl FromStr for Ipv4Addr {
-    type Err = AddrParseError;
-    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+impl Ipv4Addr {
+    /// Parse an IPv4 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv4Addr;
+    ///
+    /// let localhost = Ipv4Addr::new(127, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv4Addr::parse_ascii(b"127.0.0.1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
         // don't try to parse if too long
-        if s.len() > 15 {
+        if b.len() > 15 {
             Err(AddrParseError(AddrKind::Ipv4))
         } else {
-            Parser::new(s).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
+            Parser::new(b).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
         }
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
+impl FromStr for Ipv4Addr {
+    type Err = AddrParseError;
+    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl Ipv6Addr {
+    /// Parse an IPv6 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv6Addr;
+    ///
+    /// let localhost = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv6Addr::parse_ascii(b"::1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
 impl FromStr for Ipv6Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV4 {
+    /// Parse an IPv4 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv4Addr, SocketAddrV4};
+    ///
+    /// let socket = SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), 8080);
+    ///
+    /// assert_eq!(SocketAddrV4::parse_ascii(b"127.0.0.1:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
     }
 }
 
@@ -306,7 +380,25 @@ impl FromStr for Ipv6Addr {
 impl FromStr for SocketAddrV4 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV6 {
+    /// Parse an IPv6 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv6Addr, SocketAddrV6};
+    ///
+    /// let socket = SocketAddrV6::new(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 1), 8080, 0, 0);
+    ///
+    /// assert_eq!(SocketAddrV6::parse_ascii(b"[2001:db8::1]:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
     }
 }
 
@@ -314,7 +406,27 @@ impl FromStr for SocketAddrV4 {
 impl FromStr for SocketAddrV6 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddr {
+    /// Parse a socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr};
+    ///
+    /// let socket_v4 = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 8080);
+    /// let socket_v6 = SocketAddr::new(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1)), 8080);
+    ///
+    /// assert_eq!(SocketAddr::parse_ascii(b"127.0.0.1:8080"), Ok(socket_v4));
+    /// assert_eq!(SocketAddr::parse_ascii(b"[::1]:8080"), Ok(socket_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
     }
 }
 
@@ -322,7 +434,7 @@ impl FromStr for SocketAddrV6 {
 impl FromStr for SocketAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
diff --git a/library/std/src/net/addr.rs b/library/std/src/net/socket_addr.rs
index 53fee952a..33b0dfa03 100644
--- a/library/std/src/net/addr.rs
+++ b/library/std/src/net/socket_addr.rs
@@ -2,9 +2,9 @@
 mod tests;
 
 use crate::cmp::Ordering;
-use crate::fmt;
+use crate::fmt::{self, Write};
 use crate::hash;
-use crate::io::{self, Write};
+use crate::io;
 use crate::iter;
 use crate::mem;
 use crate::net::{IpAddr, Ipv4Addr, Ipv6Addr};
@@ -15,6 +15,8 @@ use crate::sys_common::net::LookupHost;
 use crate::sys_common::{FromInner, IntoInner};
 use crate::vec;
 
+use super::display_buffer::DisplayBuffer;
+
 /// An internet socket address, either IPv4 or IPv6.
 ///
 /// Internet socket addresses consist of an [IP address], a 16-bit port number, as well
@@ -616,25 +618,18 @@ impl fmt::Debug for SocketAddr {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for SocketAddrV4 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Fast path: if there's no alignment stuff, write to the output buffer
-        // directly
+        // If there are no alignment requirements, write the socket address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             write!(f, "{}:{}", self.ip(), self.port())
         } else {
-            const IPV4_SOCKET_BUF_LEN: usize = (3 * 4)  // the segments
-                + 3  // the separators
-                + 1 + 5; // the port
-            let mut buf = [0; IPV4_SOCKET_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
-
-            // Unwrap is fine because writing to a sufficiently-sized
-            // buffer is infallible
-            write!(buf_slice, "{}:{}", self.ip(), self.port()).unwrap();
-            let len = IPV4_SOCKET_BUF_LEN - buf_slice.len();
-
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            const LONGEST_IPV4_SOCKET_ADDR: &str = "255.255.255.255:65536";
+
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV4_SOCKET_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv4 socket address, so this should never fail.
+            write!(buf, "{}:{}", self.ip(), self.port()).unwrap();
+
+            f.pad(buf.as_str())
         }
     }
 }
@@ -649,35 +644,26 @@ impl fmt::Debug for SocketAddrV4 {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for SocketAddrV6 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Fast path: if there's no alignment stuff, write to the output
-        // buffer directly
+        // If there are no alignment requirements, write the socket address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             match self.scope_id() {
                 0 => write!(f, "[{}]:{}", self.ip(), self.port()),
                 scope_id => write!(f, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
             }
         } else {
-            const IPV6_SOCKET_BUF_LEN: usize = (4 * 8)  // The address
-            + 7  // The colon separators
-            + 2  // The brackets
-            + 1 + 10 // The scope id
-            + 1 + 5; // The port
-
-            let mut buf = [0; IPV6_SOCKET_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
+            const LONGEST_IPV6_SOCKET_ADDR: &str =
+                "[ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%4294967296]:65536";
 
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV6_SOCKET_ADDR.len() }>::new();
             match self.scope_id() {
-                0 => write!(buf_slice, "[{}]:{}", self.ip(), self.port()),
-                scope_id => write!(buf_slice, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
+                0 => write!(buf, "[{}]:{}", self.ip(), self.port()),
+                scope_id => write!(buf, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
             }
-            // Unwrap is fine because writing to a sufficiently-sized
-            // buffer is infallible
+            // Buffer is long enough for the longest possible IPv6 socket address, so this should never fail.
             .unwrap();
-            let len = IPV6_SOCKET_BUF_LEN - buf_slice.len();
 
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            f.pad(buf.as_str())
         }
     }
 }
diff --git a/library/std/src/net/addr/tests.rs b/library/std/src/net/socket_addr/tests.rs
index 585a17451..15211f819 100644
--- a/library/std/src/net/addr/tests.rs
+++ b/library/std/src/net/socket_addr/tests.rs
@@ -52,6 +52,75 @@ fn to_socket_addr_string() {
 }
 
 #[test]
+fn ipv4_socket_addr_to_string() {
+    // Shortest possible IPv4 length.
+    assert_eq!(SocketAddrV4::new(Ipv4Addr::new(0, 0, 0, 0), 0).to_string(), "0.0.0.0:0");
+
+    // Longest possible IPv4 length.
+    assert_eq!(
+        SocketAddrV4::new(Ipv4Addr::new(255, 255, 255, 255), u16::MAX).to_string(),
+        "255.255.255.255:65535"
+    );
+
+    // Test padding.
+    assert_eq!(
+        &format!("{:16}", SocketAddrV4::new(Ipv4Addr::new(1, 1, 1, 1), 53)),
+        "1.1.1.1:53      "
+    );
+    assert_eq!(
+        &format!("{:>16}", SocketAddrV4::new(Ipv4Addr::new(1, 1, 1, 1), 53)),
+        "      1.1.1.1:53"
+    );
+}
+
+#[test]
+fn ipv6_socket_addr_to_string() {
+    // IPv4-mapped address.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc000, 0x280), 8080, 0, 0)
+            .to_string(),
+        "[::ffff:192.0.2.128]:8080"
+    );
+
+    // IPv4-compatible address.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0xc000, 0x280), 8080, 0, 0).to_string(),
+        "[::192.0.2.128]:8080"
+    );
+
+    // IPv6 address with no zero segments.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(8, 9, 10, 11, 12, 13, 14, 15), 80, 0, 0).to_string(),
+        "[8:9:a:b:c:d:e:f]:80"
+    );
+
+    // Shortest possible IPv6 length.
+    assert_eq!(SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, 0, 0, 0).to_string(), "[::]:0");
+
+    // Longest possible IPv6 length.
+    assert_eq!(
+        SocketAddrV6::new(
+            Ipv6Addr::new(0x1111, 0x2222, 0x3333, 0x4444, 0x5555, 0x6666, 0x7777, 0x8888),
+            u16::MAX,
+            u32::MAX,
+            u32::MAX,
+        )
+        .to_string(),
+        "[1111:2222:3333:4444:5555:6666:7777:8888%4294967295]:65535"
+    );
+
+    // Test padding.
+    assert_eq!(
+        &format!("{:22}", SocketAddrV6::new(Ipv6Addr::new(1, 2, 3, 4, 5, 6, 7, 8), 9, 0, 0)),
+        "[1:2:3:4:5:6:7:8]:9   "
+    );
+    assert_eq!(
+        &format!("{:>22}", SocketAddrV6::new(Ipv6Addr::new(1, 2, 3, 4, 5, 6, 7, 8), 9, 0, 0)),
+        "   [1:2:3:4:5:6:7:8]:9"
+    );
+}
+
+#[test]
 fn bind_udp_socket_bad() {
     // rust-lang/rust#53957: This is a regression test for a parsing problem
     // discovered as part of issue rust-lang/rust#23076, where we were
diff --git a/library/std/src/os/android/mod.rs b/library/std/src/os/android/mod.rs
index dbb0127f3..5adcb82b6 100644
--- a/library/std/src/os/android/mod.rs
+++ b/library/std/src/os/android/mod.rs
@@ -3,4 +3,5 @@
 #![stable(feature = "raw_ext", since = "1.1.0")]
 
 pub mod fs;
+pub mod net;
 pub mod raw;
diff --git a/library/std/src/os/android/net.rs b/library/std/src/os/android/net.rs
new file mode 100644
index 000000000..ff96125c3
--- /dev/null
+++ b/library/std/src/os/android/net.rs
@@ -0,0 +1,4 @@
+//! Linux and Android-specific definitions for socket options.
+
+#![unstable(feature = "tcp_quickack", issue = "96256")]
+pub use crate::os::net::tcp::TcpStreamExt;
diff --git a/library/std/src/os/fd/owned.rs b/library/std/src/os/fd/owned.rs
index a463bc41d..71e33fb9e 100644
--- a/library/std/src/os/fd/owned.rs
+++ b/library/std/src/os/fd/owned.rs
@@ -104,7 +104,8 @@ impl BorrowedFd<'_> {
         #[cfg(target_os = "espidf")]
         let cmd = libc::F_DUPFD;
 
-        let fd = cvt(unsafe { libc::fcntl(self.as_raw_fd(), cmd, 0) })?;
+        // Avoid using file descriptors below 3 as they are used for stdio
+        let fd = cvt(unsafe { libc::fcntl(self.as_raw_fd(), cmd, 3) })?;
         Ok(unsafe { OwnedFd::from_raw_fd(fd) })
     }
 
diff --git a/library/std/src/os/fd/raw.rs b/library/std/src/os/fd/raw.rs
index 081915ed1..1b3d11042 100644
--- a/library/std/src/os/fd/raw.rs
+++ b/library/std/src/os/fd/raw.rs
@@ -14,7 +14,7 @@ use crate::os::wasi::io::OwnedFd;
 use crate::sys_common::{AsInner, IntoInner};
 
 /// Raw file descriptors.
-#[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
+#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub type RawFd = raw::c_int;
 
@@ -23,7 +23,7 @@ pub type RawFd = raw::c_int;
 /// This is only available on unix and WASI platforms and must be imported in
 /// order to call the method. Windows platforms have a corresponding
 /// `AsRawHandle` and `AsRawSocket` set of traits.
-#[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
+#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait AsRawFd {
     /// Extracts the raw file descriptor.
@@ -59,7 +59,7 @@ pub trait AsRawFd {
 
 /// A trait to express the ability to construct an object from a raw file
 /// descriptor.
-#[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
+#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "from_raw_os", since = "1.1.0")]
 pub trait FromRawFd {
     /// Constructs a new instance of `Self` from the given raw file
@@ -103,7 +103,7 @@ pub trait FromRawFd {
 
 /// A trait to express the ability to consume an object and acquire ownership of
 /// its raw file descriptor.
-#[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
+#[rustc_allowed_through_unstable_modules]
 #[stable(feature = "into_raw_os", since = "1.4.0")]
 pub trait IntoRawFd {
     /// Consumes this object, returning the raw underlying file descriptor.
diff --git a/library/std/src/os/fortanix_sgx/mod.rs b/library/std/src/os/fortanix_sgx/mod.rs
index a40dabe19..39a42f4e1 100644
--- a/library/std/src/os/fortanix_sgx/mod.rs
+++ b/library/std/src/os/fortanix_sgx/mod.rs
@@ -26,10 +26,13 @@ pub mod usercalls {
             free, insecure_time, launch_thread, read, read_alloc, send, wait, write,
         };
         pub use crate::sys::abi::usercalls::raw::{do_usercall, Usercalls as UsercallNrs};
+        pub use crate::sys::abi::usercalls::raw::{Register, RegisterArgument, ReturnValue};
 
         // fortanix-sgx-abi re-exports
         pub use crate::sys::abi::usercalls::raw::Error;
-        pub use crate::sys::abi::usercalls::raw::{ByteBuffer, FifoDescriptor, Return, Usercall};
+        pub use crate::sys::abi::usercalls::raw::{
+            ByteBuffer, Cancel, FifoDescriptor, Return, Usercall,
+        };
         pub use crate::sys::abi::usercalls::raw::{Fd, Result, Tcs};
         pub use crate::sys::abi::usercalls::raw::{
             EV_RETURNQ_NOT_EMPTY, EV_UNPARK, EV_USERCALLQ_NOT_FULL, FD_STDERR, FD_STDIN, FD_STDOUT,
diff --git a/library/std/src/os/linux/mod.rs b/library/std/src/os/linux/mod.rs
index 8e7776f66..c17053011 100644
--- a/library/std/src/os/linux/mod.rs
+++ b/library/std/src/os/linux/mod.rs
@@ -4,5 +4,6 @@
 #![doc(cfg(target_os = "linux"))]
 
 pub mod fs;
+pub mod net;
 pub mod process;
 pub mod raw;
diff --git a/library/std/src/os/linux/net.rs b/library/std/src/os/linux/net.rs
new file mode 100644
index 000000000..ff96125c3
--- /dev/null
+++ b/library/std/src/os/linux/net.rs
@@ -0,0 +1,4 @@
+//! Linux and Android-specific definitions for socket options.
+
+#![unstable(feature = "tcp_quickack", issue = "96256")]
+pub use crate::os::net::tcp::TcpStreamExt;
diff --git a/library/std/src/os/mod.rs b/library/std/src/os/mod.rs
index 6fbaa42c7..18c64b510 100644
--- a/library/std/src/os/mod.rs
+++ b/library/std/src/os/mod.rs
@@ -148,3 +148,6 @@ pub mod vxworks;
 
 #[cfg(any(unix, target_os = "wasi", doc))]
 mod fd;
+
+#[cfg(any(target_os = "linux", target_os = "android", doc))]
+mod net;
diff --git a/library/std/src/os/net/mod.rs b/library/std/src/os/net/mod.rs
new file mode 100644
index 000000000..d6d84d24e
--- /dev/null
+++ b/library/std/src/os/net/mod.rs
@@ -0,0 +1,7 @@
+//! Linux and Android-specific definitions for socket options.
+
+#![unstable(feature = "tcp_quickack", issue = "96256")]
+#![doc(cfg(any(target_os = "linux", target_os = "android",)))]
+pub mod tcp;
+#[cfg(test)]
+mod tests;
diff --git a/library/std/src/os/net/tcp.rs b/library/std/src/os/net/tcp.rs
new file mode 100644
index 000000000..5e9ee65a4
--- /dev/null
+++ b/library/std/src/os/net/tcp.rs
@@ -0,0 +1,70 @@
+//! Linux and Android-specific tcp extensions to primitives in the [`std::net`] module.
+//!
+//! [`std::net`]: crate::net
+
+use crate::io;
+use crate::net;
+use crate::sealed::Sealed;
+use crate::sys_common::AsInner;
+
+/// Os-specific extensions for [`TcpStream`]
+///
+/// [`TcpStream`]: net::TcpStream
+#[unstable(feature = "tcp_quickack", issue = "96256")]
+pub trait TcpStreamExt: Sealed {
+    /// Enable or disable `TCP_QUICKACK`.
+    ///
+    /// This flag causes Linux to eagerly send ACKs rather than delaying them.
+    /// Linux may reset this flag after further operations on the socket.
+    ///
+    /// See [`man 7 tcp`](https://man7.org/linux/man-pages/man7/tcp.7.html) and
+    /// [TCP delayed acknowledgement](https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment)
+    /// for more information.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(tcp_quickack)]
+    /// use std::net::TcpStream;
+    /// use std::os::linux::net::TcpStreamExt;
+    ///
+    /// let stream = TcpStream::connect("127.0.0.1:8080")
+    ///         .expect("Couldn't connect to the server...");
+    /// stream.set_quickack(true).expect("set_quickack call failed");
+    /// ```
+    #[unstable(feature = "tcp_quickack", issue = "96256")]
+    fn set_quickack(&self, quickack: bool) -> io::Result<()>;
+
+    /// Gets the value of the `TCP_QUICKACK` option on this socket.
+    ///
+    /// For more information about this option, see [`TcpStreamExt::set_quickack`].
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(tcp_quickack)]
+    /// use std::net::TcpStream;
+    /// use std::os::linux::net::TcpStreamExt;
+    ///
+    /// let stream = TcpStream::connect("127.0.0.1:8080")
+    ///         .expect("Couldn't connect to the server...");
+    /// stream.set_quickack(true).expect("set_quickack call failed");
+    /// assert_eq!(stream.quickack().unwrap_or(false), true);
+    /// ```
+    #[unstable(feature = "tcp_quickack", issue = "96256")]
+    fn quickack(&self) -> io::Result<bool>;
+}
+
+#[unstable(feature = "tcp_quickack", issue = "96256")]
+impl Sealed for net::TcpStream {}
+
+#[unstable(feature = "tcp_quickack", issue = "96256")]
+impl TcpStreamExt for net::TcpStream {
+    fn set_quickack(&self, quickack: bool) -> io::Result<()> {
+        self.as_inner().as_inner().set_quickack(quickack)
+    }
+
+    fn quickack(&self) -> io::Result<bool> {
+        self.as_inner().as_inner().quickack()
+    }
+}
diff --git a/library/std/src/os/net/tests.rs b/library/std/src/os/net/tests.rs
new file mode 100644
index 000000000..4704e3156
--- /dev/null
+++ b/library/std/src/os/net/tests.rs
@@ -0,0 +1,29 @@
+#[cfg(any(target_os = "android", target_os = "linux",))]
+#[test]
+fn quickack() {
+    use crate::{
+        net::{test::next_test_ip4, TcpListener, TcpStream},
+        os::net::tcp::TcpStreamExt,
+    };
+
+    macro_rules! t {
+        ($e:expr) => {
+            match $e {
+                Ok(t) => t,
+                Err(e) => panic!("received error for `{}`: {}", stringify!($e), e),
+            }
+        };
+    }
+
+    let addr = next_test_ip4();
+    let _listener = t!(TcpListener::bind(&addr));
+
+    let stream = t!(TcpStream::connect(&("localhost", addr.port())));
+
+    t!(stream.set_quickack(false));
+    assert_eq!(false, t!(stream.quickack()));
+    t!(stream.set_quickack(true));
+    assert_eq!(true, t!(stream.quickack()));
+    t!(stream.set_quickack(false));
+    assert_eq!(false, t!(stream.quickack()));
+}
diff --git a/library/std/src/os/unix/net/addr.rs b/library/std/src/os/unix/net/addr.rs
index 9aeae4b2c..094085e19 100644
--- a/library/std/src/os/unix/net/addr.rs
+++ b/library/std/src/os/unix/net/addr.rs
@@ -2,7 +2,7 @@ use crate::ffi::OsStr;
 use crate::os::unix::ffi::OsStrExt;
 use crate::path::Path;
 use crate::sys::cvt;
-use crate::{ascii, fmt, io, mem, ptr};
+use crate::{fmt, io, mem, ptr};
 
 // FIXME(#43348): Make libc adapt #[doc(cfg(...))] so we don't need these fake definitions here?
 #[cfg(not(unix))]
@@ -64,18 +64,6 @@ enum AddressKind<'a> {
     Abstract(&'a [u8]),
 }
 
-struct AsciiEscaped<'a>(&'a [u8]);
-
-impl<'a> fmt::Display for AsciiEscaped<'a> {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(fmt, "\"")?;
-        for byte in self.0.iter().cloned().flat_map(ascii::escape_default) {
-            write!(fmt, "{}", byte as char)?;
-        }
-        write!(fmt, "\"")
-    }
-}
-
 /// An address associated with a Unix socket.
 ///
 /// # Examples
@@ -329,7 +317,7 @@ impl SocketAddr {
 
             crate::ptr::copy_nonoverlapping(
                 namespace.as_ptr(),
-                addr.sun_path.as_mut_ptr().offset(1) as *mut u8,
+                addr.sun_path.as_mut_ptr().add(1) as *mut u8,
                 namespace.len(),
             );
             let len = (sun_path_offset(&addr) + 1 + namespace.len()) as libc::socklen_t;
@@ -343,7 +331,7 @@ impl fmt::Debug for SocketAddr {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self.address() {
             AddressKind::Unnamed => write!(fmt, "(unnamed)"),
-            AddressKind::Abstract(name) => write!(fmt, "{} (abstract)", AsciiEscaped(name)),
+            AddressKind::Abstract(name) => write!(fmt, "\"{}\" (abstract)", name.escape_ascii()),
             AddressKind::Pathname(path) => write!(fmt, "{path:?} (pathname)"),
         }
     }
diff --git a/library/std/src/os/unix/net/datagram.rs b/library/std/src/os/unix/net/datagram.rs
index 8008acfd1..f758f88d0 100644
--- a/library/std/src/os/unix/net/datagram.rs
+++ b/library/std/src/os/unix/net/datagram.rs
@@ -838,6 +838,31 @@ impl UnixDatagram {
         self.0.passcred()
     }
 
+    /// Set the id of the socket for network filtering purpose
+    ///
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
+    /// #![feature(unix_set_mark)]
+    /// use std::os::unix::net::UnixDatagram;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let sock = UnixDatagram::unbound()?;
+    ///     sock.set_mark(32)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(any(doc, target_os = "linux", target_os = "freebsd", target_os = "openbsd",))]
+    #[unstable(feature = "unix_set_mark", issue = "96467")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        self.0.set_mark(mark)
+    }
+
     /// Returns the value of the `SO_ERROR` option.
     ///
     /// # Examples
diff --git a/library/std/src/os/unix/net/listener.rs b/library/std/src/os/unix/net/listener.rs
index 7c0d53950..02090afc8 100644
--- a/library/std/src/os/unix/net/listener.rs
+++ b/library/std/src/os/unix/net/listener.rs
@@ -73,9 +73,11 @@ impl UnixListener {
         unsafe {
             let inner = Socket::new_raw(libc::AF_UNIX, libc::SOCK_STREAM)?;
             let (addr, len) = sockaddr_un(path.as_ref())?;
+            const backlog: libc::c_int =
+                if cfg!(any(target_os = "linux", target_os = "freebsd")) { -1 } else { 128 };
 
             cvt(libc::bind(inner.as_inner().as_raw_fd(), &addr as *const _ as *const _, len as _))?;
-            cvt(libc::listen(inner.as_inner().as_raw_fd(), 128))?;
+            cvt(libc::listen(inner.as_inner().as_raw_fd(), backlog))?;
 
             Ok(UnixListener(inner))
         }
@@ -109,12 +111,16 @@ impl UnixListener {
     pub fn bind_addr(socket_addr: &SocketAddr) -> io::Result<UnixListener> {
         unsafe {
             let inner = Socket::new_raw(libc::AF_UNIX, libc::SOCK_STREAM)?;
+            #[cfg(target_os = "linux")]
+            const backlog: libc::c_int = -1;
+            #[cfg(not(target_os = "linux"))]
+            const backlog: libc::c_int = 128;
             cvt(libc::bind(
                 inner.as_raw_fd(),
                 &socket_addr.addr as *const _ as *const _,
                 socket_addr.len as _,
             ))?;
-            cvt(libc::listen(inner.as_raw_fd(), 128))?;
+            cvt(libc::listen(inner.as_raw_fd(), backlog))?;
             Ok(UnixListener(inner))
         }
     }
diff --git a/library/std/src/os/unix/net/stream.rs b/library/std/src/os/unix/net/stream.rs
index cc3a88587..dff8f6e85 100644
--- a/library/std/src/os/unix/net/stream.rs
+++ b/library/std/src/os/unix/net/stream.rs
@@ -427,6 +427,31 @@ impl UnixStream {
         self.0.passcred()
     }
 
+    /// Set the id of the socket for network filtering purpose
+    ///
+    #[cfg_attr(
+        any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"),
+        doc = "```no_run"
+    )]
+    #[cfg_attr(
+        not(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd")),
+        doc = "```ignore"
+    )]
+    /// #![feature(unix_set_mark)]
+    /// use std::os::unix::net::UnixStream;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let sock = UnixStream::connect("/tmp/sock")?;
+    ///     sock.set_mark(32)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(any(doc, target_os = "linux", target_os = "freebsd", target_os = "openbsd",))]
+    #[unstable(feature = "unix_set_mark", issue = "96467")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        self.0.set_mark(mark)
+    }
+
     /// Returns the value of the `SO_ERROR` option.
     ///
     /// # Examples
diff --git a/library/std/src/os/wasi/io/fd.rs b/library/std/src/os/wasi/io/fd.rs
index 930aca887..75703af6a 100644
--- a/library/std/src/os/wasi/io/fd.rs
+++ b/library/std/src/os/wasi/io/fd.rs
@@ -1,9 +1,10 @@
 //! Owned and borrowed file descriptors.
 
-#![unstable(feature = "wasi_ext", issue = "71213")]
+#![stable(feature = "io_safety_wasi", since = "1.65.0")]
 
 // Tests for this module
 #[cfg(test)]
 mod tests;
 
+#[stable(feature = "io_safety_wasi", since = "1.65.0")]
 pub use crate::os::fd::owned::*;
diff --git a/library/std/src/os/wasi/io/mod.rs b/library/std/src/os/wasi/io/mod.rs
index 6c884e2ea..4f5cfbf9a 100644
--- a/library/std/src/os/wasi/io/mod.rs
+++ b/library/std/src/os/wasi/io/mod.rs
@@ -1,12 +1,12 @@
 //! WASI-specific extensions to general I/O primitives.
 
 #![deny(unsafe_op_in_unsafe_fn)]
-#![unstable(feature = "wasi_ext", issue = "71213")]
+#![stable(feature = "io_safety_wasi", since = "1.65.0")]
 
 mod fd;
 mod raw;
 
-#[unstable(feature = "wasi_ext", issue = "71213")]
+#[stable(feature = "io_safety_wasi", since = "1.65.0")]
 pub use fd::*;
-#[unstable(feature = "wasi_ext", issue = "71213")]
+#[stable(feature = "io_safety_wasi", since = "1.65.0")]
 pub use raw::*;
diff --git a/library/std/src/os/wasi/io/raw.rs b/library/std/src/os/wasi/io/raw.rs
index da3b36ada..4ac792ee8 100644
--- a/library/std/src/os/wasi/io/raw.rs
+++ b/library/std/src/os/wasi/io/raw.rs
@@ -1,20 +1,6 @@
 //! WASI-specific extensions to general I/O primitives.
 
-#![unstable(feature = "wasi_ext", issue = "71213")]
+#![stable(feature = "io_safety_wasi", since = "1.65.0")]
 
-// NOTE: despite the fact that this module is unstable,
-// stable Rust had the capability to access the stable
-// re-exported items from os::fd::raw through this
-// unstable module.
-// In PR #95956 the stability checker was changed to check
-// all path segments of an item rather than just the last,
-// which caused the aforementioned stable usage to regress
-// (see issue #99502).
-// As a result, the items in os::fd::raw were given the
-// rustc_allowed_through_unstable_modules attribute.
-// No regression tests were added to ensure this property,
-// as CI is not configured to test wasm32-wasi.
-// If this module is stabilized,
-// you may want to remove those attributes
-// (assuming no other unstable modules need them).
+#[stable(feature = "io_safety_wasi", since = "1.65.0")]
 pub use crate::os::fd::raw::*;
diff --git a/library/std/src/panic.rs b/library/std/src/panic.rs
index 45bc56efb..c4f022de0 100644
--- a/library/std/src/panic.rs
+++ b/library/std/src/panic.rs
@@ -295,23 +295,22 @@ pub fn get_backtrace_style() -> Option<BacktraceStyle> {
         return Some(style);
     }
 
-    // Setting environment variables for Fuchsia components isn't a standard
-    // or easily supported workflow. For now, display backtraces by default.
-    let format = if cfg!(target_os = "fuchsia") {
-        BacktraceStyle::Full
-    } else {
-        crate::env::var_os("RUST_BACKTRACE")
-            .map(|x| {
-                if &x == "0" {
-                    BacktraceStyle::Off
-                } else if &x == "full" {
-                    BacktraceStyle::Full
-                } else {
-                    BacktraceStyle::Short
-                }
-            })
-            .unwrap_or(BacktraceStyle::Off)
-    };
+    let format = crate::env::var_os("RUST_BACKTRACE")
+        .map(|x| {
+            if &x == "0" {
+                BacktraceStyle::Off
+            } else if &x == "full" {
+                BacktraceStyle::Full
+            } else {
+                BacktraceStyle::Short
+            }
+        })
+        .unwrap_or(if cfg!(target_os = "fuchsia") {
+            // Fuchsia components default to full backtrace.
+            BacktraceStyle::Full
+        } else {
+            BacktraceStyle::Off
+        });
     set_backtrace_style(format);
     Some(format)
 }
diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index 351cf6988..dd307022c 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -1768,6 +1768,7 @@ fn test_windows_absolute() {
 }
 
 #[bench]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_path_cmp_fast_path_buf_sort(b: &mut test::Bencher) {
     let prefix = "my/home";
     let mut paths: Vec<_> =
@@ -1781,6 +1782,7 @@ fn bench_path_cmp_fast_path_buf_sort(b: &mut test::Bencher) {
 }
 
 #[bench]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_path_cmp_fast_path_long(b: &mut test::Bencher) {
     let prefix = "/my/home/is/my/castle/and/my/castle/has/a/rusty/workbench/";
     let paths: Vec<_> =
@@ -1799,6 +1801,7 @@ fn bench_path_cmp_fast_path_long(b: &mut test::Bencher) {
 }
 
 #[bench]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_path_cmp_fast_path_short(b: &mut test::Bencher) {
     let prefix = "my/home";
     let paths: Vec<_> =
@@ -1817,6 +1820,7 @@ fn bench_path_cmp_fast_path_short(b: &mut test::Bencher) {
 }
 
 #[bench]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_path_hashset(b: &mut test::Bencher) {
     let prefix = "/my/home/is/my/castle/and/my/castle/has/a/rusty/workbench/";
     let paths: Vec<_> =
@@ -1835,6 +1839,7 @@ fn bench_path_hashset(b: &mut test::Bencher) {
 }
 
 #[bench]
+#[cfg_attr(miri, ignore)] // Miri isn't fast...
 fn bench_path_hashset_miss(b: &mut test::Bencher) {
     let prefix = "/my/home/is/my/castle/and/my/castle/has/a/rusty/workbench/";
     let paths: Vec<_> =
diff --git a/library/std/src/personality.rs b/library/std/src/personality.rs
new file mode 100644
index 000000000..63f0ad4f1
--- /dev/null
+++ b/library/std/src/personality.rs
@@ -0,0 +1,46 @@
+//! This module contains the implementation of the `eh_personality` lang item.
+//!
+//! The actual implementation is heavily dependent on the target since Rust
+//! tries to use the native stack unwinding mechanism whenever possible.
+//!
+//! This personality function is still required with `-C panic=abort` because
+//! it is used to catch foreign exceptions from `extern "C-unwind"` and turn
+//! them into aborts.
+//!
+//! Additionally, ARM EHABI uses the personality function when generating
+//! backtraces.
+
+mod dwarf;
+
+#[cfg(not(test))]
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "emscripten")] {
+        mod emcc;
+    } else if #[cfg(target_env = "msvc")] {
+        // 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 is the personality function that is always used.
+        // Hence this is just an aborting stub.
+        #[lang = "eh_personality"]
+        fn rust_eh_personality() {
+            core::intrinsics::abort()
+        }
+    } else if #[cfg(any(
+        all(target_family = "windows", target_env = "gnu"),
+        target_os = "psp",
+        target_os = "solid_asp3",
+        all(target_family = "unix", not(target_os = "espidf")),
+        all(target_vendor = "fortanix", target_env = "sgx"),
+    ))] {
+        mod gcc;
+    } else {
+        // Targets that don't support unwinding.
+        // - family=wasm
+        // - os=none ("bare metal" targets)
+        // - os=uefi
+        // - os=espidf
+        // - os=hermit
+        // - nvptx64-nvidia-cuda
+        // - arch=avr
+    }
+}
diff --git a/library/std/src/personality/dwarf/eh.rs b/library/std/src/personality/dwarf/eh.rs
new file mode 100644
index 000000000..8799137b7
--- /dev/null
+++ b/library/std/src/personality/dwarf/eh.rs
@@ -0,0 +1,192 @@
+//! Parsing of GCC-style Language-Specific Data Area (LSDA)
+//! For details see:
+//!  * <https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html>
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/exceptions.pdf>
+//!  * <https://www.airs.com/blog/archives/460>
+//!  * <https://www.airs.com/blog/archives/464>
+//!
+//! A reference implementation may be found in the GCC source tree
+//! (`<root>/libgcc/unwind-c.c` as of this writing).
+
+#![allow(non_upper_case_globals)]
+#![allow(unused)]
+
+use super::DwarfReader;
+use core::mem;
+
+pub const DW_EH_PE_omit: u8 = 0xFF;
+pub const DW_EH_PE_absptr: u8 = 0x00;
+
+pub const DW_EH_PE_uleb128: u8 = 0x01;
+pub const DW_EH_PE_udata2: u8 = 0x02;
+pub const DW_EH_PE_udata4: u8 = 0x03;
+pub const DW_EH_PE_udata8: u8 = 0x04;
+pub const DW_EH_PE_sleb128: u8 = 0x09;
+pub const DW_EH_PE_sdata2: u8 = 0x0A;
+pub const DW_EH_PE_sdata4: u8 = 0x0B;
+pub const DW_EH_PE_sdata8: u8 = 0x0C;
+
+pub const DW_EH_PE_pcrel: u8 = 0x10;
+pub const DW_EH_PE_textrel: u8 = 0x20;
+pub const DW_EH_PE_datarel: u8 = 0x30;
+pub const DW_EH_PE_funcrel: u8 = 0x40;
+pub const DW_EH_PE_aligned: u8 = 0x50;
+
+pub const DW_EH_PE_indirect: u8 = 0x80;
+
+#[derive(Copy, Clone)]
+pub struct EHContext<'a> {
+    pub ip: usize,                             // Current instruction pointer
+    pub func_start: usize,                     // Address of the current function
+    pub get_text_start: &'a dyn Fn() -> usize, // Get address of the code section
+    pub get_data_start: &'a dyn Fn() -> usize, // Get address of the data section
+}
+
+pub enum EHAction {
+    None,
+    Cleanup(usize),
+    Catch(usize),
+    Terminate,
+}
+
+pub const USING_SJLJ_EXCEPTIONS: bool = cfg!(all(target_os = "ios", target_arch = "arm"));
+
+pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>) -> Result<EHAction, ()> {
+    if lsda.is_null() {
+        return Ok(EHAction::None);
+    }
+
+    let func_start = context.func_start;
+    let mut reader = DwarfReader::new(lsda);
+
+    let start_encoding = reader.read::<u8>();
+    // base address for landing pad offsets
+    let lpad_base = if start_encoding != DW_EH_PE_omit {
+        read_encoded_pointer(&mut reader, context, start_encoding)?
+    } else {
+        func_start
+    };
+
+    let ttype_encoding = reader.read::<u8>();
+    if ttype_encoding != DW_EH_PE_omit {
+        // Rust doesn't analyze exception types, so we don't care about the type table
+        reader.read_uleb128();
+    }
+
+    let call_site_encoding = reader.read::<u8>();
+    let call_site_table_length = reader.read_uleb128();
+    let action_table = reader.ptr.add(call_site_table_length as usize);
+    let ip = context.ip;
+
+    if !USING_SJLJ_EXCEPTIONS {
+        while reader.ptr < action_table {
+            let cs_start = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_len = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_lpad = read_encoded_pointer(&mut reader, context, call_site_encoding)?;
+            let cs_action = reader.read_uleb128();
+            // Callsite table is sorted by cs_start, so if we've passed the ip, we
+            // may stop searching.
+            if ip < func_start + cs_start {
+                break;
+            }
+            if ip < func_start + cs_start + cs_len {
+                if cs_lpad == 0 {
+                    return Ok(EHAction::None);
+                } else {
+                    let lpad = lpad_base + cs_lpad;
+                    return Ok(interpret_cs_action(cs_action, lpad));
+                }
+            }
+        }
+        // Ip is not present in the table.  This should not happen... but it does: issue #35011.
+        // So rather than returning EHAction::Terminate, we do this.
+        Ok(EHAction::None)
+    } else {
+        // SjLj version:
+        // The "IP" is an index into the call-site table, with two exceptions:
+        // -1 means 'no-action', and 0 means 'terminate'.
+        match ip as isize {
+            -1 => return Ok(EHAction::None),
+            0 => return Ok(EHAction::Terminate),
+            _ => (),
+        }
+        let mut idx = ip;
+        loop {
+            let cs_lpad = reader.read_uleb128();
+            let cs_action = reader.read_uleb128();
+            idx -= 1;
+            if idx == 0 {
+                // Can never have null landing pad for sjlj -- that would have
+                // been indicated by a -1 call site index.
+                let lpad = (cs_lpad + 1) as usize;
+                return Ok(interpret_cs_action(cs_action, lpad));
+            }
+        }
+    }
+}
+
+fn interpret_cs_action(cs_action: u64, lpad: usize) -> EHAction {
+    if cs_action == 0 {
+        // If cs_action is 0 then this is a cleanup (Drop::drop). We run these
+        // for both Rust panics and foreign exceptions.
+        EHAction::Cleanup(lpad)
+    } else {
+        // Stop unwinding Rust panics at catch_unwind.
+        EHAction::Catch(lpad)
+    }
+}
+
+#[inline]
+fn round_up(unrounded: usize, align: usize) -> Result<usize, ()> {
+    if align.is_power_of_two() { Ok((unrounded + align - 1) & !(align - 1)) } else { Err(()) }
+}
+
+unsafe fn read_encoded_pointer(
+    reader: &mut DwarfReader,
+    context: &EHContext<'_>,
+    encoding: u8,
+) -> Result<usize, ()> {
+    if encoding == DW_EH_PE_omit {
+        return Err(());
+    }
+
+    // DW_EH_PE_aligned implies it's an absolute pointer value
+    if encoding == DW_EH_PE_aligned {
+        reader.ptr = round_up(reader.ptr as usize, mem::size_of::<usize>())? as *const u8;
+        return Ok(reader.read::<usize>());
+    }
+
+    let mut result = match encoding & 0x0F {
+        DW_EH_PE_absptr => reader.read::<usize>(),
+        DW_EH_PE_uleb128 => reader.read_uleb128() as usize,
+        DW_EH_PE_udata2 => reader.read::<u16>() as usize,
+        DW_EH_PE_udata4 => reader.read::<u32>() as usize,
+        DW_EH_PE_udata8 => reader.read::<u64>() as usize,
+        DW_EH_PE_sleb128 => reader.read_sleb128() as usize,
+        DW_EH_PE_sdata2 => reader.read::<i16>() as usize,
+        DW_EH_PE_sdata4 => reader.read::<i32>() as usize,
+        DW_EH_PE_sdata8 => reader.read::<i64>() as usize,
+        _ => return Err(()),
+    };
+
+    result += match encoding & 0x70 {
+        DW_EH_PE_absptr => 0,
+        // relative to address of the encoded value, despite the name
+        DW_EH_PE_pcrel => reader.ptr as usize,
+        DW_EH_PE_funcrel => {
+            if context.func_start == 0 {
+                return Err(());
+            }
+            context.func_start
+        }
+        DW_EH_PE_textrel => (*context.get_text_start)(),
+        DW_EH_PE_datarel => (*context.get_data_start)(),
+        _ => return Err(()),
+    };
+
+    if encoding & DW_EH_PE_indirect != 0 {
+        result = *(result as *const usize);
+    }
+
+    Ok(result)
+}
diff --git a/library/std/src/personality/dwarf/mod.rs b/library/std/src/personality/dwarf/mod.rs
new file mode 100644
index 000000000..652fbe95a
--- /dev/null
+++ b/library/std/src/personality/dwarf/mod.rs
@@ -0,0 +1,73 @@
+//! Utilities for parsing DWARF-encoded data streams.
+//! See <http://www.dwarfstd.org>,
+//! DWARF-4 standard, Section 7 - "Data Representation"
+
+// This module is used only by x86_64-pc-windows-gnu for now, but we
+// are compiling it everywhere to avoid regressions.
+#![allow(unused)]
+
+#[cfg(test)]
+mod tests;
+
+pub mod eh;
+
+use core::mem;
+
+pub struct DwarfReader {
+    pub ptr: *const u8,
+}
+
+#[repr(C, packed)]
+struct Unaligned<T>(T);
+
+impl DwarfReader {
+    pub fn new(ptr: *const u8) -> DwarfReader {
+        DwarfReader { ptr }
+    }
+
+    // DWARF streams are packed, so e.g., a u32 would not necessarily be aligned
+    // on a 4-byte boundary. This may cause problems on platforms with strict
+    // alignment requirements. By wrapping data in a "packed" struct, we are
+    // telling the backend to generate "misalignment-safe" code.
+    pub unsafe fn read<T: Copy>(&mut self) -> T {
+        let Unaligned(result) = *(self.ptr as *const Unaligned<T>);
+        self.ptr = self.ptr.add(mem::size_of::<T>());
+        result
+    }
+
+    // ULEB128 and SLEB128 encodings are defined in Section 7.6 - "Variable
+    // Length Data".
+    pub unsafe fn read_uleb128(&mut self) -> u64 {
+        let mut shift: usize = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        result
+    }
+
+    pub unsafe fn read_sleb128(&mut self) -> i64 {
+        let mut shift: u32 = 0;
+        let mut result: u64 = 0;
+        let mut byte: u8;
+        loop {
+            byte = self.read::<u8>();
+            result |= ((byte & 0x7F) as u64) << shift;
+            shift += 7;
+            if byte & 0x80 == 0 {
+                break;
+            }
+        }
+        // sign-extend
+        if shift < u64::BITS && (byte & 0x40) != 0 {
+            result |= (!0 as u64) << shift;
+        }
+        result as i64
+    }
+}
diff --git a/library/std/src/personality/dwarf/tests.rs b/library/std/src/personality/dwarf/tests.rs
new file mode 100644
index 000000000..1644f3708
--- /dev/null
+++ b/library/std/src/personality/dwarf/tests.rs
@@ -0,0 +1,19 @@
+use super::*;
+
+#[test]
+fn dwarf_reader() {
+    let encoded: &[u8] = &[1, 2, 3, 4, 5, 6, 7, 0xE5, 0x8E, 0x26, 0x9B, 0xF1, 0x59, 0xFF, 0xFF];
+
+    let mut reader = DwarfReader::new(encoded.as_ptr());
+
+    unsafe {
+        assert!(reader.read::<u8>() == u8::to_be(1u8));
+        assert!(reader.read::<u16>() == u16::to_be(0x0203));
+        assert!(reader.read::<u32>() == u32::to_be(0x04050607));
+
+        assert!(reader.read_uleb128() == 624485);
+        assert!(reader.read_sleb128() == -624485);
+
+        assert!(reader.read::<i8>() == i8::to_be(-1));
+    }
+}
diff --git a/library/std/src/personality/emcc.rs b/library/std/src/personality/emcc.rs
new file mode 100644
index 000000000..f942bdf18
--- /dev/null
+++ b/library/std/src/personality/emcc.rs
@@ -0,0 +1,20 @@
+//! On Emscripten Rust panics are wrapped in C++ exceptions, so we just forward
+//! to `__gxx_personality_v0` which is provided by Emscripten.
+
+use libc::c_int;
+use unwind as uw;
+
+// This is required by the compiler to exist (e.g., it's a lang item), but it's
+// never actually called by the compiler.  Emscripten EH doesn't use a
+// personality function at all, it instead uses __cxa_find_matching_catch.
+// Wasm error handling would use __gxx_personality_wasm0.
+#[lang = "eh_personality"]
+unsafe extern "C" fn rust_eh_personality(
+    _version: c_int,
+    _actions: uw::_Unwind_Action,
+    _exception_class: uw::_Unwind_Exception_Class,
+    _exception_object: *mut uw::_Unwind_Exception,
+    _context: *mut uw::_Unwind_Context,
+) -> uw::_Unwind_Reason_Code {
+    core::intrinsics::abort()
+}
diff --git a/library/std/src/personality/gcc.rs b/library/std/src/personality/gcc.rs
new file mode 100644
index 000000000..7f0b0439c
--- /dev/null
+++ b/library/std/src/personality/gcc.rs
@@ -0,0 +1,279 @@
+//! Implementation of panics backed by libgcc/libunwind (in some form).
+//!
+//! For background on exception handling and stack unwinding please see
+//! "Exception Handling in LLVM" (llvm.org/docs/ExceptionHandling.html) and
+//! documents linked from it.
+//! These are also good reads:
+//!  * <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html>
+//!  * <https://monoinfinito.wordpress.com/series/exception-handling-in-c/>
+//!  * <https://www.airs.com/blog/index.php?s=exception+frames>
+//!
+//! ## A brief summary
+//!
+//! Exception handling happens in two phases: a search phase and a cleanup
+//! phase.
+//!
+//! In both phases the unwinder walks stack frames from top to bottom using
+//! information from the stack frame unwind sections of the current process's
+//! modules ("module" here refers to an OS module, i.e., an executable or a
+//! dynamic library).
+//!
+//! For each stack frame, it invokes the associated "personality routine", whose
+//! address is also stored in the unwind info section.
+//!
+//! In the search phase, the job of a personality routine is to examine
+//! exception object being thrown, and to decide whether it should be caught at
+//! that stack frame. Once the handler frame has been identified, cleanup phase
+//! begins.
+//!
+//! In the cleanup phase, the unwinder invokes each personality routine again.
+//! This time it decides which (if any) cleanup code needs to be run for
+//! the current stack frame. If so, the control is transferred to a special
+//! branch in the function body, the "landing pad", which invokes destructors,
+//! frees memory, etc. At the end of the landing pad, control is transferred
+//! back to the unwinder and unwinding resumes.
+//!
+//! Once stack has been unwound down to the handler frame level, unwinding stops
+//! and the last personality routine transfers control to the catch block.
+
+use super::dwarf::eh::{self, EHAction, EHContext};
+use libc::{c_int, uintptr_t};
+use unwind as uw;
+
+// Register ids were lifted from LLVM's TargetLowering::getExceptionPointerRegister()
+// and TargetLowering::getExceptionSelectorRegister() for each architecture,
+// then mapped to DWARF register numbers via register definition tables
+// (typically <arch>RegisterInfo.td, search for "DwarfRegNum").
+// See also https://llvm.org/docs/WritingAnLLVMBackend.html#defining-a-register.
+
+#[cfg(target_arch = "x86")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 2); // EAX, EDX
+
+#[cfg(target_arch = "x86_64")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // RAX, RDX
+
+#[cfg(any(target_arch = "arm", target_arch = "aarch64"))]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1 / X0, X1
+
+#[cfg(target_arch = "m68k")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // D0, D1
+
+#[cfg(any(target_arch = "mips", target_arch = "mips64"))]
+const UNWIND_DATA_REG: (i32, i32) = (4, 5); // A0, A1
+
+#[cfg(any(target_arch = "powerpc", target_arch = "powerpc64"))]
+const UNWIND_DATA_REG: (i32, i32) = (3, 4); // R3, R4 / X3, X4
+
+#[cfg(target_arch = "s390x")]
+const UNWIND_DATA_REG: (i32, i32) = (6, 7); // R6, R7
+
+#[cfg(any(target_arch = "sparc", target_arch = "sparc64"))]
+const UNWIND_DATA_REG: (i32, i32) = (24, 25); // I0, I1
+
+#[cfg(target_arch = "hexagon")]
+const UNWIND_DATA_REG: (i32, i32) = (0, 1); // R0, R1
+
+#[cfg(any(target_arch = "riscv64", target_arch = "riscv32"))]
+const UNWIND_DATA_REG: (i32, i32) = (10, 11); // x10, x11
+
+// The following code is based on GCC's C and C++ personality routines.  For reference, see:
+// 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(target_arch = "arm", not(target_os = "ios"), not(target_os = "watchos"), not(target_os = "netbsd")))] {
+        // ARM EHABI personality routine.
+        // https://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf
+        //
+        // iOS uses the default routine instead since it uses SjLj unwinding.
+        #[lang = "eh_personality"]
+        unsafe extern "C" fn rust_eh_personality(
+            state: uw::_Unwind_State,
+            exception_object: *mut uw::_Unwind_Exception,
+            context: *mut uw::_Unwind_Context,
+        ) -> uw::_Unwind_Reason_Code {
+            let state = state as c_int;
+            let action = state & uw::_US_ACTION_MASK as c_int;
+            let search_phase = if action == uw::_US_VIRTUAL_UNWIND_FRAME as c_int {
+                // Backtraces on ARM will call the personality routine with
+                // state == _US_VIRTUAL_UNWIND_FRAME | _US_FORCE_UNWIND. In those cases
+                // we want to continue unwinding the stack, otherwise all our backtraces
+                // would end at __rust_try
+                if state & uw::_US_FORCE_UNWIND as c_int != 0 {
+                    return continue_unwind(exception_object, context);
+                }
+                true
+            } else if action == uw::_US_UNWIND_FRAME_STARTING as c_int {
+                false
+            } else if action == uw::_US_UNWIND_FRAME_RESUME as c_int {
+                return continue_unwind(exception_object, context);
+            } else {
+                return uw::_URC_FAILURE;
+            };
+
+            // The DWARF unwinder assumes that _Unwind_Context holds things like the function
+            // and LSDA pointers, however ARM EHABI places them into the exception object.
+            // To preserve signatures of functions like _Unwind_GetLanguageSpecificData(), which
+            // take only the context pointer, GCC personality routines stash a pointer to
+            // exception_object in the context, using location reserved for ARM's
+            // "scratch register" (r12).
+            uw::_Unwind_SetGR(context, uw::UNWIND_POINTER_REG, exception_object as uw::_Unwind_Ptr);
+            // ...A more principled approach would be to provide the full definition of ARM's
+            // _Unwind_Context in our libunwind bindings and fetch the required data from there
+            // directly, bypassing DWARF compatibility functions.
+
+            let eh_action = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FAILURE,
+            };
+            if search_phase {
+                match eh_action {
+                    EHAction::None | EHAction::Cleanup(_) => {
+                        return continue_unwind(exception_object, context);
+                    }
+                    EHAction::Catch(_) => {
+                        // EHABI requires the personality routine to update the
+                        // SP value in the barrier cache of the exception object.
+                        (*exception_object).private[5] =
+                            uw::_Unwind_GetGR(context, uw::UNWIND_SP_REG);
+                        return uw::_URC_HANDLER_FOUND;
+                    }
+                    EHAction::Terminate => return uw::_URC_FAILURE,
+                }
+            } else {
+                match eh_action {
+                    EHAction::None => return continue_unwind(exception_object, context),
+                    EHAction::Cleanup(lpad) | EHAction::Catch(lpad) => {
+                        uw::_Unwind_SetGR(
+                            context,
+                            UNWIND_DATA_REG.0,
+                            exception_object as uintptr_t,
+                        );
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
+                        uw::_Unwind_SetIP(context, lpad);
+                        return uw::_URC_INSTALL_CONTEXT;
+                    }
+                    EHAction::Terminate => return uw::_URC_FAILURE,
+                }
+            }
+
+            // On ARM EHABI the personality routine is responsible for actually
+            // unwinding a single stack frame before returning (ARM EHABI Sec. 6.1).
+            unsafe fn continue_unwind(
+                exception_object: *mut uw::_Unwind_Exception,
+                context: *mut uw::_Unwind_Context,
+            ) -> uw::_Unwind_Reason_Code {
+                if __gnu_unwind_frame(exception_object, context) == uw::_URC_NO_REASON {
+                    uw::_URC_CONTINUE_UNWIND
+                } else {
+                    uw::_URC_FAILURE
+                }
+            }
+            // defined in libgcc
+            extern "C" {
+                fn __gnu_unwind_frame(
+                    exception_object: *mut uw::_Unwind_Exception,
+                    context: *mut uw::_Unwind_Context,
+                ) -> uw::_Unwind_Reason_Code;
+            }
+        }
+    } else {
+        // Default personality routine, which is used directly on most targets
+        // and indirectly on Windows x86_64 via SEH.
+        unsafe extern "C" fn rust_eh_personality_impl(
+            version: c_int,
+            actions: uw::_Unwind_Action,
+            _exception_class: uw::_Unwind_Exception_Class,
+            exception_object: *mut uw::_Unwind_Exception,
+            context: *mut uw::_Unwind_Context,
+        ) -> uw::_Unwind_Reason_Code {
+            if version != 1 {
+                return uw::_URC_FATAL_PHASE1_ERROR;
+            }
+            let eh_action = match find_eh_action(context) {
+                Ok(action) => action,
+                Err(_) => return uw::_URC_FATAL_PHASE1_ERROR,
+            };
+            if actions as i32 & uw::_UA_SEARCH_PHASE as i32 != 0 {
+                match eh_action {
+                    EHAction::None | EHAction::Cleanup(_) => uw::_URC_CONTINUE_UNWIND,
+                    EHAction::Catch(_) => uw::_URC_HANDLER_FOUND,
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE1_ERROR,
+                }
+            } else {
+                match eh_action {
+                    EHAction::None => uw::_URC_CONTINUE_UNWIND,
+                    EHAction::Cleanup(lpad) | EHAction::Catch(lpad) => {
+                        uw::_Unwind_SetGR(
+                            context,
+                            UNWIND_DATA_REG.0,
+                            exception_object as uintptr_t,
+                        );
+                        uw::_Unwind_SetGR(context, UNWIND_DATA_REG.1, 0);
+                        uw::_Unwind_SetIP(context, lpad);
+                        uw::_URC_INSTALL_CONTEXT
+                    }
+                    EHAction::Terminate => uw::_URC_FATAL_PHASE2_ERROR,
+                }
+            }
+        }
+
+        cfg_if::cfg_if! {
+            if #[cfg(all(windows, target_arch = "x86_64", target_env = "gnu"))] {
+                // On x86_64 MinGW targets, the unwinding mechanism is SEH however the unwind
+                // handler data (aka LSDA) uses GCC-compatible encoding.
+                #[lang = "eh_personality"]
+                #[allow(nonstandard_style)]
+                unsafe extern "C" fn rust_eh_personality(
+                    exceptionRecord: *mut uw::EXCEPTION_RECORD,
+                    establisherFrame: uw::LPVOID,
+                    contextRecord: *mut uw::CONTEXT,
+                    dispatcherContext: *mut uw::DISPATCHER_CONTEXT,
+                ) -> uw::EXCEPTION_DISPOSITION {
+                    uw::_GCC_specific_handler(
+                        exceptionRecord,
+                        establisherFrame,
+                        contextRecord,
+                        dispatcherContext,
+                        rust_eh_personality_impl,
+                    )
+                }
+            } else {
+                // The personality routine for most of our targets.
+                #[lang = "eh_personality"]
+                unsafe extern "C" fn rust_eh_personality(
+                    version: c_int,
+                    actions: uw::_Unwind_Action,
+                    exception_class: uw::_Unwind_Exception_Class,
+                    exception_object: *mut uw::_Unwind_Exception,
+                    context: *mut uw::_Unwind_Context,
+                ) -> uw::_Unwind_Reason_Code {
+                    rust_eh_personality_impl(
+                        version,
+                        actions,
+                        exception_class,
+                        exception_object,
+                        context,
+                    )
+                }
+            }
+        }
+    }
+}
+
+unsafe fn find_eh_action(context: *mut uw::_Unwind_Context) -> Result<EHAction, ()> {
+    let lsda = uw::_Unwind_GetLanguageSpecificData(context) as *const u8;
+    let mut ip_before_instr: c_int = 0;
+    let ip = uw::_Unwind_GetIPInfo(context, &mut ip_before_instr);
+    let eh_context = EHContext {
+        // The return address points 1 byte past the call instruction,
+        // which could be in the next IP range in LSDA range table.
+        //
+        // `ip = -1` has special meaning, so use wrapping sub to allow for that
+        ip: if ip_before_instr != 0 { ip } else { ip.wrapping_sub(1) },
+        func_start: uw::_Unwind_GetRegionStart(context),
+        get_text_start: &|| uw::_Unwind_GetTextRelBase(context),
+        get_data_start: &|| uw::_Unwind_GetDataRelBase(context),
+    };
+    eh::find_eh_action(lsda, &eh_context)
+}
diff --git a/library/std/src/primitive_docs.rs b/library/std/src/primitive_docs.rs
index b8e546164..242f44ade 100644
--- a/library/std/src/primitive_docs.rs
+++ b/library/std/src/primitive_docs.rs
@@ -801,11 +801,53 @@ mod prim_array {}
 /// assert_eq!(2 * pointer_size, std::mem::size_of::<Box<[u8]>>());
 /// assert_eq!(2 * pointer_size, std::mem::size_of::<Rc<[u8]>>());
 /// ```
+///
+/// ## Trait Implementations
+///
+/// Some traits are implemented for slices if the element type implements
+/// that trait. This includes [`Eq`], [`Hash`] and [`Ord`].
+///
+/// ## Iteration
+///
+/// The slices implement `IntoIterator`. The iterator yields references to the
+/// slice elements.
+///
+/// ```
+/// let numbers: &[i32] = &[0, 1, 2];
+/// for n in numbers {
+///     println!("{n} is a number!");
+/// }
+/// ```
+///
+/// The mutable slice yields mutable references to the elements:
+///
+/// ```
+/// let mut scores: &mut [i32] = &mut [7, 8, 9];
+/// for score in scores {
+///     *score += 1;
+/// }
+/// ```
+///
+/// This iterator yields mutable references to the slice's elements, so while
+/// the element type of the slice is `i32`, the element type of the iterator is
+/// `&mut i32`.
+///
+/// * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
+///   iterators.
+/// * Further methods that return iterators are [`.split`], [`.splitn`],
+///   [`.chunks`], [`.windows`] and more.
+///
+/// [`Hash`]: core::hash::Hash
+/// [`.iter`]: slice::iter
+/// [`.iter_mut`]: slice::iter_mut
+/// [`.split`]: slice::split
+/// [`.splitn`]: slice::splitn
+/// [`.chunks`]: slice::chunks
+/// [`.windows`]: slice::windows
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_slice {}
 
 #[doc(primitive = "str")]
-//
 /// String slices.
 ///
 /// *[See also the `std::str` module](crate::str).*
@@ -816,19 +858,22 @@ mod prim_slice {}
 ///
 /// String slices are always valid UTF-8.
 ///
-/// # Examples
+/// # Basic Usage
 ///
 /// String literals are string slices:
 ///
 /// ```
-/// let hello = "Hello, world!";
-///
-/// // with an explicit type annotation
-/// let hello: &'static str = "Hello, world!";
+/// let hello_world = "Hello, World!";
 /// ```
 ///
-/// They are `'static` because they're stored directly in the final binary, and
-/// so will be valid for the `'static` duration.
+/// Here we have declared a string slice initialized with a string literal.
+/// String literals have a static lifetime, which means the string `hello_world`
+/// is guaranteed to be valid for the duration of the entire program.
+/// We can explicitly specify `hello_world`'s lifetime as well:
+///
+/// ```
+/// let hello_world: &'static str = "Hello, world!";
+/// ```
 ///
 /// # Representation
 ///
@@ -996,7 +1041,7 @@ impl<T> (T,) {}
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), doc(fake_variadic))]
+#[doc(fake_variadic)]
 /// This trait is implemented on arbitrary-length tuples.
 impl<T: Clone> Clone for (T,) {
     fn clone(&self) -> Self {
@@ -1007,7 +1052,7 @@ impl<T: Clone> Clone for (T,) {
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), doc(fake_variadic))]
+#[doc(fake_variadic)]
 /// This trait is implemented on arbitrary-length tuples.
 impl<T: Copy> Copy for (T,) {
     // empty
@@ -1178,7 +1223,7 @@ mod prim_usize {}
 #[doc(alias = "&")]
 #[doc(alias = "&mut")]
 //
-/// References, both shared and mutable.
+/// References, `&T` and `&mut T`.
 ///
 /// A reference represents a borrow of some owned value. You can get one by using the `&` or `&mut`
 /// operators on a value, or by using a [`ref`](../std/keyword.ref.html) or
@@ -1484,13 +1529,12 @@ mod prim_fn {}
 // Required to make auto trait impls render.
 // See src/librustdoc/passes/collect_trait_impls.rs:collect_trait_impls
 #[doc(hidden)]
-#[cfg(not(bootstrap))]
 impl<Ret, T> fn(T) -> Ret {}
 
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), doc(fake_variadic))]
+#[doc(fake_variadic)]
 /// This trait is implemented on function pointers with any number of arguments.
 impl<Ret, T> Clone for fn(T) -> Ret {
     fn clone(&self) -> Self {
@@ -1501,7 +1545,7 @@ impl<Ret, T> Clone for fn(T) -> Ret {
 // Fake impl that's only really used for docs.
 #[cfg(doc)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), doc(fake_variadic))]
+#[doc(fake_variadic)]
 /// This trait is implemented on function pointers with any number of arguments.
 impl<Ret, T> Copy for fn(T) -> Ret {
     // empty
diff --git a/library/std/src/process.rs b/library/std/src/process.rs
index d6cba7e75..d91d4fa64 100644
--- a/library/std/src/process.rs
+++ b/library/std/src/process.rs
@@ -169,15 +169,15 @@ use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 pub struct Child {
     pub(crate) handle: imp::Process,
 
-    /// The handle for writing to the child's standard input (stdin), if it has
-    /// been captured. To avoid partially moving
-    /// the `child` and thus blocking yourself from calling
-    /// functions on `child` while using `stdin`,
-    /// you might find it helpful:
+    /// The handle for writing to the child's standard input (stdin), if it
+    /// has been captured. You might find it helpful to do
     ///
     /// ```compile_fail,E0425
     /// let stdin = child.stdin.take().unwrap();
     /// ```
+    ///
+    /// to avoid partially moving the `child` and thus blocking yourself from calling
+    /// functions on `child` while using `stdin`.
     #[stable(feature = "process", since = "1.0.0")]
     pub stdin: Option<ChildStdin>,
 
diff --git a/library/std/src/rt.rs b/library/std/src/rt.rs
index 663537a05..98f6cc7aa 100644
--- a/library/std/src/rt.rs
+++ b/library/std/src/rt.rs
@@ -72,10 +72,29 @@ macro_rules! rtunwrap {
 // Runs before `main`.
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
+//
+// # The `sigpipe` parameter
+//
+// Since 2014, the Rust runtime on Unix has set the `SIGPIPE` handler to
+// `SIG_IGN`. Applications have good reasons to want a different behavior
+// though, so there is a `#[unix_sigpipe = "..."]` attribute on `fn main()` that
+// can be used to select how `SIGPIPE` shall be setup (if changed at all) before
+// `fn main()` is called. See <https://github.com/rust-lang/rust/issues/97889>
+// for more info.
+//
+// The `sigpipe` parameter to this function gets its value via the code that
+// rustc generates to invoke `fn lang_start()`. The reason we have `sigpipe` for
+// all platforms and not only Unix, is because std is not allowed to have `cfg`
+// directives as this high level. See the module docs in
+// `src/tools/tidy/src/pal.rs` for more info. On all other platforms, `sigpipe`
+// has a value, but its value is ignored.
+//
+// Even though it is an `u8`, it only ever has 3 values. These are documented in
+// `compiler/rustc_session/src/config/sigpipe.rs`.
 #[cfg_attr(test, allow(dead_code))]
-unsafe fn init(argc: isize, argv: *const *const u8) {
+unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     unsafe {
-        sys::init(argc, argv);
+        sys::init(argc, argv, sigpipe);
 
         let main_guard = sys::thread::guard::init();
         // Next, set up the current Thread with the guard information we just
@@ -107,6 +126,7 @@ fn lang_start_internal(
     main: &(dyn Fn() -> i32 + Sync + crate::panic::RefUnwindSafe),
     argc: isize,
     argv: *const *const u8,
+    sigpipe: u8,
 ) -> Result<isize, !> {
     use crate::{mem, panic};
     let rt_abort = move |e| {
@@ -124,7 +144,7 @@ fn lang_start_internal(
     // prevent libstd from accidentally introducing a panic to these functions. Another is from
     // user code from `main` or, more nefariously, as described in e.g. issue #86030.
     // SAFETY: Only called once during runtime initialization.
-    panic::catch_unwind(move || unsafe { init(argc, argv) }).map_err(rt_abort)?;
+    panic::catch_unwind(move || unsafe { init(argc, argv, sigpipe) }).map_err(rt_abort)?;
     let ret_code = panic::catch_unwind(move || panic::catch_unwind(main).unwrap_or(101) as isize)
         .map_err(move |e| {
             mem::forget(e);
@@ -140,11 +160,16 @@ fn lang_start<T: crate::process::Termination + 'static>(
     main: fn() -> T,
     argc: isize,
     argv: *const *const u8,
+    #[cfg(not(bootstrap))] sigpipe: u8,
 ) -> isize {
     let Ok(v) = lang_start_internal(
         &move || crate::sys_common::backtrace::__rust_begin_short_backtrace(main).report().to_i32(),
         argc,
         argv,
+        #[cfg(bootstrap)]
+        2, // Temporary inlining of sigpipe::DEFAULT until bootstrap stops being special
+        #[cfg(not(bootstrap))]
+        sigpipe,
     );
     v
 }
diff --git a/library/std/src/sync/mpsc/mpsc_queue/tests.rs b/library/std/src/sync/mpsc/mpsc_queue/tests.rs
index 9f4f31ed0..34b2a9a98 100644
--- a/library/std/src/sync/mpsc/mpsc_queue/tests.rs
+++ b/library/std/src/sync/mpsc/mpsc_queue/tests.rs
@@ -13,7 +13,7 @@ fn test_full() {
 #[test]
 fn test() {
     let nthreads = 8;
-    let nmsgs = 1000;
+    let nmsgs = if cfg!(miri) { 100 } else { 1000 };
     let q = Queue::new();
     match q.pop() {
         Empty => {}
diff --git a/library/std/src/sync/mpsc/spsc_queue/tests.rs b/library/std/src/sync/mpsc/spsc_queue/tests.rs
index 467ef3dbd..eb6d5c2cf 100644
--- a/library/std/src/sync/mpsc/spsc_queue/tests.rs
+++ b/library/std/src/sync/mpsc/spsc_queue/tests.rs
@@ -77,12 +77,13 @@ fn stress() {
     }
 
     unsafe fn stress_bound(bound: usize) {
+        let count = if cfg!(miri) { 1000 } else { 100000 };
         let q = Arc::new(Queue::with_additions(bound, (), ()));
 
         let (tx, rx) = channel();
         let q2 = q.clone();
         let _t = thread::spawn(move || {
-            for _ in 0..100000 {
+            for _ in 0..count {
                 loop {
                     match q2.pop() {
                         Some(1) => break,
@@ -93,7 +94,7 @@ fn stress() {
             }
             tx.send(()).unwrap();
         });
-        for _ in 0..100000 {
+        for _ in 0..count {
             q.push(1);
         }
         rx.recv().unwrap();
diff --git a/library/std/src/sync/mpsc/sync_tests.rs b/library/std/src/sync/mpsc/sync_tests.rs
index e58649bab..63c794369 100644
--- a/library/std/src/sync/mpsc/sync_tests.rs
+++ b/library/std/src/sync/mpsc/sync_tests.rs
@@ -113,23 +113,25 @@ fn chan_gone_concurrent() {
 
 #[test]
 fn stress() {
+    let count = if cfg!(miri) { 100 } else { 10000 };
     let (tx, rx) = sync_channel::<i32>(0);
     thread::spawn(move || {
-        for _ in 0..10000 {
+        for _ in 0..count {
             tx.send(1).unwrap();
         }
     });
-    for _ in 0..10000 {
+    for _ in 0..count {
         assert_eq!(rx.recv().unwrap(), 1);
     }
 }
 
 #[test]
 fn stress_recv_timeout_two_threads() {
+    let count = if cfg!(miri) { 100 } else { 10000 };
     let (tx, rx) = sync_channel::<i32>(0);
 
     thread::spawn(move || {
-        for _ in 0..10000 {
+        for _ in 0..count {
             tx.send(1).unwrap();
         }
     });
@@ -146,12 +148,12 @@ fn stress_recv_timeout_two_threads() {
         }
     }
 
-    assert_eq!(recv_count, 10000);
+    assert_eq!(recv_count, count);
 }
 
 #[test]
 fn stress_recv_timeout_shared() {
-    const AMT: u32 = 1000;
+    const AMT: u32 = if cfg!(miri) { 100 } else { 1000 };
     const NTHREADS: u32 = 8;
     let (tx, rx) = sync_channel::<i32>(0);
     let (dtx, drx) = sync_channel::<()>(0);
@@ -191,7 +193,7 @@ fn stress_recv_timeout_shared() {
 
 #[test]
 fn stress_shared() {
-    const AMT: u32 = 1000;
+    const AMT: u32 = if cfg!(miri) { 100 } else { 1000 };
     const NTHREADS: u32 = 8;
     let (tx, rx) = sync_channel::<i32>(0);
     let (dtx, drx) = sync_channel::<()>(0);
@@ -438,12 +440,13 @@ fn stream_send_recv_stress() {
 
 #[test]
 fn recv_a_lot() {
+    let count = if cfg!(miri) { 1000 } else { 10000 };
     // Regression test that we don't run out of stack in scheduler context
-    let (tx, rx) = sync_channel(10000);
-    for _ in 0..10000 {
+    let (tx, rx) = sync_channel(count);
+    for _ in 0..count {
         tx.send(()).unwrap();
     }
-    for _ in 0..10000 {
+    for _ in 0..count {
         rx.recv().unwrap();
     }
 }
diff --git a/library/std/src/sync/mpsc/tests.rs b/library/std/src/sync/mpsc/tests.rs
index 4deb3e596..f6d0796f6 100644
--- a/library/std/src/sync/mpsc/tests.rs
+++ b/library/std/src/sync/mpsc/tests.rs
@@ -120,13 +120,14 @@ fn chan_gone_concurrent() {
 
 #[test]
 fn stress() {
+    let count = if cfg!(miri) { 100 } else { 10000 };
     let (tx, rx) = channel::<i32>();
     let t = thread::spawn(move || {
-        for _ in 0..10000 {
+        for _ in 0..count {
             tx.send(1).unwrap();
         }
     });
-    for _ in 0..10000 {
+    for _ in 0..count {
         assert_eq!(rx.recv().unwrap(), 1);
     }
     t.join().ok().expect("thread panicked");
@@ -134,7 +135,7 @@ fn stress() {
 
 #[test]
 fn stress_shared() {
-    const AMT: u32 = 10000;
+    const AMT: u32 = if cfg!(miri) { 100 } else { 10000 };
     const NTHREADS: u32 = 8;
     let (tx, rx) = channel::<i32>();
 
@@ -504,12 +505,13 @@ fn very_long_recv_timeout_wont_panic() {
 
 #[test]
 fn recv_a_lot() {
+    let count = if cfg!(miri) { 1000 } else { 10000 };
     // Regression test that we don't run out of stack in scheduler context
     let (tx, rx) = channel();
-    for _ in 0..10000 {
+    for _ in 0..count {
         tx.send(()).unwrap();
     }
-    for _ in 0..10000 {
+    for _ in 0..count {
         rx.recv().unwrap();
     }
 }
diff --git a/library/std/src/sync/mutex.rs b/library/std/src/sync/mutex.rs
index e0d13cd64..de851c8fb 100644
--- a/library/std/src/sync/mutex.rs
+++ b/library/std/src/sync/mutex.rs
@@ -192,6 +192,7 @@ unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
                       and cause Futures to not implement `Send`"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[clippy::has_significant_drop]
+#[cfg_attr(not(test), rustc_diagnostic_item = "MutexGuard")]
 pub struct MutexGuard<'a, T: ?Sized + 'a> {
     lock: &'a Mutex<T>,
     poison: poison::Guard,
diff --git a/library/std/src/sync/once_lock.rs b/library/std/src/sync/once_lock.rs
index 813516040..37413ec62 100644
--- a/library/std/src/sync/once_lock.rs
+++ b/library/std/src/sync/once_lock.rs
@@ -3,7 +3,6 @@ use crate::fmt;
 use crate::marker::PhantomData;
 use crate::mem::MaybeUninit;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
-use crate::pin::Pin;
 use crate::sync::Once;
 
 /// A synchronization primitive which can be written to only once.
@@ -223,60 +222,6 @@ impl<T> OnceLock<T> {
         Ok(unsafe { self.get_unchecked() })
     }
 
-    /// Internal-only API that gets the contents of the cell, initializing it
-    /// in two steps with `f` and `g` if the cell was empty.
-    ///
-    /// `f` is called to construct the value, which is then moved into the cell
-    /// and given as a (pinned) mutable reference to `g` to finish
-    /// initialization.
-    ///
-    /// This allows `g` to inspect an manipulate the value after it has been
-    /// moved into its final place in the cell, but before the cell is
-    /// considered initialized.
-    ///
-    /// # Panics
-    ///
-    /// If `f` or `g` panics, the panic is propagated to the caller, and the
-    /// cell remains uninitialized.
-    ///
-    /// With the current implementation, if `g` panics, the value from `f` will
-    /// not be dropped. This should probably be fixed if this is ever used for
-    /// a type where this matters.
-    ///
-    /// It is an error to reentrantly initialize the cell from `f`. The exact
-    /// outcome is unspecified. Current implementation deadlocks, but this may
-    /// be changed to a panic in the future.
-    pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
-    where
-        F: FnOnce() -> T,
-        G: FnOnce(Pin<&mut T>),
-    {
-        if let Some(value) = self.get_ref().get() {
-            // SAFETY: The inner value was already initialized, and will not be
-            // moved anymore.
-            return unsafe { Pin::new_unchecked(value) };
-        }
-
-        let slot = &self.value;
-
-        // Ignore poisoning from other threads
-        // If another thread panics, then we'll be able to run our closure
-        self.once.call_once_force(|_| {
-            let value = f();
-            // SAFETY: We use the Once (self.once) to guarantee unique access
-            // to the UnsafeCell (slot).
-            let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
-            // SAFETY: The value has been written to its final place in
-            // self.value. We do not to move it anymore, which we promise here
-            // with a Pin<&mut T>.
-            g(unsafe { Pin::new_unchecked(value) });
-        });
-
-        // SAFETY: The inner value has been initialized, and will not be moved
-        // anymore.
-        unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
-    }
-
     /// Consumes the `OnceLock`, returning the wrapped value. Returns
     /// `None` if the cell was empty.
     ///
diff --git a/library/std/src/sync/rwlock.rs b/library/std/src/sync/rwlock.rs
index 6e4a2cfc8..9ab781561 100644
--- a/library/std/src/sync/rwlock.rs
+++ b/library/std/src/sync/rwlock.rs
@@ -101,6 +101,7 @@ unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
                       and cause Futures to not implement `Send`"]
 #[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
     // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
@@ -130,6 +131,7 @@ unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}
                       and cause Future's to not implement `Send`"]
 #[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>,
     poison: poison::Guard,
diff --git a/library/std/src/sync/rwlock/tests.rs b/library/std/src/sync/rwlock/tests.rs
index 08255c985..b5b3ad989 100644
--- a/library/std/src/sync/rwlock/tests.rs
+++ b/library/std/src/sync/rwlock/tests.rs
@@ -19,7 +19,7 @@ fn smoke() {
 #[test]
 fn frob() {
     const N: u32 = 10;
-    const M: usize = 1000;
+    const M: usize = if cfg!(miri) { 100 } else { 1000 };
 
     let r = Arc::new(RwLock::new(()));
 
diff --git a/library/std/src/sys/hermit/condvar.rs b/library/std/src/sys/hermit/condvar.rs
deleted file mode 100644
index 22059ca0d..000000000
--- a/library/std/src/sys/hermit/condvar.rs
+++ /dev/null
@@ -1,90 +0,0 @@
-use crate::ffi::c_void;
-use crate::ptr;
-use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
-use crate::sys::hermit::abi;
-use crate::sys::locks::Mutex;
-use crate::sys_common::lazy_box::{LazyBox, LazyInit};
-use crate::time::Duration;
-
-// The implementation is inspired by Andrew D. Birrell's paper
-// "Implementing Condition Variables with Semaphores"
-
-pub struct Condvar {
-    counter: AtomicUsize,
-    sem1: *const c_void,
-    sem2: *const c_void,
-}
-
-pub(crate) type MovableCondvar = LazyBox<Condvar>;
-
-impl LazyInit for Condvar {
-    fn init() -> Box<Self> {
-        Box::new(Self::new())
-    }
-}
-
-unsafe impl Send for Condvar {}
-unsafe impl Sync for Condvar {}
-
-impl Condvar {
-    pub fn new() -> Self {
-        let mut condvar =
-            Self { counter: AtomicUsize::new(0), sem1: ptr::null(), sem2: ptr::null() };
-        unsafe {
-            let _ = abi::sem_init(&mut condvar.sem1, 0);
-            let _ = abi::sem_init(&mut condvar.sem2, 0);
-        }
-        condvar
-    }
-
-    pub unsafe fn notify_one(&self) {
-        if self.counter.load(SeqCst) > 0 {
-            self.counter.fetch_sub(1, SeqCst);
-            abi::sem_post(self.sem1);
-            abi::sem_timedwait(self.sem2, 0);
-        }
-    }
-
-    pub unsafe fn notify_all(&self) {
-        let counter = self.counter.swap(0, SeqCst);
-        for _ in 0..counter {
-            abi::sem_post(self.sem1);
-        }
-        for _ in 0..counter {
-            abi::sem_timedwait(self.sem2, 0);
-        }
-    }
-
-    pub unsafe fn wait(&self, mutex: &Mutex) {
-        self.counter.fetch_add(1, SeqCst);
-        mutex.unlock();
-        abi::sem_timedwait(self.sem1, 0);
-        abi::sem_post(self.sem2);
-        mutex.lock();
-    }
-
-    pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
-        self.counter.fetch_add(1, SeqCst);
-        mutex.unlock();
-        let millis = dur.as_millis().min(u32::MAX as u128) as u32;
-
-        let res = if millis > 0 {
-            abi::sem_timedwait(self.sem1, millis)
-        } else {
-            abi::sem_trywait(self.sem1)
-        };
-
-        abi::sem_post(self.sem2);
-        mutex.lock();
-        res == 0
-    }
-}
-
-impl Drop for Condvar {
-    fn drop(&mut self) {
-        unsafe {
-            let _ = abi::sem_destroy(self.sem1);
-            let _ = abi::sem_destroy(self.sem2);
-        }
-    }
-}
diff --git a/library/std/src/sys/hermit/fs.rs b/library/std/src/sys/hermit/fs.rs
index fa9a7fb19..f921839cf 100644
--- a/library/std/src/sys/hermit/fs.rs
+++ b/library/std/src/sys/hermit/fs.rs
@@ -2,7 +2,7 @@ use crate::ffi::{CStr, CString, OsString};
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
 use crate::io::{self, Error, ErrorKind};
-use crate::io::{IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{BorrowedCursor, IoSlice, IoSliceMut, SeekFrom};
 use crate::os::unix::ffi::OsStrExt;
 use crate::path::{Path, PathBuf};
 use crate::sys::cvt;
@@ -41,6 +41,9 @@ pub struct OpenOptions {
     mode: i32,
 }
 
+#[derive(Copy, Clone, Debug, Default)]
+pub struct FileTimes {}
+
 pub struct FilePermissions(!);
 
 pub struct FileType(!);
@@ -110,6 +113,11 @@ impl fmt::Debug for FilePermissions {
     }
 }
 
+impl FileTimes {
+    pub fn set_accessed(&mut self, _t: SystemTime) {}
+    pub fn set_modified(&mut self, _t: SystemTime) {}
+}
+
 impl FileType {
     pub fn is_dir(&self) -> bool {
         self.0
@@ -312,8 +320,8 @@ impl File {
         false
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        crate::io::default_read_buf(|buf| self.read(buf), buf)
+    pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        crate::io::default_read_buf(|buf| self.read(buf), cursor)
     }
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
@@ -344,6 +352,10 @@ impl File {
     pub fn set_permissions(&self, _perm: FilePermissions) -> io::Result<()> {
         Err(Error::from_raw_os_error(22))
     }
+
+    pub fn set_times(&self, _times: FileTimes) -> io::Result<()> {
+        Err(Error::from_raw_os_error(22))
+    }
 }
 
 impl DirBuilder {
diff --git a/library/std/src/sys/hermit/futex.rs b/library/std/src/sys/hermit/futex.rs
new file mode 100644
index 000000000..b64c174b0
--- /dev/null
+++ b/library/std/src/sys/hermit/futex.rs
@@ -0,0 +1,39 @@
+use super::abi;
+use crate::ptr::null;
+use crate::sync::atomic::AtomicU32;
+use crate::time::Duration;
+
+pub fn futex_wait(futex: &AtomicU32, expected: u32, timeout: Option<Duration>) -> bool {
+    // Calculate the timeout as a relative timespec.
+    //
+    // Overflows are rounded up to an infinite timeout (None).
+    let timespec = timeout.and_then(|dur| {
+        Some(abi::timespec {
+            tv_sec: dur.as_secs().try_into().ok()?,
+            tv_nsec: dur.subsec_nanos().into(),
+        })
+    });
+
+    let r = unsafe {
+        abi::futex_wait(
+            futex.as_mut_ptr(),
+            expected,
+            timespec.as_ref().map_or(null(), |t| t as *const abi::timespec),
+            abi::FUTEX_RELATIVE_TIMEOUT,
+        )
+    };
+
+    r != -abi::errno::ETIMEDOUT
+}
+
+#[inline]
+pub fn futex_wake(futex: &AtomicU32) -> bool {
+    unsafe { abi::futex_wake(futex.as_mut_ptr(), 1) > 0 }
+}
+
+#[inline]
+pub fn futex_wake_all(futex: &AtomicU32) {
+    unsafe {
+        abi::futex_wake(futex.as_mut_ptr(), i32::MAX);
+    }
+}
diff --git a/library/std/src/sys/hermit/mod.rs b/library/std/src/sys/hermit/mod.rs
index 60b7a973c..827d82900 100644
--- a/library/std/src/sys/hermit/mod.rs
+++ b/library/std/src/sys/hermit/mod.rs
@@ -25,6 +25,7 @@ pub mod cmath;
 pub mod env;
 pub mod fd;
 pub mod fs;
+pub mod futex;
 #[path = "../unsupported/io.rs"]
 pub mod io;
 pub mod memchr;
@@ -45,14 +46,14 @@ pub mod thread_local_dtor;
 pub mod thread_local_key;
 pub mod time;
 
-mod condvar;
-mod mutex;
-mod rwlock;
-
+#[path = "../unix/locks"]
 pub mod locks {
-    pub use super::condvar::*;
-    pub use super::mutex::*;
-    pub use super::rwlock::*;
+    mod futex_condvar;
+    mod futex_mutex;
+    mod futex_rwlock;
+    pub(crate) use futex_condvar::MovableCondvar;
+    pub(crate) use futex_mutex::{MovableMutex, Mutex};
+    pub(crate) use futex_rwlock::{MovableRwLock, RwLock};
 }
 
 use crate::io::ErrorKind;
@@ -98,7 +99,7 @@ pub extern "C" fn __rust_abort() {
 
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(argc: isize, argv: *const *const u8) {
+pub unsafe fn init(argc: isize, argv: *const *const u8, _sigpipe: u8) {
     let _ = net::init();
     args::init(argc, argv);
 }
diff --git a/library/std/src/sys/hermit/mutex.rs b/library/std/src/sys/hermit/mutex.rs
deleted file mode 100644
index eb15a04ff..000000000
--- a/library/std/src/sys/hermit/mutex.rs
+++ /dev/null
@@ -1,216 +0,0 @@
-use crate::cell::UnsafeCell;
-use crate::collections::VecDeque;
-use crate::hint;
-use crate::ops::{Deref, DerefMut, Drop};
-use crate::ptr;
-use crate::sync::atomic::{AtomicUsize, Ordering};
-use crate::sys::hermit::abi;
-
-/// This type provides a lock based on busy waiting to realize mutual exclusion
-///
-/// # Description
-///
-/// This structure behaves a lot like a common mutex. There are some differences:
-///
-/// - By using busy waiting, it can be used outside the runtime.
-/// - It is a so called ticket lock and is completely fair.
-#[cfg_attr(target_arch = "x86_64", repr(align(128)))]
-#[cfg_attr(not(target_arch = "x86_64"), repr(align(64)))]
-struct Spinlock<T: ?Sized> {
-    queue: AtomicUsize,
-    dequeue: AtomicUsize,
-    data: UnsafeCell<T>,
-}
-
-unsafe impl<T: ?Sized + Send> Sync for Spinlock<T> {}
-unsafe impl<T: ?Sized + Send> Send for Spinlock<T> {}
-
-/// A guard to which the protected data can be accessed
-///
-/// When the guard falls out of scope it will release the lock.
-struct SpinlockGuard<'a, T: ?Sized + 'a> {
-    dequeue: &'a AtomicUsize,
-    data: &'a mut T,
-}
-
-impl<T> Spinlock<T> {
-    pub const fn new(user_data: T) -> Spinlock<T> {
-        Spinlock {
-            queue: AtomicUsize::new(0),
-            dequeue: AtomicUsize::new(1),
-            data: UnsafeCell::new(user_data),
-        }
-    }
-
-    #[inline]
-    fn obtain_lock(&self) {
-        let ticket = self.queue.fetch_add(1, Ordering::SeqCst) + 1;
-        let mut counter: u16 = 0;
-        while self.dequeue.load(Ordering::SeqCst) != ticket {
-            counter += 1;
-            if counter < 100 {
-                hint::spin_loop();
-            } else {
-                counter = 0;
-                unsafe {
-                    abi::yield_now();
-                }
-            }
-        }
-    }
-
-    #[inline]
-    pub unsafe fn lock(&self) -> SpinlockGuard<'_, T> {
-        self.obtain_lock();
-        SpinlockGuard { dequeue: &self.dequeue, data: &mut *self.data.get() }
-    }
-}
-
-impl<T: ?Sized + Default> Default for Spinlock<T> {
-    fn default() -> Spinlock<T> {
-        Spinlock::new(Default::default())
-    }
-}
-
-impl<'a, T: ?Sized> Deref for SpinlockGuard<'a, T> {
-    type Target = T;
-    fn deref(&self) -> &T {
-        &*self.data
-    }
-}
-
-impl<'a, T: ?Sized> DerefMut for SpinlockGuard<'a, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        &mut *self.data
-    }
-}
-
-impl<'a, T: ?Sized> Drop for SpinlockGuard<'a, T> {
-    /// The dropping of the SpinlockGuard will release the lock it was created from.
-    fn drop(&mut self) {
-        self.dequeue.fetch_add(1, Ordering::SeqCst);
-    }
-}
-
-/// Realize a priority queue for tasks
-struct PriorityQueue {
-    queues: [Option<VecDeque<abi::Tid>>; abi::NO_PRIORITIES],
-    prio_bitmap: u64,
-}
-
-impl PriorityQueue {
-    pub const fn new() -> PriorityQueue {
-        PriorityQueue {
-            queues: [
-                None, None, None, None, None, None, None, None, None, None, None, None, None, None,
-                None, None, None, None, None, None, None, None, None, None, None, None, None, None,
-                None, None, None,
-            ],
-            prio_bitmap: 0,
-        }
-    }
-
-    /// Add a task id by its priority to the queue
-    pub fn push(&mut self, prio: abi::Priority, id: abi::Tid) {
-        let i: usize = prio.into().into();
-        self.prio_bitmap |= (1 << i) as u64;
-        if let Some(queue) = &mut self.queues[i] {
-            queue.push_back(id);
-        } else {
-            let mut queue = VecDeque::new();
-            queue.push_back(id);
-            self.queues[i] = Some(queue);
-        }
-    }
-
-    fn pop_from_queue(&mut self, queue_index: usize) -> Option<abi::Tid> {
-        if let Some(queue) = &mut self.queues[queue_index] {
-            let id = queue.pop_front();
-
-            if queue.is_empty() {
-                self.prio_bitmap &= !(1 << queue_index as u64);
-            }
-
-            id
-        } else {
-            None
-        }
-    }
-
-    /// Pop the task handle with the highest priority from the queue
-    pub fn pop(&mut self) -> Option<abi::Tid> {
-        for i in 0..abi::NO_PRIORITIES {
-            if self.prio_bitmap & (1 << i) != 0 {
-                return self.pop_from_queue(i);
-            }
-        }
-
-        None
-    }
-}
-
-struct MutexInner {
-    locked: bool,
-    blocked_task: PriorityQueue,
-}
-
-impl MutexInner {
-    pub const fn new() -> MutexInner {
-        MutexInner { locked: false, blocked_task: PriorityQueue::new() }
-    }
-}
-
-pub struct Mutex {
-    inner: Spinlock<MutexInner>,
-}
-
-pub type MovableMutex = Mutex;
-
-unsafe impl Send for Mutex {}
-unsafe impl Sync for Mutex {}
-
-impl Mutex {
-    pub const fn new() -> Mutex {
-        Mutex { inner: Spinlock::new(MutexInner::new()) }
-    }
-
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
-    pub unsafe fn lock(&self) {
-        loop {
-            let mut guard = self.inner.lock();
-            if guard.locked == false {
-                guard.locked = true;
-                return;
-            } else {
-                let prio = abi::get_priority();
-                let id = abi::getpid();
-
-                guard.blocked_task.push(prio, id);
-                abi::block_current_task();
-                drop(guard);
-                abi::yield_now();
-            }
-        }
-    }
-
-    #[inline]
-    pub unsafe fn unlock(&self) {
-        let mut guard = self.inner.lock();
-        guard.locked = false;
-        if let Some(tid) = guard.blocked_task.pop() {
-            abi::wakeup_task(tid);
-        }
-    }
-
-    #[inline]
-    pub unsafe fn try_lock(&self) -> bool {
-        let mut guard = self.inner.lock();
-        if guard.locked == false {
-            guard.locked = true;
-        }
-        guard.locked
-    }
-}
diff --git a/library/std/src/sys/hermit/net.rs b/library/std/src/sys/hermit/net.rs
index 745476171..8a13879d8 100644
--- a/library/std/src/sys/hermit/net.rs
+++ b/library/std/src/sys/hermit/net.rs
@@ -487,6 +487,4 @@ pub mod netc {
 
     #[derive(Copy, Clone)]
     pub struct sockaddr {}
-
-    pub type socklen_t = usize;
 }
diff --git a/library/std/src/sys/hermit/rwlock.rs b/library/std/src/sys/hermit/rwlock.rs
deleted file mode 100644
index 9701bab1f..000000000
--- a/library/std/src/sys/hermit/rwlock.rs
+++ /dev/null
@@ -1,144 +0,0 @@
-use crate::cell::UnsafeCell;
-use crate::sys::locks::{MovableCondvar, Mutex};
-use crate::sys_common::lazy_box::{LazyBox, LazyInit};
-
-pub struct RwLock {
-    lock: Mutex,
-    cond: MovableCondvar,
-    state: UnsafeCell<State>,
-}
-
-pub type MovableRwLock = RwLock;
-
-enum State {
-    Unlocked,
-    Reading(usize),
-    Writing,
-}
-
-unsafe impl Send for RwLock {}
-unsafe impl Sync for RwLock {}
-
-// This rwlock implementation is a relatively simple implementation which has a
-// condition variable for readers/writers as well as a mutex protecting the
-// internal state of the lock. A current downside of the implementation is that
-// unlocking the lock will notify *all* waiters rather than just readers or just
-// writers. This can cause lots of "thundering stampede" problems. While
-// hopefully correct this implementation is very likely to want to be changed in
-// the future.
-
-impl RwLock {
-    pub const fn new() -> RwLock {
-        RwLock {
-            lock: Mutex::new(),
-            cond: MovableCondvar::new(),
-            state: UnsafeCell::new(State::Unlocked),
-        }
-    }
-
-    #[inline]
-    pub unsafe fn read(&self) {
-        self.lock.lock();
-        while !(*self.state.get()).inc_readers() {
-            self.cond.wait(&self.lock);
-        }
-        self.lock.unlock();
-    }
-
-    #[inline]
-    pub unsafe fn try_read(&self) -> bool {
-        self.lock.lock();
-        let ok = (*self.state.get()).inc_readers();
-        self.lock.unlock();
-        return ok;
-    }
-
-    #[inline]
-    pub unsafe fn write(&self) {
-        self.lock.lock();
-        while !(*self.state.get()).inc_writers() {
-            self.cond.wait(&self.lock);
-        }
-        self.lock.unlock();
-    }
-
-    #[inline]
-    pub unsafe fn try_write(&self) -> bool {
-        self.lock.lock();
-        let ok = (*self.state.get()).inc_writers();
-        self.lock.unlock();
-        return ok;
-    }
-
-    #[inline]
-    pub unsafe fn read_unlock(&self) {
-        self.lock.lock();
-        let notify = (*self.state.get()).dec_readers();
-        self.lock.unlock();
-        if notify {
-            // FIXME: should only wake up one of these some of the time
-            self.cond.notify_all();
-        }
-    }
-
-    #[inline]
-    pub unsafe fn write_unlock(&self) {
-        self.lock.lock();
-        (*self.state.get()).dec_writers();
-        self.lock.unlock();
-        // FIXME: should only wake up one of these some of the time
-        self.cond.notify_all();
-    }
-}
-
-impl State {
-    fn inc_readers(&mut self) -> bool {
-        match *self {
-            State::Unlocked => {
-                *self = State::Reading(1);
-                true
-            }
-            State::Reading(ref mut cnt) => {
-                *cnt += 1;
-                true
-            }
-            State::Writing => false,
-        }
-    }
-
-    fn inc_writers(&mut self) -> bool {
-        match *self {
-            State::Unlocked => {
-                *self = State::Writing;
-                true
-            }
-            State::Reading(_) | State::Writing => false,
-        }
-    }
-
-    fn dec_readers(&mut self) -> bool {
-        let zero = match *self {
-            State::Reading(ref mut cnt) => {
-                *cnt -= 1;
-                *cnt == 0
-            }
-            State::Unlocked | State::Writing => invalid(),
-        };
-        if zero {
-            *self = State::Unlocked;
-        }
-        zero
-    }
-
-    fn dec_writers(&mut self) {
-        match *self {
-            State::Writing => {}
-            State::Unlocked | State::Reading(_) => invalid(),
-        }
-        *self = State::Unlocked;
-    }
-}
-
-fn invalid() -> ! {
-    panic!("inconsistent rwlock");
-}
diff --git a/library/std/src/sys/itron/mutex.rs b/library/std/src/sys/itron/mutex.rs
index 715e94c3b..085662e6d 100644
--- a/library/std/src/sys/itron/mutex.rs
+++ b/library/std/src/sys/itron/mutex.rs
@@ -31,12 +31,6 @@ impl Mutex {
         Mutex { mtx: SpinIdOnceCell::new() }
     }
 
-    pub unsafe fn init(&mut self) {
-        // Initialize `self.mtx` eagerly
-        let id = new_mtx().unwrap_or_else(|e| fail(e, &"acre_mtx"));
-        unsafe { self.mtx.set_unchecked((id, ())) };
-    }
-
     /// Get the inner mutex's ID, which is lazily created.
     fn raw(&self) -> abi::ID {
         match self.mtx.get_or_try_init(|| new_mtx().map(|id| (id, ()))) {
diff --git a/library/std/src/sys/sgx/abi/thread.rs b/library/std/src/sys/sgx/abi/thread.rs
index ef55b821a..2b23e368c 100644
--- a/library/std/src/sys/sgx/abi/thread.rs
+++ b/library/std/src/sys/sgx/abi/thread.rs
@@ -7,7 +7,11 @@ use fortanix_sgx_abi::Tcs;
 #[unstable(feature = "sgx_platform", issue = "56975")]
 pub fn current() -> Tcs {
     extern "C" {
-        fn get_tcs_addr() -> Tcs;
+        fn get_tcs_addr() -> *mut u8;
+    }
+    let addr = unsafe { get_tcs_addr() };
+    match Tcs::new(addr) {
+        Some(tcs) => tcs,
+        None => rtabort!("TCS must not be placed at address zero (this is a linker error)"),
     }
-    unsafe { get_tcs_addr() }
 }
diff --git a/library/std/src/sys/sgx/abi/usercalls/alloc.rs b/library/std/src/sys/sgx/abi/usercalls/alloc.rs
index ea24fedd0..5409bd177 100644
--- a/library/std/src/sys/sgx/abi/usercalls/alloc.rs
+++ b/library/std/src/sys/sgx/abi/usercalls/alloc.rs
@@ -56,6 +56,8 @@ unsafe impl UserSafeSized for Usercall {}
 #[unstable(feature = "sgx_platform", issue = "56975")]
 unsafe impl UserSafeSized for Return {}
 #[unstable(feature = "sgx_platform", issue = "56975")]
+unsafe impl UserSafeSized for Cancel {}
+#[unstable(feature = "sgx_platform", issue = "56975")]
 unsafe impl<T: UserSafeSized> UserSafeSized for [T; 2] {}
 
 /// A type that can be represented in memory as one or more `UserSafeSized`s.
@@ -115,7 +117,7 @@ pub unsafe trait UserSafe {
     /// * the pointer is null.
     /// * the pointed-to range is not in user memory.
     unsafe fn check_ptr(ptr: *const Self) {
-        let is_aligned = |p| -> bool { 0 == (p as usize) & (Self::align_of() - 1) };
+        let is_aligned = |p: *const u8| -> bool { p.is_aligned_to(Self::align_of()) };
 
         assert!(is_aligned(ptr as *const u8));
         assert!(is_user_range(ptr as _, mem::size_of_val(unsafe { &*ptr })));
@@ -305,6 +307,34 @@ where
     }
 }
 
+// Split a memory region ptr..ptr + len into three parts:
+//   +--------+
+//   | small0 | Chunk smaller than 8 bytes
+//   +--------+
+//   |   big  | Chunk 8-byte aligned, and size a multiple of 8 bytes
+//   +--------+
+//   | small1 | Chunk smaller than 8 bytes
+//   +--------+
+fn region_as_aligned_chunks(ptr: *const u8, len: usize) -> (usize, usize, usize) {
+    let small0_size = if ptr as usize % 8 == 0 { 0 } else { 8 - ptr as usize % 8 };
+    let small1_size = (len - small0_size as usize) % 8;
+    let big_size = len - small0_size as usize - small1_size as usize;
+
+    (small0_size, big_size, small1_size)
+}
+
+unsafe fn copy_quadwords(src: *const u8, dst: *mut u8, len: usize) {
+    unsafe {
+        asm!(
+            "rep movsq (%rsi), (%rdi)",
+            inout("rcx") len / 8 => _,
+            inout("rdi") dst => _,
+            inout("rsi") src => _,
+            options(att_syntax, nostack, preserves_flags)
+        );
+    }
+}
+
 /// Copies `len` bytes of data from enclave pointer `src` to userspace `dst`
 ///
 /// This function mitigates stale data vulnerabilities by ensuring all writes to untrusted memory are either:
@@ -343,17 +373,6 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
         }
     }
 
-    unsafe fn copy_aligned_quadwords_to_userspace(src: *const u8, dst: *mut u8, len: usize) {
-        unsafe {
-            asm!(
-                "rep movsq (%rsi), (%rdi)",
-                inout("rcx") len / 8 => _,
-                inout("rdi") dst => _,
-                inout("rsi") src => _,
-                options(att_syntax, nostack, preserves_flags)
-            );
-        }
-    }
     assert!(!src.is_null());
     assert!(!dst.is_null());
     assert!(is_enclave_range(src, len));
@@ -367,10 +386,10 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
         unsafe {
             copy_bytewise_to_userspace(src, dst, len);
         }
-    } else if len % 8 == 0 && dst as usize % 8 == 0 {
+    } else if len % 8 == 0 && dst.is_aligned_to(8) {
         // Copying 8-byte aligned quadwords: copy quad word per quad word
         unsafe {
-            copy_aligned_quadwords_to_userspace(src, dst, len);
+            copy_quadwords(src, dst, len);
         }
     } else {
         // Split copies into three parts:
@@ -381,20 +400,16 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
         //   +--------+
         //   | small1 | Chunk smaller than 8 bytes
         //   +--------+
+        let (small0_size, big_size, small1_size) = region_as_aligned_chunks(dst, len);
 
         unsafe {
             // Copy small0
-            let small0_size = (8 - dst as usize % 8) as u8;
-            let small0_src = src;
-            let small0_dst = dst;
-            copy_bytewise_to_userspace(small0_src as _, small0_dst, small0_size as _);
+            copy_bytewise_to_userspace(src, dst, small0_size as _);
 
             // Copy big
-            let small1_size = ((len - small0_size as usize) % 8) as u8;
-            let big_size = len - small0_size as usize - small1_size as usize;
             let big_src = src.offset(small0_size as _);
             let big_dst = dst.offset(small0_size as _);
-            copy_aligned_quadwords_to_userspace(big_src as _, big_dst, big_size);
+            copy_quadwords(big_src as _, big_dst, big_size);
 
             // Copy small1
             let small1_src = src.offset(big_size as isize + small0_size as isize);
@@ -404,6 +419,106 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
     }
 }
 
+/// Copies `len` bytes of data from userspace pointer `src` to enclave pointer `dst`
+///
+/// This function mitigates AEPIC leak vulnerabilities by ensuring all reads from untrusted memory are 8-byte aligned
+///
+/// # Panics
+/// This function panics if:
+///
+/// * The `src` pointer is null
+/// * The `dst` pointer is null
+/// * The `src` memory range is not in user memory
+/// * The `dst` memory range is not in enclave memory
+///
+/// # References
+///  - https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00657.html
+///  - https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/stale-data-read-from-xapic.html
+pub(crate) unsafe fn copy_from_userspace(src: *const u8, dst: *mut u8, len: usize) {
+    // Copies memory region `src..src + len` to the enclave at `dst`. The source memory region
+    // is:
+    //  - strictly less than 8 bytes in size and may be
+    //  - located at a misaligned memory location
+    fn copy_misaligned_chunk_to_enclave(src: *const u8, dst: *mut u8, len: usize) {
+        let mut tmp_buff = [0u8; 16];
+
+        unsafe {
+            // Compute an aligned memory region to read from
+            // +--------+ <-- aligned_src + aligned_len (8B-aligned)
+            // |  pad1  |
+            // +--------+ <-- src + len (misaligned)
+            // |        |
+            // |        |
+            // |        |
+            // +--------+ <-- src (misaligned)
+            // |  pad0  |
+            // +--------+ <-- aligned_src (8B-aligned)
+            let pad0_size = src as usize % 8;
+            let aligned_src = src.sub(pad0_size);
+
+            let pad1_size = 8 - (src.add(len) as usize % 8);
+            let aligned_len = pad0_size + len + pad1_size;
+
+            debug_assert!(len < 8);
+            debug_assert_eq!(aligned_src as usize % 8, 0);
+            debug_assert_eq!(aligned_len % 8, 0);
+            debug_assert!(aligned_len <= 16);
+
+            // Copy the aligned buffer to a temporary buffer
+            // Note: copying from a slightly different memory location is a bit odd. In this case it
+            // can't lead to page faults or inadvertent copying from the enclave as we only ensured
+            // that the `src` pointer is aligned at an 8 byte boundary. As pages are 4096 bytes
+            // aligned, `aligned_src` must be on the same page as `src`. A similar argument can be made
+            // for `src + len`
+            copy_quadwords(aligned_src as _, tmp_buff.as_mut_ptr(), aligned_len);
+
+            // Copy the correct parts of the temporary buffer to the destination
+            ptr::copy(tmp_buff.as_ptr().add(pad0_size), dst, len);
+        }
+    }
+
+    assert!(!src.is_null());
+    assert!(!dst.is_null());
+    assert!(is_user_range(src, len));
+    assert!(is_enclave_range(dst, len));
+    assert!(!(src as usize).overflowing_add(len + 8).1);
+    assert!(!(dst as usize).overflowing_add(len + 8).1);
+
+    if len < 8 {
+        copy_misaligned_chunk_to_enclave(src, dst, len);
+    } else if len % 8 == 0 && src as usize % 8 == 0 {
+        // Copying 8-byte aligned quadwords: copy quad word per quad word
+        unsafe {
+            copy_quadwords(src, dst, len);
+        }
+    } else {
+        // Split copies into three parts:
+        //   +--------+
+        //   | small0 | Chunk smaller than 8 bytes
+        //   +--------+
+        //   |   big  | Chunk 8-byte aligned, and size a multiple of 8 bytes
+        //   +--------+
+        //   | small1 | Chunk smaller than 8 bytes
+        //   +--------+
+        let (small0_size, big_size, small1_size) = region_as_aligned_chunks(dst, len);
+
+        unsafe {
+            // Copy small0
+            copy_misaligned_chunk_to_enclave(src, dst, small0_size);
+
+            // Copy big
+            let big_src = src.add(small0_size);
+            let big_dst = dst.add(small0_size);
+            copy_quadwords(big_src, big_dst, big_size);
+
+            // Copy small1
+            let small1_src = src.add(big_size + small0_size);
+            let small1_dst = dst.add(big_size + small0_size);
+            copy_misaligned_chunk_to_enclave(small1_src, small1_dst, small1_size);
+        }
+    }
+}
+
 #[unstable(feature = "sgx_platform", issue = "56975")]
 impl<T: ?Sized> UserRef<T>
 where
@@ -468,7 +583,7 @@ where
     pub fn copy_to_enclave(&self, dest: &mut T) {
         unsafe {
             assert_eq!(mem::size_of_val(dest), mem::size_of_val(&*self.0.get()));
-            ptr::copy(
+            copy_from_userspace(
                 self.0.get() as *const T as *const u8,
                 dest as *mut T as *mut u8,
                 mem::size_of_val(dest),
@@ -494,7 +609,11 @@ where
 {
     /// Copies the value from user memory into enclave memory.
     pub fn to_enclave(&self) -> T {
-        unsafe { ptr::read(self.0.get()) }
+        unsafe {
+            let mut data: T = mem::MaybeUninit::uninit().assume_init();
+            copy_from_userspace(self.0.get() as _, &mut data as *mut T as _, mem::size_of::<T>());
+            data
+        }
     }
 }
 
diff --git a/library/std/src/sys/sgx/abi/usercalls/mod.rs b/library/std/src/sys/sgx/abi/usercalls/mod.rs
index 79d1db5e1..e19e84326 100644
--- a/library/std/src/sys/sgx/abi/usercalls/mod.rs
+++ b/library/std/src/sys/sgx/abi/usercalls/mod.rs
@@ -292,12 +292,17 @@ fn check_os_error(err: Result) -> i32 {
     }
 }
 
-trait FromSgxResult {
+/// Translate the raw result of an SGX usercall.
+#[unstable(feature = "sgx_platform", issue = "56975")]
+pub trait FromSgxResult {
+    /// Return type
     type Return;
 
+    /// Translate the raw result of an SGX usercall.
     fn from_sgx_result(self) -> IoResult<Self::Return>;
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl<T> FromSgxResult for (Result, T) {
     type Return = T;
 
@@ -310,6 +315,7 @@ impl<T> FromSgxResult for (Result, T) {
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl FromSgxResult for Result {
     type Return = ();
 
diff --git a/library/std/src/sys/sgx/abi/usercalls/raw.rs b/library/std/src/sys/sgx/abi/usercalls/raw.rs
index 4267b96cc..10c1456d4 100644
--- a/library/std/src/sys/sgx/abi/usercalls/raw.rs
+++ b/library/std/src/sys/sgx/abi/usercalls/raw.rs
@@ -37,14 +37,23 @@ pub unsafe fn do_usercall(
     (a, b)
 }
 
-type Register = u64;
+/// A value passed or returned in a CPU register.
+#[unstable(feature = "sgx_platform", issue = "56975")]
+pub type Register = u64;
 
-trait RegisterArgument {
+/// Translate a type from/to Register to be used as an argument.
+#[unstable(feature = "sgx_platform", issue = "56975")]
+pub trait RegisterArgument {
+    /// Translate a Register to Self.
     fn from_register(_: Register) -> Self;
+    /// Translate self to a Register.
     fn into_register(self) -> Register;
 }
 
-trait ReturnValue {
+/// Translate a pair of Registers to the raw usercall return value.
+#[unstable(feature = "sgx_platform", issue = "56975")]
+pub trait ReturnValue {
+    /// Translate a pair of Registers to the raw usercall return value.
     fn from_registers(call: &'static str, regs: (Register, Register)) -> Self;
 }
 
@@ -68,6 +77,7 @@ macro_rules! define_usercalls {
 
 macro_rules! define_ra {
     (< $i:ident > $t:ty) => {
+        #[unstable(feature = "sgx_platform", issue = "56975")]
         impl<$i> RegisterArgument for $t {
             fn from_register(a: Register) -> Self {
                 a as _
@@ -78,6 +88,7 @@ macro_rules! define_ra {
         }
     };
     ($i:ty as $t:ty) => {
+        #[unstable(feature = "sgx_platform", issue = "56975")]
         impl RegisterArgument for $t {
             fn from_register(a: Register) -> Self {
                 a as $i as _
@@ -88,6 +99,7 @@ macro_rules! define_ra {
         }
     };
     ($t:ty) => {
+        #[unstable(feature = "sgx_platform", issue = "56975")]
         impl RegisterArgument for $t {
             fn from_register(a: Register) -> Self {
                 a as _
@@ -112,6 +124,7 @@ define_ra!(usize as isize);
 define_ra!(<T> *const T);
 define_ra!(<T> *mut T);
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl RegisterArgument for bool {
     fn from_register(a: Register) -> bool {
         if a != 0 { true } else { false }
@@ -121,6 +134,7 @@ impl RegisterArgument for bool {
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl<T: RegisterArgument> RegisterArgument for Option<NonNull<T>> {
     fn from_register(a: Register) -> Option<NonNull<T>> {
         NonNull::new(a as _)
@@ -130,12 +144,14 @@ impl<T: RegisterArgument> RegisterArgument for Option<NonNull<T>> {
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl ReturnValue for ! {
     fn from_registers(call: &'static str, _regs: (Register, Register)) -> Self {
         rtabort!("Usercall {call}: did not expect to be re-entered");
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl ReturnValue for () {
     fn from_registers(call: &'static str, usercall_retval: (Register, Register)) -> Self {
         rtassert!(usercall_retval.0 == 0);
@@ -144,6 +160,7 @@ impl ReturnValue for () {
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl<T: RegisterArgument> ReturnValue for T {
     fn from_registers(call: &'static str, usercall_retval: (Register, Register)) -> Self {
         rtassert!(usercall_retval.1 == 0);
@@ -151,6 +168,7 @@ impl<T: RegisterArgument> ReturnValue for T {
     }
 }
 
+#[unstable(feature = "sgx_platform", issue = "56975")]
 impl<T: RegisterArgument, U: RegisterArgument> ReturnValue for (T, U) {
     fn from_registers(_call: &'static str, regs: (Register, Register)) -> Self {
         (T::from_register(regs.0), U::from_register(regs.1))
diff --git a/library/std/src/sys/sgx/abi/usercalls/tests.rs b/library/std/src/sys/sgx/abi/usercalls/tests.rs
index cbf7d7d54..58b8eb215 100644
--- a/library/std/src/sys/sgx/abi/usercalls/tests.rs
+++ b/library/std/src/sys/sgx/abi/usercalls/tests.rs
@@ -1,8 +1,8 @@
-use super::alloc::copy_to_userspace;
 use super::alloc::User;
+use super::alloc::{copy_from_userspace, copy_to_userspace};
 
 #[test]
-fn test_copy_function() {
+fn test_copy_to_userspace_function() {
     let mut src = [0u8; 100];
     let mut dst = User::<[u8]>::uninitialized(100);
 
@@ -17,12 +17,38 @@ fn test_copy_function() {
             dst.copy_from_enclave(&[0u8; 100]);
 
             // Copy src[0..size] to dst + offset
-            unsafe { copy_to_userspace(src.as_ptr(), dst.as_mut_ptr().offset(offset), size) };
+            unsafe { copy_to_userspace(src.as_ptr(), dst.as_mut_ptr().add(offset), size) };
 
             // Verify copy
             for byte in 0..size {
                 unsafe {
-                    assert_eq!(*dst.as_ptr().offset(offset + byte as isize), src[byte as usize]);
+                    assert_eq!(*dst.as_ptr().add(offset + byte), src[byte as usize]);
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn test_copy_from_userspace_function() {
+    let mut dst = [0u8; 100];
+    let mut src = User::<[u8]>::uninitialized(100);
+
+    src.copy_from_enclave(&[0u8; 100]);
+
+    for size in 0..48 {
+        // For all possible alignment
+        for offset in 0..8 {
+            // overwrite complete dst
+            dst = [0u8; 100];
+
+            // Copy src[0..size] to dst + offset
+            unsafe { copy_from_userspace(src.as_ptr().offset(offset), dst.as_mut_ptr(), size) };
+
+            // Verify copy
+            for byte in 0..size {
+                unsafe {
+                    assert_eq!(dst[byte as usize], *src.as_ptr().offset(offset + byte as isize));
                 }
             }
         }
diff --git a/library/std/src/sys/sgx/mod.rs b/library/std/src/sys/sgx/mod.rs
index 696400670..b1d32929e 100644
--- a/library/std/src/sys/sgx/mod.rs
+++ b/library/std/src/sys/sgx/mod.rs
@@ -47,7 +47,7 @@ pub mod locks {
 
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(argc: isize, argv: *const *const u8) {
+pub unsafe fn init(argc: isize, argv: *const *const u8, _sigpipe: u8) {
     unsafe {
         args::init(argc, argv);
     }
diff --git a/library/std/src/sys/sgx/mutex.rs b/library/std/src/sys/sgx/mutex.rs
index 513cd77fd..aa747d56b 100644
--- a/library/std/src/sys/sgx/mutex.rs
+++ b/library/std/src/sys/sgx/mutex.rs
@@ -21,9 +21,6 @@ impl Mutex {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn lock(&self) {
         let mut guard = self.inner.lock();
         if *guard.lock_var() {
diff --git a/library/std/src/sys/solid/fs.rs b/library/std/src/sys/solid/fs.rs
index a2cbee4dc..969222253 100644
--- a/library/std/src/sys/solid/fs.rs
+++ b/library/std/src/sys/solid/fs.rs
@@ -2,7 +2,7 @@ use super::{abi, error};
 use crate::{
     ffi::{CStr, CString, OsStr, OsString},
     fmt,
-    io::{self, IoSlice, IoSliceMut, ReadBuf, SeekFrom},
+    io::{self, BorrowedCursor, IoSlice, IoSliceMut, SeekFrom},
     mem::MaybeUninit,
     os::raw::{c_int, c_short},
     os::solid::ffi::OsStrExt,
@@ -77,6 +77,9 @@ pub struct OpenOptions {
     custom_flags: i32,
 }
 
+#[derive(Copy, Clone, Debug, Default)]
+pub struct FileTimes {}
+
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct FilePermissions(c_short);
 
@@ -126,6 +129,11 @@ impl FilePermissions {
     }
 }
 
+impl FileTimes {
+    pub fn set_accessed(&mut self, _t: SystemTime) {}
+    pub fn set_modified(&mut self, _t: SystemTime) {}
+}
+
 impl FileType {
     pub fn is_dir(&self) -> bool {
         self.is(abi::S_IFDIR)
@@ -358,13 +366,13 @@ impl File {
         }
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    pub fn read_buf(&self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
         unsafe {
-            let len = buf.remaining();
+            let len = cursor.capacity();
             let mut out_num_bytes = MaybeUninit::uninit();
             error::SolidError::err_if_negative(abi::SOLID_FS_Read(
                 self.fd.raw(),
-                buf.unfilled_mut().as_mut_ptr() as *mut u8,
+                cursor.as_mut().as_mut_ptr() as *mut u8,
                 len,
                 out_num_bytes.as_mut_ptr(),
             ))
@@ -376,9 +384,7 @@ impl File {
 
             // Safety: `num_bytes_read` bytes were written to the unfilled
             // portion of the buffer
-            buf.assume_init(num_bytes_read);
-
-            buf.add_filled(num_bytes_read);
+            cursor.advance(num_bytes_read);
 
             Ok(())
         }
@@ -452,6 +458,10 @@ impl File {
     pub fn set_permissions(&self, _perm: FilePermissions) -> io::Result<()> {
         unsupported()
     }
+
+    pub fn set_times(&self, _times: FileTimes) -> io::Result<()> {
+        unsupported()
+    }
 }
 
 impl Drop for File {
diff --git a/library/std/src/sys/solid/mod.rs b/library/std/src/sys/solid/mod.rs
index 778a589d1..5867979a2 100644
--- a/library/std/src/sys/solid/mod.rs
+++ b/library/std/src/sys/solid/mod.rs
@@ -56,7 +56,7 @@ pub mod locks {
 
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(_argc: isize, _argv: *const *const u8) {}
+pub unsafe fn init(_argc: isize, _argv: *const *const u8, _sigpipe: u8) {}
 
 // SAFETY: must be called only once during runtime cleanup.
 pub unsafe fn cleanup() {}
diff --git a/library/std/src/sys/unix/fd.rs b/library/std/src/sys/unix/fd.rs
index 30812dabb..dbaa3c33e 100644
--- a/library/std/src/sys/unix/fd.rs
+++ b/library/std/src/sys/unix/fd.rs
@@ -4,7 +4,7 @@
 mod tests;
 
 use crate::cmp;
-use crate::io::{self, IoSlice, IoSliceMut, Read, ReadBuf};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, Read};
 use crate::os::unix::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::sys::cvt;
 use crate::sys_common::{AsInner, FromInner, IntoInner};
@@ -131,20 +131,19 @@ impl FileDesc {
         }
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    pub fn read_buf(&self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
         let ret = cvt(unsafe {
             libc::read(
                 self.as_raw_fd(),
-                buf.unfilled_mut().as_mut_ptr() as *mut libc::c_void,
-                cmp::min(buf.remaining(), READ_LIMIT),
+                cursor.as_mut().as_mut_ptr() as *mut libc::c_void,
+                cmp::min(cursor.capacity(), READ_LIMIT),
             )
         })?;
 
         // Safety: `ret` bytes were written to the initialized portion of the buffer
         unsafe {
-            buf.assume_init(ret as usize);
+            cursor.advance(ret as usize);
         }
-        buf.add_filled(ret as usize);
         Ok(())
     }
 
diff --git a/library/std/src/sys/unix/fs.rs b/library/std/src/sys/unix/fs.rs
index b5cc8038c..cc347e358 100644
--- a/library/std/src/sys/unix/fs.rs
+++ b/library/std/src/sys/unix/fs.rs
@@ -2,7 +2,7 @@ use crate::os::unix::prelude::*;
 
 use crate::ffi::{CStr, CString, OsStr, OsString};
 use crate::fmt;
-use crate::io::{self, Error, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{self, BorrowedCursor, Error, IoSlice, IoSliceMut, SeekFrom};
 use crate::mem;
 use crate::os::unix::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd};
 use crate::path::{Path, PathBuf};
@@ -544,11 +544,11 @@ impl Default for FileTimes {
     fn default() -> Self {
         // Redox doesn't appear to support `UTIME_OMIT`, so we stub it out here, and always return
         // an error in `set_times`.
-        // ESP-IDF does not support `futimens` at all and the behavior for that OS is therefore
+        // ESP-IDF and HorizonOS do not support `futimens` at all and the behavior for those OS is therefore
         // the same as for Redox.
-        #[cfg(any(target_os = "redox", target_os = "espidf"))]
+        #[cfg(any(target_os = "redox", target_os = "espidf", target_os = "horizon"))]
         let omit = libc::timespec { tv_sec: 0, tv_nsec: 0 };
-        #[cfg(not(any(target_os = "redox", target_os = "espidf")))]
+        #[cfg(not(any(target_os = "redox", target_os = "espidf", target_os = "horizon")))]
         let omit = libc::timespec { tv_sec: 0, tv_nsec: libc::UTIME_OMIT as _ };
         Self([omit; 2])
     }
@@ -687,7 +687,11 @@ impl Iterator for ReadDir {
 impl Drop for Dir {
     fn drop(&mut self) {
         let r = unsafe { libc::closedir(self.0) };
-        debug_assert_eq!(r, 0);
+        assert!(
+            r == 0 || crate::io::Error::last_os_error().kind() == crate::io::ErrorKind::Interrupted,
+            "unexpected error during closedir: {:?}",
+            crate::io::Error::last_os_error()
+        );
     }
 }
 
@@ -825,6 +829,7 @@ impl DirEntry {
         target_os = "fuchsia",
         target_os = "redox"
     )))]
+    #[cfg_attr(miri, allow(unused))]
     fn name_cstr(&self) -> &CStr {
         unsafe { CStr::from_ptr(self.entry.d_name.as_ptr()) }
     }
@@ -836,6 +841,7 @@ impl DirEntry {
         target_os = "fuchsia",
         target_os = "redox"
     ))]
+    #[cfg_attr(miri, allow(unused))]
     fn name_cstr(&self) -> &CStr {
         &self.name
     }
@@ -1031,8 +1037,8 @@ impl File {
         self.0.read_at(buf, offset)
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        self.0.read_buf(buf)
+    pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.0.read_buf(cursor)
     }
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
@@ -1079,9 +1085,9 @@ impl File {
 
     pub fn set_times(&self, times: FileTimes) -> io::Result<()> {
         cfg_if::cfg_if! {
-            if #[cfg(any(target_os = "redox", target_os = "espidf"))] {
+            if #[cfg(any(target_os = "redox", target_os = "espidf", target_os = "horizon"))] {
                 // Redox doesn't appear to support `UTIME_OMIT`.
-                // ESP-IDF does not support `futimens` at all and the behavior for that OS is therefore
+                // ESP-IDF and HorizonOS do not support `futimens` at all and the behavior for those OS is therefore
                 // the same as for Redox.
                 drop(times);
                 Err(io::const_io_error!(
diff --git a/library/std/src/sys/unix/locks/fuchsia_mutex.rs b/library/std/src/sys/unix/locks/fuchsia_mutex.rs
index ce427599c..117611ce4 100644
--- a/library/std/src/sys/unix/locks/fuchsia_mutex.rs
+++ b/library/std/src/sys/unix/locks/fuchsia_mutex.rs
@@ -86,9 +86,6 @@ impl Mutex {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn try_lock(&self) -> bool {
         let thread_self = zx_thread_self();
         self.futex.compare_exchange(UNLOCKED, to_state(thread_self), Acquire, Relaxed).is_ok()
@@ -138,7 +135,7 @@ impl Mutex {
                 }
             }
 
-            // The state has changed or a wakeup occured, try to lock the mutex.
+            // The state has changed or a wakeup occurred, try to lock the mutex.
             match self.futex.compare_exchange(UNLOCKED, owned_state, Acquire, Relaxed) {
                 Ok(_) => return,
                 Err(updated) => state = updated,
diff --git a/library/std/src/sys/unix/locks/futex_mutex.rs b/library/std/src/sys/unix/locks/futex_mutex.rs
index 99ba86e5f..33b13dad4 100644
--- a/library/std/src/sys/unix/locks/futex_mutex.rs
+++ b/library/std/src/sys/unix/locks/futex_mutex.rs
@@ -20,9 +20,6 @@ impl Mutex {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn try_lock(&self) -> bool {
         self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()
     }
@@ -53,7 +50,7 @@ impl Mutex {
             // We avoid an unnecessary write if it as already set to 2,
             // to be friendlier for the caches.
             if state != 2 && self.futex.swap(2, Acquire) == 0 {
-                // We changed it from 0 to 2, so we just succesfully locked it.
+                // We changed it from 0 to 2, so we just successfully locked it.
                 return;
             }
 
diff --git a/library/std/src/sys/unix/locks/futex_rwlock.rs b/library/std/src/sys/unix/locks/futex_rwlock.rs
index b3bbbf743..0cc92244e 100644
--- a/library/std/src/sys/unix/locks/futex_rwlock.rs
+++ b/library/std/src/sys/unix/locks/futex_rwlock.rs
@@ -54,7 +54,7 @@ fn is_read_lockable(state: u32) -> bool {
     // We don't allow read-locking if there's readers waiting, even if the lock is unlocked
     // and there's no writers waiting. The only situation when this happens is after unlocking,
     // at which point the unlocking thread might be waking up writers, which have priority over readers.
-    // The unlocking thread will clear the readers waiting bit and wake up readers, if necssary.
+    // The unlocking thread will clear the readers waiting bit and wake up readers, if necessary.
     state & MASK < MAX_READERS && !has_readers_waiting(state) && !has_writers_waiting(state)
 }
 
diff --git a/library/std/src/sys/unix/locks/pthread_condvar.rs b/library/std/src/sys/unix/locks/pthread_condvar.rs
index abf27e7db..4741c0c67 100644
--- a/library/std/src/sys/unix/locks/pthread_condvar.rs
+++ b/library/std/src/sys/unix/locks/pthread_condvar.rs
@@ -172,7 +172,7 @@ impl Condvar {
         let mut sys_now = libc::timeval { tv_sec: 0, tv_usec: 0 };
         let stable_now = Instant::now();
         let r = libc::gettimeofday(&mut sys_now, ptr::null_mut());
-        debug_assert_eq!(r, 0);
+        assert_eq!(r, 0, "unexpected error: {:?}", crate::io::Error::last_os_error());
 
         let nsec = dur.subsec_nanos() as libc::c_long + (sys_now.tv_usec * 1000) as libc::c_long;
         let extra = (nsec / 1_000_000_000) as libc::time_t;
diff --git a/library/std/src/sys/unix/locks/pthread_mutex.rs b/library/std/src/sys/unix/locks/pthread_mutex.rs
index 98afee69b..5964935dd 100644
--- a/library/std/src/sys/unix/locks/pthread_mutex.rs
+++ b/library/std/src/sys/unix/locks/pthread_mutex.rs
@@ -52,7 +52,7 @@ impl Mutex {
         Mutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
     }
     #[inline]
-    pub unsafe fn init(&mut self) {
+    unsafe fn init(&mut self) {
         // Issue #33770
         //
         // A pthread mutex initialized with PTHREAD_MUTEX_INITIALIZER will have
diff --git a/library/std/src/sys/unix/mod.rs b/library/std/src/sys/unix/mod.rs
index 3d0d91460..c84e292ea 100644
--- a/library/std/src/sys/unix/mod.rs
+++ b/library/std/src/sys/unix/mod.rs
@@ -44,12 +44,13 @@ pub mod thread_parker;
 pub mod time;
 
 #[cfg(target_os = "espidf")]
-pub fn init(argc: isize, argv: *const *const u8) {}
+pub fn init(argc: isize, argv: *const *const u8, _sigpipe: u8) {}
 
 #[cfg(not(target_os = "espidf"))]
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(argc: isize, argv: *const *const u8) {
+// See `fn init()` in `library/std/src/rt.rs` for docs on `sigpipe`.
+pub unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     // The standard streams might be closed on application startup. To prevent
     // std::io::{stdin, stdout,stderr} objects from using other unrelated file
     // resources opened later, we reopen standards streams when they are closed.
@@ -61,8 +62,9 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
     // want!
     //
     // Hence, we set SIGPIPE to ignore when the program starts up in order
-    // to prevent this problem.
-    reset_sigpipe();
+    // to prevent this problem. Add `#[unix_sigpipe = "..."]` above `fn main()` to
+    // alter this behavior.
+    reset_sigpipe(sigpipe);
 
     stack_overflow::init();
     args::init(argc, argv);
@@ -151,9 +153,31 @@ pub unsafe fn init(argc: isize, argv: *const *const u8) {
         }
     }
 
-    unsafe fn reset_sigpipe() {
+    unsafe fn reset_sigpipe(#[allow(unused_variables)] sigpipe: u8) {
         #[cfg(not(any(target_os = "emscripten", target_os = "fuchsia", target_os = "horizon")))]
-        rtassert!(signal(libc::SIGPIPE, libc::SIG_IGN) != libc::SIG_ERR);
+        {
+            // We don't want to add this as a public type to libstd, nor do we
+            // want to `include!` a file from the compiler (which would break
+            // Miri and xargo for example), so we choose to duplicate these
+            // constants from `compiler/rustc_session/src/config/sigpipe.rs`.
+            // See the other file for docs. NOTE: Make sure to keep them in
+            // sync!
+            mod sigpipe {
+                pub const INHERIT: u8 = 1;
+                pub const SIG_IGN: u8 = 2;
+                pub const SIG_DFL: u8 = 3;
+            }
+
+            let handler = match sigpipe {
+                sigpipe::INHERIT => None,
+                sigpipe::SIG_IGN => Some(libc::SIG_IGN),
+                sigpipe::SIG_DFL => Some(libc::SIG_DFL),
+                _ => unreachable!(),
+            };
+            if let Some(handler) = handler {
+                rtassert!(signal(libc::SIGPIPE, handler) != libc::SIG_ERR);
+            }
+        }
     }
 }
 
@@ -295,8 +319,10 @@ pub fn abort_internal() -> ! {
 
 cfg_if::cfg_if! {
     if #[cfg(target_os = "android")] {
-        #[link(name = "dl")]
-        #[link(name = "log")]
+        #[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")))]
         extern "C" {}
     } else if #[cfg(target_os = "freebsd")] {
         #[link(name = "execinfo")]
diff --git a/library/std/src/sys/unix/net.rs b/library/std/src/sys/unix/net.rs
index 462a45b01..b84bf8f92 100644
--- a/library/std/src/sys/unix/net.rs
+++ b/library/std/src/sys/unix/net.rs
@@ -393,6 +393,17 @@ impl Socket {
     }
 
     #[cfg(any(target_os = "android", target_os = "linux",))]
+    pub fn set_quickack(&self, quickack: bool) -> io::Result<()> {
+        setsockopt(self, libc::IPPROTO_TCP, libc::TCP_QUICKACK, quickack as c_int)
+    }
+
+    #[cfg(any(target_os = "android", target_os = "linux",))]
+    pub fn quickack(&self) -> io::Result<bool> {
+        let raw: c_int = getsockopt(self, libc::IPPROTO_TCP, libc::TCP_QUICKACK)?;
+        Ok(raw != 0)
+    }
+
+    #[cfg(any(target_os = "android", target_os = "linux",))]
     pub fn set_passcred(&self, passcred: bool) -> io::Result<()> {
         setsockopt(self, libc::SOL_SOCKET, libc::SO_PASSCRED, passcred as libc::c_int)
     }
@@ -427,6 +438,17 @@ impl Socket {
         self.0.set_nonblocking(nonblocking)
     }
 
+    #[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"))]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        #[cfg(target_os = "linux")]
+        let option = libc::SO_MARK;
+        #[cfg(target_os = "freebsd")]
+        let option = libc::SO_USER_COOKIE;
+        #[cfg(target_os = "openbsd")]
+        let option = libc::SO_RTABLE;
+        setsockopt(self, libc::SOL_SOCKET, option, mark as libc::c_int)
+    }
+
     pub fn take_error(&self) -> io::Result<Option<io::Error>> {
         let raw: c_int = getsockopt(self, libc::SOL_SOCKET, libc::SO_ERROR)?;
         if raw == 0 { Ok(None) } else { Ok(Some(io::Error::from_raw_os_error(raw as i32))) }
diff --git a/library/std/src/sys/unix/os_str.rs b/library/std/src/sys/unix/os_str.rs
index ccbc18224..017e2af29 100644
--- a/library/std/src/sys/unix/os_str.rs
+++ b/library/std/src/sys/unix/os_str.rs
@@ -11,7 +11,7 @@ use crate::str;
 use crate::sync::Arc;
 use crate::sys_common::{AsInner, IntoInner};
 
-use core::str::lossy::{Utf8Lossy, Utf8LossyChunk};
+use core::str::Utf8Chunks;
 
 #[cfg(test)]
 #[path = "../unix/os_str/tests.rs"]
@@ -29,26 +29,32 @@ pub struct Slice {
 }
 
 impl fmt::Debug for Slice {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Writes out a valid unicode string with the correct escape sequences
-
-        formatter.write_str("\"")?;
-        for Utf8LossyChunk { valid, broken } in Utf8Lossy::from_bytes(&self.inner).chunks() {
-            for c in valid.chars().flat_map(|c| c.escape_debug()) {
-                formatter.write_char(c)?
-            }
-
-            for b in broken {
-                write!(formatter, "\\x{:02X}", b)?;
-            }
-        }
-        formatter.write_str("\"")
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&Utf8Chunks::new(&self.inner).debug(), f)
     }
 }
 
 impl fmt::Display for Slice {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&Utf8Lossy::from_bytes(&self.inner), formatter)
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // If we're the empty string then our iterator won't actually yield
+        // anything, so perform the formatting manually
+        if self.inner.is_empty() {
+            return "".fmt(f);
+        }
+
+        for chunk in Utf8Chunks::new(&self.inner) {
+            let valid = chunk.valid();
+            // If we successfully decoded the whole chunk as a valid string then
+            // we can return a direct formatting of the string which will also
+            // respect various formatting flags if possible.
+            if chunk.invalid().is_empty() {
+                return valid.fmt(f);
+            }
+
+            f.write_str(valid)?;
+            f.write_char(char::REPLACEMENT_CHARACTER)?;
+        }
+        Ok(())
     }
 }
 
diff --git a/library/std/src/sys/unix/os_str/tests.rs b/library/std/src/sys/unix/os_str/tests.rs
index 213277f01..22ba0c923 100644
--- a/library/std/src/sys/unix/os_str/tests.rs
+++ b/library/std/src/sys/unix/os_str/tests.rs
@@ -8,3 +8,11 @@ fn slice_debug_output() {
 
     assert_eq!(output, expected);
 }
+
+#[test]
+fn display() {
+    assert_eq!(
+        "Hello\u{FFFD}\u{FFFD} There\u{FFFD} Goodbye",
+        Slice::from_u8_slice(b"Hello\xC0\x80 There\xE6\x83 Goodbye").to_string(),
+    );
+}
diff --git a/library/std/src/sys/unix/process/process_common.rs b/library/std/src/sys/unix/process/process_common.rs
index bca1b65a7..2834ee0ac 100644
--- a/library/std/src/sys/unix/process/process_common.rs
+++ b/library/std/src/sys/unix/process/process_common.rs
@@ -45,11 +45,31 @@ cfg_if::cfg_if! {
         }
         #[allow(dead_code)]
         pub unsafe fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int {
-            use crate::{slice, mem};
+            use crate::{
+                mem::{align_of, size_of},
+                slice,
+            };
+            use libc::{c_ulong, sigset_t};
+
+            // The implementations from bionic (android libc) type pun `sigset_t` as an
+            // array of `c_ulong`. This works, but lets add a smoke check to make sure
+            // that doesn't change.
+            const _: () = assert!(
+                align_of::<c_ulong>() == align_of::<sigset_t>()
+                    && (size_of::<sigset_t>() % size_of::<c_ulong>()) == 0
+            );
 
-            let raw = slice::from_raw_parts_mut(set as *mut u8, mem::size_of::<libc::sigset_t>());
             let bit = (signum - 1) as usize;
-            raw[bit / 8] |= 1 << (bit % 8);
+            if set.is_null() || bit >= (8 * size_of::<sigset_t>()) {
+                crate::sys::unix::os::set_errno(libc::EINVAL);
+                return -1;
+            }
+            let raw = slice::from_raw_parts_mut(
+                set as *mut c_ulong,
+                size_of::<sigset_t>() / size_of::<c_ulong>(),
+            );
+            const LONG_BIT: usize = size_of::<c_ulong>() * 8;
+            raw[bit / LONG_BIT] |= 1 << (bit % LONG_BIT);
             return 0;
         }
     } else {
@@ -72,6 +92,7 @@ pub struct Command {
     argv: Argv,
     env: CommandEnv,
 
+    program_kind: ProgramKind,
     cwd: Option<CString>,
     uid: Option<uid_t>,
     gid: Option<gid_t>,
@@ -128,15 +149,40 @@ pub enum Stdio {
     Fd(FileDesc),
 }
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum ProgramKind {
+    /// A program that would be looked up on the PATH (e.g. `ls`)
+    PathLookup,
+    /// A relative path (e.g. `my-dir/foo`, `../foo`, `./foo`)
+    Relative,
+    /// An absolute path.
+    Absolute,
+}
+
+impl ProgramKind {
+    fn new(program: &OsStr) -> Self {
+        if program.bytes().starts_with(b"/") {
+            Self::Absolute
+        } else if program.bytes().contains(&b'/') {
+            // If the program has more than one component in it, it is a relative path.
+            Self::Relative
+        } else {
+            Self::PathLookup
+        }
+    }
+}
+
 impl Command {
     #[cfg(not(target_os = "linux"))]
     pub fn new(program: &OsStr) -> Command {
         let mut saw_nul = false;
+        let program_kind = ProgramKind::new(program.as_ref());
         let program = os2c(program, &mut saw_nul);
         Command {
             argv: Argv(vec![program.as_ptr(), ptr::null()]),
             args: vec![program.clone()],
             program,
+            program_kind,
             env: Default::default(),
             cwd: None,
             uid: None,
@@ -154,11 +200,13 @@ impl Command {
     #[cfg(target_os = "linux")]
     pub fn new(program: &OsStr) -> Command {
         let mut saw_nul = false;
+        let program_kind = ProgramKind::new(program.as_ref());
         let program = os2c(program, &mut saw_nul);
         Command {
             argv: Argv(vec![program.as_ptr(), ptr::null()]),
             args: vec![program.clone()],
             program,
+            program_kind,
             env: Default::default(),
             cwd: None,
             uid: None,
@@ -234,6 +282,11 @@ impl Command {
         OsStr::from_bytes(self.program.as_bytes())
     }
 
+    #[allow(dead_code)]
+    pub fn get_program_kind(&self) -> ProgramKind {
+        self.program_kind
+    }
+
     pub fn get_args(&self) -> CommandArgs<'_> {
         let mut iter = self.args.iter();
         iter.next();
diff --git a/library/std/src/sys/unix/process/process_common/tests.rs b/library/std/src/sys/unix/process/process_common/tests.rs
index 1956b3692..d176b3401 100644
--- a/library/std/src/sys/unix/process/process_common/tests.rs
+++ b/library/std/src/sys/unix/process/process_common/tests.rs
@@ -122,3 +122,27 @@ fn test_process_group_no_posix_spawn() {
         t!(cat.wait());
     }
 }
+
+#[test]
+fn test_program_kind() {
+    let vectors = &[
+        ("foo", ProgramKind::PathLookup),
+        ("foo.out", ProgramKind::PathLookup),
+        ("./foo", ProgramKind::Relative),
+        ("../foo", ProgramKind::Relative),
+        ("dir/foo", ProgramKind::Relative),
+        // Note that paths on Unix can't contain / in them, so this is actually the directory "fo\\"
+        // followed by the file "o".
+        ("fo\\/o", ProgramKind::Relative),
+        ("/foo", ProgramKind::Absolute),
+        ("/dir/../foo", ProgramKind::Absolute),
+    ];
+
+    for (program, expected_kind) in vectors {
+        assert_eq!(
+            ProgramKind::new(program.as_ref()),
+            *expected_kind,
+            "actual != expected program kind for input {program}",
+        );
+    }
+}
diff --git a/library/std/src/sys/unix/process/process_unix.rs b/library/std/src/sys/unix/process/process_unix.rs
index 75bb92437..26ae62817 100644
--- a/library/std/src/sys/unix/process/process_unix.rs
+++ b/library/std/src/sys/unix/process/process_unix.rs
@@ -453,7 +453,9 @@ impl Command {
                     // successfully launch the program, but erroneously return
                     // ENOENT when used with posix_spawn_file_actions_addchdir_np
                     // which was introduced in macOS 10.15.
-                    return Ok(None);
+                    if self.get_program_kind() == ProgramKind::Relative {
+                        return Ok(None);
+                    }
                 }
                 match posix_spawn_file_actions_addchdir_np.get() {
                     Some(f) => Some((f, cwd)),
diff --git a/library/std/src/sys/unix/rand.rs b/library/std/src/sys/unix/rand.rs
index bf4920488..a6fe07873 100644
--- a/library/std/src/sys/unix/rand.rs
+++ b/library/std/src/sys/unix/rand.rs
@@ -1,13 +1,13 @@
-use crate::mem;
-use crate::slice;
-
 pub fn hashmap_random_keys() -> (u64, u64) {
-    let mut v = (0, 0);
-    unsafe {
-        let view = slice::from_raw_parts_mut(&mut v as *mut _ as *mut u8, mem::size_of_val(&v));
-        imp::fill_bytes(view);
-    }
-    v
+    const KEY_LEN: usize = core::mem::size_of::<u64>();
+
+    let mut v = [0u8; KEY_LEN * 2];
+    imp::fill_bytes(&mut v);
+
+    let key1 = v[0..KEY_LEN].try_into().unwrap();
+    let key2 = v[KEY_LEN..].try_into().unwrap();
+
+    (u64::from_ne_bytes(key1), u64::from_ne_bytes(key2))
 }
 
 #[cfg(all(
diff --git a/library/std/src/sys/unix/thread.rs b/library/std/src/sys/unix/thread.rs
index 36a3fa602..f6b627afc 100644
--- a/library/std/src/sys/unix/thread.rs
+++ b/library/std/src/sys/unix/thread.rs
@@ -116,11 +116,9 @@ impl Thread {
         debug_assert_eq!(ret, 0);
     }
 
-    #[cfg(any(target_os = "linux", target_os = "android"))]
+    #[cfg(target_os = "android")]
     pub fn set_name(name: &CStr) {
         const PR_SET_NAME: libc::c_int = 15;
-        // pthread wrapper only appeared in glibc 2.12, so we use syscall
-        // directly.
         unsafe {
             libc::prctl(
                 PR_SET_NAME,
@@ -132,6 +130,17 @@ impl Thread {
         }
     }
 
+    #[cfg(target_os = "linux")]
+    pub fn set_name(name: &CStr) {
+        const TASK_COMM_LEN: usize = 16;
+
+        unsafe {
+            // Available since glibc 2.12, musl 1.1.16, and uClibc 1.0.20.
+            let name = truncate_cstr(name, TASK_COMM_LEN);
+            libc::pthread_setname_np(libc::pthread_self(), name.as_ptr());
+        }
+    }
+
     #[cfg(any(target_os = "freebsd", target_os = "dragonfly", target_os = "openbsd"))]
     pub fn set_name(name: &CStr) {
         unsafe {
@@ -142,6 +151,7 @@ impl Thread {
     #[cfg(any(target_os = "macos", target_os = "ios", target_os = "watchos"))]
     pub fn set_name(name: &CStr) {
         unsafe {
+            let name = truncate_cstr(name, libc::MAXTHREADNAMESIZE);
             libc::pthread_setname_np(name.as_ptr());
         }
     }
@@ -271,6 +281,20 @@ impl Drop for Thread {
     }
 }
 
+#[cfg(any(target_os = "linux", target_os = "macos", target_os = "ios", target_os = "watchos"))]
+fn truncate_cstr(cstr: &CStr, max_with_nul: usize) -> crate::borrow::Cow<'_, CStr> {
+    use crate::{borrow::Cow, ffi::CString};
+
+    if cstr.to_bytes_with_nul().len() > max_with_nul {
+        let bytes = cstr.to_bytes()[..max_with_nul - 1].to_vec();
+        // SAFETY: the non-nul bytes came straight from a CStr.
+        // (CString will add the terminating nul.)
+        Cow::Owned(unsafe { CString::from_vec_unchecked(bytes) })
+    } else {
+        Cow::Borrowed(cstr)
+    }
+}
+
 pub fn available_parallelism() -> io::Result<NonZeroUsize> {
     cfg_if::cfg_if! {
         if #[cfg(any(
@@ -423,7 +447,7 @@ mod cgroups {
                         Some(b"") => Cgroup::V2,
                         Some(controllers)
                             if from_utf8(controllers)
-                                .is_ok_and(|c| c.split(",").any(|c| c == "cpu")) =>
+                                .is_ok_and(|c| c.split(',').any(|c| c == "cpu")) =>
                         {
                             Cgroup::V1
                         }
diff --git a/library/std/src/sys/unix/thread_parker/mod.rs b/library/std/src/sys/unix/thread_parker/mod.rs
new file mode 100644
index 000000000..e2453580d
--- /dev/null
+++ b/library/std/src/sys/unix/thread_parker/mod.rs
@@ -0,0 +1,21 @@
+//! Thread parking on systems without futex support.
+
+#![cfg(not(any(
+    target_os = "linux",
+    target_os = "android",
+    all(target_os = "emscripten", target_feature = "atomics"),
+    target_os = "freebsd",
+    target_os = "openbsd",
+    target_os = "dragonfly",
+    target_os = "fuchsia",
+)))]
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "netbsd")] {
+        mod netbsd;
+        pub use netbsd::Parker;
+    } else {
+        mod pthread;
+        pub use pthread::Parker;
+    }
+}
diff --git a/library/std/src/sys/unix/thread_parker/netbsd.rs b/library/std/src/sys/unix/thread_parker/netbsd.rs
new file mode 100644
index 000000000..7657605b5
--- /dev/null
+++ b/library/std/src/sys/unix/thread_parker/netbsd.rs
@@ -0,0 +1,113 @@
+use crate::ffi::{c_int, c_void};
+use crate::pin::Pin;
+use crate::ptr::{null, null_mut};
+use crate::sync::atomic::{
+    AtomicU64,
+    Ordering::{Acquire, Relaxed, Release},
+};
+use crate::time::Duration;
+use libc::{_lwp_self, clockid_t, lwpid_t, time_t, timespec, CLOCK_MONOTONIC};
+
+extern "C" {
+    fn ___lwp_park60(
+        clock_id: clockid_t,
+        flags: c_int,
+        ts: *mut timespec,
+        unpark: lwpid_t,
+        hint: *const c_void,
+        unparkhint: *const c_void,
+    ) -> c_int;
+    fn _lwp_unpark(lwp: lwpid_t, hint: *const c_void) -> c_int;
+}
+
+/// The thread is not parked and the token is not available.
+///
+/// Zero cannot be a valid LWP id, since it is used as empty value for the unpark
+/// argument in _lwp_park.
+const EMPTY: u64 = 0;
+/// The token is available. Do not park anymore.
+const NOTIFIED: u64 = u64::MAX;
+
+pub struct Parker {
+    /// The parker state. Contains either one of the two state values above or the LWP
+    /// id of the parked thread.
+    state: AtomicU64,
+}
+
+impl Parker {
+    pub unsafe fn new(parker: *mut Parker) {
+        parker.write(Parker { state: AtomicU64::new(EMPTY) })
+    }
+
+    // Does not actually need `unsafe` or `Pin`, but the pthread implementation does.
+    pub unsafe fn park(self: Pin<&Self>) {
+        // If the token has already been made available, we can skip
+        // a bit of work, so check for it here.
+        if self.state.load(Acquire) != NOTIFIED {
+            let parked = _lwp_self() as u64;
+            let hint = self.state.as_mut_ptr().cast();
+            if self.state.compare_exchange(EMPTY, parked, Relaxed, Acquire).is_ok() {
+                // Loop to guard against spurious wakeups.
+                loop {
+                    ___lwp_park60(0, 0, null_mut(), 0, hint, null());
+                    if self.state.load(Acquire) == NOTIFIED {
+                        break;
+                    }
+                }
+            }
+        }
+
+        // At this point, the change to NOTIFIED has always been observed with acquire
+        // ordering, so we can just use a relaxed store here (instead of a swap).
+        self.state.store(EMPTY, Relaxed);
+    }
+
+    // Does not actually need `unsafe` or `Pin`, but the pthread implementation does.
+    pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) {
+        if self.state.load(Acquire) != NOTIFIED {
+            let parked = _lwp_self() as u64;
+            let hint = self.state.as_mut_ptr().cast();
+            let mut timeout = timespec {
+                // Saturate so that the operation will definitely time out
+                // (even if it is after the heat death of the universe).
+                tv_sec: dur.as_secs().try_into().ok().unwrap_or(time_t::MAX),
+                tv_nsec: dur.subsec_nanos().into(),
+            };
+
+            if self.state.compare_exchange(EMPTY, parked, Relaxed, Acquire).is_ok() {
+                // Timeout needs to be mutable since it is modified on NetBSD 9.0 and
+                // above.
+                ___lwp_park60(CLOCK_MONOTONIC, 0, &mut timeout, 0, hint, null());
+                // Use a swap to get acquire ordering even if the token was set after
+                // the timeout occurred.
+                self.state.swap(EMPTY, Acquire);
+                return;
+            }
+        }
+
+        self.state.store(EMPTY, Relaxed);
+    }
+
+    // Does not actually need `Pin`, but the pthread implementation does.
+    pub fn unpark(self: Pin<&Self>) {
+        let state = self.state.swap(NOTIFIED, Release);
+        if !matches!(state, EMPTY | NOTIFIED) {
+            let lwp = state as lwpid_t;
+            let hint = self.state.as_mut_ptr().cast();
+
+            // If the parking thread terminated and did not actually park, this will
+            // probably return an error, which is OK. In the worst case, another
+            // thread has received the same LWP id. It will then receive a spurious
+            // wakeup, but those are allowable per the API contract. The same reasoning
+            // applies if a timeout occurred before this call, but the state was not
+            // yet reset.
+
+            // SAFETY:
+            // The syscall has no invariants to hold. Only unsafe because it is an
+            // extern function.
+            unsafe {
+                _lwp_unpark(lwp, hint);
+            }
+        }
+    }
+}
diff --git a/library/std/src/sys/unix/thread_parker.rs b/library/std/src/sys/unix/thread_parker/pthread.rs
index ca1a7138f..3dfc0026e 100644
--- a/library/std/src/sys/unix/thread_parker.rs
+++ b/library/std/src/sys/unix/thread_parker/pthread.rs
@@ -1,15 +1,5 @@
 //! Thread parking without `futex` using the `pthread` synchronization primitives.
 
-#![cfg(not(any(
-    target_os = "linux",
-    target_os = "android",
-    all(target_os = "emscripten", target_feature = "atomics"),
-    target_os = "freebsd",
-    target_os = "openbsd",
-    target_os = "dragonfly",
-    target_os = "fuchsia",
-)))]
-
 use crate::cell::UnsafeCell;
 use crate::marker::PhantomPinned;
 use crate::pin::Pin;
@@ -59,8 +49,8 @@ unsafe fn wait_timeout(
         target_os = "espidf"
     ))]
     let (now, dur) = {
-        use super::time::SystemTime;
         use crate::cmp::min;
+        use crate::sys::time::SystemTime;
 
         // OSX implementation of `pthread_cond_timedwait` is buggy
         // with super long durations. When duration is greater than
@@ -85,7 +75,7 @@ unsafe fn wait_timeout(
         target_os = "espidf"
     )))]
     let (now, dur) = {
-        use super::time::Timespec;
+        use crate::sys::time::Timespec;
 
         (Timespec::now(libc::CLOCK_MONOTONIC), dur)
     };
diff --git a/library/std/src/sys/unsupported/alloc.rs b/library/std/src/sys/unsupported/alloc.rs
index 8d5d0a2f5..d715ae454 100644
--- a/library/std/src/sys/unsupported/alloc.rs
+++ b/library/std/src/sys/unsupported/alloc.rs
@@ -1,15 +1,16 @@
 use crate::alloc::{GlobalAlloc, Layout, System};
+use crate::ptr::null_mut;
 
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
 unsafe impl GlobalAlloc for System {
     #[inline]
     unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 
     #[inline]
     unsafe fn alloc_zeroed(&self, _layout: Layout) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 
     #[inline]
@@ -17,6 +18,6 @@ unsafe impl GlobalAlloc for System {
 
     #[inline]
     unsafe fn realloc(&self, _ptr: *mut u8, _layout: Layout, _new_size: usize) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 }
diff --git a/library/std/src/sys/unsupported/common.rs b/library/std/src/sys/unsupported/common.rs
index 4c9ade4a8..5cd9e57de 100644
--- a/library/std/src/sys/unsupported/common.rs
+++ b/library/std/src/sys/unsupported/common.rs
@@ -6,7 +6,7 @@ pub mod memchr {
 
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(_argc: isize, _argv: *const *const u8) {}
+pub unsafe fn init(_argc: isize, _argv: *const *const u8, _sigpipe: u8) {}
 
 // SAFETY: must be called only once during runtime cleanup.
 // NOTE: this is not guaranteed to run, for example when the program aborts.
diff --git a/library/std/src/sys/unsupported/fs.rs b/library/std/src/sys/unsupported/fs.rs
index 0e1a6257e..6ac1b5d2b 100644
--- a/library/std/src/sys/unsupported/fs.rs
+++ b/library/std/src/sys/unsupported/fs.rs
@@ -1,7 +1,7 @@
 use crate::ffi::OsString;
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
-use crate::io::{self, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, SeekFrom};
 use crate::path::{Path, PathBuf};
 use crate::sys::time::SystemTime;
 use crate::sys::unsupported;
@@ -214,7 +214,7 @@ impl File {
         self.0
     }
 
-    pub fn read_buf(&self, _buf: &mut ReadBuf<'_>) -> io::Result<()> {
+    pub fn read_buf(&self, _cursor: BorrowedCursor<'_>) -> io::Result<()> {
         self.0
     }
 
diff --git a/library/std/src/sys/unsupported/locks/mutex.rs b/library/std/src/sys/unsupported/locks/mutex.rs
index d7cb12e0c..2be0b34b9 100644
--- a/library/std/src/sys/unsupported/locks/mutex.rs
+++ b/library/std/src/sys/unsupported/locks/mutex.rs
@@ -17,9 +17,6 @@ impl Mutex {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn lock(&self) {
         assert_eq!(self.locked.replace(true), false, "cannot recursively acquire mutex");
     }
diff --git a/library/std/src/sys/unsupported/process.rs b/library/std/src/sys/unsupported/process.rs
index 42a1ff730..633f17c05 100644
--- a/library/std/src/sys/unsupported/process.rs
+++ b/library/std/src/sys/unsupported/process.rs
@@ -200,6 +200,9 @@ impl<'a> Iterator for CommandArgs<'a> {
     fn next(&mut self) -> Option<&'a OsStr> {
         None
     }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 impl<'a> ExactSizeIterator for CommandArgs<'a> {}
diff --git a/library/std/src/sys/wasi/fs.rs b/library/std/src/sys/wasi/fs.rs
index 6614ae397..510cf36b1 100644
--- a/library/std/src/sys/wasi/fs.rs
+++ b/library/std/src/sys/wasi/fs.rs
@@ -3,7 +3,7 @@
 use super::fd::WasiFd;
 use crate::ffi::{CStr, CString, OsStr, OsString};
 use crate::fmt;
-use crate::io::{self, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, SeekFrom};
 use crate::iter;
 use crate::mem::{self, ManuallyDrop};
 use crate::os::raw::c_int;
@@ -439,8 +439,8 @@ impl File {
         true
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        crate::io::default_read_buf(|buf| self.read(buf), buf)
+    pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        crate::io::default_read_buf(|buf| self.read(buf), cursor)
     }
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
diff --git a/library/std/src/sys/wasi/stdio.rs b/library/std/src/sys/wasi/stdio.rs
index 4cc0e4ed5..d2081771b 100644
--- a/library/std/src/sys/wasi/stdio.rs
+++ b/library/std/src/sys/wasi/stdio.rs
@@ -4,7 +4,7 @@ use super::fd::WasiFd;
 use crate::io::{self, IoSlice, IoSliceMut};
 use crate::mem::ManuallyDrop;
 use crate::os::raw;
-use crate::os::wasi::io::{AsRawFd, FromRawFd};
+use crate::os::wasi::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd};
 
 pub struct Stdin;
 pub struct Stdout;
@@ -23,6 +23,13 @@ impl AsRawFd for Stdin {
     }
 }
 
+impl AsFd for Stdin {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(0) }
+    }
+}
+
 impl io::Read for Stdin {
     fn read(&mut self, data: &mut [u8]) -> io::Result<usize> {
         self.read_vectored(&mut [IoSliceMut::new(data)])
@@ -51,6 +58,13 @@ impl AsRawFd for Stdout {
     }
 }
 
+impl AsFd for Stdout {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(1) }
+    }
+}
+
 impl io::Write for Stdout {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         self.write_vectored(&[IoSlice::new(data)])
@@ -82,6 +96,13 @@ impl AsRawFd for Stderr {
     }
 }
 
+impl AsFd for Stderr {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(2) }
+    }
+}
+
 impl io::Write for Stderr {
     fn write(&mut self, data: &[u8]) -> io::Result<usize> {
         self.write_vectored(&[IoSlice::new(data)])
diff --git a/library/std/src/sys/windows/alloc.rs b/library/std/src/sys/windows/alloc.rs
index fdc81cdea..d53ea1600 100644
--- a/library/std/src/sys/windows/alloc.rs
+++ b/library/std/src/sys/windows/alloc.rs
@@ -16,6 +16,7 @@ mod tests;
 // Flag to indicate that the memory returned by `HeapAlloc` should be zeroed.
 const HEAP_ZERO_MEMORY: c::DWORD = 0x00000008;
 
+#[link(name = "kernel32")]
 extern "system" {
     // Get a handle to the default heap of the current process, or null if the operation fails.
     //
@@ -168,7 +169,7 @@ unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
         // SAFETY: Because the size and alignment of a header is <= `MIN_ALIGN` and `aligned`
         // is aligned to at least `MIN_ALIGN` and has at least `MIN_ALIGN` bytes of padding before
         // it, it is safe to write a header directly before it.
-        unsafe { ptr::write((aligned as *mut Header).offset(-1), Header(ptr)) };
+        unsafe { ptr::write((aligned as *mut Header).sub(1), Header(ptr)) };
 
         // SAFETY: The returned pointer does not point to the to the start of an allocated block,
         // but there is a header readable directly before it containing the location of the start
@@ -213,7 +214,7 @@ unsafe impl GlobalAlloc for System {
 
                 // SAFETY: Because of the contract of `System`, `ptr` is guaranteed to be non-null
                 // and have a header readable directly before it.
-                unsafe { ptr::read((ptr as *mut Header).offset(-1)).0 }
+                unsafe { ptr::read((ptr as *mut Header).sub(1)).0 }
             }
         };
 
diff --git a/library/std/src/sys/windows/c.rs b/library/std/src/sys/windows/c.rs
index 478068c73..89d0ab59b 100644
--- a/library/std/src/sys/windows/c.rs
+++ b/library/std/src/sys/windows/c.rs
@@ -66,6 +66,7 @@ pub type LPSYSTEM_INFO = *mut SYSTEM_INFO;
 pub type LPWSABUF = *mut WSABUF;
 pub type LPWSAOVERLAPPED = *mut c_void;
 pub type LPWSAOVERLAPPED_COMPLETION_ROUTINE = *mut c_void;
+pub type BCRYPT_ALG_HANDLE = LPVOID;
 
 pub type PCONDITION_VARIABLE = *mut CONDITION_VARIABLE;
 pub type PLARGE_INTEGER = *mut c_longlong;
@@ -278,6 +279,7 @@ pub const STATUS_INVALID_PARAMETER: NTSTATUS = 0xc000000d_u32 as _;
 pub const STATUS_PENDING: NTSTATUS = 0x103 as _;
 pub const STATUS_END_OF_FILE: NTSTATUS = 0xC0000011_u32 as _;
 pub const STATUS_NOT_IMPLEMENTED: NTSTATUS = 0xC0000002_u32 as _;
+pub const STATUS_NOT_SUPPORTED: NTSTATUS = 0xC00000BB_u32 as _;
 
 // Equivalent to the `NT_SUCCESS` C preprocessor macro.
 // See: https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/using-ntstatus-values
@@ -285,7 +287,8 @@ pub fn nt_success(status: NTSTATUS) -> bool {
     status >= 0
 }
 
-pub const BCRYPT_USE_SYSTEM_PREFERRED_RNG: DWORD = 0x00000002;
+// "RNG\0"
+pub const BCRYPT_RNG_ALGORITHM: &[u16] = &[b'R' as u16, b'N' as u16, b'G' as u16, 0];
 
 #[repr(C)]
 pub struct UNICODE_STRING {
@@ -455,6 +458,12 @@ pub enum FILE_INFO_BY_HANDLE_CLASS {
 }
 
 #[repr(C)]
+pub struct FILE_ATTRIBUTE_TAG_INFO {
+    pub FileAttributes: DWORD,
+    pub ReparseTag: DWORD,
+}
+
+#[repr(C)]
 pub struct FILE_DISPOSITION_INFO {
     pub DeleteFile: BOOLEAN,
 }
@@ -501,6 +510,8 @@ pub struct FILE_END_OF_FILE_INFO {
     pub EndOfFile: LARGE_INTEGER,
 }
 
+/// NB: Use carefully! In general using this as a reference is likely to get the
+/// provenance wrong for the `rest` field!
 #[repr(C)]
 pub struct REPARSE_DATA_BUFFER {
     pub ReparseTag: c_uint,
@@ -509,6 +520,8 @@ pub struct REPARSE_DATA_BUFFER {
     pub rest: (),
 }
 
+/// NB: Use carefully! In general using this as a reference is likely to get the
+/// provenance wrong for the `PathBuffer` field!
 #[repr(C)]
 pub struct SYMBOLIC_LINK_REPARSE_BUFFER {
     pub SubstituteNameOffset: c_ushort,
@@ -519,6 +532,8 @@ pub struct SYMBOLIC_LINK_REPARSE_BUFFER {
     pub PathBuffer: WCHAR,
 }
 
+/// NB: Use carefully! In general using this as a reference is likely to get the
+/// provenance wrong for the `PathBuffer` field!
 #[repr(C)]
 pub struct MOUNT_POINT_REPARSE_BUFFER {
     pub SubstituteNameOffset: c_ushort,
@@ -1217,11 +1232,18 @@ extern "system" {
     // >= Vista / Server 2008
     // https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
     pub fn BCryptGenRandom(
-        hAlgorithm: LPVOID,
+        hAlgorithm: BCRYPT_ALG_HANDLE,
         pBuffer: *mut u8,
         cbBuffer: ULONG,
         dwFlags: ULONG,
     ) -> NTSTATUS;
+    pub fn BCryptOpenAlgorithmProvider(
+        phalgorithm: *mut BCRYPT_ALG_HANDLE,
+        pszAlgId: LPCWSTR,
+        pszimplementation: LPCWSTR,
+        dwflags: ULONG,
+    ) -> NTSTATUS;
+    pub fn BCryptCloseAlgorithmProvider(hAlgorithm: BCRYPT_ALG_HANDLE, dwFlags: ULONG) -> NTSTATUS;
 }
 
 // Functions that aren't available on every version of Windows that we support,
@@ -1251,17 +1273,14 @@ compat_fn_with_fallback! {
 }
 
 compat_fn_optional! {
-    pub static SYNCH_API: &CStr = ansi_str!("api-ms-win-core-synch-l1-2-0");
-
-    // >= Windows 8 / Server 2012
-    // https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitonaddress
+    crate::sys::compat::load_synch_functions();
     pub fn WaitOnAddress(
         Address: LPVOID,
         CompareAddress: LPVOID,
         AddressSize: SIZE_T,
         dwMilliseconds: DWORD
-    ) -> BOOL;
-    pub fn WakeByAddressSingle(Address: LPVOID) -> ();
+    );
+    pub fn WakeByAddressSingle(Address: LPVOID);
 }
 
 compat_fn_with_fallback! {
diff --git a/library/std/src/sys/windows/cmath.rs b/library/std/src/sys/windows/cmath.rs
index 1a5421fac..43ab8c7ee 100644
--- a/library/std/src/sys/windows/cmath.rs
+++ b/library/std/src/sys/windows/cmath.rs
@@ -44,7 +44,7 @@ mod shims {
 }
 
 // On 32-bit x86 MSVC these functions aren't defined, so we just define shims
-// which promote everything fo f64, perform the calculation, and then demote
+// 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(all(target_env = "msvc", target_arch = "x86"))]
 mod shims {
diff --git a/library/std/src/sys/windows/compat.rs b/library/std/src/sys/windows/compat.rs
index ccc90177a..7dff81ecb 100644
--- a/library/std/src/sys/windows/compat.rs
+++ b/library/std/src/sys/windows/compat.rs
@@ -7,52 +7,66 @@
 //! `GetModuleHandle` and `GetProcAddress` to look up DLL entry points at
 //! runtime.
 //!
-//! This implementation uses a static initializer to look up the DLL entry
-//! points. The CRT (C runtime) executes static initializers before `main`
-//! is called (for binaries) and before `DllMain` is called (for DLLs).
-//! This is the ideal time to look up DLL imports, because we are guaranteed
-//! that no other threads will attempt to call these entry points. Thus,
-//! we can look up the imports and store them in `static mut` fields
-//! without any synchronization.
+//! This is implemented simply by storing a function pointer in an atomic.
+//! Loading and calling this function will have little or no overhead
+//! compared with calling any other dynamically imported function.
 //!
-//! This has an additional advantage: Because the DLL import lookup happens
-//! at module initialization, the cost of these lookups is deterministic,
-//! and is removed from the code paths that actually call the DLL imports.
-//! That is, there is no unpredictable "cache miss" that occurs when calling
-//! a DLL import. For applications that benefit from predictable delays,
-//! this is a benefit. This also eliminates the comparison-and-branch
-//! from the hot path.
-//!
-//! Currently, the standard library uses only a small number of dynamic
-//! DLL imports. If this number grows substantially, then the cost of
-//! performing all of the lookups at initialization time might become
-//! substantial.
-//!
-//! The mechanism of registering a static initializer with the CRT is
-//! documented in
-//! [CRT Initialization](https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-initialization?view=msvc-160).
-//! It works by contributing a global symbol to the `.CRT$XCU` section.
-//! The linker builds a table of all static initializer functions.
-//! The CRT startup code then iterates that table, calling each
-//! initializer function.
-//!
-//! # **WARNING!!*
-//! The environment that a static initializer function runs in is highly
-//! constrained. There are **many** restrictions on what static initializers
-//! can safely do. Static initializer functions **MUST NOT** do any of the
-//! following (this list is not comprehensive):
-//! * touch any other static field that is used by a different static
-//!   initializer, because the order that static initializers run in
-//!   is not defined.
-//! * call `LoadLibrary` or any other function that acquires the DLL
-//!   loader lock.
-//! * call any Rust function or CRT function that touches any static
-//!   (global) state.
+//! The stored function pointer starts out as an importer function which will
+//! swap itself with the real function when it's called for the first time. If
+//! the real function can't be imported then a fallback function is used in its
+//! place. While this is low cost for the happy path (where the function is
+//! already loaded) it does mean there's some overhead the first time the
+//! function is called. In the worst case, multiple threads may all end up
+//! importing the same function unnecessarily.
 
 use crate::ffi::{c_void, CStr};
 use crate::ptr::NonNull;
+use crate::sync::atomic::Ordering;
 use crate::sys::c;
 
+// This uses a static initializer to preload some imported functions.
+// The CRT (C runtime) executes static initializers before `main`
+// is called (for binaries) and before `DllMain` is called (for DLLs).
+//
+// It works by contributing a global symbol to the `.CRT$XCT` section.
+// The linker builds a table of all static initializer functions.
+// The CRT startup code then iterates that table, calling each
+// initializer function.
+//
+// NOTE: User code should instead use .CRT$XCU to reliably run after std's initializer.
+// If you're reading this and would like a guarantee here, please
+// file an issue for discussion; currently we don't guarantee any functionality
+// before main.
+// See https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-initialization?view=msvc-170
+#[used]
+#[link_section = ".CRT$XCT"]
+static INIT_TABLE_ENTRY: unsafe extern "C" fn() = init;
+
+/// Preload some imported functions.
+///
+/// Note that any functions included here will be unconditionally loaded in
+/// the final binary, regardless of whether or not they're actually used.
+///
+/// Therefore, this should be limited to `compat_fn_optional` functions which
+/// must be preloaded or any functions where lazier loading demonstrates a
+/// negative performance impact in practical situations.
+///
+/// Currently we only preload `WaitOnAddress` and `WakeByAddressSingle`.
+unsafe extern "C" fn init() {
+    // In an exe this code is executed before main() so is single threaded.
+    // In a DLL the system's loader lock will be held thereby synchronizing
+    // access. So the same best practices apply here as they do to running in DllMain:
+    // https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices
+    //
+    // DO NOT do anything interesting or complicated in this function! DO NOT call
+    // any Rust functions or CRT functions if those functions touch any global state,
+    // because this function runs during global initialization. For example, DO NOT
+    // do any dynamic allocation, don't call LoadLibrary, etc.
+
+    // Attempt to preload the synch functions.
+    load_synch_functions();
+}
+
 /// Helper macro for creating CStrs from literals and symbol names.
 macro_rules! ansi_str {
     (sym $ident:ident) => {{
@@ -85,39 +99,6 @@ pub(crate) const fn const_cstr_from_bytes(bytes: &'static [u8]) -> &'static CStr
     unsafe { crate::ffi::CStr::from_bytes_with_nul_unchecked(bytes) }
 }
 
-#[used]
-#[link_section = ".CRT$XCU"]
-static INIT_TABLE_ENTRY: unsafe extern "C" fn() = init;
-
-/// This is where the magic preloading of symbols happens.
-///
-/// Note that any functions included here will be unconditionally included in
-/// the final binary, regardless of whether or not they're actually used.
-///
-/// Therefore, this is limited to `compat_fn_optional` functions which must be
-/// preloaded and any functions which may be more time sensitive, even for the first call.
-unsafe extern "C" fn init() {
-    // There is no locking here. This code is executed before main() is entered, and
-    // is guaranteed to be single-threaded.
-    //
-    // DO NOT do anything interesting or complicated in this function! DO NOT call
-    // any Rust functions or CRT functions if those functions touch any global state,
-    // because this function runs during global initialization. For example, DO NOT
-    // do any dynamic allocation, don't call LoadLibrary, etc.
-
-    if let Some(synch) = Module::new(c::SYNCH_API) {
-        // These are optional and so we must manually attempt to load them
-        // before they can be used.
-        c::WaitOnAddress::preload(synch);
-        c::WakeByAddressSingle::preload(synch);
-    }
-
-    if let Some(kernel32) = Module::new(c::KERNEL32) {
-        // Preloading this means getting a precise time will be as fast as possible.
-        c::GetSystemTimePreciseAsFileTime::preload(kernel32);
-    }
-}
-
 /// Represents a loaded module.
 ///
 /// Note that the modules std depends on must not be unloaded.
@@ -151,7 +132,7 @@ impl Module {
 macro_rules! compat_fn_with_fallback {
     (pub static $module:ident: &CStr = $name:expr; $(
         $(#[$meta:meta])*
-        pub fn $symbol:ident($($argname:ident: $argtype:ty),*) -> $rettype:ty $fallback_body:block
+        $vis:vis fn $symbol:ident($($argname:ident: $argtype:ty),*) -> $rettype:ty $fallback_body:block
     )*) => (
         pub static $module: &CStr = $name;
     $(
@@ -196,11 +177,6 @@ macro_rules! compat_fn_with_fallback {
                 $fallback_body
             }
 
-            #[allow(unused)]
-            pub(in crate::sys) fn preload(module: Module) {
-                load_from_module(Some(module));
-            }
-
             #[inline(always)]
             pub unsafe fn call($($argname: $argtype),*) -> $rettype {
                 let func: F = mem::transmute(PTR.load(Ordering::Relaxed));
@@ -208,66 +184,60 @@ macro_rules! compat_fn_with_fallback {
             }
         }
         $(#[$meta])*
-        pub use $symbol::call as $symbol;
+        $vis use $symbol::call as $symbol;
     )*)
 }
 
-/// A function that either exists or doesn't.
+/// Optionally loaded functions.
 ///
-/// NOTE: Optional functions must be preloaded in the `init` function above, or they will always be None.
+/// Actual loading of the function defers to $load_functions.
 macro_rules! compat_fn_optional {
-    (pub static $module:ident: &CStr = $name:expr; $(
-        $(#[$meta:meta])*
-        pub fn $symbol:ident($($argname:ident: $argtype:ty),*) -> $rettype:ty;
-    )*) => (
-        pub static $module: &CStr = $name;
+    ($load_functions:expr;
     $(
-        $(#[$meta])*
-        pub mod $symbol {
-            #[allow(unused_imports)]
-            use super::*;
-            use crate::mem;
-            use crate::sync::atomic::{AtomicPtr, Ordering};
-            use crate::sys::compat::Module;
-            use crate::ptr::{self, NonNull};
-
-            type F = unsafe extern "system" fn($($argtype),*) -> $rettype;
-
-            /// `PTR` will either be `null()` or set to the loaded function.
-            static PTR: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
-
-            /// Only allow access to the function if it has loaded successfully.
-            #[inline(always)]
-            #[cfg(not(miri))]
-            pub fn option() -> Option<F> {
-                unsafe {
-                    NonNull::new(PTR.load(Ordering::Relaxed)).map(|f| mem::transmute(f))
+        $(#[$meta:meta])*
+        $vis:vis fn $symbol:ident($($argname:ident: $argtype:ty),*) $(-> $rettype:ty)?;
+    )+) => (
+        $(
+            pub mod $symbol {
+                use super::*;
+                use crate::ffi::c_void;
+                use crate::mem;
+                use crate::ptr::{self, NonNull};
+                use crate::sync::atomic::{AtomicPtr, Ordering};
+
+                pub(in crate::sys) static PTR: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
+
+                type F = unsafe extern "system" fn($($argtype),*) $(-> $rettype)?;
+
+                #[inline(always)]
+                pub fn option() -> Option<F> {
+                    // Miri does not understand the way we do preloading
+                    // therefore load the function here instead.
+                    #[cfg(miri)] $load_functions;
+                    NonNull::new(PTR.load(Ordering::Relaxed)).map(|f| unsafe { mem::transmute(f) })
                 }
             }
+        )+
+    )
+}
 
-            // Miri does not understand the way we do preloading
-            // therefore load the function here instead.
-            #[cfg(miri)]
-            pub fn option() -> Option<F> {
-                let mut func = NonNull::new(PTR.load(Ordering::Relaxed));
-                if func.is_none() {
-                    unsafe { Module::new($module).map(preload) };
-                    func = NonNull::new(PTR.load(Ordering::Relaxed));
-                }
-                unsafe {
-                    func.map(|f| mem::transmute(f))
-                }
-            }
+/// Load all needed functions from "api-ms-win-core-synch-l1-2-0".
+pub(super) fn load_synch_functions() {
+    fn try_load() -> Option<()> {
+        const MODULE_NAME: &CStr = ansi_str!("api-ms-win-core-synch-l1-2-0");
+        const WAIT_ON_ADDRESS: &CStr = ansi_str!("WaitOnAddress");
+        const WAKE_BY_ADDRESS_SINGLE: &CStr = ansi_str!("WakeByAddressSingle");
+
+        // Try loading the library and all the required functions.
+        // If any step fails, then they all fail.
+        let library = unsafe { Module::new(MODULE_NAME) }?;
+        let wait_on_address = library.proc_address(WAIT_ON_ADDRESS)?;
+        let wake_by_address_single = library.proc_address(WAKE_BY_ADDRESS_SINGLE)?;
+
+        c::WaitOnAddress::PTR.store(wait_on_address.as_ptr(), Ordering::Relaxed);
+        c::WakeByAddressSingle::PTR.store(wake_by_address_single.as_ptr(), Ordering::Relaxed);
+        Some(())
+    }
 
-            #[allow(unused)]
-            pub(in crate::sys) fn preload(module: Module) {
-                unsafe {
-                    static SYMBOL_NAME: &CStr = ansi_str!(sym $symbol);
-                    if let Some(f) = module.proc_address(SYMBOL_NAME) {
-                        PTR.store(f.as_ptr(), Ordering::Relaxed);
-                    }
-                }
-            }
-        }
-    )*)
+    try_load();
 }
diff --git a/library/std/src/sys/windows/fs.rs b/library/std/src/sys/windows/fs.rs
index aed082b3e..155d0297e 100644
--- a/library/std/src/sys/windows/fs.rs
+++ b/library/std/src/sys/windows/fs.rs
@@ -2,8 +2,8 @@ use crate::os::windows::prelude::*;
 
 use crate::ffi::OsString;
 use crate::fmt;
-use crate::io::{self, Error, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
-use crate::mem;
+use crate::io::{self, BorrowedCursor, Error, IoSlice, IoSliceMut, SeekFrom};
+use crate::mem::{self, MaybeUninit};
 use crate::os::windows::io::{AsHandle, BorrowedHandle};
 use crate::path::{Path, PathBuf};
 use crate::ptr;
@@ -11,7 +11,7 @@ use crate::slice;
 use crate::sync::Arc;
 use crate::sys::handle::Handle;
 use crate::sys::time::SystemTime;
-use crate::sys::{c, cvt};
+use crate::sys::{c, cvt, Align8};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::thread;
 
@@ -326,9 +326,15 @@ impl File {
             cvt(c::GetFileInformationByHandle(self.handle.as_raw_handle(), &mut info))?;
             let mut reparse_tag = 0;
             if info.dwFileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
-                let mut b = [0; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    reparse_tag = buf.ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(FileAttr {
@@ -389,9 +395,15 @@ impl File {
             attr.file_size = info.AllocationSize as u64;
             attr.number_of_links = Some(info.NumberOfLinks);
             if attr.file_type().is_reparse_point() {
-                let mut b = [0; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
-                if let Ok((_, buf)) = self.reparse_point(&mut b) {
-                    attr.reparse_tag = buf.ReparseTag;
+                let mut attr_tag: c::FILE_ATTRIBUTE_TAG_INFO = mem::zeroed();
+                cvt(c::GetFileInformationByHandleEx(
+                    self.handle.as_raw_handle(),
+                    c::FileAttributeTagInfo,
+                    ptr::addr_of_mut!(attr_tag).cast(),
+                    mem::size_of::<c::FILE_ATTRIBUTE_TAG_INFO>().try_into().unwrap(),
+                ))?;
+                if attr_tag.FileAttributes & c::FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+                    attr.reparse_tag = attr_tag.ReparseTag;
                 }
             }
             Ok(attr)
@@ -415,8 +427,8 @@ impl File {
         self.handle.read_at(buf, offset)
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        self.handle.read_buf(buf)
+    pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        self.handle.read_buf(cursor)
     }
 
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
@@ -458,38 +470,46 @@ impl File {
         Ok(Self { handle: self.handle.try_clone()? })
     }
 
-    fn reparse_point<'a>(
+    // NB: returned pointer is derived from `space`, and has provenance to
+    // match. A raw pointer is returned rather than a reference in order to
+    // avoid narrowing provenance to the actual `REPARSE_DATA_BUFFER`.
+    fn reparse_point(
         &self,
-        space: &'a mut [u8; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE],
-    ) -> io::Result<(c::DWORD, &'a c::REPARSE_DATA_BUFFER)> {
+        space: &mut Align8<[MaybeUninit<u8>]>,
+    ) -> io::Result<(c::DWORD, *const c::REPARSE_DATA_BUFFER)> {
         unsafe {
             let mut bytes = 0;
             cvt({
+                // Grab this in advance to avoid it invalidating the pointer
+                // we get from `space.0.as_mut_ptr()`.
+                let len = space.0.len();
                 c::DeviceIoControl(
                     self.handle.as_raw_handle(),
                     c::FSCTL_GET_REPARSE_POINT,
                     ptr::null_mut(),
                     0,
-                    space.as_mut_ptr() as *mut _,
-                    space.len() as c::DWORD,
+                    space.0.as_mut_ptr().cast(),
+                    len as c::DWORD,
                     &mut bytes,
                     ptr::null_mut(),
                 )
             })?;
-            Ok((bytes, &*(space.as_ptr() as *const c::REPARSE_DATA_BUFFER)))
+            const _: () = assert!(core::mem::align_of::<c::REPARSE_DATA_BUFFER>() <= 8);
+            Ok((bytes, space.0.as_ptr().cast::<c::REPARSE_DATA_BUFFER>()))
         }
     }
 
     fn readlink(&self) -> io::Result<PathBuf> {
-        let mut space = [0u8; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
+        let mut space = Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
         let (_bytes, buf) = self.reparse_point(&mut space)?;
         unsafe {
-            let (path_buffer, subst_off, subst_len, relative) = match buf.ReparseTag {
+            let (path_buffer, subst_off, subst_len, relative) = match (*buf).ReparseTag {
                 c::IO_REPARSE_TAG_SYMLINK => {
                     let info: *const c::SYMBOLIC_LINK_REPARSE_BUFFER =
-                        &buf.rest as *const _ as *const _;
+                        ptr::addr_of!((*buf).rest).cast();
+                    assert!(info.is_aligned());
                     (
-                        &(*info).PathBuffer as *const _ as *const u16,
+                        ptr::addr_of!((*info).PathBuffer).cast::<u16>(),
                         (*info).SubstituteNameOffset / 2,
                         (*info).SubstituteNameLength / 2,
                         (*info).Flags & c::SYMLINK_FLAG_RELATIVE != 0,
@@ -497,9 +517,10 @@ impl File {
                 }
                 c::IO_REPARSE_TAG_MOUNT_POINT => {
                     let info: *const c::MOUNT_POINT_REPARSE_BUFFER =
-                        &buf.rest as *const _ as *const _;
+                        ptr::addr_of!((*buf).rest).cast();
+                    assert!(info.is_aligned());
                     (
-                        &(*info).PathBuffer as *const _ as *const u16,
+                        ptr::addr_of!((*info).PathBuffer).cast::<u16>(),
                         (*info).SubstituteNameOffset / 2,
                         (*info).SubstituteNameLength / 2,
                         false,
@@ -512,7 +533,7 @@ impl File {
                     ));
                 }
             };
-            let subst_ptr = path_buffer.offset(subst_off as isize);
+            let subst_ptr = path_buffer.add(subst_off.into());
             let mut subst = slice::from_raw_parts(subst_ptr, subst_len as usize);
             // Absolute paths start with an NT internal namespace prefix `\??\`
             // We should not let it leak through.
@@ -649,27 +670,31 @@ impl File {
 
 /// A buffer for holding directory entries.
 struct DirBuff {
-    buffer: Vec<u8>,
+    buffer: Box<Align8<[MaybeUninit<u8>; Self::BUFFER_SIZE]>>,
 }
 impl DirBuff {
+    const BUFFER_SIZE: usize = 1024;
     fn new() -> Self {
-        const BUFFER_SIZE: usize = 1024;
-        Self { buffer: vec![0_u8; BUFFER_SIZE] }
+        Self {
+            // Safety: `Align8<[MaybeUninit<u8>; N]>` does not need
+            // initialization.
+            buffer: unsafe { Box::new_uninit().assume_init() },
+        }
     }
     fn capacity(&self) -> usize {
-        self.buffer.len()
+        self.buffer.0.len()
     }
     fn as_mut_ptr(&mut self) -> *mut u8 {
-        self.buffer.as_mut_ptr().cast()
+        self.buffer.0.as_mut_ptr().cast()
     }
     /// Returns a `DirBuffIter`.
     fn iter(&self) -> DirBuffIter<'_> {
         DirBuffIter::new(self)
     }
 }
-impl AsRef<[u8]> for DirBuff {
-    fn as_ref(&self) -> &[u8] {
-        &self.buffer
+impl AsRef<[MaybeUninit<u8>]> for DirBuff {
+    fn as_ref(&self) -> &[MaybeUninit<u8>] {
+        &self.buffer.0
     }
 }
 
@@ -677,7 +702,7 @@ impl AsRef<[u8]> for DirBuff {
 ///
 /// Currently only returns file names (UTF-16 encoded).
 struct DirBuffIter<'a> {
-    buffer: Option<&'a [u8]>,
+    buffer: Option<&'a [MaybeUninit<u8>]>,
     cursor: usize,
 }
 impl<'a> DirBuffIter<'a> {
@@ -692,14 +717,21 @@ impl<'a> Iterator for DirBuffIter<'a> {
         let buffer = &self.buffer?[self.cursor..];
 
         // Get the name and next entry from the buffer.
-        // SAFETY: The buffer contains a `FILE_ID_BOTH_DIR_INFO` struct but the
-        // last field (the file name) is unsized. So an offset has to be
-        // used to get the file name slice.
+        // SAFETY:
+        // - The buffer contains a `FILE_ID_BOTH_DIR_INFO` struct but the last
+        //   field (the file name) is unsized. So an offset has to be used to
+        //   get the file name slice.
+        // - The OS has guaranteed initialization of the fields of
+        //   `FILE_ID_BOTH_DIR_INFO` and the trailing filename (for at least
+        //   `FileNameLength` bytes)
         let (name, is_directory, next_entry) = unsafe {
             let info = buffer.as_ptr().cast::<c::FILE_ID_BOTH_DIR_INFO>();
+            // Guaranteed to be aligned in documentation for
+            // https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-file_id_both_dir_info
+            assert!(info.is_aligned());
             let next_entry = (*info).NextEntryOffset as usize;
             let name = crate::slice::from_raw_parts(
-                (*info).FileName.as_ptr().cast::<u16>(),
+                ptr::addr_of!((*info).FileName).cast::<u16>(),
                 (*info).FileNameLength as usize / size_of::<u16>(),
             );
             let is_directory = ((*info).FileAttributes & c::FILE_ATTRIBUTE_DIRECTORY) != 0;
@@ -1337,18 +1369,19 @@ fn symlink_junction_inner(original: &Path, junction: &Path) -> io::Result<()> {
     let h = f.as_inner().as_raw_handle();
 
     unsafe {
-        let mut data = [0u8; c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
-        let db = data.as_mut_ptr() as *mut c::REPARSE_MOUNTPOINT_DATA_BUFFER;
-        let buf = &mut (*db).ReparseTarget as *mut c::WCHAR;
+        let mut data = Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
+        let data_ptr = data.0.as_mut_ptr();
+        let db = data_ptr.cast::<c::REPARSE_MOUNTPOINT_DATA_BUFFER>();
+        let buf = ptr::addr_of_mut!((*db).ReparseTarget).cast::<c::WCHAR>();
         let mut i = 0;
         // FIXME: this conversion is very hacky
         let v = br"\??\";
         let v = v.iter().map(|x| *x as u16);
         for c in v.chain(original.as_os_str().encode_wide()) {
-            *buf.offset(i) = c;
+            *buf.add(i) = c;
             i += 1;
         }
-        *buf.offset(i) = 0;
+        *buf.add(i) = 0;
         i += 1;
         (*db).ReparseTag = c::IO_REPARSE_TAG_MOUNT_POINT;
         (*db).ReparseTargetMaximumLength = (i * 2) as c::WORD;
@@ -1359,7 +1392,7 @@ fn symlink_junction_inner(original: &Path, junction: &Path) -> io::Result<()> {
         cvt(c::DeviceIoControl(
             h as *mut _,
             c::FSCTL_SET_REPARSE_POINT,
-            data.as_ptr() as *mut _,
+            data_ptr.cast(),
             (*db).ReparseDataLength + 8,
             ptr::null_mut(),
             0,
diff --git a/library/std/src/sys/windows/handle.rs b/library/std/src/sys/windows/handle.rs
index e24b09cc9..ae33d48c6 100644
--- a/library/std/src/sys/windows/handle.rs
+++ b/library/std/src/sys/windows/handle.rs
@@ -4,7 +4,7 @@
 mod tests;
 
 use crate::cmp;
-use crate::io::{self, ErrorKind, IoSlice, IoSliceMut, Read, ReadBuf};
+use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, Read};
 use crate::mem;
 use crate::os::windows::io::{
     AsHandle, AsRawHandle, BorrowedHandle, FromRawHandle, IntoRawHandle, OwnedHandle, RawHandle,
@@ -112,18 +112,16 @@ impl Handle {
         }
     }
 
-    pub fn read_buf(&self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
-        let res = unsafe {
-            self.synchronous_read(buf.unfilled_mut().as_mut_ptr(), buf.remaining(), None)
-        };
+    pub fn read_buf(&self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        let res =
+            unsafe { self.synchronous_read(cursor.as_mut().as_mut_ptr(), cursor.capacity(), None) };
 
         match res {
             Ok(read) => {
                 // Safety: `read` bytes were written to the initialized portion of the buffer
                 unsafe {
-                    buf.assume_init(read as usize);
+                    cursor.advance(read as usize);
                 }
-                buf.add_filled(read as usize);
                 Ok(())
             }
 
diff --git a/library/std/src/sys/windows/locks/mutex.rs b/library/std/src/sys/windows/locks/mutex.rs
index f91e8f9f5..91207f5f4 100644
--- a/library/std/src/sys/windows/locks/mutex.rs
+++ b/library/std/src/sys/windows/locks/mutex.rs
@@ -37,8 +37,6 @@ impl Mutex {
     pub const fn new() -> Mutex {
         Mutex { srwlock: UnsafeCell::new(c::SRWLOCK_INIT) }
     }
-    #[inline]
-    pub unsafe fn init(&mut self) {}
 
     #[inline]
     pub unsafe fn lock(&self) {
diff --git a/library/std/src/sys/windows/mod.rs b/library/std/src/sys/windows/mod.rs
index b3f6d2d0a..eab9b9612 100644
--- a/library/std/src/sys/windows/mod.rs
+++ b/library/std/src/sys/windows/mod.rs
@@ -2,6 +2,7 @@
 
 use crate::ffi::{CStr, OsStr, OsString};
 use crate::io::ErrorKind;
+use crate::mem::MaybeUninit;
 use crate::os::windows::ffi::{OsStrExt, OsStringExt};
 use crate::path::PathBuf;
 use crate::time::Duration;
@@ -47,7 +48,7 @@ cfg_if::cfg_if! {
 
 // SAFETY: must be called only once during runtime initialization.
 // NOTE: this is not guaranteed to run, for example when Rust code is called externally.
-pub unsafe fn init(_argc: isize, _argv: *const *const u8) {
+pub unsafe fn init(_argc: isize, _argv: *const *const u8, _sigpipe: u8) {
     stack_overflow::init();
 
     // Normally, `thread::spawn` will call `Thread::set_name` but since this thread already
@@ -204,8 +205,8 @@ where
     // This initial size also works around `GetFullPathNameW` returning
     // incorrect size hints for some short paths:
     // https://github.com/dylni/normpath/issues/5
-    let mut stack_buf = [0u16; 512];
-    let mut heap_buf = Vec::new();
+    let mut stack_buf: [MaybeUninit<u16>; 512] = MaybeUninit::uninit_array();
+    let mut heap_buf: Vec<MaybeUninit<u16>> = Vec::new();
     unsafe {
         let mut n = stack_buf.len();
         loop {
@@ -214,6 +215,11 @@ where
             } else {
                 let extra = n - heap_buf.len();
                 heap_buf.reserve(extra);
+                // We used `reserve` and not `reserve_exact`, so in theory we
+                // may have gotten more than requested. If so, we'd like to use
+                // it... so long as we won't cause overflow.
+                n = heap_buf.capacity().min(c::DWORD::MAX as usize);
+                // Safety: MaybeUninit<u16> does not need initialization
                 heap_buf.set_len(n);
                 &mut heap_buf[..]
             };
@@ -228,13 +234,13 @@ where
             // error" is still 0 then we interpret it as a 0 length buffer and
             // not an actual error.
             c::SetLastError(0);
-            let k = match f1(buf.as_mut_ptr(), n as c::DWORD) {
+            let k = match f1(buf.as_mut_ptr().cast::<u16>(), n as c::DWORD) {
                 0 if c::GetLastError() == 0 => 0,
                 0 => return Err(crate::io::Error::last_os_error()),
                 n => n,
             } as usize;
             if k == n && c::GetLastError() == c::ERROR_INSUFFICIENT_BUFFER {
-                n *= 2;
+                n = n.saturating_mul(2).min(c::DWORD::MAX as usize);
             } else if k > n {
                 n = k;
             } else if k == n {
@@ -244,7 +250,9 @@ where
                 // Therefore k never equals n.
                 unreachable!();
             } else {
-                return Ok(f2(&buf[..k]));
+                // Safety: First `k` values are initialized.
+                let slice: &[u16] = MaybeUninit::slice_assume_init_ref(&buf[..k]);
+                return Ok(f2(slice));
             }
         }
     }
@@ -321,3 +329,11 @@ pub fn abort_internal() -> ! {
     }
     crate::intrinsics::abort();
 }
+
+/// Align the inner value to 8 bytes.
+///
+/// This is enough for almost all of the buffers we're likely to work with in
+/// the Windows APIs we use.
+#[repr(C, align(8))]
+#[derive(Copy, Clone)]
+pub(crate) struct Align8<T: ?Sized>(pub T);
diff --git a/library/std/src/sys/windows/os.rs b/library/std/src/sys/windows/os.rs
index bcac996c0..352337ba3 100644
--- a/library/std/src/sys/windows/os.rs
+++ b/library/std/src/sys/windows/os.rs
@@ -99,11 +99,11 @@ impl Iterator for Env {
                 }
                 let p = self.cur as *const u16;
                 let mut len = 0;
-                while *p.offset(len) != 0 {
+                while *p.add(len) != 0 {
                     len += 1;
                 }
-                let s = slice::from_raw_parts(p, len as usize);
-                self.cur = self.cur.offset(len + 1);
+                let s = slice::from_raw_parts(p, len);
+                self.cur = self.cur.add(len + 1);
 
                 // Windows allows environment variables to start with an equals
                 // symbol (in any other position, this is the separator between
diff --git a/library/std/src/sys/windows/os_str.rs b/library/std/src/sys/windows/os_str.rs
index 11883f150..4bdd8c505 100644
--- a/library/std/src/sys/windows/os_str.rs
+++ b/library/std/src/sys/windows/os_str.rs
@@ -164,9 +164,7 @@ impl Slice {
     }
 
     pub fn to_owned(&self) -> Buf {
-        let mut buf = Wtf8Buf::with_capacity(self.inner.len());
-        buf.push_wtf8(&self.inner);
-        Buf { inner: buf }
+        Buf { inner: self.inner.to_owned() }
     }
 
     pub fn clone_into(&self, buf: &mut Buf) {
diff --git a/library/std/src/sys/windows/path/tests.rs b/library/std/src/sys/windows/path/tests.rs
index 6eab38cab..623c62361 100644
--- a/library/std/src/sys/windows/path/tests.rs
+++ b/library/std/src/sys/windows/path/tests.rs
@@ -105,7 +105,7 @@ fn test_parse_prefix_verbatim_device() {
     assert_eq!(prefix, parse_prefix(r"\\?/C:\windows\system32\notepad.exe"));
 }
 
-// See #93586 for more infomation.
+// See #93586 for more information.
 #[test]
 fn test_windows_prefix_components() {
     use crate::path::Path;
diff --git a/library/std/src/sys/windows/rand.rs b/library/std/src/sys/windows/rand.rs
index f8fd93a73..d6cd8f802 100644
--- a/library/std/src/sys/windows/rand.rs
+++ b/library/std/src/sys/windows/rand.rs
@@ -1,35 +1,126 @@
-use crate::io;
+//! # Random key generation
+//!
+//! This module wraps the RNG provided by the OS. There are a few different
+//! ways to interface with the OS RNG so it's worth exploring each of the options.
+//! Note that at the time of writing these all go through the (undocumented)
+//! `bcryptPrimitives.dll` but they use different route to get there.
+//!
+//! Originally we were using [`RtlGenRandom`], however that function is
+//! deprecated and warns it "may be altered or unavailable in subsequent versions".
+//!
+//! So we switched to [`BCryptGenRandom`] with the `BCRYPT_USE_SYSTEM_PREFERRED_RNG`
+//! flag to query and find the system configured RNG. However, this change caused a small
+//! but significant number of users to experience panics caused by a failure of
+//! this function. See [#94098].
+//!
+//! The current version changes this to use the `BCRYPT_RNG_ALG_HANDLE`
+//! [Pseudo-handle], which gets the default RNG algorithm without querying the
+//! system preference thus hopefully avoiding the previous issue.
+//! This is only supported on Windows 10+ so a fallback is used for older versions.
+//!
+//! [#94098]: https://github.com/rust-lang/rust/issues/94098
+//! [`RtlGenRandom`]: https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom
+//! [`BCryptGenRandom`]: https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
+//! [Pseudo-handle]: https://docs.microsoft.com/en-us/windows/win32/seccng/cng-algorithm-pseudo-handles
 use crate::mem;
 use crate::ptr;
 use crate::sys::c;
 
+/// Generates high quality secure random keys for use by [`HashMap`].
+///
+/// This is used to seed the default [`RandomState`].
+///
+/// [`HashMap`]: crate::collections::HashMap
+/// [`RandomState`]: crate::collections::hash_map::RandomState
 pub fn hashmap_random_keys() -> (u64, u64) {
-    let mut v = (0, 0);
-    let ret = unsafe {
-        c::BCryptGenRandom(
-            ptr::null_mut(),
-            &mut v as *mut _ as *mut u8,
-            mem::size_of_val(&v) as c::ULONG,
-            c::BCRYPT_USE_SYSTEM_PREFERRED_RNG,
-        )
-    };
-    if ret != 0 { fallback_rng() } else { v }
+    Rng::open().and_then(|rng| rng.gen_random_keys()).unwrap_or_else(fallback_rng)
+}
+
+struct Rng(c::BCRYPT_ALG_HANDLE);
+impl Rng {
+    #[cfg(miri)]
+    fn open() -> Result<Self, c::NTSTATUS> {
+        const BCRYPT_RNG_ALG_HANDLE: c::BCRYPT_ALG_HANDLE = ptr::invalid_mut(0x81);
+        let _ = (
+            c::BCryptOpenAlgorithmProvider,
+            c::BCryptCloseAlgorithmProvider,
+            c::BCRYPT_RNG_ALGORITHM,
+            c::STATUS_NOT_SUPPORTED,
+        );
+        Ok(Self(BCRYPT_RNG_ALG_HANDLE))
+    }
+    #[cfg(not(miri))]
+    // Open a handle to the RNG algorithm.
+    fn open() -> Result<Self, c::NTSTATUS> {
+        use crate::sync::atomic::AtomicPtr;
+        use crate::sync::atomic::Ordering::{Acquire, Release};
+        const ERROR_VALUE: c::LPVOID = ptr::invalid_mut(usize::MAX);
+
+        // An atomic is used so we don't need to reopen the handle every time.
+        static HANDLE: AtomicPtr<crate::ffi::c_void> = AtomicPtr::new(ptr::null_mut());
+
+        let mut handle = HANDLE.load(Acquire);
+        // We use a sentinel value to designate an error occurred last time.
+        if handle == ERROR_VALUE {
+            Err(c::STATUS_NOT_SUPPORTED)
+        } else if handle.is_null() {
+            let status = unsafe {
+                c::BCryptOpenAlgorithmProvider(
+                    &mut handle,
+                    c::BCRYPT_RNG_ALGORITHM.as_ptr(),
+                    ptr::null(),
+                    0,
+                )
+            };
+            if c::nt_success(status) {
+                // If another thread opens a handle first then use that handle instead.
+                let result = HANDLE.compare_exchange(ptr::null_mut(), handle, Release, Acquire);
+                if let Err(previous_handle) = result {
+                    // Close our handle and return the previous one.
+                    unsafe { c::BCryptCloseAlgorithmProvider(handle, 0) };
+                    handle = previous_handle;
+                }
+                Ok(Self(handle))
+            } else {
+                HANDLE.store(ERROR_VALUE, Release);
+                Err(status)
+            }
+        } else {
+            Ok(Self(handle))
+        }
+    }
+
+    fn gen_random_keys(self) -> Result<(u64, u64), c::NTSTATUS> {
+        let mut v = (0, 0);
+        let status = unsafe {
+            let size = mem::size_of_val(&v).try_into().unwrap();
+            c::BCryptGenRandom(self.0, ptr::addr_of_mut!(v).cast(), size, 0)
+        };
+        if c::nt_success(status) { Ok(v) } else { Err(status) }
+    }
 }
 
 /// Generate random numbers using the fallback RNG function (RtlGenRandom)
 #[cfg(not(target_vendor = "uwp"))]
 #[inline(never)]
-fn fallback_rng() -> (u64, u64) {
+fn fallback_rng(rng_status: c::NTSTATUS) -> (u64, u64) {
     let mut v = (0, 0);
     let ret =
         unsafe { c::RtlGenRandom(&mut v as *mut _ as *mut u8, mem::size_of_val(&v) as c::ULONG) };
 
-    if ret != 0 { v } else { panic!("fallback RNG broken: {}", io::Error::last_os_error()) }
+    if ret != 0 {
+        v
+    } else {
+        panic!(
+            "RNG broken: {rng_status:#x}, fallback RNG broken: {}",
+            crate::io::Error::last_os_error()
+        )
+    }
 }
 
 /// We can't use RtlGenRandom with UWP, so there is no fallback
 #[cfg(target_vendor = "uwp")]
 #[inline(never)]
-fn fallback_rng() -> (u64, u64) {
-    panic!("fallback RNG broken: RtlGenRandom() not supported on UWP");
+fn fallback_rng(rng_status: c::NTSTATUS) -> (u64, u64) {
+    panic!("RNG broken: {rng_status:#x} fallback RNG broken: RtlGenRandom() not supported on UWP");
 }
diff --git a/library/std/src/sys/windows/stdio.rs b/library/std/src/sys/windows/stdio.rs
index a001d6b98..70c9b14a0 100644
--- a/library/std/src/sys/windows/stdio.rs
+++ b/library/std/src/sys/windows/stdio.rs
@@ -3,6 +3,7 @@
 use crate::char::decode_utf16;
 use crate::cmp;
 use crate::io;
+use crate::mem::MaybeUninit;
 use crate::os::windows::io::{FromRawHandle, IntoRawHandle};
 use crate::ptr;
 use crate::str;
@@ -169,13 +170,14 @@ fn write(
 }
 
 fn write_valid_utf8_to_console(handle: c::HANDLE, utf8: &str) -> io::Result<usize> {
-    let mut utf16 = [0u16; MAX_BUFFER_SIZE / 2];
+    let mut utf16 = [MaybeUninit::<u16>::uninit(); MAX_BUFFER_SIZE / 2];
     let mut len_utf16 = 0;
     for (chr, dest) in utf8.encode_utf16().zip(utf16.iter_mut()) {
-        *dest = chr;
+        *dest = MaybeUninit::new(chr);
         len_utf16 += 1;
     }
-    let utf16 = &utf16[..len_utf16];
+    // Safety: We've initialized `len_utf16` values.
+    let utf16: &[u16] = unsafe { MaybeUninit::slice_assume_init_ref(&utf16[..len_utf16]) };
 
     let mut written = write_u16s(handle, &utf16)?;
 
@@ -250,11 +252,14 @@ impl io::Read for Stdin {
             return Ok(bytes_copied);
         } else if buf.len() - bytes_copied < 4 {
             // Not enough space to get a UTF-8 byte. We will use the incomplete UTF8.
-            let mut utf16_buf = [0u16; 1];
+            let mut utf16_buf = [MaybeUninit::new(0); 1];
             // Read one u16 character.
             let read = read_u16s_fixup_surrogates(handle, &mut utf16_buf, 1, &mut self.surrogate)?;
             // Read bytes, using the (now-empty) self.incomplete_utf8 as extra space.
-            let read_bytes = utf16_to_utf8(&utf16_buf[..read], &mut self.incomplete_utf8.bytes)?;
+            let read_bytes = utf16_to_utf8(
+                unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) },
+                &mut self.incomplete_utf8.bytes,
+            )?;
 
             // Read in the bytes from incomplete_utf8 until the buffer is full.
             self.incomplete_utf8.len = read_bytes as u8;
@@ -262,15 +267,18 @@ impl io::Read for Stdin {
             bytes_copied += self.incomplete_utf8.read(&mut buf[bytes_copied..]);
             Ok(bytes_copied)
         } else {
-            let mut utf16_buf = [0u16; MAX_BUFFER_SIZE / 2];
+            let mut utf16_buf = [MaybeUninit::<u16>::uninit(); MAX_BUFFER_SIZE / 2];
+
             // In the worst case, a UTF-8 string can take 3 bytes for every `u16` of a UTF-16. So
             // we can read at most a third of `buf.len()` chars and uphold the guarantee no data gets
             // lost.
             let amount = cmp::min(buf.len() / 3, utf16_buf.len());
             let read =
                 read_u16s_fixup_surrogates(handle, &mut utf16_buf, amount, &mut self.surrogate)?;
-
-            match utf16_to_utf8(&utf16_buf[..read], buf) {
+            // Safety `read_u16s_fixup_surrogates` returns the number of items
+            // initialized.
+            let utf16s = unsafe { MaybeUninit::slice_assume_init_ref(&utf16_buf[..read]) };
+            match utf16_to_utf8(utf16s, buf) {
                 Ok(value) => return Ok(bytes_copied + value),
                 Err(e) => return Err(e),
             }
@@ -283,14 +291,14 @@ impl io::Read for Stdin {
 // This is a best effort, and might not work if we are not the only reader on Stdin.
 fn read_u16s_fixup_surrogates(
     handle: c::HANDLE,
-    buf: &mut [u16],
+    buf: &mut [MaybeUninit<u16>],
     mut amount: usize,
     surrogate: &mut u16,
 ) -> io::Result<usize> {
     // Insert possibly remaining unpaired surrogate from last read.
     let mut start = 0;
     if *surrogate != 0 {
-        buf[0] = *surrogate;
+        buf[0] = MaybeUninit::new(*surrogate);
         *surrogate = 0;
         start = 1;
         if amount == 1 {
@@ -303,7 +311,10 @@ fn read_u16s_fixup_surrogates(
     let mut amount = read_u16s(handle, &mut buf[start..amount])? + start;
 
     if amount > 0 {
-        let last_char = buf[amount - 1];
+        // Safety: The returned `amount` is the number of values initialized,
+        // and it is not 0, so we know that `buf[amount - 1]` have been
+        // initialized.
+        let last_char = unsafe { buf[amount - 1].assume_init() };
         if last_char >= 0xD800 && last_char <= 0xDBFF {
             // high surrogate
             *surrogate = last_char;
@@ -313,7 +324,8 @@ fn read_u16s_fixup_surrogates(
     Ok(amount)
 }
 
-fn read_u16s(handle: c::HANDLE, buf: &mut [u16]) -> io::Result<usize> {
+// Returns `Ok(n)` if it initialized `n` values in `buf`.
+fn read_u16s(handle: c::HANDLE, buf: &mut [MaybeUninit<u16>]) -> io::Result<usize> {
     // Configure the `pInputControl` parameter to not only return on `\r\n` but also Ctrl-Z, the
     // traditional DOS method to indicate end of character stream / user input (SUB).
     // See #38274 and https://stackoverflow.com/questions/43836040/win-api-readconsole.
@@ -346,8 +358,9 @@ fn read_u16s(handle: c::HANDLE, buf: &mut [u16]) -> io::Result<usize> {
         }
         break;
     }
-
-    if amount > 0 && buf[amount as usize - 1] == CTRL_Z {
+    // Safety: if `amount > 0`, then that many bytes were written, so
+    // `buf[amount as usize - 1]` has been initialized.
+    if amount > 0 && unsafe { buf[amount as usize - 1].assume_init() } == CTRL_Z {
         amount -= 1;
     }
     Ok(amount as usize)
diff --git a/library/std/src/sys/windows/thread_local_dtor.rs b/library/std/src/sys/windows/thread_local_dtor.rs
index 25d1c6e8e..9707a95df 100644
--- a/library/std/src/sys/windows/thread_local_dtor.rs
+++ b/library/std/src/sys/windows/thread_local_dtor.rs
@@ -8,10 +8,14 @@
 #[thread_local]
 static mut DESTRUCTORS: Vec<(*mut u8, unsafe extern "C" fn(*mut u8))> = Vec::new();
 
+// Ensure this can never be inlined because otherwise this may break in dylibs.
+// See #44391.
+#[inline(never)]
 pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
     DESTRUCTORS.push((t, dtor));
 }
 
+#[inline(never)] // See comment above
 /// Runs destructors. This should not be called until thread exit.
 pub unsafe fn run_keyless_dtors() {
     // Drop all the destructors.
diff --git a/library/std/src/sys/windows/thread_parker.rs b/library/std/src/sys/windows/thread_parker.rs
index d876e0f6f..2f7ae863b 100644
--- a/library/std/src/sys/windows/thread_parker.rs
+++ b/library/std/src/sys/windows/thread_parker.rs
@@ -197,19 +197,17 @@ impl Parker {
         // purpose, to make sure every unpark() has a release-acquire ordering
         // with park().
         if self.state.swap(NOTIFIED, Release) == PARKED {
-            if let Some(wake_by_address_single) = c::WakeByAddressSingle::option() {
-                unsafe {
+            unsafe {
+                if let Some(wake_by_address_single) = c::WakeByAddressSingle::option() {
                     wake_by_address_single(self.ptr());
-                }
-            } else {
-                // If we run NtReleaseKeyedEvent before the waiting thread runs
-                // NtWaitForKeyedEvent, this (shortly) blocks until we can wake it up.
-                // If the waiting thread wakes up before we run NtReleaseKeyedEvent
-                // (e.g. due to a timeout), this blocks until we do wake up a thread.
-                // To prevent this thread from blocking indefinitely in that case,
-                // park_impl() will, after seeing the state set to NOTIFIED after
-                // waking up, call NtWaitForKeyedEvent again to unblock us.
-                unsafe {
+                } else {
+                    // If we run NtReleaseKeyedEvent before the waiting thread runs
+                    // NtWaitForKeyedEvent, this (shortly) blocks until we can wake it up.
+                    // If the waiting thread wakes up before we run NtReleaseKeyedEvent
+                    // (e.g. due to a timeout), this blocks until we do wake up a thread.
+                    // To prevent this thread from blocking indefinitely in that case,
+                    // park_impl() will, after seeing the state set to NOTIFIED after
+                    // waking up, call NtWaitForKeyedEvent again to unblock us.
                     c::NtReleaseKeyedEvent(keyed_event_handle(), self.ptr(), 0, ptr::null_mut());
                 }
             }
diff --git a/library/std/src/sys_common/net.rs b/library/std/src/sys_common/net.rs
index 33d336c43..3ad802afa 100644
--- a/library/std/src/sys_common/net.rs
+++ b/library/std/src/sys_common/net.rs
@@ -10,7 +10,7 @@ use crate::net::{Ipv4Addr, Ipv6Addr, Shutdown, SocketAddr};
 use crate::ptr;
 use crate::sys::net::netc as c;
 use crate::sys::net::{cvt, cvt_gai, cvt_r, init, wrlen_t, Socket};
-use crate::sys_common::{FromInner, IntoInner};
+use crate::sys_common::{AsInner, FromInner, IntoInner};
 use crate::time::Duration;
 
 use libc::{c_int, c_void};
@@ -345,6 +345,12 @@ impl TcpStream {
     }
 }
 
+impl AsInner<Socket> for TcpStream {
+    fn as_inner(&self) -> &Socket {
+        &self.inner
+    }
+}
+
 impl FromInner<Socket> for TcpStream {
     fn from_inner(socket: Socket) -> TcpStream {
         TcpStream { inner: socket }
diff --git a/library/std/src/sys_common/remutex.rs b/library/std/src/sys_common/remutex.rs
index 8921af311..b448ae3a9 100644
--- a/library/std/src/sys_common/remutex.rs
+++ b/library/std/src/sys_common/remutex.rs
@@ -1,13 +1,11 @@
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests;
 
+use super::mutex as sys;
 use crate::cell::UnsafeCell;
-use crate::marker::PhantomPinned;
 use crate::ops::Deref;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
-use crate::pin::Pin;
 use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
-use crate::sys::locks as sys;
 
 /// A re-entrant mutual exclusion
 ///
@@ -41,11 +39,10 @@ use crate::sys::locks as sys;
 /// synchronization is left to the mutex, making relaxed memory ordering for
 /// the `owner` field fine in all cases.
 pub struct ReentrantMutex<T> {
-    mutex: sys::Mutex,
+    mutex: sys::MovableMutex,
     owner: AtomicUsize,
     lock_count: UnsafeCell<u32>,
     data: T,
-    _pinned: PhantomPinned,
 }
 
 unsafe impl<T: Send> Send for ReentrantMutex<T> {}
@@ -68,39 +65,22 @@ impl<T> RefUnwindSafe for ReentrantMutex<T> {}
 /// guarded data.
 #[must_use = "if unused the ReentrantMutex will immediately unlock"]
 pub struct ReentrantMutexGuard<'a, T: 'a> {
-    lock: Pin<&'a ReentrantMutex<T>>,
+    lock: &'a ReentrantMutex<T>,
 }
 
 impl<T> !Send for ReentrantMutexGuard<'_, T> {}
 
 impl<T> ReentrantMutex<T> {
     /// Creates a new reentrant mutex in an unlocked state.
-    ///
-    /// # Unsafety
-    ///
-    /// This function is unsafe because it is required that `init` is called
-    /// once this mutex is in its final resting place, and only then are the
-    /// lock/unlock methods safe.
-    pub const unsafe fn new(t: T) -> ReentrantMutex<T> {
+    pub const fn new(t: T) -> ReentrantMutex<T> {
         ReentrantMutex {
-            mutex: sys::Mutex::new(),
+            mutex: sys::MovableMutex::new(),
             owner: AtomicUsize::new(0),
             lock_count: UnsafeCell::new(0),
             data: t,
-            _pinned: PhantomPinned,
         }
     }
 
-    /// Initializes this mutex so it's ready for use.
-    ///
-    /// # Unsafety
-    ///
-    /// Unsafe to call more than once, and must be called after this will no
-    /// longer move in memory.
-    pub unsafe fn init(self: Pin<&mut Self>) {
-        self.get_unchecked_mut().mutex.init()
-    }
-
     /// Acquires a mutex, blocking the current thread until it is able to do so.
     ///
     /// This function will block the caller until it is available to acquire the mutex.
@@ -113,15 +93,14 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn lock(self: Pin<&Self>) -> ReentrantMutexGuard<'_, T> {
+    pub fn lock(&self) -> ReentrantMutexGuard<'_, T> {
         let this_thread = current_thread_unique_ptr();
-        // Safety: We only touch lock_count when we own the lock,
-        // and since self is pinned we can safely call the lock() on the mutex.
+        // Safety: We only touch lock_count when we own the lock.
         unsafe {
             if self.owner.load(Relaxed) == this_thread {
                 self.increment_lock_count();
             } else {
-                self.mutex.lock();
+                self.mutex.raw_lock();
                 self.owner.store(this_thread, Relaxed);
                 debug_assert_eq!(*self.lock_count.get(), 0);
                 *self.lock_count.get() = 1;
@@ -142,10 +121,9 @@ impl<T> ReentrantMutex<T> {
     /// If another user of this mutex panicked while holding the mutex, then
     /// this call will return failure if the mutex would otherwise be
     /// acquired.
-    pub fn try_lock(self: Pin<&Self>) -> Option<ReentrantMutexGuard<'_, T>> {
+    pub fn try_lock(&self) -> Option<ReentrantMutexGuard<'_, T>> {
         let this_thread = current_thread_unique_ptr();
-        // Safety: We only touch lock_count when we own the lock,
-        // and since self is pinned we can safely call the try_lock on the mutex.
+        // Safety: We only touch lock_count when we own the lock.
         unsafe {
             if self.owner.load(Relaxed) == this_thread {
                 self.increment_lock_count();
@@ -179,12 +157,12 @@ impl<T> Deref for ReentrantMutexGuard<'_, T> {
 impl<T> Drop for ReentrantMutexGuard<'_, T> {
     #[inline]
     fn drop(&mut self) {
-        // Safety: We own the lock, and the lock is pinned.
+        // Safety: We own the lock.
         unsafe {
             *self.lock.lock_count.get() -= 1;
             if *self.lock.lock_count.get() == 0 {
                 self.lock.owner.store(0, Relaxed);
-                self.lock.mutex.unlock();
+                self.lock.mutex.raw_unlock();
             }
         }
     }
diff --git a/library/std/src/sys_common/remutex/tests.rs b/library/std/src/sys_common/remutex/tests.rs
index 64873b850..8e97ce11c 100644
--- a/library/std/src/sys_common/remutex/tests.rs
+++ b/library/std/src/sys_common/remutex/tests.rs
@@ -1,18 +1,11 @@
-use crate::boxed::Box;
 use crate::cell::RefCell;
-use crate::pin::Pin;
 use crate::sync::Arc;
 use crate::sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
 use crate::thread;
 
 #[test]
 fn smoke() {
-    let m = unsafe {
-        let mut m = Box::pin(ReentrantMutex::new(()));
-        m.as_mut().init();
-        m
-    };
-    let m = m.as_ref();
+    let m = ReentrantMutex::new(());
     {
         let a = m.lock();
         {
@@ -29,20 +22,15 @@ fn smoke() {
 
 #[test]
 fn is_mutex() {
-    let m = unsafe {
-        // FIXME: Simplify this if Arc gets an Arc::get_pin_mut.
-        let mut m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
-        Pin::new_unchecked(Arc::get_mut_unchecked(&mut m)).init();
-        Pin::new_unchecked(m)
-    };
+    let m = Arc::new(ReentrantMutex::new(RefCell::new(0)));
     let m2 = m.clone();
-    let lock = m.as_ref().lock();
+    let lock = m.lock();
     let child = thread::spawn(move || {
-        let lock = m2.as_ref().lock();
+        let lock = m2.lock();
         assert_eq!(*lock.borrow(), 4950);
     });
     for i in 0..100 {
-        let lock = m.as_ref().lock();
+        let lock = m.lock();
         *lock.borrow_mut() += i;
     }
     drop(lock);
@@ -51,22 +39,17 @@ fn is_mutex() {
 
 #[test]
 fn trylock_works() {
-    let m = unsafe {
-        // FIXME: Simplify this if Arc gets an Arc::get_pin_mut.
-        let mut m = Arc::new(ReentrantMutex::new(()));
-        Pin::new_unchecked(Arc::get_mut_unchecked(&mut m)).init();
-        Pin::new_unchecked(m)
-    };
+    let m = Arc::new(ReentrantMutex::new(()));
     let m2 = m.clone();
-    let _lock = m.as_ref().try_lock();
-    let _lock2 = m.as_ref().try_lock();
+    let _lock = m.try_lock();
+    let _lock2 = m.try_lock();
     thread::spawn(move || {
-        let lock = m2.as_ref().try_lock();
+        let lock = m2.try_lock();
         assert!(lock.is_none());
     })
     .join()
     .unwrap();
-    let _lock3 = m.as_ref().try_lock();
+    let _lock3 = m.try_lock();
 }
 
 pub struct Answer<'a>(pub ReentrantMutexGuard<'a, RefCell<u32>>);
diff --git a/library/std/src/sys_common/thread_local_key.rs b/library/std/src/sys_common/thread_local_key.rs
index 70beebe86..032bf604d 100644
--- a/library/std/src/sys_common/thread_local_key.rs
+++ b/library/std/src/sys_common/thread_local_key.rs
@@ -69,8 +69,10 @@ use crate::sys_common::mutex::StaticMutex;
 /// ```ignore (cannot-doctest-private-modules)
 /// use tls::os::{StaticKey, INIT};
 ///
+/// // Use a regular global static to store the key.
 /// static KEY: StaticKey = INIT;
 ///
+/// // The state provided via `get` and `set` is thread-local.
 /// unsafe {
 ///     assert!(KEY.get().is_null());
 ///     KEY.set(1 as *mut u8);
diff --git a/library/std/src/sys_common/thread_local_key/tests.rs b/library/std/src/sys_common/thread_local_key/tests.rs
index 968738a41..6f32b858f 100644
--- a/library/std/src/sys_common/thread_local_key/tests.rs
+++ b/library/std/src/sys_common/thread_local_key/tests.rs
@@ -1,4 +1,5 @@
 use super::{Key, StaticKey};
+use core::ptr;
 
 fn assert_sync<T: Sync>() {}
 fn assert_send<T: Send>() {}
@@ -12,8 +13,8 @@ fn smoke() {
     let k2 = Key::new(None);
     assert!(k1.get().is_null());
     assert!(k2.get().is_null());
-    k1.set(1 as *mut _);
-    k2.set(2 as *mut _);
+    k1.set(ptr::invalid_mut(1));
+    k2.set(ptr::invalid_mut(2));
     assert_eq!(k1.get() as usize, 1);
     assert_eq!(k2.get() as usize, 2);
 }
@@ -26,8 +27,8 @@ fn statik() {
     unsafe {
         assert!(K1.get().is_null());
         assert!(K2.get().is_null());
-        K1.set(1 as *mut _);
-        K2.set(2 as *mut _);
+        K1.set(ptr::invalid_mut(1));
+        K2.set(ptr::invalid_mut(2));
         assert_eq!(K1.get() as usize, 1);
         assert_eq!(K2.get() as usize, 2);
     }
diff --git a/library/std/src/sys_common/thread_parker/mod.rs b/library/std/src/sys_common/thread_parker/mod.rs
index cbd7832eb..f86a9a555 100644
--- a/library/std/src/sys_common/thread_parker/mod.rs
+++ b/library/std/src/sys_common/thread_parker/mod.rs
@@ -7,6 +7,7 @@ cfg_if::cfg_if! {
         target_os = "openbsd",
         target_os = "dragonfly",
         target_os = "fuchsia",
+        target_os = "hermit",
     ))] {
         mod futex;
         pub use futex::Parker;
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
index 57fa49893..dd53767d4 100644
--- a/library/std/src/sys_common/wtf8.rs
+++ b/library/std/src/sys_common/wtf8.rs
@@ -89,6 +89,24 @@ impl CodePoint {
         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).
@@ -117,6 +135,14 @@ impl CodePoint {
 #[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 {
@@ -147,13 +173,13 @@ impl Wtf8Buf {
     /// Creates a new, empty WTF-8 string.
     #[inline]
     pub fn new() -> Wtf8Buf {
-        Wtf8Buf { bytes: Vec::new() }
+        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) }
+        Wtf8Buf { bytes: Vec::with_capacity(capacity), is_known_utf8: true }
     }
 
     /// Creates a WTF-8 string from a UTF-8 `String`.
@@ -163,7 +189,7 @@ impl Wtf8Buf {
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
     pub fn from_string(string: String) -> Wtf8Buf {
-        Wtf8Buf { bytes: string.into_bytes() }
+        Wtf8Buf { bytes: string.into_bytes(), is_known_utf8: true }
     }
 
     /// Creates a WTF-8 string from a UTF-8 `&str` slice.
@@ -173,11 +199,12 @@ impl Wtf8Buf {
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
     pub fn from_str(str: &str) -> Wtf8Buf {
-        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()) }
+        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()), is_known_utf8: true }
     }
 
     pub fn clear(&mut self) {
-        self.bytes.clear()
+        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.
@@ -193,9 +220,11 @@ impl Wtf8Buf {
                     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.push_code_point_unchecked(code_point);
                 }
             }
         }
@@ -203,7 +232,7 @@ impl Wtf8Buf {
     }
 
     /// Copied from String::push
-    /// This does **not** include the WTF-8 concatenation check.
+    /// This does **not** include the WTF-8 concatenation check or `is_known_utf8` check.
     fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
         let mut bytes = [0; 4];
         let bytes = char::encode_utf8_raw(code_point.value, &mut bytes);
@@ -217,6 +246,9 @@ impl Wtf8Buf {
 
     #[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) }
     }
 
@@ -236,7 +268,8 @@ impl Wtf8Buf {
     /// 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.
+    /// capacity is already sufficient. This method preserves the contents even
+    /// if an error occurs.
     ///
     /// # Errors
     ///
@@ -313,7 +346,15 @@ impl Wtf8Buf {
                 self.push_char(decode_surrogate_pair(lead, trail));
                 self.bytes.extend_from_slice(other_without_trail_surrogate);
             }
-            _ => self.bytes.extend_from_slice(&other.bytes),
+            _ => {
+                // If we'll be pushing a string containing a surrogate, we may
+                // no longer have UTF-8.
+                if other.next_surrogate(0).is_some() {
+                    self.is_known_utf8 = false;
+                }
+
+                self.bytes.extend_from_slice(&other.bytes);
+            }
         }
     }
 
@@ -330,13 +371,19 @@ impl Wtf8Buf {
     /// like concatenating ill-formed UTF-16 strings effectively would.
     #[inline]
     pub fn push(&mut self, code_point: CodePoint) {
-        if let trail @ 0xDC00..=0xDFFF = code_point.to_u32() {
+        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 as u16));
+                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.
@@ -363,9 +410,10 @@ impl Wtf8Buf {
     /// (that is, if the string contains surrogates),
     /// the original WTF-8 string is returned instead.
     pub fn into_string(self) -> Result<String, Wtf8Buf> {
-        match self.next_surrogate(0) {
-            None => Ok(unsafe { String::from_utf8_unchecked(self.bytes) }),
-            Some(_) => Err(self),
+        if self.is_known_utf8 || self.next_surrogate(0).is_none() {
+            Ok(unsafe { String::from_utf8_unchecked(self.bytes) })
+        } else {
+            Err(self)
         }
     }
 
@@ -375,6 +423,11 @@ impl Wtf8Buf {
     ///
     /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
     pub fn into_string_lossy(mut self) -> String {
+        // Fast path: If we already have UTF-8, we can return it immediately.
+        if self.is_known_utf8 {
+            return unsafe { String::from_utf8_unchecked(self.bytes) };
+        }
+
         let mut pos = 0;
         loop {
             match self.next_surrogate(pos) {
@@ -397,7 +450,7 @@ impl Wtf8Buf {
     /// 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() }
+        Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
     }
 }
 
@@ -575,6 +628,11 @@ impl Wtf8 {
         }
     }
 
+    /// 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.
     ///
@@ -664,7 +722,8 @@ impl Wtf8 {
     }
 
     pub fn clone_into(&self, buf: &mut Wtf8Buf) {
-        self.bytes.clone_into(&mut buf.bytes)
+        buf.is_known_utf8 = false;
+        self.bytes.clone_into(&mut buf.bytes);
     }
 
     /// Boxes this `Wtf8`.
@@ -704,12 +763,12 @@ impl Wtf8 {
 
     #[inline]
     pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase() }
+        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() }
+        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase(), is_known_utf8: false }
     }
 
     #[inline]
diff --git a/library/std/src/sys_common/wtf8/tests.rs b/library/std/src/sys_common/wtf8/tests.rs
index 931996791..1a302d646 100644
--- a/library/std/src/sys_common/wtf8/tests.rs
+++ b/library/std/src/sys_common/wtf8/tests.rs
@@ -20,6 +20,36 @@ fn code_point_to_u32() {
 }
 
 #[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);
@@ -70,35 +100,66 @@ fn wtf8buf_from_string() {
 
 #[test]
 fn wtf8buf_from_wide() {
-    assert_eq!(Wtf8Buf::from_wide(&[]).bytes, b"");
-    assert_eq!(
-        Wtf8Buf::from_wide(&[0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]).bytes,
-        b"a\xC3\xA9 \xED\xA0\xBD\xF0\x9F\x92\xA9"
-    );
+    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()
@@ -106,37 +167,46 @@ fn wtf8buf_push() {
 
     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");
 }
 
@@ -146,6 +216,7 @@ fn wtf8buf_push_wtf8() {
     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) }
@@ -161,37 +232,68 @@ fn wtf8buf_push_wtf8() {
     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]
@@ -209,10 +311,36 @@ fn wtf8buf_truncate_fail_longer() {
 }
 
 #[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));
 }
 
@@ -229,15 +357,33 @@ 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]).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    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]).bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-    assert_eq!(f(&[0xD800, 0xDBFF]).bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-    assert_eq!(f(&[0xD800, 0xE000]).bytes, b"\xED\xA0\x80\xEE\x80\x80");
-    assert_eq!(f(&[0xD7FF, 0xDC00]).bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-    assert_eq!(f(&[0x61, 0xDC00]).bytes, b"\x61\xED\xB0\x80");
-    assert_eq!(f(&[0xDC00]).bytes, b"\xED\xB0\x80");
+    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]
@@ -251,15 +397,36 @@ fn wtf8buf_extend() {
         string
     }
 
-    assert_eq!(e(&[0x61, 0xE9], &[0x20, 0x1F4A9]).bytes, b"a\xC3\xA9 \xF0\x9F\x92\xA9");
+    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]).bytes, b"\xED\xA0\xBD \xED\xB2\xA9");
-    assert_eq!(e(&[0xD800], &[0xDBFF]).bytes, b"\xED\xA0\x80\xED\xAF\xBF");
-    assert_eq!(e(&[0xD800], &[0xE000]).bytes, b"\xED\xA0\x80\xEE\x80\x80");
-    assert_eq!(e(&[0xD7FF], &[0xDC00]).bytes, b"\xED\x9F\xBF\xED\xB0\x80");
-    assert_eq!(e(&[0x61], &[0xDC00]).bytes, b"\x61\xED\xB0\x80");
-    assert_eq!(e(&[], &[0xDC00]).bytes, b"\xED\xB0\x80");
+    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]
@@ -407,3 +574,93 @@ fn wtf8_encode_wide_size_hint() {
     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);
+}
diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs
index f4750cdf7..8aedfc4a6 100644
--- a/library/std/src/thread/local.rs
+++ b/library/std/src/thread/local.rs
@@ -1036,6 +1036,7 @@ pub mod fast {
 }
 
 #[doc(hidden)]
+#[cfg(not(target_thread_local))]
 pub mod os {
     use super::lazy::LazyKeyInner;
     use crate::cell::Cell;
@@ -1044,6 +1045,8 @@ pub mod os {
     use crate::ptr;
     use crate::sys_common::thread_local_key::StaticKey as OsStaticKey;
 
+    /// Use a regular global static to store this key; the state provided will then be
+    /// thread-local.
     pub struct Key<T> {
         // OS-TLS key that we'll use to key off.
         os: OsStaticKey,
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 44c8a50fd..ceea6986e 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -116,7 +116,7 @@
 //! Threads are able to have associated names for identification purposes. By default, spawned
 //! threads are unnamed. To specify a name for a thread, build the thread with [`Builder`] and pass
 //! the desired thread name to [`Builder::name`]. To retrieve the thread name from within the
-//! thread, use [`Thread::name`]. A couple examples of where the name of a thread gets used:
+//! thread, use [`Thread::name`]. A couple of examples where the name of a thread gets used:
 //!
 //! * If a panic occurs in a named thread, the thread name will be printed in the panic message.
 //! * The thread name is provided to the OS where applicable (e.g., `pthread_setname_np` in
@@ -170,7 +170,6 @@ use crate::ptr::addr_of_mut;
 use crate::str;
 use crate::sync::Arc;
 use crate::sys::thread as imp;
-use crate::sys_common::mutex;
 use crate::sys_common::thread;
 use crate::sys_common::thread_info;
 use crate::sys_common::thread_parker::Parker;
@@ -193,21 +192,31 @@ pub use scoped::{scope, Scope, ScopedJoinHandle};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::local::{AccessError, LocalKey};
 
-// The types used by the thread_local! macro to access TLS keys. Note that there
-// are two types, the "OS" type and the "fast" type. The OS thread local key
-// type is accessed via platform-specific API calls and is slow, while the fast
+// Select the type used by the thread_local! macro to access TLS keys. There
+// are three types: "static", "fast", "OS". The "OS" thread local key
+// type is accessed via platform-specific API calls and is slow, while the "fast"
 // key type is accessed via code generated via LLVM, where TLS keys are set up
-// by the elf linker. Note that the OS TLS type is always available: on macOS
-// the standard library is compiled with support for older platform versions
-// where fast TLS was not available; end-user code is compiled with fast TLS
-// where available, but both are needed.
+// by the elf linker. "static" is for single-threaded platforms where a global
+// static is sufficient.
 
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
 #[cfg(target_thread_local)]
+#[cfg(not(test))]
 #[doc(hidden)]
 pub use self::local::fast::Key as __FastLocalKeyInner;
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[cfg(target_thread_local)]
+#[cfg(test)] // when building for tests, use real std's key
+pub use realstd::thread::__FastLocalKeyInner;
+
+#[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[cfg(target_thread_local)]
+#[cfg(test)]
+pub use self::local::fast::Key as __FastLocalKeyInnerUnused; // we import this anyway to silence 'unused' warnings
+
+#[unstable(feature = "libstd_thread_internals", issue = "none")]
 #[doc(hidden)]
+#[cfg(not(target_thread_local))]
 pub use self::local::os::Key as __OsLocalKeyInner;
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
 #[cfg(all(target_family = "wasm", not(target_feature = "atomics")))]
@@ -1033,24 +1042,48 @@ pub struct ThreadId(NonZeroU64);
 impl ThreadId {
     // Generate a new unique thread ID.
     fn new() -> ThreadId {
-        // It is UB to attempt to acquire this mutex reentrantly!
-        static GUARD: mutex::StaticMutex = mutex::StaticMutex::new();
-        static mut COUNTER: u64 = 1;
-
-        unsafe {
-            let guard = GUARD.lock();
-
-            // If we somehow use up all our bits, panic so that we're not
-            // covering up subtle bugs of IDs being reused.
-            if COUNTER == u64::MAX {
-                drop(guard); // in case the panic handler ends up calling `ThreadId::new()`, avoid reentrant lock acquire.
-                panic!("failed to generate unique thread ID: bitspace exhausted");
-            }
-
-            let id = COUNTER;
-            COUNTER += 1;
+        #[cold]
+        fn exhausted() -> ! {
+            panic!("failed to generate unique thread ID: bitspace exhausted")
+        }
 
-            ThreadId(NonZeroU64::new(id).unwrap())
+        cfg_if::cfg_if! {
+            if #[cfg(target_has_atomic = "64")] {
+                use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
+
+                static COUNTER: AtomicU64 = AtomicU64::new(0);
+
+                let mut last = COUNTER.load(Relaxed);
+                loop {
+                    let Some(id) = last.checked_add(1) else {
+                        exhausted();
+                    };
+
+                    match COUNTER.compare_exchange_weak(last, id, Relaxed, Relaxed) {
+                        Ok(_) => return ThreadId(NonZeroU64::new(id).unwrap()),
+                        Err(id) => last = id,
+                    }
+                }
+            } else {
+                use crate::sys_common::mutex::StaticMutex;
+
+                // It is UB to attempt to acquire this mutex reentrantly!
+                static GUARD: StaticMutex = StaticMutex::new();
+                static mut COUNTER: u64 = 0;
+
+                unsafe {
+                    let guard = GUARD.lock();
+
+                    let Some(id) = COUNTER.checked_add(1) else {
+                        drop(guard); // in case the panic handler ends up calling `ThreadId::new()`, avoid reentrant lock acquire.
+                        exhausted();
+                    };
+
+                    COUNTER = id;
+                    drop(guard);
+                    ThreadId(NonZeroU64::new(id).unwrap())
+                }
+            }
         }
     }
 
diff --git a/library/std/src/thread/tests.rs b/library/std/src/thread/tests.rs
index ec68b5291..777964f04 100644
--- a/library/std/src/thread/tests.rs
+++ b/library/std/src/thread/tests.rs
@@ -37,6 +37,37 @@ fn test_named_thread() {
         .unwrap();
 }
 
+#[cfg(any(
+    // Note: musl didn't add pthread_getname_np until 1.2.3
+    all(target_os = "linux", target_env = "gnu"),
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "watchos"
+))]
+#[test]
+fn test_named_thread_truncation() {
+    use crate::ffi::CStr;
+
+    let long_name = crate::iter::once("test_named_thread_truncation")
+        .chain(crate::iter::repeat(" yada").take(100))
+        .collect::<String>();
+
+    let result = Builder::new().name(long_name.clone()).spawn(move || {
+        // Rust remembers the full thread name itself.
+        assert_eq!(thread::current().name(), Some(long_name.as_str()));
+
+        // But the system is limited -- make sure we successfully set a truncation.
+        let mut buf = vec![0u8; long_name.len() + 1];
+        unsafe {
+            libc::pthread_getname_np(libc::pthread_self(), buf.as_mut_ptr().cast(), buf.len());
+        }
+        let cstr = CStr::from_bytes_until_nul(&buf).unwrap();
+        assert!(cstr.to_bytes().len() > 0);
+        assert!(long_name.as_bytes().starts_with(cstr.to_bytes()));
+    });
+    result.unwrap().join().unwrap();
+}
+
 #[test]
 #[should_panic]
 fn test_invalid_named_thread() {
@@ -329,3 +360,22 @@ fn test_scoped_threads_nll() {
     let x = 42_u8;
     foo(&x);
 }
+
+// Regression test for https://github.com/rust-lang/rust/issues/98498.
+#[test]
+#[cfg(miri)] // relies on Miri's data race detector
+fn scope_join_race() {
+    for _ in 0..100 {
+        let a_bool = AtomicBool::new(false);
+
+        thread::scope(|s| {
+            for _ in 0..5 {
+                s.spawn(|| a_bool.load(Ordering::Relaxed));
+            }
+
+            for _ in 0..5 {
+                s.spawn(|| a_bool.load(Ordering::Relaxed));
+            }
+        });
+    }
+}
diff --git a/library/std/src/time/tests.rs b/library/std/src/time/tests.rs
index d710a5744..6229556c8 100644
--- a/library/std/src/time/tests.rs
+++ b/library/std/src/time/tests.rs
@@ -31,7 +31,8 @@ fn instant_monotonic_concurrent() -> crate::thread::Result<()> {
         .map(|_| {
             crate::thread::spawn(|| {
                 let mut old = Instant::now();
-                for _ in 0..5_000_000 {
+                let count = if cfg!(miri) { 1_000 } else { 5_000_000 };
+                for _ in 0..count {
                     let new = Instant::now();
                     assert!(new >= old);
                     old = new;