diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 2cee23a5c752..9a23b54dfa0a 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -1423,13 +1423,14 @@ pub(crate) fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
 }
 
 /// Checks whether the regions of memory starting at `src` and `dst` of size
-/// `count * size_of::<T>()` overlap.
-fn overlaps<T>(src: *const T, dst: *const T, count: usize) -> bool {
+/// `count * size_of::<T>()` do *not* overlap.
+pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -> bool {
     let src_usize = src as usize;
     let dst_usize = dst as usize;
     let size = mem::size_of::<T>().checked_mul(count).unwrap();
     let diff = if src_usize > dst_usize { src_usize - dst_usize } else { dst_usize - src_usize };
-    size > diff
+    let overlaps = size > diff;
+    !overlaps
 }
 
 /// Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
@@ -1524,7 +1525,7 @@ pub unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
 
     debug_assert!(is_aligned_and_not_null(src), "attempt to copy from unaligned or null pointer");
     debug_assert!(is_aligned_and_not_null(dst), "attempt to copy to unaligned or null pointer");
-    debug_assert!(!overlaps(src, dst, count), "attempt to copy to overlapping memory");
+    debug_assert!(is_nonoverlapping(src, dst, count), "attempt to copy to overlapping memory");
     copy_nonoverlapping(src, dst, count)
 }
 
diff --git a/src/libcore/ptr/mod.rs b/src/libcore/ptr/mod.rs
index 0ee50966f968..3d41a158b6d8 100644
--- a/src/libcore/ptr/mod.rs
+++ b/src/libcore/ptr/mod.rs
@@ -72,7 +72,7 @@
 use crate::cmp::Ordering;
 use crate::fmt;
 use crate::hash;
-use crate::intrinsics;
+use crate::intrinsics::{self, is_aligned_and_not_null, is_nonoverlapping};
 use crate::mem::{self, MaybeUninit};
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -119,10 +119,13 @@ mod mut_ptr;
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `to_drop` must be [valid] for reads.
+/// * `to_drop` must be [valid] for both reads and writes.
 ///
 /// * `to_drop` must be properly aligned.
 ///
+/// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
+///   additional invariants - this is type-dependent.
+///
 /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
 /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
 /// foo` counts as a use because it will cause the value to be dropped
@@ -289,7 +292,7 @@ pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * Both `x` and `y` must be [valid] for reads and writes.
+/// * Both `x` and `y` must be [valid] for both reads and writes.
 ///
 /// * Both `x` and `y` must be properly aligned.
 ///
@@ -355,7 +358,7 @@ pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * Both `x` and `y` must be [valid] for reads and writes of `count *
+/// * Both `x` and `y` must be [valid] for both reads and writes of `count *
 ///   size_of::<T>()` bytes.
 ///
 /// * Both `x` and `y` must be properly aligned.
@@ -389,6 +392,10 @@ pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
 #[inline]
 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
 pub unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
+    debug_assert!(is_aligned_and_not_null(x), "attempt to swap unaligned or null pointer");
+    debug_assert!(is_aligned_and_not_null(y), "attempt to swap unaligned or null pointer");
+    debug_assert!(is_nonoverlapping(x, y, count), "attempt to swap overlapping memory");
+
     let x = x as *mut u8;
     let y = y as *mut u8;
     let len = mem::size_of::<T>() * count;
@@ -471,10 +478,12 @@ unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `dst` must be [valid] for writes.
+/// * `dst` must be [valid] for both reads and writes.
 ///
 /// * `dst` must be properly aligned.
 ///
+/// * `dst` must point to a properly initialized value of type `T`.
+///
 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
 ///
 /// [valid]: ../ptr/index.html#safety
@@ -514,6 +523,8 @@ pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
 ///   case.
 ///
+/// * `src` must point to a properly initialized value of type `T`.
+///
 /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
 ///
 /// # Examples
@@ -612,6 +623,7 @@ pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn read<T>(src: *const T) -> T {
+    // `copy_nonoverlapping` takes care of debug_assert.
     let mut tmp = MaybeUninit::<T>::uninit();
     copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
     tmp.assume_init()
@@ -628,6 +640,8 @@ pub unsafe fn read<T>(src: *const T) -> T {
 ///
 /// * `src` must be [valid] for reads.
 ///
+/// * `src` must point to a properly initialized value of type `T`.
+///
 /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
 /// value and the value at `*src` can [violate memory safety][read-ownership].
@@ -703,6 +717,7 @@ pub unsafe fn read<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
 pub unsafe fn read_unaligned<T>(src: *const T) -> T {
+    // `copy_nonoverlapping` takes care of debug_assert.
     let mut tmp = MaybeUninit::<T>::uninit();
     copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
     tmp.assume_init()
@@ -795,6 +810,7 @@ pub unsafe fn read_unaligned<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn write<T>(dst: *mut T, src: T) {
+    debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
     intrinsics::move_val_init(&mut *dst, src)
 }
 
@@ -887,6 +903,7 @@ pub unsafe fn write<T>(dst: *mut T, src: T) {
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
 pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
+    // `copy_nonoverlapping` takes care of debug_assert.
     copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
     mem::forget(src);
 }
@@ -922,6 +939,8 @@ pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 ///
 /// * `src` must be properly aligned.
 ///
+/// * `src` must point to a properly initialized value of type `T`.
+///
 /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
 /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
 /// value and the value at `*src` can [violate memory safety][read-ownership].
@@ -956,6 +975,7 @@ pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 #[inline]
 #[stable(feature = "volatile", since = "1.9.0")]
 pub unsafe fn read_volatile<T>(src: *const T) -> T {
+    debug_assert!(is_aligned_and_not_null(src), "attempt to read from unaligned or null pointer");
     intrinsics::volatile_load(src)
 }
 
@@ -1024,6 +1044,7 @@ pub unsafe fn read_volatile<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "volatile", since = "1.9.0")]
 pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
+    debug_assert!(is_aligned_and_not_null(dst), "attempt to write to unaligned or null pointer");
     intrinsics::volatile_store(dst, src);
 }
 
diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 734b3ba7c6bb..e5b8412e117e 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -2658,7 +2658,8 @@ impl str {
     ///
     /// It's important to remember that [`char`] represents a Unicode Scalar
     /// Value, and may not match your idea of what a 'character' is. Iteration
-    /// over grapheme clusters may be what you actually want.
+    /// over grapheme clusters may be what you actually want. This functionality
+    /// is not provided by Rust's standard library, check crates.io instead.
     ///
     /// # Examples
     ///
diff --git a/src/librustc_error_codes/error_codes/E0309.md b/src/librustc_error_codes/error_codes/E0309.md
index 73ce7407476f..e719ee590aba 100644
--- a/src/librustc_error_codes/error_codes/E0309.md
+++ b/src/librustc_error_codes/error_codes/E0309.md
@@ -1,9 +1,7 @@
-The type definition contains some field whose type
-requires an outlives annotation. Outlives annotations
-(e.g., `T: 'a`) are used to guarantee that all the data in T is valid
-for at least the lifetime `'a`. This scenario most commonly
-arises when the type contains an associated type reference
-like `<T as SomeTrait<'a>>::Output`, as shown in this example:
+A parameter type is missing an explicit lifetime bound and may not live long
+enough.
+
+Erroneous code example:
 
 ```compile_fail,E0309
 // This won't compile because the applicable impl of
@@ -25,9 +23,15 @@ where
 }
 ```
 
-Here, the where clause `T: 'a` that appears on the impl is not known to be
-satisfied on the struct. To make this example compile, you have to add
-a where-clause like `T: 'a` to the struct definition:
+The type definition contains some field whose type requires an outlives
+annotation. Outlives annotations (e.g., `T: 'a`) are used to guarantee that all
+the data in T is valid for at least the lifetime `'a`. This scenario most
+commonly arises when the type contains an associated type reference like
+`<T as SomeTrait<'a>>::Output`, as shown in the previous code.
+
+There, the where clause `T: 'a` that appears on the impl is not known to be
+satisfied on the struct. To make this example compile, you have to add a
+where-clause like `T: 'a` to the struct definition:
 
 ```
 struct Foo<'a, T>
diff --git a/src/librustc_expand/mbe/macro_check.rs b/src/librustc_expand/mbe/macro_check.rs
index 47865b2fb9fc..10cdceefdf57 100644
--- a/src/librustc_expand/mbe/macro_check.rs
+++ b/src/librustc_expand/mbe/macro_check.rs
@@ -109,7 +109,7 @@ use crate::mbe::{KleeneToken, TokenTree};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_session::lint::builtin::META_VARIABLE_MISUSE;
 use rustc_session::parse::ParseSess;
-use rustc_span::symbol::{kw, sym};
+use rustc_span::symbol::kw;
 use rustc_span::{symbol::Ident, MultiSpan, Span};
 use syntax::ast::NodeId;
 use syntax::token::{DelimToken, Token, TokenKind};
@@ -392,7 +392,7 @@ fn check_nested_occurrences(
                 NestedMacroState::Empty,
                 &TokenTree::Token(Token { kind: TokenKind::Ident(name, false), .. }),
             ) => {
-                if name == sym::macro_rules {
+                if name == kw::MacroRules {
                     state = NestedMacroState::MacroRules;
                 } else if name == kw::Macro {
                     state = NestedMacroState::Macro;
diff --git a/src/librustc_expand/parse/tests.rs b/src/librustc_expand/parse/tests.rs
index 3641f03cb30c..4713c8dcd9a3 100644
--- a/src/librustc_expand/parse/tests.rs
+++ b/src/librustc_expand/parse/tests.rs
@@ -65,7 +65,7 @@ fn string_to_tts_macro() {
 
         match tts {
             [TokenTree::Token(Token { kind: token::Ident(name_macro_rules, false), .. }), TokenTree::Token(Token { kind: token::Not, .. }), TokenTree::Token(Token { kind: token::Ident(name_zip, false), .. }), TokenTree::Delimited(_, macro_delim, macro_tts)]
-                if name_macro_rules == &sym::macro_rules && name_zip.as_str() == "zip" =>
+                if name_macro_rules == &kw::MacroRules && name_zip.as_str() == "zip" =>
             {
                 let tts = &macro_tts.trees().collect::<Vec<_>>();
                 match &tts[..] {
diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index 500aaaf43b92..d7b8d9778f0d 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -1343,14 +1343,14 @@ impl<'a> Parser<'a> {
 
     /// Is this unambiguously the start of a `macro_rules! foo` item defnition?
     fn is_macro_rules_item(&mut self) -> bool {
-        self.check_keyword(sym::macro_rules)
+        self.check_keyword(kw::MacroRules)
             && self.look_ahead(1, |t| *t == token::Not)
             && self.look_ahead(2, |t| t.is_ident())
     }
 
     /// Parses a legacy `macro_rules! foo { ... }` declarative macro.
     fn parse_item_macro_rules(&mut self, vis: &Visibility) -> PResult<'a, ItemInfo> {
-        self.expect_keyword(sym::macro_rules)?; // `macro_rules`
+        self.expect_keyword(kw::MacroRules)?; // `macro_rules`
         self.expect(&token::Not)?; // `!`
 
         let ident = self.parse_ident()?;
diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
index 1cc4a2778809..3c419334d421 100644
--- a/src/librustc_span/symbol.rs
+++ b/src/librustc_span/symbol.rs
@@ -97,6 +97,7 @@ symbols! {
         Auto:               "auto",
         Catch:              "catch",
         Default:            "default",
+        MacroRules:         "macro_rules",
         Raw:                "raw",
         Union:              "union",
     }
@@ -429,7 +430,6 @@ symbols! {
         macro_lifetime_matcher,
         macro_literal_matcher,
         macro_reexport,
-        macro_rules,
         macros_in_extern,
         macro_use,
         macro_vis_matcher,
@@ -1071,6 +1071,9 @@ pub mod sym {
 
     symbols!();
 
+    // Used from a macro in `librustc_feature/accepted.rs`
+    pub use super::kw::MacroRules as macro_rules;
+
     // Get the symbol for an integer. The first few non-negative integers each
     // have a static symbol and therefore are fast.
     pub fn integer<N: TryInto<usize> + Copy + ToString>(n: N) -> Symbol {
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index 3ca778354e41..674d4c711380 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -255,8 +255,9 @@ pub mod guard {
 
     #[cfg(target_os = "macos")]
     unsafe fn get_stack_start() -> Option<*mut libc::c_void> {
-        let stackaddr = libc::pthread_get_stackaddr_np(libc::pthread_self()) as usize
-            - libc::pthread_get_stacksize_np(libc::pthread_self());
+        let th = libc::pthread_self();
+        let stackaddr =
+            libc::pthread_get_stackaddr_np(th) as usize - libc::pthread_get_stacksize_np(th);
         Some(stackaddr as *mut libc::c_void)
     }