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-rw-r--r--library/alloc/src/alloc.rs7
-rw-r--r--library/alloc/src/collections/btree/set.rs8
-rw-r--r--library/alloc/src/collections/linked_list/tests.rs147
-rw-r--r--library/alloc/src/collections/vec_deque/tests.rs38
-rw-r--r--library/alloc/src/ffi/c_str.rs111
-rw-r--r--library/alloc/src/rc.rs1
-rw-r--r--library/alloc/src/string.rs53
-rw-r--r--library/alloc/src/sync.rs1
-rw-r--r--library/alloc/src/vec/mod.rs32
-rw-r--r--library/alloc/src/vec/peek_mut.rs55
10 files changed, 300 insertions, 153 deletions
diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index e1cc4ba25c4..b4176e9c1f4 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -31,8 +31,9 @@ unsafe extern "Rust" {
     #[rustc_std_internal_symbol]
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
 
+    #[rustc_nounwind]
     #[rustc_std_internal_symbol]
-    static __rust_no_alloc_shim_is_unstable: u8;
+    fn __rust_no_alloc_shim_is_unstable_v2();
 }
 
 /// The global memory allocator.
@@ -88,7 +89,7 @@ pub unsafe fn alloc(layout: Layout) -> *mut u8 {
     unsafe {
         // Make sure we don't accidentally allow omitting the allocator shim in
         // stable code until it is actually stabilized.
-        core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
+        __rust_no_alloc_shim_is_unstable_v2();
 
         __rust_alloc(layout.size(), layout.align())
     }
@@ -171,7 +172,7 @@ pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
     unsafe {
         // Make sure we don't accidentally allow omitting the allocator shim in
         // stable code until it is actually stabilized.
-        core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
+        __rust_no_alloc_shim_is_unstable_v2();
 
         __rust_alloc_zeroed(layout.size(), layout.align())
     }
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 51418036f42..aa9e5fce1d4 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -1220,11 +1220,11 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// assert_eq!(high.into_iter().collect::<Vec<_>>(), [4, 5, 6, 7]);
     /// ```
     #[unstable(feature = "btree_extract_if", issue = "70530")]
-    pub fn extract_if<'a, F, R>(&'a mut self, range: R, pred: F) -> ExtractIf<'a, T, R, F, A>
+    pub fn extract_if<F, R>(&mut self, range: R, pred: F) -> ExtractIf<'_, T, R, F, A>
     where
         T: Ord,
         R: RangeBounds<T>,
-        F: 'a + FnMut(&T) -> bool,
+        F: FnMut(&T) -> bool,
     {
         let (inner, alloc) = self.map.extract_if_inner(range);
         ExtractIf { pred, inner, alloc }
@@ -1585,11 +1585,11 @@ where
 }
 
 #[unstable(feature = "btree_extract_if", issue = "70530")]
-impl<'a, T, R, F, A: Allocator + Clone> Iterator for ExtractIf<'_, T, R, F, A>
+impl<T, R, F, A: Allocator + Clone> Iterator for ExtractIf<'_, T, R, F, A>
 where
     T: PartialOrd,
     R: RangeBounds<T>,
-    F: 'a + FnMut(&T) -> bool,
+    F: FnMut(&T) -> bool,
 {
     type Item = T;
 
diff --git a/library/alloc/src/collections/linked_list/tests.rs b/library/alloc/src/collections/linked_list/tests.rs
index 812fe229a0f..3d6c740e80b 100644
--- a/library/alloc/src/collections/linked_list/tests.rs
+++ b/library/alloc/src/collections/linked_list/tests.rs
@@ -1,6 +1,3 @@
-// FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
-#![allow(static_mut_refs)]
-
 use std::panic::{AssertUnwindSafe, catch_unwind};
 use std::thread;
 
@@ -8,6 +5,7 @@ use rand::RngCore;
 
 use super::*;
 use crate::testing::crash_test::{CrashTestDummy, Panic};
+use crate::testing::macros::struct_with_counted_drop;
 use crate::vec::Vec;
 
 #[test]
@@ -58,48 +56,33 @@ fn list_from<T: Clone>(v: &[T]) -> LinkedList<T> {
     v.iter().cloned().collect()
 }
 
+/// Starting from the head of the LinkedList,
+/// follow the next links, while checking the prev links,
+/// and check that length equals the count of visited nodes.
 fn check_links<T>(list: &LinkedList<T>) {
-    unsafe {
-        let mut len = 0;
-        let mut last_ptr: Option<&Node<T>> = None;
-        let mut node_ptr: &Node<T>;
-        match list.head {
-            None => {
-                // tail node should also be None.
-                assert!(list.tail.is_none());
-                assert_eq!(0, list.len);
-                return;
-            }
-            Some(node) => node_ptr = &*node.as_ptr(),
-        }
-        loop {
-            match (last_ptr, node_ptr.prev) {
-                (None, None) => {}
-                (None, _) => panic!("prev link for head"),
-                (Some(p), Some(pptr)) => {
-                    assert_eq!(p as *const Node<T>, pptr.as_ptr() as *const Node<T>);
-                }
-                _ => panic!("prev link is none, not good"),
-            }
-            match node_ptr.next {
-                Some(next) => {
-                    last_ptr = Some(node_ptr);
-                    node_ptr = &*next.as_ptr();
-                    len += 1;
-                }
-                None => {
-                    len += 1;
-                    break;
-                }
-            }
-        }
-
-        // verify that the tail node points to the last node.
-        let tail = list.tail.as_ref().expect("some tail node").as_ref();
-        assert_eq!(tail as *const Node<T>, node_ptr as *const Node<T>);
-        // check that len matches interior links.
-        assert_eq!(len, list.len);
+    let mut node: &Node<T> = if let Some(node) = list.head {
+        // SAFETY: depends on correctness of LinkedList
+        unsafe { &*node.as_ptr() }
+    } else {
+        assert!(list.tail.is_none(), "empty list should have no tail node");
+        assert_eq!(list.len, 0, "empty list should have length 0");
+        return;
+    };
+
+    assert!(node.prev.is_none(), "head node should not have a prev link");
+    let mut prev;
+    let mut len = 1;
+    while let Some(next) = node.next {
+        prev = node;
+        // SAFETY: depends on correctness of LinkedList
+        node = unsafe { &*next.as_ptr() };
+        len += 1;
+        assert_eq!(node.prev.expect("missing prev link"), prev.into(), "bad prev link");
     }
+
+    let tail = list.tail.expect("list is non-empty, so there should be a tail node");
+    assert_eq!(tail, node.into(), "tail node points to the last node");
+    assert_eq!(len, list.len, "len matches interior links");
 }
 
 #[test]
@@ -1030,18 +1013,7 @@ fn extract_if_drop_panic_leak() {
 #[test]
 #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn extract_if_pred_panic_leak() {
-    static mut DROPS: i32 = 0;
-
-    #[derive(Debug)]
-    struct D(u32);
-
-    impl Drop for D {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-        }
-    }
+    struct_with_counted_drop!(D(u32), DROPS);
 
     let mut q = LinkedList::new();
     q.push_back(D(3));
@@ -1053,26 +1025,17 @@ fn extract_if_pred_panic_leak() {
     q.push_front(D(1));
     q.push_front(D(0));
 
-    catch_unwind(AssertUnwindSafe(|| {
+    _ = catch_unwind(AssertUnwindSafe(|| {
         q.extract_if(|item| if item.0 >= 2 { panic!() } else { true }).for_each(drop)
-    }))
-    .ok();
+    }));
 
-    assert_eq!(unsafe { DROPS }, 2); // 0 and 1
+    assert_eq!(DROPS.get(), 2); // 0 and 1
     assert_eq!(q.len(), 6);
 }
 
 #[test]
 fn test_drop() {
-    static mut DROPS: i32 = 0;
-    struct Elem;
-    impl Drop for Elem {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-        }
-    }
+    struct_with_counted_drop!(Elem, DROPS);
 
     let mut ring = LinkedList::new();
     ring.push_back(Elem);
@@ -1081,20 +1044,12 @@ fn test_drop() {
     ring.push_front(Elem);
     drop(ring);
 
-    assert_eq!(unsafe { DROPS }, 4);
+    assert_eq!(DROPS.get(), 4);
 }
 
 #[test]
 fn test_drop_with_pop() {
-    static mut DROPS: i32 = 0;
-    struct Elem;
-    impl Drop for Elem {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-        }
-    }
+    struct_with_counted_drop!(Elem, DROPS);
 
     let mut ring = LinkedList::new();
     ring.push_back(Elem);
@@ -1104,23 +1059,15 @@ fn test_drop_with_pop() {
 
     drop(ring.pop_back());
     drop(ring.pop_front());
-    assert_eq!(unsafe { DROPS }, 2);
+    assert_eq!(DROPS.get(), 2);
 
     drop(ring);
-    assert_eq!(unsafe { DROPS }, 4);
+    assert_eq!(DROPS.get(), 4);
 }
 
 #[test]
 fn test_drop_clear() {
-    static mut DROPS: i32 = 0;
-    struct Elem;
-    impl Drop for Elem {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-        }
-    }
+    struct_with_counted_drop!(Elem, DROPS);
 
     let mut ring = LinkedList::new();
     ring.push_back(Elem);
@@ -1128,30 +1075,16 @@ fn test_drop_clear() {
     ring.push_back(Elem);
     ring.push_front(Elem);
     ring.clear();
-    assert_eq!(unsafe { DROPS }, 4);
+    assert_eq!(DROPS.get(), 4);
 
     drop(ring);
-    assert_eq!(unsafe { DROPS }, 4);
+    assert_eq!(DROPS.get(), 4);
 }
 
 #[test]
 #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_drop_panic() {
-    static mut DROPS: i32 = 0;
-
-    struct D(bool);
-
-    impl Drop for D {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-
-            if self.0 {
-                panic!("panic in `drop`");
-            }
-        }
-    }
+    struct_with_counted_drop!(D(bool), DROPS => |this: &D| if this.0 { panic!("panic in `drop`"); } );
 
     let mut q = LinkedList::new();
     q.push_back(D(false));
@@ -1165,7 +1098,7 @@ fn test_drop_panic() {
 
     catch_unwind(move || drop(q)).ok();
 
-    assert_eq!(unsafe { DROPS }, 8);
+    assert_eq!(DROPS.get(), 8);
 }
 
 #[test]
diff --git a/library/alloc/src/collections/vec_deque/tests.rs b/library/alloc/src/collections/vec_deque/tests.rs
index c90679f1797..ad76cb14deb 100644
--- a/library/alloc/src/collections/vec_deque/tests.rs
+++ b/library/alloc/src/collections/vec_deque/tests.rs
@@ -1,9 +1,7 @@
-// FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
-#![allow(static_mut_refs)]
-
 use core::iter::TrustedLen;
 
 use super::*;
+use crate::testing::macros::struct_with_counted_drop;
 
 #[bench]
 fn bench_push_back_100(b: &mut test::Bencher) {
@@ -1086,36 +1084,24 @@ fn test_clone_from() {
 
 #[test]
 fn test_vec_deque_truncate_drop() {
-    static mut DROPS: u32 = 0;
-    #[derive(Clone)]
-    struct Elem(#[allow(dead_code)] i32);
-    impl Drop for Elem {
-        fn drop(&mut self) {
-            unsafe {
-                DROPS += 1;
-            }
-        }
-    }
+    struct_with_counted_drop!(Elem, DROPS);
 
-    let v = vec![Elem(1), Elem(2), Elem(3), Elem(4), Elem(5)];
-    for push_front in 0..=v.len() {
-        let v = v.clone();
-        let mut tester = VecDeque::with_capacity(5);
-        for (index, elem) in v.into_iter().enumerate() {
+    const LEN: usize = 5;
+    for push_front in 0..=LEN {
+        let mut tester = VecDeque::with_capacity(LEN);
+        for index in 0..LEN {
             if index < push_front {
-                tester.push_front(elem);
+                tester.push_front(Elem);
             } else {
-                tester.push_back(elem);
+                tester.push_back(Elem);
             }
         }
-        assert_eq!(unsafe { DROPS }, 0);
+        assert_eq!(DROPS.get(), 0);
         tester.truncate(3);
-        assert_eq!(unsafe { DROPS }, 2);
+        assert_eq!(DROPS.get(), 2);
         tester.truncate(0);
-        assert_eq!(unsafe { DROPS }, 5);
-        unsafe {
-            DROPS = 0;
-        }
+        assert_eq!(DROPS.get(), 5);
+        DROPS.set(0);
     }
 }
 
diff --git a/library/alloc/src/ffi/c_str.rs b/library/alloc/src/ffi/c_str.rs
index 8b448a18402..93bdad75380 100644
--- a/library/alloc/src/ffi/c_str.rs
+++ b/library/alloc/src/ffi/c_str.rs
@@ -714,6 +714,8 @@ impl ops::Deref for CString {
     }
 }
 
+/// Delegates to the [`CStr`] implementation of [`fmt::Debug`],
+/// showing invalid UTF-8 as hex escapes.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for CString {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1097,6 +1099,46 @@ impl From<&CStr> for CString {
     }
 }
 
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CStr> for CString {
+    #[inline]
+    fn eq(&self, other: &CStr) -> bool {
+        **self == *other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CStr) -> bool {
+        **self != *other
+    }
+}
+
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<&CStr> for CString {
+    #[inline]
+    fn eq(&self, other: &&CStr) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &&CStr) -> bool {
+        **self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<Cow<'_, CStr>> for CString {
+    #[inline]
+    fn eq(&self, other: &Cow<'_, CStr>) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Cow<'_, CStr>) -> bool {
+        **self != **other
+    }
+}
+
 #[stable(feature = "cstring_asref", since = "1.7.0")]
 impl ops::Index<ops::RangeFull> for CString {
     type Output = CStr;
@@ -1179,6 +1221,75 @@ impl CStr {
     }
 }
 
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CString> for CStr {
+    #[inline]
+    fn eq(&self, other: &CString) -> bool {
+        *self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CString) -> bool {
+        *self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<Cow<'_, Self>> for CStr {
+    #[inline]
+    fn eq(&self, other: &Cow<'_, Self>) -> bool {
+        *self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Cow<'_, Self>) -> bool {
+        *self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CStr> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &CStr) -> bool {
+        **self == *other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CStr) -> bool {
+        **self != *other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<&CStr> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &&CStr) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &&CStr) -> bool {
+        **self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CString> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &CString) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CString) -> bool {
+        **self != **other
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl core::error::Error for NulError {
     #[allow(deprecated)]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 4b8ea708e7e..010d17f7476 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -3004,7 +3004,6 @@ pub struct Weak<
     // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
     // to allocate space on the heap. That's not a value a real pointer
     // will ever have because RcInner has alignment at least 2.
-    // This is only possible when `T: Sized`; unsized `T` never dangle.
     ptr: NonNull<RcInner<T>>,
     alloc: A,
 }
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 37614a7ca45..5f69f699867 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1105,6 +1105,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_confusables("append", "push")]
     #[rustc_diagnostic_item = "string_push_str"]
@@ -1135,6 +1136,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "string_extend_from_within", since = "1.87.0")]
+    #[track_caller]
     pub fn extend_from_within<R>(&mut self, src: R)
     where
         R: RangeBounds<usize>,
@@ -1206,6 +1208,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn reserve(&mut self, additional: usize) {
         self.vec.reserve(additional)
@@ -1257,6 +1260,7 @@ impl String {
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[track_caller]
     pub fn reserve_exact(&mut self, additional: usize) {
         self.vec.reserve_exact(additional)
     }
@@ -1352,6 +1356,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn shrink_to_fit(&mut self) {
         self.vec.shrink_to_fit()
@@ -1379,6 +1384,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "shrink_to", since = "1.56.0")]
     pub fn shrink_to(&mut self, min_capacity: usize) {
         self.vec.shrink_to(min_capacity)
@@ -1400,6 +1406,7 @@ impl String {
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[track_caller]
     pub fn push(&mut self, ch: char) {
         let len = self.len();
         let ch_len = ch.len_utf8();
@@ -1456,6 +1463,7 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[track_caller]
     pub fn truncate(&mut self, new_len: usize) {
         if new_len <= self.len() {
             assert!(self.is_char_boundary(new_len));
@@ -1489,10 +1497,11 @@ impl String {
         Some(ch)
     }
 
-    /// Removes a [`char`] from this `String` at a byte position and returns it.
+    /// Removes a [`char`] from this `String` at byte position `idx` and returns it.
     ///
-    /// This is an *O*(*n*) operation, as it requires copying every element in the
-    /// buffer.
+    /// Copies all bytes after the removed char to new positions.
+    ///
+    /// Note that calling this in a loop can result in quadratic behavior.
     ///
     /// # Panics
     ///
@@ -1510,6 +1519,7 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[track_caller]
     #[rustc_confusables("delete", "take")]
     pub fn remove(&mut self, idx: usize) -> char {
         let ch = match self[idx..].chars().next() {
@@ -1678,10 +1688,13 @@ impl String {
         drop(guard);
     }
 
-    /// Inserts a character into this `String` at a byte position.
+    /// Inserts a character into this `String` at byte position `idx`.
+    ///
+    /// Reallocates if `self.capacity()` is insufficient, which may involve copying all
+    /// `self.capacity()` bytes. Makes space for the insertion by copying all bytes of
+    /// `&self[idx..]` to new positions.
     ///
-    /// This is an *O*(*n*) operation as it requires copying every element in the
-    /// buffer.
+    /// Note that calling this in a loop can result in quadratic behavior.
     ///
     /// # Panics
     ///
@@ -1701,6 +1714,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_confusables("set")]
     pub fn insert(&mut self, idx: usize, ch: char) {
@@ -1733,10 +1747,13 @@ impl String {
         }
     }
 
-    /// Inserts a string slice into this `String` at a byte position.
+    /// Inserts a string slice into this `String` at byte position `idx`.
+    ///
+    /// Reallocates if `self.capacity()` is insufficient, which may involve copying all
+    /// `self.capacity()` bytes. Makes space for the insertion by copying all bytes of
+    /// `&self[idx..]` to new positions.
     ///
-    /// This is an *O*(*n*) operation as it requires copying every element in the
-    /// buffer.
+    /// Note that calling this in a loop can result in quadratic behavior.
     ///
     /// # Panics
     ///
@@ -1754,6 +1771,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "insert_str", since = "1.16.0")]
     #[rustc_diagnostic_item = "string_insert_str"]
     pub fn insert_str(&mut self, idx: usize, string: &str) {
@@ -1882,6 +1900,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[inline]
+    #[track_caller]
     #[stable(feature = "string_split_off", since = "1.16.0")]
     #[must_use = "use `.truncate()` if you don't need the other half"]
     pub fn split_off(&mut self, at: usize) -> String {
@@ -1946,6 +1965,7 @@ impl String {
     /// assert_eq!(s, "");
     /// ```
     #[stable(feature = "drain", since = "1.6.0")]
+    #[track_caller]
     pub fn drain<R>(&mut self, range: R) -> Drain<'_>
     where
         R: RangeBounds<usize>,
@@ -2045,6 +2065,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "splice", since = "1.27.0")]
+    #[track_caller]
     pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
     where
         R: RangeBounds<usize>,
@@ -2094,6 +2115,7 @@ impl String {
     #[stable(feature = "box_str", since = "1.4.0")]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
+    #[track_caller]
     pub fn into_boxed_str(self) -> Box<str> {
         let slice = self.vec.into_boxed_slice();
         unsafe { from_boxed_utf8_unchecked(slice) }
@@ -2281,6 +2303,7 @@ impl Error for FromUtf16Error {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Clone for String {
+    #[track_caller]
     fn clone(&self) -> Self {
         String { vec: self.vec.clone() }
     }
@@ -2289,6 +2312,7 @@ impl Clone for String {
     ///
     /// This method is preferred over simply assigning `source.clone()` to `self`,
     /// as it avoids reallocation if possible.
+    #[track_caller]
     fn clone_from(&mut self, source: &Self) {
         self.vec.clone_from(&source.vec);
     }
@@ -2462,11 +2486,14 @@ impl<'a> Extend<Cow<'a, str>> for String {
 #[cfg(not(no_global_oom_handling))]
 #[unstable(feature = "ascii_char", issue = "110998")]
 impl Extend<core::ascii::Char> for String {
+    #[inline]
+    #[track_caller]
     fn extend<I: IntoIterator<Item = core::ascii::Char>>(&mut self, iter: I) {
         self.vec.extend(iter.into_iter().map(|c| c.to_u8()));
     }
 
     #[inline]
+    #[track_caller]
     fn extend_one(&mut self, c: core::ascii::Char) {
         self.vec.push(c.to_u8());
     }
@@ -2475,11 +2502,14 @@ impl Extend<core::ascii::Char> for String {
 #[cfg(not(no_global_oom_handling))]
 #[unstable(feature = "ascii_char", issue = "110998")]
 impl<'a> Extend<&'a core::ascii::Char> for String {
+    #[inline]
+    #[track_caller]
     fn extend<I: IntoIterator<Item = &'a core::ascii::Char>>(&mut self, iter: I) {
         self.extend(iter.into_iter().cloned());
     }
 
     #[inline]
+    #[track_caller]
     fn extend_one(&mut self, c: &'a core::ascii::Char) {
         self.vec.push(c.to_u8());
     }
@@ -2834,7 +2864,7 @@ macro_rules! impl_to_string {
         impl SpecToString for $signed {
             #[inline]
             fn spec_to_string(&self) -> String {
-                const SIZE: usize = $signed::MAX.ilog(10) as usize + 1;
+                const SIZE: usize = $signed::MAX.ilog10() as usize + 1;
                 let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
                 // Only difference between signed and unsigned are these 8 lines.
                 let mut out;
@@ -2854,7 +2884,7 @@ macro_rules! impl_to_string {
         impl SpecToString for $unsigned {
             #[inline]
             fn spec_to_string(&self) -> String {
-                const SIZE: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                const SIZE: usize = $unsigned::MAX.ilog10() as usize + 1;
                 let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
 
                 self._fmt(&mut buf).to_string()
@@ -2870,6 +2900,7 @@ impl_to_string! {
     i32, u32,
     i64, u64,
     isize, usize,
+    i128, u128,
 }
 
 #[cfg(not(no_global_oom_handling))]
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 17090925cfa..1e3c03977bd 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -342,7 +342,6 @@ pub struct Weak<
     // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
     // to allocate space on the heap. That's not a value a real pointer
     // will ever have because RcInner has alignment at least 2.
-    // This is only possible when `T: Sized`; unsized `T` never dangle.
     ptr: NonNull<ArcInner<T>>,
     alloc: A,
 }
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index ce7321544b6..5bd82560da7 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -109,6 +109,11 @@ mod in_place_collect;
 
 mod partial_eq;
 
+#[unstable(feature = "vec_peek_mut", issue = "122742")]
+pub use self::peek_mut::PeekMut;
+
+mod peek_mut;
+
 #[cfg(not(no_global_oom_handling))]
 use self::spec_from_elem::SpecFromElem;
 
@@ -729,6 +734,33 @@ impl<T> Vec<T> {
     pub unsafe fn from_parts(ptr: NonNull<T>, length: usize, capacity: usize) -> Self {
         unsafe { Self::from_parts_in(ptr, length, capacity, Global) }
     }
+
+    /// Returns a mutable reference to the last item in the vector, or
+    /// `None` if it is empty.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(vec_peek_mut)]
+    /// let mut vec = Vec::new();
+    /// assert!(vec.peek_mut().is_none());
+    ///
+    /// vec.push(1);
+    /// vec.push(5);
+    /// vec.push(2);
+    /// assert_eq!(vec.last(), Some(&2));
+    /// if let Some(mut val) = vec.peek_mut() {
+    ///     *val = 0;
+    /// }
+    /// assert_eq!(vec.last(), Some(&0));
+    /// ```
+    #[inline]
+    #[unstable(feature = "vec_peek_mut", issue = "122742")]
+    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
+        PeekMut::new(self)
+    }
 }
 
 impl<T, A: Allocator> Vec<T, A> {
diff --git a/library/alloc/src/vec/peek_mut.rs b/library/alloc/src/vec/peek_mut.rs
new file mode 100644
index 00000000000..c0dd941ed39
--- /dev/null
+++ b/library/alloc/src/vec/peek_mut.rs
@@ -0,0 +1,55 @@
+use core::ops::{Deref, DerefMut};
+
+use super::Vec;
+use crate::fmt;
+
+/// Structure wrapping a mutable reference to the last item in a
+/// `Vec`.
+///
+/// This `struct` is created by the [`peek_mut`] method on [`Vec`]. See
+/// its documentation for more.
+///
+/// [`peek_mut`]: Vec::peek_mut
+#[unstable(feature = "vec_peek_mut", issue = "122742")]
+pub struct PeekMut<'a, T> {
+    vec: &'a mut Vec<T>,
+}
+
+#[unstable(feature = "vec_peek_mut", issue = "122742")]
+impl<T: fmt::Debug> fmt::Debug for PeekMut<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("PeekMut").field(self.deref()).finish()
+    }
+}
+
+impl<'a, T> PeekMut<'a, T> {
+    pub(crate) fn new(vec: &'a mut Vec<T>) -> Option<Self> {
+        if vec.is_empty() { None } else { Some(Self { vec }) }
+    }
+
+    /// Removes the peeked value from the vector and returns it.
+    #[unstable(feature = "vec_peek_mut", issue = "122742")]
+    pub fn pop(self) -> T {
+        // SAFETY: PeekMut is only constructed if the vec is non-empty
+        unsafe { self.vec.pop().unwrap_unchecked() }
+    }
+}
+
+#[unstable(feature = "vec_peek_mut", issue = "122742")]
+impl<'a, T> Deref for PeekMut<'a, T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: PeekMut is only constructed if the vec is non-empty
+        unsafe { self.vec.get_unchecked(self.vec.len() - 1) }
+    }
+}
+
+#[unstable(feature = "vec_peek_mut", issue = "122742")]
+impl<'a, T> DerefMut for PeekMut<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        let idx = self.vec.len() - 1;
+        // SAFETY: PeekMut is only constructed if the vec is non-empty
+        unsafe { self.vec.get_unchecked_mut(idx) }
+    }
+}
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2025-11-01 19:56:33 +0000