Merge subtree update for toolchain nightly-2026-08-21

[github-actions]

This is an automated PR to merge library subtree updates from 2026-02-05 (rust-lang/rust@db3e99b) to 2026-08-21 (rust-lang/rust@8925ea3) (inclusive) into main. git merge resulted in conflicts, which require manual resolution. Files were commited with merge conflict markers. Do not remove or edit the following annotations:
git-subtree-dir: library
git-subtree-split: 767896b

[jrey8343]

The failures on this PR are not tool regressions: the subtree merge left unresolved conflict markers (<<<<<<< HEAD / >>>>>>> subtree/library) committed in 29 files under library/, starting with library/alloc/Cargo.toml and library/core/Cargo.toml (hence key with no value, expected \=`from cargo in every Kani job) andlibrary/alloc/src/lib.rs`. Full list:

library/alloc/Cargo.toml
library/alloc/src/lib.rs
library/core/Cargo.toml
library/core/src/alloc/layout.rs
library/core/src/convert/num.rs
library/core/src/ffi/c_str.rs
library/core/src/intrinsics/mod.rs
library/core/src/iter/adapters/cloned.rs
library/core/src/iter/adapters/fuse.rs
library/core/src/iter/adapters/map.rs
library/core/src/iter/adapters/map_windows.rs
library/core/src/iter/adapters/step_by.rs
library/core/src/iter/adapters/zip.rs
library/core/src/iter/range.rs
library/core/src/iter/traits/iterator.rs
library/core/src/mem/mod.rs
library/core/src/num/int_macros.rs
library/core/src/num/mod.rs
library/core/src/num/niche_types.rs
library/core/src/num/uint_macros.rs
library/core/src/ptr/mod.rs
library/core/src/ptr/non_null.rs
library/core/src/slice/iter.rs
library/core/src/slice/iter/macros.rs
library/core/src/slice/memchr.rs
library/core/src/slice/mod.rs
library/core/src/str/lossy.rs
library/core/src/str/pattern.rs
library/std/src/alloc.rs

In each block the HEAD side is this repo's verification additions (cfg(kani), safety contracts, mod verify) and the subtree/library side is the new upstream text; the two generally need to be combined rather than either side taken.

For what it is worth, the VecDeque changes in this update (the WrappedIndex newtype) rebase onto #681 cleanly once the tree compiles; I have that ready on my side.

[github-advanced-security]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.