- [Guarantee layout and ABI of `Result` in some scenarios](https://github.com/rust-lang/rust/pull/124870)
Compiler
--------
- [Update cc crate to v1.0.97 allowing additional spectre mitigations on MSVC targets](https://github.com/rust-lang/rust/pull/124892/)
- [Allow field reordering on types marked `repr(packed(1))`](https://github.com/rust-lang/rust/pull/125360/)
- [Add a lint against never type fallback affecting unsafe code](https://github.com/rust-lang/rust/pull/123939/)
- [Disallow cast with trailing braced macro in let-else](https://github.com/rust-lang/rust/pull/125049/)
- [Expand `for_loops_over_fallibles` lint to lint on fallibles behind references.](https://github.com/rust-lang/rust/pull/125156/)
- [self-contained linker: retry linking without `-fuse-ld=lld` on CCs that don't support it](https://github.com/rust-lang/rust/pull/125417/)
- [Do not parse CVarArgs (`...`) as a type in trait bounds](https://github.com/rust-lang/rust/pull/125863/)
- Improvements to LLDB formatting [#124458](https://github.com/rust-lang/rust/pull/124458) [#124500](https://github.com/rust-lang/rust/pull/124500)
- [For the wasm32-wasip2 target default to PIC and do not use `-fuse-ld=lld`](https://github.com/rust-lang/rust/pull/124858/)
- [Add x86_64-unknown-linux-none as a tier 3 target](https://github.com/rust-lang/rust/pull/125023/)
- [Lint on `foo.into_iter()` resolving to `&Box<[T]>: IntoIterator`](https://github.com/rust-lang/rust/pull/124097/)
Libraries
---------
- [Add `size_of` and `size_of_val` and `align_of` and `align_of_val` to the prelude](https://github.com/rust-lang/rust/pull/123168/)
- [Abort a process when FD ownership is violated](https://github.com/rust-lang/rust/pull/124210/)
- [io::Write::write_fmt: panic if the formatter fails when the stream does not fail](https://github.com/rust-lang/rust/pull/125012/)
- [Panic if `PathBuf::set_extension` would add a path separator](https://github.com/rust-lang/rust/pull/125070/)
- [Add assert_unsafe_precondition to unchecked_{add,sub,neg,mul,shl,shr} methods](https://github.com/rust-lang/rust/pull/121571/)
- [Update `c_char` on AIX to use the correct type](https://github.com/rust-lang/rust/pull/122986/)
- [`offset_of!` no longer returns a temporary](https://github.com/rust-lang/rust/pull/124484/)
- [Handle sigma in `str.to_lowercase` correctly](https://github.com/rust-lang/rust/pull/124773/)
- [Raise `DEFAULT_MIN_STACK_SIZE` to at least 64KiB](https://github.com/rust-lang/rust/pull/126059/)
Stabilized APIs
---------------
- [`impl Default for Rc<CStr>`](https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3CCStr%3E)
- [`impl Default for Rc<str>`](https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3Cstr%3E)
- [`impl Default for Rc<[T]>`](https://doc.rust-lang.org/beta/alloc/rc/struct.Rc.html#impl-Default-for-Rc%3C%5BT%5D%3E)
- [`impl Default for Arc<str>`](https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3Cstr%3E)
- [`impl Default for Arc<CStr>`](https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3CCStr%3E)
- [`impl Default for Arc<[T]>`](https://doc.rust-lang.org/beta/alloc/sync/struct.Arc.html#impl-Default-for-Arc%3C%5BT%5D%3E)
- [`impl IntoIterator for Box<[T]>`](https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-IntoIterator-for-Box%3C%5BI%5D,+A%3E)
- [`impl FromIterator<String> for Box<str>`](https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-FromIterator%3CString%3E-for-Box%3Cstr%3E)
- [`impl FromIterator<char> for Box<str>`](https://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#impl-FromIterator%3Cchar%3E-for-Box%3Cstr%3E)
- [fix: In cross-crate scenarios show enum variants on type aliases of enums](https://github.com/rust-lang/rust/pull/125300/)
Compatibility Notes
-------------------
- [rustfmt estimates line lengths differently when using non-ascii characters](https://github.com/rust-lang/rustfmt/issues/6203)
- [Type aliases are now handled correctly in orphan check](https://github.com/rust-lang/rust/pull/117164/)
- [Allow instructing rustdoc to read from stdin via `-`](https://github.com/rust-lang/rust/pull/124611/)
- [`std::env::{set_var, remove_var}` can no longer be converted to safe function pointers and no longer implement the `Fn` family of traits](https://github.com/rust-lang/rust/pull/124636)
- [Warn (or error) when `Self` constructor from outer item is referenced in inner nested item](https://github.com/rust-lang/rust/pull/124187/)
- [Turn `indirect_structural_match` and `pointer_structural_match` lints into hard errors](https://github.com/rust-lang/rust/pull/124661/)
- [Make `where_clause_object_safety` lint a regular object safety violation](https://github.com/rust-lang/rust/pull/125380/)
- [Turn `proc_macro_back_compat` lint into a hard error.](https://github.com/rust-lang/rust/pull/125596/)
- [Detect unused structs even when implementing private traits](https://github.com/rust-lang/rust/pull/122382/)
- [`std::sync::ReentrantLockGuard<T>` is no longer `Sync` if `T: !Sync`](https://github.com/rust-lang/rust/pull/125527) which means [`std::io::StdoutLock` and `std::io::StderrLock` are no longer Sync](https://github.com/rust-lang/rust/issues/127340)
Internal Changes
----------------
These changes do not affect any public interfaces of Rust, but they represent
significant improvements to the performance or internals of rustc and related
tools.
- Misc improvements to size of generated html by rustdoc e.g. [#124738](https://github.com/rust-lang/rust/pull/124738/) and [#123734](https://github.com/rust-lang/rust/pull/123734/)
- [MSVC targets no longer depend on libc](https://github.com/rust-lang/rust/pull/124050/)