Rust 1.81: what's new, what was stabilized

• Usage of the wasm32-wasi target will now issue a compiler warning and request users switch to the wasm32-wasip1 target instead. Both targets are the same, wasm32-wasi is only being renamed, and this change to the WASI target is being done to enable removing wasm32-wasi in January 2025.
• We have renamed std::panic::PanicInfo to std::panic::PanicHookInfo. The old name will continue to work as an alias, but will result in a deprecation warning starting in Rust 1.82.0.
• The new sort implementations may panic if a type's implementation of Ord (or the given comparison function) does not implement a total order as the trait requires. Ord's supertraits (PartialOrd, Eq, and PartialEq) must also be consistent. The previous implementations would not "notice" any problem, but the new implementations have a good chance of detecting inconsistencies, throwing a panic rather than returning knowingly unsorted data.
• [In very rare cases, a change in the internal evaluation order of the trait

Language

• Abort on uncaught panics in extern "C" functions.
• Fix ambiguous cases of multiple & in elided self lifetimes.
• [Stabilize #[expect] for lints (RFC 2383),](https://github.com/rust-lang/rust/pull/120924/) like #[allow] with a warning if the lint is _not_ fulfilled.
• Change method resolution to constrain hidden types instead of rejecting method candidates.
• Bump elided_lifetimes_in_associated_constant to deny.
• offset_from: always allow pointers to point to the same address.
• Allow constraining opaque types during subtyping in the trait system.
• Allow constraining opaque types during various unsizing casts.
• Deny keyword lifetimes pre-expansion.

<a id="1.81.0-Compiler"></a>

Compiler

• Make casts of pointers to trait objects stricter.
• Check alias args for well-formedness even if they have escaping bound vars.
• Deprecate no-op codegen option -Cinline-threshold=....
• Re-implement a type-size based limit.
• Properly account for alignment in transmute size checks.
• Remove the box_pointers lint.
• Ensure the interpreter checks bool/char for validity when they are used in a cast.
• Improve coverage instrumentation for functions containing nested items.
• Target changes:
• Add Tier 3 no_std Xtensa targets: xtensa-esp32-none-elf, xtensa-esp32s2-none-elf, xtensa-esp32s3-none-elf
• Add Tier 3 std Xtensa targets: xtensa-esp32-espidf, xtensa-esp32s2-espidf, xtensa-esp32s3-espidf
• Add Tier 3 i686 Redox OS target: i686-unknown-redox
• Promote arm64ec-pc-windows-msvc to Tier 2.
• Promote loongarch64-unknown-linux-musl to Tier 2 with host tools.
• Enable full tools and profiler for LoongArch Linux targets.
• Unconditionally warn on usage of wasm32-wasi. (see compatibility note below)
• Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support.

<a id="1.81.0-Libraries"></a>

Libraries

• Split core's PanicInfo and std's PanicInfo. (see compatibility note below)
• Generalize {Rc,Arc}::make_mut() to unsized types.
• Replace sort implementations with stable driftsort and unstable ipnsort. All slice::sort* and slice::select_nth* methods are expected to see significant performance improvements. See the research project for more details.
• Document behavior of create_dir_all with respect to empty paths.
• Fix interleaved output in the default panic hook when multiple threads panic simultaneously.
• Fix Command's batch files argument escaping not working when file name has trailing whitespace or periods (CVE-2024-43402).

<a id="1.81.0-Stabilized-APIs"></a>

Stabilized APIs

• core::error
• hint::assert_unchecked
• fs::exists
• AtomicBool::fetch_not
• Duration::abs_diff
• IoSlice::advance
• IoSlice::advance_slices
• IoSliceMut::advance
• IoSliceMut::advance_slices
• PanicHookInfo
• PanicInfo::message
• PanicMessage

These APIs are now stable in const contexts:

• char::from_u32_unchecked (function)
• char::from_u32_unchecked (method)
• CStr::count_bytes
• CStr::from_ptr

<a id="1.81.0-Cargo"></a>

Cargo

• Generated .cargo_vcs_info.json is always included, even when --allow-dirty is passed.
• Disallow package.license-file and package.readme pointing to non-existent files during packaging.
• Disallow passing --release/--debug flag along with the --profile flag.
• Remove lib.plugin key support in Cargo.toml. Rust plugin support has been deprecated for four years and was removed in 1.75.0.

<a id="1.81.0-Compatibility-Notes"></a>

Compatibility Notes

• Usage of the wasm32-wasi target will now issue a compiler warning and request users switch to the wasm32-wasip1 target instead. Both targets are the same, wasm32-wasi is only being renamed, and this change to the WASI target is being done to enable removing wasm32-wasi in January 2025.

• We have renamed std::panic::PanicInfo to std::panic::PanicHookInfo. The old name will continue to work as an alias, but will result in a deprecation warning starting in Rust 1.82.0.

core::panic::PanicInfo will remain unchanged, however, as this is now a *different type*.

The reason is that these types have different roles: std::panic::PanicHookInfo is the argument to the panic hook in std context (where panics can have an arbitrary payload), while core::panic::PanicInfo is the argument to the [#[panic_handler]](https://doc.rust-lang.org/nomicon/panic-handler.html) in no_std context (where panics always carry a formatted *message*). Separating these types allows us to add more useful methods to these types, such as std::panic::PanicHookInfo::payload_as_str() and core::panic::PanicInfo::message().

• The new sort implementations may panic if a type's implementation of Ord (or the given comparison function) does not implement a total order as the trait requires. Ord's supertraits (PartialOrd, Eq, and PartialEq) must also be consistent. The previous implementations would not "notice" any problem, but the new implementations have a good chance of detecting inconsistencies, throwing a panic rather than returning knowingly unsorted data.
• In very rare cases, a change in the internal evaluation order of the trait solver may result in new fatal overflow errors.

<a id="1.81.0-Internal-Changes"></a>

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.

• Add a Rust-for-Linux auto CI job to check kernel builds.