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1.84.0 release post
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‎posts/2025-01-09-Rust-1.84.0.md

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---
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layout: post
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title: "Announcing Rust 1.84.0"
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author: The Rust Release Team
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release: true
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---
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The Rust team is happy to announce a new version of Rust, 1.84.0. Rust is a programming language empowering everyone to build reliable and efficient software.
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If you have a previous version of Rust installed via `rustup`, you can get 1.84.0 with:
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```console
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$ rustup update stable
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```
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If you don't have it already, you can [get `rustup`](https://www.rust-lang.org/install.html) from the appropriate page on our website, and check out the [detailed release notes for 1.84.0](https://doc.rust-lang.org/stable/releases.html#version-1840-2025-01-09).
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If you'd like to help us out by testing future releases, you might consider updating locally to use the beta channel (`rustup default beta`) or the nightly channel (`rustup default nightly`). Please [report](https://github.com/rust-lang/rust/issues/new/choose) any bugs you might come across!
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## What's in 1.84.0 stable
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### Cargo considers Rust versions for dependency version selection
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1.84.0 stabilizes the minimum supported Rust version (MSRV) aware resolver,
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which prefers dependency versions compatible with the project's declared
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[MSRV](https://doc.rust-lang.org/cargo/reference/rust-version.html).
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With MSRV-aware version selection, the toil is reduced for maintainers to
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support older toolchains by not needing to manually select older versions for
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each dependency.
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You can opt-in to the MSRV-aware resolver via [`.cargo/config.toml`](https://doc.rust-lang.org/cargo/reference/config.html#resolverincompatible-rust-versions):
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```toml
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[resolver]
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incompatible-rust-versions = "fallback"
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```
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Then when adding a dependency:
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```console
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$ cargo add clap
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Updating crates.io index
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warning: ignoring clap@4.5.23 (which requires rustc 1.74) to maintain demo's rust-version of 1.60
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Adding clap v4.0.32 to dependencies
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Updating crates.io index
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Locking 33 packages to latest Rust 1.60 compatible versions
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Adding clap v4.0.32 (available: v4.5.23, requires Rust 1.74)
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```
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When [verifying the latest dependencies in CI](https://doc.rust-lang.org/cargo/guide/continuous-integration.html#verifying-latest-dependencies), you can override this:
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```console
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$ CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS=allow cargo update
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Updating crates.io index
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Locking 12 packages to latest compatible versions
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Updating clap v4.0.32 -> v4.5.23
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```
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You can also opt-in by setting [`package.resolver = "3"`](https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions) in the Cargo.toml manifest file though that will require raising your MSRV to 1.84. The new resolver will be enabled by default for projects using the 2024 edition
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(which will stabilize in 1.85).
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This gives library authors more flexibility when deciding
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their policy on adopting new Rust toolchain features. Previously, a library
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adopting features from a new Rust toolchain would force downstream users of
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that library who have an older Rust version to either upgrade their toolchain
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or manually select an old version of the library compatible with their
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toolchain (and avoid running `cargo update`). Now, those users will be able to
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automatically use older library versions compatible with their older toolchain.
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See the [documentation](https://doc.rust-lang.org/cargo/reference/rust-version.html#setting-and-updating-rust-version) for more considerations when deciding on an MSRV policy.
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### Migration to the new trait solver begins
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The Rust compiler is in the process of moving to a new implementation for the
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trait solver. The next-generation trait solver is a reimplementation of a core
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component of Rust's type system. It is not only responsible for checking
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whether trait-bounds - e.g. `Vec<T>: Clone` - hold, but is also used by many
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other parts of the type system, such as normalization - figuring out the
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underlying type of `<Vec<T> as IntoIterator>::Item` - and equating types
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(checking whether `T` and `U` are the same).
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In 1.84, the new solver is used for checking coherence of trait impls. At a
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high level, coherence is responsible for ensuring that there is at most one
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implementation of a trait for a given type while considering not yet written
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or visible code from other crates.
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This stabilization fixes a few mostly theoretical correctness issues of the
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old implementation, resulting in potential "conflicting implementations of trait ..."
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errors that were not previously reported. We expect the affected patterns to be
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very rare based on evaluation of available code through [Crater]. The stabilization
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also improves our ability to prove that impls do *not* overlap, allowing more code
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to be written in some cases.
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For more details, see a [previous blog post](https://blog.rust-lang.org/inside-rust/2024/12/04/trait-system-refactor-initiative.html)
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and the [stabilization report](https://github.com/rust-lang/rust/pull/130654).
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[Crater]: https://github.com/rust-lang/crater/
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### Strict provenance APIs
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In Rust, [pointers are not simply an "integer" or
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"address"](https://rust-lang.github.io/rfcs/3559-rust-has-provenance.html). For
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instance, a "use after free" is undefined behavior even if you "get lucky" and the freed memory gets
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reallocated before your read/write. As another example, writing
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through a pointer derived from an `&i32` reference is undefined behavior, even
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if writing to the same address via a different pointer is legal. The underlying
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pattern here is that *the way a pointer is computed matters*, not just the
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address that results from this computation. For this reason, we say that
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pointers have **provenance**: to fully characterize pointer-related undefined
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behavior in Rust, we have to know not only the address the pointer points to,
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but also track which other pointer(s) it is "derived from".
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Most of the time, programmers do not need to worry much about provenance, and
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it is very clear how a pointer got derived. However, when casting pointers to
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integers and back, the provenance of the resulting pointer is underspecified.
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With this release, Rust is adding a set of APIs that can in many cases replace
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the use of integer-pointer-casts, and therefore avoid the ambiguities inherent
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to such casts. In particular, the pattern of using the lowest bits of an
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aligned pointer to store extra information can now be implemented without ever
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casting a pointer to an integer or back. This makes the code easier to reason
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about, easier to analyze for the compiler, and also benefits tools like
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[Miri](https://github.com/rust-lang/miri) and architectures like
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[CHERI](https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/) that aim to
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detect and diagnose pointer misuse.
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For more details, see the standard library [documentation on provenance](https://doc.rust-lang.org/std/ptr/index.html#provenance).
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### Stabilized APIs
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- [`Ipv6Addr::is_unique_local`](https://doc.rust-lang.org/stable/core/net/struct.Ipv6Addr.html#method.is_unique_local)
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- [`Ipv6Addr::is_unicast_link_local`](https://doc.rust-lang.org/stable/core/net/struct.Ipv6Addr.html#method.is_unicast_link_local)
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- [`core::ptr::with_exposed_provenance`](https://doc.rust-lang.org/stable/core/ptr/fn.with_exposed_provenance.html)
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- [`core::ptr::with_exposed_provenance_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.with_exposed_provenance_mut.html)
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- [`<ptr>::addr`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.addr)
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- [`<ptr>::expose_provenance`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.expose_provenance)
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- [`<ptr>::with_addr`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.with_addr)
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- [`<ptr>::map_addr`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.map_addr)
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- [`<int>::isqrt`](https://doc.rust-lang.org/stable/core/primitive.i32.html#method.isqrt)
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- [`<int>::checked_isqrt`](https://doc.rust-lang.org/stable/core/primitive.i32.html#method.checked_isqrt)
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- [`<uint>::isqrt`](https://doc.rust-lang.org/stable/core/primitive.u32.html#method.isqrt)
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- [`NonZero::isqrt`](https://doc.rust-lang.org/stable/core/num/struct.NonZero.html#impl-NonZero%3Cu128%3E/method.isqrt)
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- [`core::ptr::without_provenance`](https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance.html)
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- [`core::ptr::without_provenance_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance_mut.html)
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- [`core::ptr::dangling`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling.html)
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- [`core::ptr::dangling_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling_mut.html)
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- [`Pin::as_deref_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.as_deref_mut)
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These APIs are now stable in const contexts
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- [`AtomicBool::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicBool.html#method.from_ptr)
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- [`AtomicPtr::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicPtr.html#method.from_ptr)
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- [`AtomicU8::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicU8.html#method.from_ptr)
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- [`AtomicU16::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicU16.html#method.from_ptr)
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- [`AtomicU32::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicU32.html#method.from_ptr)
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- [`AtomicU64::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicU64.html#method.from_ptr)
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- [`AtomicUsize::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicUsize.html#method.from_ptr)
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- [`AtomicI8::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicI8.html#method.from_ptr)
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- [`AtomicI16::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicI16.html#method.from_ptr)
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- [`AtomicI32::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicI32.html#method.from_ptr)
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- [`AtomicI64::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicI64.html#method.from_ptr)
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- [`AtomicIsize::from_ptr`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.AtomicIsize.html#method.from_ptr)
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- [`<ptr>::is_null`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.is_null-1)
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- [`<ptr>::as_ref`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.as_ref-1)
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- [`<ptr>::as_mut`](https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.as_mut)
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- [`Pin::new`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.new)
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- [`Pin::new_unchecked`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.new_unchecked)
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- [`Pin::get_ref`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.get_ref)
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- [`Pin::into_ref`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.into_ref)
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- [`Pin::get_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.get_mut)
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- [`Pin::get_unchecked_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.get_unchecked_mut)
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- [`Pin::static_ref`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.static_ref)
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- [`Pin::static_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.static_mut)
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### Other changes
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Check out everything that changed in [Rust](https://github.com/rust-lang/rust/releases/tag/1.84.0), [Cargo](https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-184-2025-01-09), and [Clippy](https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-184).
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## Contributors to 1.84.0
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Many people came together to create Rust 1.84.0. We couldn't have done it without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.84.0/)

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