The Rust Release Team has officially introduced version 1.86 of the popular Rust programming language, bringing several cutting-edge features and enhancements aimed at empowering developers to build safe and efficient software.
One of the most anticipated additions is the ability to upcast trait objects. In practical terms, if a trait depends on a supertrait, references to the “child” trait can now be converted to references to the “parent” trait. In previous versions, developers had to rely on a manually implemented method to accomplish similar behavior.
Another notable improvement is the new get_disjoint_mut method, made available for slices and HashMap. This method safely retrieves multiple mutable references at once—something that could not be done reliably through repeated calls to get_mut in the past.
With get_disjoint_mut, it is now possible to mutate distinct elements in tandem, all while satisfying Rust’s strict borrow checker rules. This addition offers a more direct and expressive way to handle scenarios involving partial updates to a collection.
Rust 1.86 also stabilizes #[target_feature] usage for safe functions (previously limited to unsafe functions only). This attribute is useful for optimizing code paths for specific CPU features, such as AVX2 instructions.
Moreover, in debug mode, Rust will now emit a runtime assertion to confirm that pointers are not null before any non-zero-sized read or write occurs. These checks apply even when references are reborrowed. Importantly, these extra checks are part of debug assertions, so they do not replace or diminish the existing compile-time guarantees.
The new version also upgrades the missing_abi lint to a “warn by default” status. In other words, developers who omit specifying "C" or another ABI explicitly in extern blocks will now receive a warning.
Meanwhile, the i586-pc-windows-msvc target will be removed in Rust 1.87. Since SSE2 instructions are mandatory on Windows 10 (the minimum supported OS for the mainstream Windows targets), users are encouraged to migrate to i686-pc-windows-msvc before the deprecation officially takes effect.
Lastly, on the library side, Rust 1.86 stabilizes a variety of APIs, including Float::next_down, Float::next_up, and the newly introduced slice::get_disjoint_mut variants. In addition, several functions—like hint::black_box and various string-splitting methods—are now available within const contexts, expanding their usability in compile-time scenarios.
Refer to the announcement for more information and a deep dive into all the novelties that Rust 1.86 brings.
