Rust 1.86, the latest update to the fast and safe programming language, has been announced, bringing with it a highly anticipated feature: trait upcasting. This capability, which allows developers to coerce a reference to a trait object to a reference to a trait object of the supertrait, has been stabilized in Rust 1.86. The update, announced on April 3, can be accessed via the rustup tool by running the command rustup update stable.

For developers, trait upcasting is especially useful when working with the Any trait, as it enables upcasting of trait objects to dyn Any to call downcast methods of Any without adding any trait methods or using external crates. This feature has been long-awaited, and its stabilization in Rust 1.86 is a significant milestone for the language.

In addition to trait upcasting, Rust 1.86 brings several other notable enhancements. HashMap and slices now support indexing multiple elements mutably, thanks to the introduction of a get_disjoint_mut helper. This feature allows developers to safely retrieve mutable references to multiple elements simultaneously, with the borrow checker preventing simultaneous usage of references obtained from repeated calls to get_mut methods.

The compiler in Rust 1.86 has also been improved, with the insertion of debug assertions that a pointer is not null upon non-zero-sized reads and writes, and also when the pointer is reborrowed into a reference. Furthermore, the target_feature_11 feature has been stabilized, allowing safe functions to be marked with the #[target_feature] attribute.

Other changes in Rust 1.86 include the deprecation of omitting the ABI in extern blocks and functions, which will now result in a warning via the missing_abi lint. Additionally, the tier-2 target i586-pc-windows-msvc will be removed in the next version of Rust, Rust 1.87.0.

Rust 1.86 also stabilizes several APIs, including {float}::next_down, {float}::next_up, <[_]>::get_disjoint_mut, and others. Furthermore, certain APIs, such as hint::black_box and io::Cursor::get_mut, are now stable in const contexts.

This latest update follows the release of Rust 1.85 in February, which introduced async closures. A point release, Rust 1.85.1, was subsequently released on March 18, addressing issues such as combined doctest compilation, which did not work as intended in the stable Rust 2024 edition.

Overall, Rust 1.86 represents a significant step forward for the language, with its stabilization of trait upcasting and other enhancements set to improve the development experience for Rust programmers.