Published on Jun 07, 2025 6 min read

Rust 1.86 Enhances Trait Objects, Borrow Checker, and More

Rust, the systems programming language known for its emphasis on memory safety and zero-cost abstractions, continues to evolve with the release of version 1.86. This update introduces several powerful improvements to the language, the compiler, and the standard library, all aimed at making Rust development more ergonomic and reliable.

Among the most impactful changes in Rust 1.86 is the addition of trait object upcasting—a long-requested feature that allows smoother conversions between trait references. Developers can now treat trait objects with greater flexibility, particularly when working with layered or complex trait hierarchies. Also included in this release are borrow checker refinements that further enforce Rust's guarantees around reference safety, helping avoid subtle runtime errors during mutable access.

This version also stabilizes target-specific features for safe functions and introduces several compiler diagnostics, deprecations, and standard library additions. Let’s explore the highlights of Rust 1.86 and how they improve the developer experience across the board.

Trait Object Upcasting Simplifies Polymorphism

One of the headline features of Rust 1.86 is support for trait object upcasting. This feature makes it possible to coerce a reference to a trait object into a reference to one of its supertraits. Previously, developers had to rely on verbose or indirect methods to achieve this behavior, which often involved manual implementation of boilerplate code.

With upcasting now built into the language, developers gain a more intuitive and concise way to work with object-safe trait hierarchies. For example, if a trait Child extends a trait Parent, it's now possible to treat a reference to dyn Child as a dyn Parent without needing intermediate layers of indirection. This not only improves readability but also aligns Rust’s trait system more closely with expectations from object-oriented programming models.

This feature is especially beneficial in projects with deeply nested trait relationships or in libraries that expose generic behavior across varying levels of abstraction. It unlocks more flexible architecture without compromising Rust’s strict safety guarantees.

Better Borrow Checker Diagnostics and New get_disjoint_mut

Rust’s borrow checker, already famous for preventing data races and undefined behavior, receives further refinement in this release. One of the key improvements targets the handling of multiple mutable references obtained through repeated calls to methods like get_mut. These cases could previously lead to undefined behavior or compiler panics if misused.

To mitigate this, Rust 1.86 introduces a new helper: get_disjoint_mut. This function allows developers to safely extract multiple mutable references from structures like slices or hash maps—provided the accesses are to non-overlapping elements. It enforces disjointness at compile time, reducing the risk of aliasing errors during development.

This change not only enhances safety but also simplifies common patterns in data manipulation, such as simultaneously updating multiple entries in a collection. Developers can now rely on clearer, more reliable logic backed by compiler guarantees, making Rust’s already robust ownership model even more trustworthy.

Stabilization of #[target_feature] for Safe Functions

Another milestone in Rust 1.86 is the stabilization of the #[target_feature] attribute for safe functions. This attribute, previously reserved for unsafe contexts, allows developers to enable specific CPU instruction sets—such as SIMD extensions—on individual functions.

With this stabilization, safe functions can now be annotated with #[target_feature] as long as they comply with the platform's safety constraints. This opens the door for performance optimizations on a per-function basis without sacrificing code safety or requiring deep unsafe blocks.

This change helps balance Rust's dual goals: enabling high-performance low-level programming while maintaining strong compile-time correctness checks. It's particularly beneficial in scenarios involving numerical computations, graphics processing, or any other domain that relies on hardware-specific enhancements.

New Compiler Warnings and Debug Assertions

The compiler in Rust 1.86 has also been updated to include several valuable diagnostics aimed at catching errors early in the development process. Notably, the compiler now emits warnings when an extern block or function omits an explicit ABI (Application Binary Interface). This clarification helps developers avoid silent inconsistencies, especially when interfacing with C code or foreign function interfaces (FFIs).

Additionally, Rust 1.86 introduces debug assertions for non-null pointers during operations involving reads, writes, or reborrowing. These checks help catch issues earlier in development and prevent elusive bugs from surfacing in release builds. They are particularly useful when dealing with low-level pointer manipulation or when working near unsafe code boundaries.

These additions reaffirm Rust’s commitment to providing meaningful feedback and guidance through its compiler, reducing the need for excessive debugging or third-party static analysis tools.

Deprecation Notice: i586-pc-windows-msvc Target

As part of its ongoing maintenance and modernization strategy, Rust 1.86 deprecates the i586-pc-windows-msvc compilation target. This 32-bit Windows target, which uses the Microsoft Visual C++ toolchain, will be removed entirely in the next stable release of Rust.

The decision reflects a broader industry trend toward 64-bit architecture as the default standard for modern development. Deprecating underused or outdated targets helps maintain the compiler’s performance and focus its resources on actively supported platforms.

Developers still relying on the deprecated target are advised to migrate to i686 or x86_64 alternatives to ensure compatibility with future Rust versions. The Rust team has provided advance notice to ensure a smooth transition and minimize disruption.

Stabilized APIs and Library Additions

As with most Rust releases, version 1.86 includes several newly stabilized APIs and improvements across the standard library. These additions expand the toolkit available to developers and often reflect real-world usage patterns that have emerged through nightly testing and community feedback.

While the full list of stabilized APIs is extensive, some highlights include enhancements to collection manipulation, iteration utilities, and string handling. These refinements help reduce the need for external crates or custom implementations, making standard library usage more efficient and expressive.

By continuously evolving its core libraries, Rust makes it easier for developers to write concise, maintainable, and high-performance code with fewer dependencies.

Conclusion

Rust 1.86 is a feature-rich release that brings meaningful enhancements to the language and tooling without compromising its core principles of safety and performance. The introduction of trait object upcasting offers greater flexibility in designing trait hierarchies, while improved borrow checker logic and the get_disjoint_mut helper further solidify Rust’s reputation for uncompromising memory safety.

Stabilizing #[target_feature] for safe functions also widens the scope for optimization, letting developers harness hardware-specific instructions with confidence. On the compiler side, new diagnostics and deprecations help ensure best practices and forward compatibility.

Whether you’re building performance-critical applications, working with complex abstractions, or simply maintaining legacy systems, Rust 1.86 brings updates that will likely streamline your workflow and catch errors before they become costly. It’s another confident step forward for one of the most rigorously engineered languages in modern software development.