Rust enables the next-generation trait solver by default on nightly and calls it the largest compiler change since the initial release

Sources: Rust blog post, HN discussion
Summary: The change reaches the nightly channel on 2026-08-22 and replaces how where-clauses are proved and associated types are normalized, so crates that depended on the old inference behavior break on nightly before the work reaches stable. The post names bevy and minijinja as existing code affected by the change to associated types under higher-ranked binders, and gives an escape hatch, -Znext-solver=coherence set through RUSTFLAGS or .cargo/config.toml. The Rust team states it knows of more than 200 GitHub issues fixed by the change, tracks remaining breakage in a pinned issue, reports that a comparison across the top 20,000 crates on crates.io found nearly all of them effectively unchanged in compile time with datafusion compiling more than 8x faster, and states that error messages for the new solver have not had much work yet.
Why it matters: Removing the old implementation is what unblocks Type Alias Impl Trait and Return Type Notation, and the nightly default puts the breakage in front of nightly users before it reaches stable.
Follow-up: Track stabilization of the next-generation trait solver past nightly.