Rust compiler completes 4-year trait solver overhaul

The Rust compiler team has enabled the next-generation trait solver by default on nightly builds after nearly four years of development. This rewrite completely replaces the compiler's handling of where-clauses, associated type normalization, and impl Trait behavior. Developers testing the new solver should watch for breaking changes, most of which are intentional improvements to type inference or removal of undesired behavior. The change already resolves over 200 known GitHub issues and impacts compile times, though detailed performance metrics remain forthcoming.