[$] Rust's next-generation trait solver

Rust's compiler team has been working on a long-term project torewrite the trait solver - the part of the compiler that determines whichconcrete function should be called when a programmer uses a trait method that isimplemented for multiple types. The rewrite is intended to simplifyfuture changes to the trait system, fix a handful of tricky soundness bugs, andprovide faster compile times. It's also nearly finished, with a relativelysmall number of remaining blocking bugs.