perf: store the fulfillment engine inline in ObligationCtxt - #47
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Every ObligationCtxt heap-allocated its fulfillment engine as a Box<dyn TraitEngine>, making it the single largest allocation site in the compiler (161k allocations on a syn check build, created per candidate probe in method resolution among others). The solver choice is a per-session constant and both engine types are small, so store them inline in a two-variant enum with static dispatch. The enum's TraitEngine impl needs both FromSolverError bounds, which ripples to the generic impl blocks and two generic users; the concrete error types used everywhere implement both. The boxed engine remains for the per-body typeck root fulfillment context.
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Every
ObligationCtxtallocated its fulfillment engine on the heap as aBox<dyn TraitEngine>. This was the single largest allocation site in the compiler: 161k allocations on asyncheck build (measured with DHAT).ObligationCtxts are created in hot paths, for example once per candidate probe during method resolution.The allocation is easy to avoid. Which solver is used never changes during a compilation session, and both engine types are small (the obligation forest allocates its own storage separately). So this PR stores the engine directly inside
ObligationCtxt, in a two-variant enum. Calls now go through a match on that enum instead of virtual dispatch.The enum's
TraitEngineimpl needs bothFromSolverErrorbounds, so a few generic impl blocks and two generic users now need both bounds as well. The concrete error types used in practice already implement both, so nothing else changes for callers. The typeck root fulfillment context keeps the boxed engine; it is created once per function body, so the allocation does not matter there.