Perf: Clear the param env for more global trait goals - #161126
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The branch in `evaluate_trait_predicate_recursively` that clears the param env, so a global goal can be cached globally, rarely fires: - it asks `obligation.is_global()`, but an `Obligation` visits its own `param_env`, so that is false whenever the env holds anything local -- including the region inference variables the branch then discards. It could only fire when the env was already empty. - it requires every bound to mention a generic parameter. An outlives bound is never a selection candidate, so it cannot change the answer either. Code using `#[async_trait]` runs into this: every method carries `where 'life0: 'async_trait, ..`, whose bounds hold region inference variables, so each method re-proves the same `Send`/`Sync` goals.
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evaluate_trait_predicate_recursivelyclears the param env when its bounds cannotaffect a global goal, so the result can go in the global cache. It almost never
fires today, for two reasons:
It asks
obligation.is_global(). AnObligationvisits its ownparam_env, sothis is false whenever the env holds anything local, including the region
inference variables the branch is about to discard. In practice it could only
fire when the env was already empty.
It requires every bound to mention a generic parameter. An outlives bound is
never a selection candidate, so it cannot change the answer either.
Code using
#[async_trait]runs into this. Every method there carrieswhere 'life0: 'async_trait, .., and those bounds hold region inferencevariables, so each method re-proves the same
Send/Syncgoals from scratch.On a generated test file with an
async_trait-shaped trait over a large concretetype graph,
evaluate_obligationself time drops:I found this while looking at compile times in DataFusion, where a few crates
spend most of their time proving
Sendfor futures returned by#[async_trait]methods.
AI Disclosure
I used LLM (Claude) to run some experiments and change the code