make the provisional cache slightly less broken #114694
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rustbot added S-waiting-on-review 
| Some changes occurred to the core trait solver cc @rust-lang/initiative-trait-system-refactor |
lcnr commented Aug 10, 2023
| | ||
| fn main() { | ||
| impls_a::<()>(); | ||
| //~^ ERROR overflow evaluating the requirement `(): A` |
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This is the issue I am talking about in the PR description
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lcnr force-pushed the provisional-cache branch 3 times, most recently from e4af0a0 to b5346a0 Compare lcnr force-pushed the provisional-cache branch from b5346a0 to 02529d2 Compare 
compiler-errors approved these changes Aug 10, 2023
| @bors r+ rollup |
bors added S-waiting-on-bors 
bors added a commit to rust-lang-ci/rust that referenced this pull request Aug 11, 2023
…mpiler-errors Rollup of 7 pull requests Successful merges: - rust-lang#114599 (Add impl trait declarations to SMIR) - rust-lang#114622 (rustc: Move `crate_types` and `stable_crate_id` from `Session` to `GlobalCtxt`) - rust-lang#114662 (Unlock trailing where-clauses for lazy type aliases) - rust-lang#114693 (Remove myself from the review rotation) - rust-lang#114694 (make the provisional cache slightly less broken) - rust-lang#114705 (Add spastorino to mailmap) - rust-lang#114712 (Fix a couple of bad comments) r? `@ghost` `@rustbot` modify labels: rollup
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It is still broken for the following cycles:
the
R -> A -> B -> Rcycle should be considered to not hold, as it is mixed, but because we first putBinto the cache from theR -> B -> Rcycle which is coinductive, it does hold.This issue will also affect our new coinduction approach. Longterm cycles are coinductive as long as one step goes through an impl where-clause, see https://github.com/rust-lang/a-mir-formality/blob/f4fc5bae36ab1a9fefddd54e5ccffc5f671467ec/crates/formality-prove/src/prove/prove_wc.rs#L51-L62. Here we would first have a fully inductive cycle
R -> B -> Rwhich is then entered by a cycle with a coinductive stepR -> A -coinductive-> B -> R.I don't know how to soundly implement a provisional cache for goals not on the stack without tracking all cycles the goal was involved in and whether they were inductive or not. We could then only use goals from the cache if the inductivity? of every cycle remained the same. This is a mess to implement. I therefore want to rip out the provisional cache entirely, but will wait with this until I talked about it with @nikomatsakis.
r? @compiler-errors