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Auto merge of #110100 - compiler-errors:no-infer-pred-must-hold, r=jackh726

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do not allow inference in `predicate_must_hold` (alternative approach)

See the FCP description for more info, but tl;dr is that we should not return `EvaluatedToOkModuloRegions` if an obligation may hold only with some choice of inference vars being constrained.

Attempts to solve this in the approach laid out by lcnr here: https://github.com/rust-lang/rust/pull/109558#discussion_r1147318134, rather than by eagerly replacing infer vars with placeholders which is a bit too restrictive.

r? `@ghost`
This commit is contained in:
bors 2023-05-19 03:36:37 +00:00
commit 19ca5692f6
4 changed files with 46 additions and 24 deletions
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14
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -537,14 +537,22 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
obligation: &PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
self.evaluation_probe(|this| {
if this.tcx().trait_solver_next() {
this.evaluate_predicates_recursively_in_new_solver([obligation.clone()])
let goal =
this.infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
let mut result = if this.tcx().trait_solver_next() {
this.evaluate_predicates_recursively_in_new_solver([obligation.clone()])?
} else {
this.evaluate_predicate_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
obligation.clone(),
)
)?
};
// If the predicate has done any inference, then downgrade the
// result to ambiguous.
if this.infcx.shallow_resolve(goal) != goal {
result = result.max(EvaluatedToAmbig);
}
Ok(result)
})
}