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A bit of cleanup

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This commit is contained in:
Aaron Hill 2018-08-02 14:34:25 -04:00
parent a0943b6bba
commit 016b586685
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GPG Key ID: B4087E510E98B164
1 changed files with 23 additions and 4 deletions
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27
src/librustc/traits/auto_trait.rs
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@ -326,9 +326,6 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
let mut user_computed_preds: FxHashSet<_> =
user_env.caller_bounds.iter().cloned().collect();
let mut new_env = param_env.clone();
let dummy_cause = ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID);
@ -362,7 +359,6 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
if self.is_of_param(pred.skip_binder().trait_ref.substs) {
already_visited.remove(&pred);
self.add_user_pred(&mut user_computed_preds, ty::Predicate::Trait(pred.clone()));
//user_computed_preds.insert(ty::Predicate::Trait(pred.clone()));
predicates.push_back(pred);
} else {
debug!(
@ -397,6 +393,29 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
return Some((new_env, final_user_env));
}
// This method is designed to work around the following issue:
// When we compute auto trait bounds, we repeatedly call SelectionContext.select,
// progressively building a ParamEnv based on the results we get.
// However, our usage of SelectionContext differs from its normal use within the compiler,
// in that we capture and re-reprocess predicates from Unimplemented errors.
//
// This can lead to a corner case when dealing with region parameters.
// During our selection loop in evaluate_predicates, we might end up with
// two trait predicates that differ only in their region parameters:
// one containing a HRTB lifetime parameter, and one containing a 'normal'
// lifetime parameter. For example:
//
// T as MyTrait<'a>
// T as MyTrait<'static>
//
// If we put both of these predicates in our computed ParamEnv, we'll
// confuse SelectionContext, since it will (correctly) view both as being applicable.
//
// To solve this, we pick the 'more strict' lifetime bound - i.e. the HRTB
// Our end goal is to generate a user-visible description of the conditions
// under which a type implements an auto trait. A trait predicate involving
// a HRTB means that the type needs to work with any choice of lifetime,
// not just one specific lifetime (e.g. 'static).
fn add_user_pred<'c>(&self, user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>, new_pred: ty::Predicate<'c>) {
let mut should_add_new = true;
user_computed_preds.retain(|&old_pred| {