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Niko Matsakis 2017-09-07 12:38:33 -04:00
parent ffd21b184c
commit c1dddcec06
1 changed files with 18 additions and 7 deletions
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25
src/librustc/traits/project.rs
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@ -624,13 +624,24 @@ fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx,
/// that `T: Trait` (we may also include some other obligations). This
/// may or may not be necessary -- in principle, all the obligations
/// that must be proven to show that `T: Trait` were also returned
/// when the cache was first populated. But there is a vague concern
/// that perhaps someone would not have proven those, but also not
/// have used a snapshot, in which case the cache could remain
/// populated even though `T: Trait` has not been shown. Returning
/// this "paranoid" obligation ensures that, no matter what has come
/// before, if you prove the subobligations, we at least know that `T:
/// Trait` is implemented.
/// when the cache was first populated. But there are some vague concerns,
/// and so we take the precatuionary measure of including `T: Trait` in
/// the result:
///
/// Concern #1. The current setup is fragile. Perhaps someone could
/// have failed to prove the concerns from when the cache was
/// populated, but also not have used a snapshot, in which case the
/// cache could remain populated even though `T: Trait` has not been
/// shown. In this case, the "other code" is at fault -- when you
/// project something, you are supposed to either have a snapshot or
/// else prove all the resulting obligations -- but it's still easy to
/// get wrong.
///
/// Concern #2. Even within the snapshot, if those original
/// obligations are not yet proven, then we are able to do projections
/// that may yet turn out to be wrong. This *may* lead to some sort
/// of trouble, though we don't have a concrete example of how that
/// can occur yet. But it seems risky at best.
fn push_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
param_env: ty::ParamEnv<'tcx>,
projection_ty: ty::ProjectionTy<'tcx>,