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Rollup merge of #98713 - nikomatsakis:issue-98693, r=jackh726

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promote placeholder bounds to 'static obligations

In NLL, when we are promoting a bound out from a closure, if we have a requirement that `T: 'a` where `'a` is in a higher universe, we were previously ignoring that, which is totally wrong. We should be promoting those constraints to `'static`, since universes are not expressible across closure boundaries.

Fixes #98693

~~(Marking as WIP because I'm still running tests, haven't add the new test, etc)~~

r? ``@jackh726``
This commit is contained in:
Matthias Krüger 2022-07-11 00:33:47 +02:00 • committed by GitHub
commit 2cb7d1c933
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5 changed files with 85 additions and 6 deletions
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38
compiler/rustc_borrowck/src/region_infer/mod.rs
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@ -917,6 +917,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// The idea then is to lower the `T: 'X` constraint into multiple
/// bounds -- e.g., if `'X` is the union of two free lifetimes,
/// `'1` and `'2`, then we would create `T: '1` and `T: '2`.
#[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))]
fn try_promote_type_test(
&self,
infcx: &InferCtxt<'_, 'tcx>,
@ -934,11 +935,41 @@ impl<'tcx> RegionInferenceContext<'tcx> {
return false;
};
debug!("subject = {:?}", subject);
let r_scc = self.constraint_sccs.scc(*lower_bound);
debug!(
"lower_bound = {:?} r_scc={:?} universe={:?}",
lower_bound, r_scc, self.scc_universes[r_scc]
);
// If the type test requires that `T: 'a` where `'a` is a
// placeholder from another universe, that effectively requires
// `T: 'static`, so we have to propagate that requirement.
//
// It doesn't matter *what* universe because the promoted `T` will
// always be in the root universe.
if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() {
debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p);
let static_r = self.universal_regions.fr_static;
propagated_outlives_requirements.push(ClosureOutlivesRequirement {
subject,
outlived_free_region: static_r,
blame_span: locations.span(body),
category: ConstraintCategory::Boring,
});
// we can return here -- the code below might push add'l constraints
// but they would all be weaker than this one.
return true;
}
// For each region outlived by lower_bound find a non-local,
// universal region (it may be the same region) and add it to
// `ClosureOutlivesRequirement`.
let r_scc = self.constraint_sccs.scc(*lower_bound);
for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
debug!("universal_region_outlived_by ur={:?}", ur);
// Check whether we can already prove that the "subject" outlives `ur`.
// If so, we don't have to propagate this requirement to our caller.
//
@ -963,8 +994,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
continue;
}
debug!("try_promote_type_test: ur={:?}", ur);
let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub);
@ -1001,6 +1030,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// will use it's *external name*, which will be a `RegionKind`
/// variant that can be used in query responses such as
/// `ReEarlyBound`.
#[instrument(level = "debug", skip(self, infcx))]
fn try_promote_type_test_subject(
&self,
infcx: &InferCtxt<'_, 'tcx>,
@ -1008,8 +1038,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
) -> Option<ClosureOutlivesSubject<'tcx>> {
let tcx = infcx.tcx;
debug!("try_promote_type_test_subject(ty = {:?})", ty);
let ty = tcx.fold_regions(ty, |r, _depth| {
let region_vid = self.to_region_vid(r);