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Avoid propagating redundant outlives constraints from closures
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@matthewjasper
matthewjasper committed Feb 13, 2019
1 parent e544947 commit 168c14a
Showing 1 changed file with 93 additions and 49 deletions.
142 changes: 93 additions & 49 deletions src/librustc_mir/borrow_check/nll/region_infer/mod.rs
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Expand Up @@ -15,7 +15,7 @@ use rustc::mir::{
ConstraintCategory, Local, Location, Mir,
};
use rustc::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable};
use rustc::util::common;
use rustc::util::common::{self, ErrorReported};
use rustc_data_structures::bit_set::BitSet;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::graph::scc::Sccs;
Expand Down Expand Up @@ -1157,63 +1157,107 @@ impl<'tcx> RegionInferenceContext<'tcx> {
.is_none()
);

// Only check all of the relations for the main representative of each
// SCC, otherwise just check that we outlive said representative. This
// reduces the number of redundant relations propagated out of
// closures.
// Note that the representative will be a universal region if there is
// one in this SCC, so we will always check the representative here.
let representative = self.scc_representatives[longer_fr_scc];
if representative != longer_fr {
self.check_universal_region_relation(
longer_fr,
representative,
infcx,
mir,
mir_def_id,
propagated_outlives_requirements,
errors_buffer,
);
return;
}

// Find every region `o` such that `fr: o`
// (because `fr` includes `end(o)`).
for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) {
// If it is known that `fr: o`, carry on.
if self.universal_region_relations
.outlives(longer_fr, shorter_fr)
{
continue;
if let Some(ErrorReported) = self.check_universal_region_relation(
longer_fr,
shorter_fr,
infcx,
mir,
mir_def_id,
propagated_outlives_requirements,
errors_buffer,
) {
// continuing to iterate just reports more errors than necessary
return;
}
}
}

debug!(
"check_universal_region: fr={:?} does not outlive shorter_fr={:?}",
longer_fr, shorter_fr,
);
fn check_universal_region_relation(
&self,
longer_fr: RegionVid,
shorter_fr: RegionVid,
infcx: &InferCtxt<'_, 'gcx, 'tcx>,
mir: &Mir<'tcx>,
mir_def_id: DefId,
propagated_outlives_requirements: &mut Option<&mut Vec<ClosureOutlivesRequirement<'gcx>>>,
errors_buffer: &mut Vec<Diagnostic>,
) -> Option<ErrorReported> {
// If it is known that `fr: o`, carry on.
if self.universal_region_relations
.outlives(longer_fr, shorter_fr)
{
return None;
}

let blame_span_category = self.find_outlives_blame_span(mir, longer_fr, shorter_fr);

if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
// Shrink `fr` until we find a non-local region (if we do).
// We'll call that `fr-` -- it's ever so slightly smaller than `fr`.
if let Some(fr_minus) = self.universal_region_relations
.non_local_lower_bound(longer_fr)
{
debug!("check_universal_region: fr_minus={:?}", fr_minus);

// Grow `shorter_fr` until we find a non-local
// region. (We always will.) We'll call that
// `shorter_fr+` -- it's ever so slightly larger than
// `fr`.
let shorter_fr_plus = self.universal_region_relations
.non_local_upper_bound(shorter_fr);
debug!(
"check_universal_region: shorter_fr_plus={:?}",
shorter_fr_plus
);
debug!(
"check_universal_region_relation: fr={:?} does not outlive shorter_fr={:?}",
longer_fr, shorter_fr,
);

// Push the constraint `fr-: shorter_fr+`
propagated_outlives_requirements.push(ClosureOutlivesRequirement {
subject: ClosureOutlivesSubject::Region(fr_minus),
outlived_free_region: shorter_fr_plus,
blame_span: blame_span_category.1,
category: blame_span_category.0,
});
continue;
}
}

// If we are not in a context where we can propagate
// errors, or we could not shrink `fr` to something
// smaller, then just report an error.
//
// Note: in this case, we use the unapproximated regions
// to report the error. This gives better error messages
// in some cases.
self.report_error(mir, infcx, mir_def_id, longer_fr, shorter_fr, errors_buffer);
return; // continuing to iterate just reports more errors than necessary
if let Some(propagated_outlives_requirements) = propagated_outlives_requirements {
// Shrink `fr` until we find a non-local region (if we do).
// We'll call that `fr-` -- it's ever so slightly smaller than `fr`.
if let Some(fr_minus) = self.universal_region_relations
.non_local_lower_bound(longer_fr)
{
debug!("check_universal_region: fr_minus={:?}", fr_minus);

let blame_span_category = self.find_outlives_blame_span(mir, longer_fr, shorter_fr);

// Grow `shorter_fr` until we find a non-local
// region. (We always will.) We'll call that
// `shorter_fr+` -- it's ever so slightly larger than
// `fr`.
let shorter_fr_plus = self.universal_region_relations
.non_local_upper_bound(shorter_fr);
debug!(
"check_universal_region: shorter_fr_plus={:?}",
shorter_fr_plus
);

// Push the constraint `fr-: shorter_fr+`
propagated_outlives_requirements.push(ClosureOutlivesRequirement {
subject: ClosureOutlivesSubject::Region(fr_minus),
outlived_free_region: shorter_fr_plus,
blame_span: blame_span_category.1,
category: blame_span_category.0,
});
return None;
}
}

// If we are not in a context where we can't propagate errors, or we
// could not shrink `fr` to something smaller, then just report an
// error.
//
// Note: in this case, we use the unapproximated regions to report the
// error. This gives better error messages in some cases.
self.report_error(mir, infcx, mir_def_id, longer_fr, shorter_fr, errors_buffer);
Some(ErrorReported)
}

fn check_bound_universal_region<'gcx>(
Expand Down

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