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rework so that we pick static as a last resort only
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@nikomatsakis
nikomatsakis committed Sep 23, 2021
1 parent b893cff commit 92b2be2
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Showing 4 changed files with 89 additions and 55 deletions.
80 changes: 47 additions & 33 deletions compiler/rustc_borrowck/src/region_infer/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -733,44 +733,58 @@ impl<'tcx> RegionInferenceContext<'tcx> {
return false;
}

// If there WERE no upper bounds (apart from static), and static is one of the options,
// then we can just pick that (c.f. #63033).
let choice = if !any_upper_bounds && choice_regions.contains(&fr_static) {
fr_static
} else {
// Otherwise, we need to find the minimum remaining choice, if
// any, and take that.
debug!("apply_member_constraint: choice_regions remaining are {:#?}", choice_regions);
let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
match (r1_outlives_r2, r2_outlives_r1) {
(true, true) => Some(r1.min(r2)),
(true, false) => Some(r2),
(false, true) => Some(r1),
(false, false) => None,
}
};
let mut min_choice = choice_regions[0];
for &other_option in &choice_regions[1..] {
debug!(
"apply_member_constraint: min_choice={:?} other_option={:?}",
min_choice, other_option,
);
match min(min_choice, other_option) {
Some(m) => min_choice = m,
None => {
debug!(
"apply_member_constraint: {:?} and {:?} are incomparable; no min choice",
min_choice, other_option,
// Otherwise, we need to find the minimum remaining choice, if
// any, and take that.
debug!("apply_member_constraint: choice_regions remaining are {:#?}", choice_regions);
let min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
match (r1_outlives_r2, r2_outlives_r1) {
(true, true) => Some(r1.min(r2)),
(true, false) => Some(r2),
(false, true) => Some(r1),
(false, false) => None,
}
};
let mut min_choice = choice_regions[0];
for &other_option in &choice_regions[1..] {
debug!(
"apply_member_constraint: min_choice={:?} other_option={:?}",
min_choice, other_option,
);
match min(min_choice, other_option) {
Some(m) => min_choice = m,
None => {
debug!(
"apply_member_constraint: {:?} and {:?} are incomparable; no min choice",
min_choice, other_option,
);

// If there is no minimum choice, then we *may* wish to use `'static`
// instead, but only if there are no upper bounds, and `'static` is a valid
// choice.
if !any_upper_bounds && choice_regions.contains(&fr_static) {
return self.apply_member_constraint_choice(
scc,
member_constraint_index,
fr_static,
);
return false;
}

// Otherwise, we give up.
return false;
}
}
min_choice
};
}
self.apply_member_constraint_choice(scc, member_constraint_index, min_choice)
}

fn apply_member_constraint_choice(
&mut self,
scc: ConstraintSccIndex,
member_constraint_index: NllMemberConstraintIndex,
choice: ty::RegionVid,
) -> bool {
let choice_scc = self.constraint_sccs.scc(choice);
debug!("apply_member_constraint: min_choice={:?} best_choice_scc={:?}", choice, choice_scc,);
if self.scc_values.add_region(scc, choice_scc) {
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3 changes: 2 additions & 1 deletion compiler/rustc_borrowck/src/universal_regions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -473,8 +473,9 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
}

let fr_fn_body = self.infcx.next_nll_region_var(FR).to_region_vid();
let num_universals = self.infcx.num_region_vars();
debug!("build: fr_fn_Body = {:?}", fr_fn_body);

let num_universals = self.infcx.num_region_vars();
debug!("build: global regions = {}..{}", FIRST_GLOBAL_INDEX, first_extern_index);
debug!("build: extern regions = {}..{}", first_extern_index, first_local_index);
debug!("build: local regions = {}..{}", first_local_index, num_universals);
Expand Down
42 changes: 21 additions & 21 deletions compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -303,16 +303,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
self.collect_bounding_regions(graph, member_vid, OUTGOING, None);
debug!("enforce_member_constraint: upper_bounds={:#?}", member_upper_bounds);

// If there are no upper bounds, and static is a choice (in practice, it always is),
// then we should just pick static.
if member_upper_bounds.is_empty()
&& member_constraint.choice_regions.contains(&&ty::RegionKind::ReStatic)
{
debug!("enforce_member_constraint: selecting 'static since there are no upper bounds",);
*var_values.value_mut(member_vid) = VarValue::Value(self.tcx().lifetimes.re_static);
return true;
}

// Get an iterator over the *available choice* -- that is,
// each choice region `c` where `lb <= c` and `c <= ub` for all the
// upper bounds `ub`.
Expand All @@ -326,19 +316,32 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
// If there is more than one option, we only make a choice if
// there is a single *least* choice -- i.e., some available
// region that is `<=` all the others.
let mut least_choice: ty::Region<'tcx> = match options.next() {
let mut choice: ty::Region<'tcx> = match options.next() {
Some(&r) => r,
None => return false,
};
debug!("enforce_member_constraint: least_choice={:?}", least_choice);
debug!("enforce_member_constraint: least_choice={:?}", choice);
for &option in options {
debug!("enforce_member_constraint: option={:?}", option);
if !self.sub_concrete_regions(least_choice, option) {
if self.sub_concrete_regions(option, least_choice) {
if !self.sub_concrete_regions(choice, option) {
if self.sub_concrete_regions(option, choice) {
debug!("enforce_member_constraint: new least choice");
least_choice = option;
choice = option;
} else {
debug!("enforce_member_constraint: no least choice");

// Pick static if:
// * There is no least choice (which we just found to be true)
// * There are no upper bounds (so the region can grow to any size)
// * Static is a choice (in practice, it always is)
if member_upper_bounds.is_empty()
&& member_constraint.choice_regions.contains(&&ty::RegionKind::ReStatic)
{
choice = self.tcx().lifetimes.re_static;
break;
}

// Otherwise give up
return false;
}
}
Expand All @@ -353,13 +356,10 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
// choice is needed here so that we don't end up in a cycle of
// `expansion` changing the region one way and the code here changing
// it back.
let lub = self.lub_concrete_regions(least_choice, member_lower_bound);
debug!(
"enforce_member_constraint: final least choice = {:?}\nlub = {:?}",
least_choice, lub
);
let lub = self.lub_concrete_regions(choice, member_lower_bound);
debug!("enforce_member_constraint: final choice = {:?}\nlub = {:?}", choice, lub);
if lub != member_lower_bound {
*var_values.value_mut(member_vid) = VarValue::Value(least_choice);
*var_values.value_mut(member_vid) = VarValue::Value(choice);
true
} else {
false
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19 changes: 19 additions & 0 deletions src/test/ui/upper-bound-from-type-param.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
// Regression test found by crater run. The scenario here is that we have
// a region variable `?0` from the `impl Future` with no upper bounds
// and a member constraint of `?0 in ['a, 'static]`. If we pick `'static`,
// however, we then fail the check that `F: ?0` (since we only know that
// F: ?a). The problem here is that our upper bound detection
// doesn't consider "type tests" like `F: 'x`. This was fixed by
// preferring to pick a least choice and only using static as a last resort.
//
// edition:2018
// check-pass

trait Future {}
impl Future for () {}

fn sink_error1<'a, F: 'a>(f: F) -> impl Future + 'a {
sink_error2(f) // error: `F` may not live long enough
}
fn sink_error2<'a, F: 'a>(_: F) -> impl Future + 'a {}
fn main() {}

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