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promote placeholder bounds to 'static obligations

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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.
This commit is contained in:
Niko Matsakis 2022-06-30 10:26:17 -04:00
parent 7425fb293f
commit 7fda86249f
5 changed files with 85 additions and 6 deletions
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38
compiler/rustc_borrowck/src/region_infer/mod.rs
View File

@ -916,6 +916,7 @@ fn check_type_tests(
/// 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>,
@ -933,11 +934,41 @@ fn try_promote_type_test(
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.
//
@ -962,8 +993,6 @@ fn try_promote_type_test(
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);
@ -1000,6 +1029,7 @@ fn try_promote_type_test(
/// 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>,
@ -1007,8 +1037,6 @@ fn try_promote_type_test_subject(
) -> 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);

1
src/test/ui/generic-associated-types/issue-91139.rs
View File

@ -22,6 +22,7 @@ fn foo<T>() {
//~| ERROR `T` does not live long enough
//~| ERROR `T` does not live long enough
//~| ERROR `T` does not live long enough
//~| ERROR `T` may not live long enough
//
// FIXME: This error is bogus, but it arises because we try to validate
// that `<() as Foo<T>>::Type<'a>` is valid, which requires proving

14
src/test/ui/generic-associated-types/issue-91139.stderr
View File

@ -34,6 +34,17 @@ error: `T` does not live long enough
LL | let _: for<'a> fn(<() as Foo<T>>::Type<'a>, &'a T) = |_, _| ();
| ^^^^^^^^^
error[E0310]: the parameter type `T` may not live long enough
--> $DIR/issue-91139.rs:16:58
|
LL | let _: for<'a> fn(<() as Foo<T>>::Type<'a>, &'a T) = |_, _| ();
| ^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
|
help: consider adding an explicit lifetime bound...
|
LL | fn foo<T: 'static>() {
| +++++++++
error: `T` does not live long enough
--> $DIR/issue-91139.rs:16:58
|
@ -46,5 +57,6 @@ error: `T` does not live long enough
LL | let _: for<'a> fn(<() as Foo<T>>::Type<'a>, &'a T) = |_, _| ();
| ^^^^^^^^^
error: aborting due to 8 previous errors
error: aborting due to 9 previous errors
For more information about this error, try `rustc --explain E0310`.

21
src/test/ui/nll/issue-98693.rs Normal file
View File

@ -0,0 +1,21 @@
// Regression test for #98693.
//
// The closure encounters an obligation that `T` must outlive `!U1`,
// a placeholder from universe U1. We were ignoring this placeholder
// when promoting the constraint to the enclosing function, and
// thus incorrectly judging the closure to be safe.
fn assert_static<T>()
where
for<'a> T: 'a,
{
}
fn test<T>() {
|| {
//~^ ERROR the parameter type `T` may not live long enough
assert_static::<T>();
};
}
fn main() {}

17
src/test/ui/nll/issue-98693.stderr Normal file
View File

@ -0,0 +1,17 @@
error[E0310]: the parameter type `T` may not live long enough
--> $DIR/issue-98693.rs:15:5
|
LL | / || {
LL | |
LL | | assert_static::<T>();
LL | | };
| |_____^ ...so that the type `T` will meet its required lifetime bounds
|
help: consider adding an explicit lifetime bound...
|
LL | fn test<T: 'static>() {
| +++++++++
error: aborting due to previous error
For more information about this error, try `rustc --explain E0310`.