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.

Author: nikomatsakis

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -916,6 +916,7 @@ 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>,
@@ -933,11 +934,41 @@ 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.
             //
@@ -962,8 +993,6 @@ 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);
 
@@ -1000,15 +1029,14 @@ 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>,
         ty: Ty<'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);
 

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

@@ -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

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

@@ -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`.

src/test/ui/nll/issue-98693.rs

@@ -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() {}

src/test/ui/nll/issue-98693.stderr

@@ -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`.