Rollup merge of #111211 - compiler-errors:negative-bounds-super, r=TaKO8Ki

Don't compute trait super bounds unless they're positive

Fixes #111207

The comment is modified to explain the rationale for why we even have this recursive call to supertraits in the first place, which doesn't apply to negative bounds since they don't elaborate at all.

Author: JohnTitor

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -657,14 +657,15 @@ pub(super) fn implied_predicates_with_filter(
         &*tcx.arena.alloc_from_iter(superbounds.predicates().chain(where_bounds_that_match));
     debug!(?implied_bounds);
 
-    // Now require that immediate supertraits are converted,
-    // which will, in turn, reach indirect supertraits.
+    // Now require that immediate supertraits are converted, which will, in
+    // turn, reach indirect supertraits, so we detect cycles now instead of
+    // overflowing during elaboration.
     if matches!(filter, PredicateFilter::SelfOnly) {
-        // Now require that immediate supertraits are converted,
-        // which will, in turn, reach indirect supertraits.
         for &(pred, span) in implied_bounds {
             debug!("superbound: {:?}", pred);
-            if let ty::PredicateKind::Clause(ty::Clause::Trait(bound)) = pred.kind().skip_binder() {
+            if let ty::PredicateKind::Clause(ty::Clause::Trait(bound)) = pred.kind().skip_binder()
+                && bound.polarity == ty::ImplPolarity::Positive
+            {
                 tcx.at(span).super_predicates_of(bound.def_id());
             }
         }

tests/ui/traits/negative-bounds/supertrait.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+#![feature(negative_bounds)]
+//~^ WARN the feature `negative_bounds` is incomplete
+
+trait A: !B {}
+trait B: !A {}
+
+fn main() {}

tests/ui/traits/negative-bounds/supertrait.stderr

@@ -0,0 +1,10 @@
+warning: the feature `negative_bounds` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/supertrait.rs:3:12
+   |
+LL | #![feature(negative_bounds)]
+   |            ^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+