Do not need to account for overflow in predicate_can_apply

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2544,7 +2544,10 @@ fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
             let obligation =
                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
 
-            self.predicate_may_hold(&obligation)
+            // We don't use `InferCtxt::predicate_may_hold` because that
+            // will re-run predicates that overflow locally, which ends up
+            // taking a really long time to compute.
+            self.evaluate_obligation(&obligation).map_or(false, |eval| eval.may_apply())
         })
     }
 

src/test/ui/traits/predicate_can_apply-hang.rs

@@ -0,0 +1,6 @@
+fn f<B>(x: Vec<[[[B; 1]; 1]; 1]>) -> impl PartialEq<B> {
+    //~^ ERROR can't compare `Vec<[[[B; 1]; 1]; 1]>` with `B`
+    x
+}
+
+fn main() {}

src/test/ui/traits/predicate_can_apply-hang.stderr

@@ -0,0 +1,21 @@
+error[E0277]: can't compare `Vec<[[[B; 1]; 1]; 1]>` with `B`
+  --> $DIR/predicate_can_apply-hang.rs:1:38
+   |
+LL | fn f<B>(x: Vec<[[[B; 1]; 1]; 1]>) -> impl PartialEq<B> {
+   |                                      ^^^^^^^^^^^^^^^^^ no implementation for `Vec<[[[B; 1]; 1]; 1]> == B`
+LL |
+LL |     x
+   |     - return type was inferred to be `Vec<[[[B; 1]; 1]; 1]>` here
+   |
+   = help: the trait `PartialEq<B>` is not implemented for `Vec<[[[B; 1]; 1]; 1]>`
+   = help: the following other types implement trait `PartialEq<Rhs>`:
+             <Vec<T, A1> as PartialEq<Vec<U, A2>>>
+             <Vec<T, A> as PartialEq<&[U; N]>>
+             <Vec<T, A> as PartialEq<&[U]>>
+             <Vec<T, A> as PartialEq<&mut [U]>>
+             <Vec<T, A> as PartialEq<[U; N]>>
+             <Vec<T, A> as PartialEq<[U]>>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.