don't bail when encountering many placeholders

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs

@@ -173,7 +173,8 @@ where
         // causing a coherence error in diesel, see #131969. We still bail with verflow
         // when later returning from the parent AliasRelate goal.
         if !self.is_normalizes_to_goal {
-            let num_non_region_vars = canonical.variables.iter().filter(|c| !c.is_region()).count();
+            let num_non_region_vars =
+                canonical.variables.iter().filter(|c| !c.is_region() && c.is_existential()).count();
             if num_non_region_vars > self.cx().recursion_limit() {
                 debug!(?num_non_region_vars, "too many inference variables -> overflow");
                 return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Overflow {

tests/ui/traits/next-solver/canonical/do-not-bail-due-to-placeholders.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// When canonicalizing responses, we bail if there are too many inference variables.
+// We previously also counted placeholders, which is incorrect.
+#![recursion_limit = "8"]
+
+fn foo<T>() {}
+
+fn bar<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9>() {
+    // The query response will contain 10 placeholders, which previously
+    // caused us to bail here.
+    foo::<(T0, T1, T2, T3, T4, T5, T6, T7, T8, T9)>();
+}
+
+fn main() {}

tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.rs

@@ -1,8 +1,8 @@
-//~ ERROR overflow evaluating the requirement `Self: Trait`
-//~^ ERROR overflow evaluating the requirement `Self well-formed`
-// This is a non-regression test for issue #115351, where a recursion limit of 0 caused an ICE.
+//@ check-pass
 //@ compile-flags: -Znext-solver --crate-type=lib
 
+// This is a non-regression test for issue #115351, where a recursion limit of 0 caused an ICE.
+
 #![recursion_limit = "0"]
 trait Trait {}
 impl Trait for u32 {}