From 919b61a6f4aa2e0002c01037d923c68c57f47f33 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Mon, 21 Oct 2024 17:51:43 +0200
Subject: [PATCH] don't bail when encountering many placeholders

---
 .../src/solve/eval_ctxt/canonical.rs             |  3 ++-
 .../canonical/do-not-bail-due-to-placeholders.rs | 16 ++++++++++++++++
 .../recursion-limit-zero-issue-115351.rs         |  6 +++---
 .../recursion-limit-zero-issue-115351.stderr     | 11 -----------
 4 files changed, 21 insertions(+), 15 deletions(-)
 create mode 100644 tests/ui/traits/next-solver/canonical/do-not-bail-due-to-placeholders.rs
 delete mode 100644 tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.stderr

diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
index bc691a8aa2b..71ce0cce772 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
+++ b/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 {
diff --git a/tests/ui/traits/next-solver/canonical/do-not-bail-due-to-placeholders.rs b/tests/ui/traits/next-solver/canonical/do-not-bail-due-to-placeholders.rs
new file mode 100644
index 00000000000..3fc11f27d1f
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.rs b/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.rs
index 86d428cc0f0..dd8ad3c2dfe 100644
--- a/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.rs
+++ b/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 {}
diff --git a/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.stderr b/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.stderr
deleted file mode 100644
index 2eed7e8f723..00000000000
--- a/tests/ui/traits/next-solver/overflow/recursion-limit-zero-issue-115351.stderr
+++ /dev/null
@@ -1,11 +0,0 @@
-error[E0275]: overflow evaluating the requirement `Self: Trait`
-   |
-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "2"]` attribute to your crate (`recursion_limit_zero_issue_115351`)
-
-error[E0275]: overflow evaluating the requirement `Self well-formed`
-   |
-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "2"]` attribute to your crate (`recursion_limit_zero_issue_115351`)
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0275`.