From 1ffc6ca9a513d1a3a928cc1d32b21d55d6326bb3 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Thu, 27 Jul 2023 04:00:18 +0000
Subject: [PATCH] Consider a goal as NOT changed if its response is identity
 modulo regions

---
 .../src/solve/eval_ctxt.rs                    |  2 +-
 tests/ui/impl-trait/autoderef.rs              |  2 +-
 ...dont-loop-fulfill-on-region-constraints.rs | 32 +++++++++++++++++++
 3 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/traits/new-solver/dont-loop-fulfill-on-region-constraints.rs

diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
index f48a992d327..9e3b0bbc4fb 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
@@ -344,7 +344,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             Ok(response) => response,
         };
 
-        let has_changed = !canonical_response.value.var_values.is_identity()
+        let has_changed = !canonical_response.value.var_values.is_identity_modulo_regions()
             || !canonical_response.value.external_constraints.opaque_types.is_empty();
         let (certainty, nested_goals) = match self.instantiate_and_apply_query_response(
             goal.param_env,
diff --git a/tests/ui/impl-trait/autoderef.rs b/tests/ui/impl-trait/autoderef.rs
index 0d07a549640..cd2cdd9e3b3 100644
--- a/tests/ui/impl-trait/autoderef.rs
+++ b/tests/ui/impl-trait/autoderef.rs
@@ -1,5 +1,5 @@
 // revisions: current next
-//[next] compile-flag: -Ztrait-solver=next
+//[next] compile-flags: -Ztrait-solver=next
 // check-pass
 
 use std::path::Path;
diff --git a/tests/ui/traits/new-solver/dont-loop-fulfill-on-region-constraints.rs b/tests/ui/traits/new-solver/dont-loop-fulfill-on-region-constraints.rs
new file mode 100644
index 00000000000..b241e3bf865
--- /dev/null
+++ b/tests/ui/traits/new-solver/dont-loop-fulfill-on-region-constraints.rs
@@ -0,0 +1,32 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+trait Eq<'a, 'b, T> {}
+
+trait Ambig {}
+impl Ambig for () {}
+
+impl<'a, T> Eq<'a, 'a, T> for () where T: Ambig {}
+
+fn eq<'a, 'b, T>(t: T)
+where
+    (): Eq<'a, 'b, T>,
+{
+}
+
+fn test<'r>() {
+    let mut x = Default::default();
+
+    // When we evaluate `(): Eq<'r, 'r, ?0>` we uniquify the regions.
+    // That leads us to evaluate `(): Eq<'?0, '?1, ?0>`. The response of this
+    // will be ambiguous (because `?0: Ambig` is ambig) and also not an "identity"
+    // response, since the region constraints will contain `'?0 == '?1` (so
+    // `is_changed` will return true). Since it's both ambig and changed,
+    // fulfillment will both re-register the goal AND loop again. This hits the
+    // overflow limit. This should neither be considered overflow, nor ICE.
+    eq::<'r, 'r, _>(x);
+
+    x = ();
+}
+
+fn main() {}