From 0e9241cb6e9499e7449973689b12527c03e5333d Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Tue, 21 Nov 2023 00:07:55 +0000
Subject: [PATCH] Always make progress on FnOnce ClosureKind goals in the old
 solver

---
 compiler/rustc_trait_selection/src/traits/fulfill.rs    | 8 +++++++-
 compiler/rustc_trait_selection/src/traits/select/mod.rs | 8 +++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index fb9cf51b513bb..28dbdadaa802d 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -420,7 +420,13 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
                             }
                         }
-                        None => ProcessResult::Unchanged,
+                        None => {
+                            if kind == ty::ClosureKind::FnOnce {
+                                ProcessResult::Changed(vec![])
+                            } else {
+                                ProcessResult::Unchanged
+                            }
+                        }
                     }
                 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 1ed0da59b64f0..e870a7950878d 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -894,7 +894,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                 Ok(EvaluatedToErr)
                             }
                         }
-                        None => Ok(EvaluatedToAmbig),
+                        None => {
+                            if kind == ty::ClosureKind::FnOnce {
+                                Ok(EvaluatedToOk)
+                            } else {
+                                Ok(EvaluatedToAmbig)
+                            }
+                        }
                     }
                 }