From cc584ba24527f16b3c88f46959fedcfc716d5503 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Tue, 27 Feb 2024 20:07:58 +0000
Subject: [PATCH] Process alias-relate obligations in CoerceUnsized loop

---
 compiler/rustc_hir_typeck/src/coercion.rs      | 14 ++++++++++++++
 .../constrain-alias-goals-in-unsize.rs         | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 tests/ui/traits/next-solver/constrain-alias-goals-in-unsize.rs

diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 8f4f028fc07f1..d07c9b10f1cbd 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -636,6 +636,20 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 {
                     self.resolve_vars_if_possible(trait_pred)
                 }
+                // Eagerly process alias-relate obligations in new trait solver,
+                // since these can be emitted in the process of solving trait goals,
+                // but we need to constrain vars before processing goals mentioning
+                // them.
+                Some(ty::PredicateKind::AliasRelate(..)) => {
+                    let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(self);
+                    fulfill_cx.register_predicate_obligation(self, obligation);
+                    let errs = fulfill_cx.select_where_possible(self);
+                    if !errs.is_empty() {
+                        return Err(TypeError::Mismatch);
+                    }
+                    coercion.obligations.extend(fulfill_cx.pending_obligations());
+                    continue;
+                }
                 _ => {
                     coercion.obligations.push(obligation);
                     continue;
diff --git a/tests/ui/traits/next-solver/constrain-alias-goals-in-unsize.rs b/tests/ui/traits/next-solver/constrain-alias-goals-in-unsize.rs
new file mode 100644
index 0000000000000..1656238bd6188
--- /dev/null
+++ b/tests/ui/traits/next-solver/constrain-alias-goals-in-unsize.rs
@@ -0,0 +1,18 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+use std::mem::ManuallyDrop;
+
+trait Foo {}
+
+struct Guard<T> {
+    value: ManuallyDrop<T>,
+}
+
+impl<T: Foo> Guard<T> {
+    fn uwu(&self) {
+        let x: &dyn Foo = &*self.value;
+    }
+}
+
+fn main() {}