From 4af6137e2202425bae7a89a6beb34824d2d14fc3 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sat, 28 Sep 2024 13:21:39 -0400
Subject: [PATCH] Don't check unsize goal in MIR validation when opaques remain

---
 compiler/rustc_mir_transform/src/validate.rs       | 11 +++++++++++
 tests/ui/impl-trait/unsize-cast-validation-rpit.rs | 12 ++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 tests/ui/impl-trait/unsize-cast-validation-rpit.rs

diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index e353be6a105f0..25e68f44456b8 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -595,6 +595,17 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         &self,
         pred: impl Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>>,
     ) -> bool {
+        let pred: ty::Predicate<'tcx> = pred.upcast(self.tcx);
+
+        // We sometimes have to use `defining_opaque_types` for predicates
+        // to succeed here and figuring out how exactly that should work
+        // is annoying. It is harmless enough to just not validate anything
+        // in that case. We still check this after analysis as all opaque
+        // types have been revealed at this point.
+        if pred.has_opaque_types() {
+            return true;
+        }
+
         let infcx = self.tcx.infer_ctxt().build();
         let ocx = ObligationCtxt::new(&infcx);
         ocx.register_obligation(Obligation::new(
diff --git a/tests/ui/impl-trait/unsize-cast-validation-rpit.rs b/tests/ui/impl-trait/unsize-cast-validation-rpit.rs
new file mode 100644
index 0000000000000..cace30aca8a6d
--- /dev/null
+++ b/tests/ui/impl-trait/unsize-cast-validation-rpit.rs
@@ -0,0 +1,12 @@
+//@ check-pass
+//@ compile-flags: -Zvalidate-mir
+
+fn hello() -> &'static [impl Sized; 0] {
+    if false {
+        let x = hello();
+        let _: &[i32] = x;
+    }
+    &[]
+}
+
+fn main() {}