From c4f829b2e5053cef753e010fa89bcc01b164a306 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Wed, 19 Oct 2022 13:33:45 +0200
Subject: [PATCH] Allow #[unstable] impl for fn() -> UnstableType.

(But not fn() -> !, which is stable.)
---
 compiler/rustc_passes/src/stability.rs        | 26 ++++++++++++++-----
 .../stability-attribute-trait-impl.rs         |  3 +++
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 78591e640e3..9591aeb881f 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -888,14 +888,26 @@ impl<'tcx> Visitor<'tcx> for CheckTraitImplStable<'tcx> {
     }
 
     fn visit_ty(&mut self, t: &'tcx Ty<'tcx>) {
-        match t.kind {
-            TyKind::Never => self.fully_stable = false,
-            TyKind::BareFn(f) => {
-                if rustc_target::spec::abi::is_stable(f.abi.name()).is_err() {
-                    self.fully_stable = false;
-                }
+        if let TyKind::Never = t.kind {
+            self.fully_stable = false;
+        }
+        if let TyKind::BareFn(f) = t.kind {
+            if rustc_target::spec::abi::is_stable(f.abi.name()).is_err() {
+                self.fully_stable = false;
+            }
+        }
+        intravisit::walk_ty(self, t)
+    }
+
+    fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl<'tcx>) {
+        for ty in fd.inputs {
+            self.visit_ty(ty)
+        }
+        if let hir::FnRetTy::Return(output_ty) = fd.output {
+            match output_ty.kind {
+                TyKind::Never => {} // `-> !` is stable
+                _ => self.visit_ty(output_ty),
             }
-            _ => intravisit::walk_ty(self, t),
         }
     }
 }
diff --git a/src/test/ui/stability-attribute/stability-attribute-trait-impl.rs b/src/test/ui/stability-attribute/stability-attribute-trait-impl.rs
index cc5bc3b6d91..0c771ae8795 100644
--- a/src/test/ui/stability-attribute/stability-attribute-trait-impl.rs
+++ b/src/test/ui/stability-attribute/stability-attribute-trait-impl.rs
@@ -37,4 +37,7 @@ impl StableTrait for StableType {}
 //~^ ERROR an `#[unstable]` annotation here has no effect [ineffective_unstable_trait_impl]
 impl StableTrait for fn() -> ! {}
 
+#[unstable(feature = "l", issue = "none")]
+impl StableTrait for fn() -> UnstableType {}
+
 fn main() {}