From 584c26f7e03e26f1acff78e72a9b44b2376b4acb Mon Sep 17 00:00:00 2001
From: Kevin Reid <kpreid@switchb.org>
Date: Mon, 17 Aug 2026 10:42:55 -0700
Subject: [PATCH] Implement `AsMut<T>` and `AsRef<T>` for `!`.
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This will allow e.g. `&[!]` to satisfy `&[T] where T: AsRef<str>`.
It follows the recommendation from the never documentation:

> When writing your own traits, `!` should have an `impl` whenever
> there is an obvious `impl` which doesn’t `panic!`.

-- <https://doc.rust-lang.org/1.97.1/std/primitive.never.html#-and-traits>

The test tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs
had to be updated because it depended on this impl not existing. I’ve
confirmed that the modified test still functions as a regression test by
compiling it in nightly-2022-08-28 and seeing it ICE.
---
 library/core/src/convert/mod.rs               | 22 +++++++++++++++++++
 ...h-implicit-hrtb-without-dyn.pre2021.stderr | 13 ++++-------
 .../generic-with-implicit-hrtb-without-dyn.rs |  4 ++--
 3 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs
index a57f3d58e3998..c21c743672014 100644
--- a/library/core/src/convert/mod.rs
+++ b/library/core/src/convert/mod.rs
@@ -863,6 +863,28 @@ const impl AsMut<str> for str {
     }
 }
 
+#[stable(feature = "never_type", since = "CURRENT_RUSTC_VERSION")]
+#[expect(
+    clippy::explicit_auto_deref,
+    reason = "https://github.com/rust-lang/rust-clippy/issues/17713"
+)]
+impl<T: ?Sized> AsRef<T> for ! {
+    fn as_ref(&self) -> &T {
+        *self
+    }
+}
+
+#[stable(feature = "never_type", since = "CURRENT_RUSTC_VERSION")]
+#[expect(
+    clippy::explicit_auto_deref,
+    reason = "https://github.com/rust-lang/rust-clippy/issues/17713"
+)]
+impl<T: ?Sized> AsMut<T> for ! {
+    fn as_mut(&mut self) -> &mut T {
+        *self
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // THE NO-ERROR ERROR TYPE
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.pre2021.stderr b/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.pre2021.stderr
index dc566ad50153a..54ea4882c4669 100644
--- a/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.pre2021.stderr
+++ b/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.pre2021.stderr
@@ -1,16 +1,11 @@
-error[E0277]: the trait bound `!: AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied
+error[E0277]: the trait bound `(): AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied
   --> $DIR/generic-with-implicit-hrtb-without-dyn.rs:7:13
    |
 LL | fn ice() -> impl AsRef<Fn(&())> {
-   |             ^^^^^^^^^^^^^^^^^^^ the trait `AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not implemented for `!`
+   |             ^^^^^^^^^^^^^^^^^^^ the trait `AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not implemented for `()`
 ...
-LL |     todo!()
-   |     ------- return type was inferred to be `!` here
-   |
-help: `!` can be coerced to any type; consider casting it to a concrete type that implements the trait
-   |
-LL |     todo!() as /* Type */
-   |             +++++++++++++
+LL |     ()
+   |     -- return type was inferred to be `()` here
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs b/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs
index 64311a474cade..6c538e8c8ee16 100644
--- a/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs
+++ b/tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs
@@ -5,10 +5,10 @@
 #![allow(warnings)]
 
 fn ice() -> impl AsRef<Fn(&())> {
-    //[pre2021]~^ ERROR: the trait bound `!: AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied [E0277]
+    //[pre2021]~^ ERROR: the trait bound `(): AsRef<(dyn for<'a> Fn(&'a ()) + 'static)>` is not satisfied [E0277]
     //[edition2021]~^^ ERROR: expected a type, found a trait [E0782]
     //[edition2021]~| ERROR: expected a type, found a trait [E0782]
-    todo!()
+    ()
 }
 
 fn main() {}