Implement AsMut<T> and AsRef<T> for !.

library/core/src/convert/mod.rs

@@ -863,6 +863,20 @@ const impl AsMut<str> for str {
     }
 }
 
+#[stable(feature = "never_type", since = "CURRENT_RUSTC_VERSION")]
+impl<T: ?Sized> AsRef<T> for ! {
+    fn as_ref(&self) -> &T {
+        match *self {}
+    }
+}
+
+#[stable(feature = "never_type", since = "CURRENT_RUSTC_VERSION")]
+impl<T: ?Sized> AsMut<T> for ! {
+    fn as_mut(&mut self) -> &mut T {
+        match *self {}
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // THE NO-ERROR ERROR TYPE
 ////////////////////////////////////////////////////////////////////////////////

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
 

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() {}