Rollup merge of #70281 - xfix:infallible-hash, r=dtolnay

Implement Hash for Infallible

https://www.reddit.com/r/rust/comments/fmllgx/never_crate_stable_alternative_to/ lists not implementing `Hash` as a reason for the `never` crate. I see no reason not to implement `Hash` for `Infallible`, so might as well do it.

No changes necessary for `!`, because `!` already implements `Hash` (see #51404).

src/libcore/convert/mod.rs

@@ -41,6 +41,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::fmt;
+use crate::hash::{Hash, Hasher};
 
 mod num;
 
@@ -746,3 +747,10 @@ impl From<!> for Infallible {
         x
     }
 }
+
+#[stable(feature = "convert_infallible_hash", since = "1.44.0")]
+impl Hash for Infallible {
+    fn hash<H: Hasher>(&self, _: &mut H) {
+        match *self {}
+    }
+}