Rollup merge of #91122 - dtolnay:not, r=m-ou-se

impl Not for !

The lack of this impl caused trouble for me in some degenerate cases of macro-generated code of the form `if !$cond {...}`, even without `feature(never_type)` on a stable compiler. Namely if `$cond` contains a `return` or `break` or similar diverging expression, which would otherwise be perfectly legal in boolean position, the code previously failed to compile with:

```console
error[E0600]: cannot apply unary operator `!` to type `!`
   --> library/core/tests/ops.rs:239:8
    |
239 |     if !return () {}
    |        ^^^^^^^^^^ cannot apply unary operator `!`
```

src/test/ui/reachable/expr_unary.rs

@@ -5,8 +5,8 @@
 #![deny(unreachable_code)]
 
 fn foo() {
-    let x: ! = ! { return; }; //~ ERROR unreachable
-    //~| ERROR cannot apply unary operator `!` to type `!`
+    let x: ! = * { return; }; //~ ERROR unreachable
+    //~| ERROR type `!` cannot be dereferenced
 }
 
 fn main() { }

src/test/ui/reachable/expr_unary.stderr

@@ -1,13 +1,13 @@
-error[E0600]: cannot apply unary operator `!` to type `!`
+error[E0614]: type `!` cannot be dereferenced
   --> $DIR/expr_unary.rs:8:16
    |
-LL |     let x: ! = ! { return; };
-   |                ^^^^^^^^^^^^^ cannot apply unary operator `!`
+LL |     let x: ! = * { return; };
+   |                ^^^^^^^^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_unary.rs:8:16
    |
-LL |     let x: ! = ! { return; };
+LL |     let x: ! = * { return; };
    |                ^^^^------^^^
    |                |   |
    |                |   any code following this expression is unreachable
@@ -21,4 +21,4 @@ LL | #![deny(unreachable_code)]
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0600`.
+For more information about this error, try `rustc --explain E0614`.