tracing::instrument doesn't work for `-> !` on stable

```rs
fn main() {}

#[tracing::instrument]
fn foo() -> ! {
    loop {}
}
```
Fails with
```
error[E0658]: the `!` type is experimental
 --> src/main.rs:4:13
  |
4 | fn foo() -> ! {
  |             ^
  |
```