Rollup merge of #136414 - estebank:expected-return-type, r=oli-obk

Shorten error message for callable with wrong return type

```
error: expected `{closure@...}` to return `Ret`, but it returns `Other`
```
instead of
```
error: expected `{closure@...}` to be a closure that returns `Ret`, but it returns `Other`
```

Author: matthiaskrgr