Box::from(&str) -> Box<Error> regression

The following code compiles fine on the stable channel but fails on the beta and nigthly channels:

```
fn test() -> Box<std::error::Error> { Box::from("test") }
fn main() { test(); }
```

The error message is:

```
error: the trait `std::error::Error` is not implemented for the type `&str`
```
