FP syntax error `use std::fmt::self;`

```rust
use std::fmt::self;
fn main () {}
```
this compiles but ra diagnostics say
```
at crate ra, file /tmp/ra/src/main.rs: Error SyntaxError from LineCol { line: 0, col: 14 } to LineCol { line: 0, col: 18 }: Syntax Error: The `self` keyword is only allowed as the first segment of a path
```