Rollup merge of #122651 - kornelski:flat-turbofish, r=spastorino,compiler-errors

Suggest `_` for missing generic arguments in turbofish

The compiler may suggest unusable generic type names for missing generic arguments in an expression context:

```rust
fn main() {
    (0..1).collect::<Vec>()
}
```

> help: add missing generic argument
>
>      (0..1).collect::<Vec<T>>()

but `T` is not a valid name in this context, and this suggestion won't compile.

I've changed it to use `_` inside method calls (turbofish), so it will suggest `(0..1).collect::<Vec<_>>()` which _may_ compile.

It's possible that the suggested `_` will be ambiguous, but there is very extensive E0283 that will help resolve that, which is more helpful than a basic "cannot find type `T` in this scope" users would get otherwise.

Out of caution to limit scope of the change I've limited it to just turbofish, but I suspect `_` could be the better choice in more cases. Perhaps in all expressions?

Author: matthiaskrgr