11173: Allow adding partially resolved types r=Veykril a=SomeoneToIgnore
Sometimes when writing something like `let foo = Arc::new(Mutex::new(CrazyGenerics::new(HashMap::new())))`, I want/have to specify an explicit type for the expression.
Using turbofish isn't very readable and not always appreciated by guidelines, so `let foo: T` has to be filled.
To ease that, the PR enables the `add_explicit_type` assist on types that contain unknown types and some generics.
Fully unresolved types, arrays with unknown types and other known cases behave the same.
`_` placeholder was chosen to replace an unknown type:
```rust
let foo = HashMap::new();
// after assist usage, turns into
let foo: HashMap<_, _> = HashMap::new();
```
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
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