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### What it does
Checks for `Rc<T>` and `Arc<T>` when `T` is a mutable buffer type such as `String` or `Vec`.
### Why is this bad?
Expressions such as `Rc<String>` usually have no advantage over `Rc<str>`, since
it is larger and involves an extra level of indirection, and doesn't implement `Borrow<str>`.
While mutating a buffer type would still be possible with `Rc::get_mut()`, it only
works if there are no additional references yet, which usually defeats the purpose of
enclosing it in a shared ownership type. Instead, additionally wrapping the inner
type with an interior mutable container (such as `RefCell` or `Mutex`) would normally
be used.
### Known problems
This pattern can be desirable to avoid the overhead of a `RefCell` or `Mutex` for
cases where mutation only happens before there are any additional references.
### Example
```
fn foo(interned: Rc<String>) { ... }
```
Better:
```
fn foo(interned: Rc<str>) { ... }
```
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