Suggest extend over for_each for element insertion

What it does

The lint should detect/convert code which uses a .for_each containing a single .insert into an .extend call.

Lint Name

manual_extend

Category

style

Advantage

The code becomes much cleaner and easier to understand.

Mentally, I read the first example as follows:

• Get all keys from self.state_machine_info.det_states
• For each key
  • Insert it in states, but first clone the key.

The second example though:

• Extend states
• Using the cloned keys from self.state_machine_info.det_states

Drawbacks

I know that not everyone will read it as I will, however the second approach feels much more in-line with "proper" Rust.

Example

In my typestate macro I have the following code:

let mut states = HashSet::new();
self.state_machine_info.det_states.keys().for_each(|k| {
    states.insert(k.clone());
});
self.state_machine_info
    .non_det_transitions
    .keys()
    .for_each(|k| {
        states.insert(k.clone());
    });

Which becomes much cleaner when written like:

let mut states = HashSet::new();
states.extend(
    self.state_machine_info.det_states.keys().cloned()
);
states.extend(
    self.state_machine_info.non_det_transitions.keys().cloned()
);

https://github.com/rustype/typestate-rs/blob/83b0ef0a6dbd1658c283ec45be0ad611e73e8ff1/typestate-proc-macro/src/visitors/transition.rs#L99-L108

Start with the manual_extend lint name and the Rust example linked in the issue, focusing on the single-insert .for_each pattern. Define the supported transformation from the first example to the .extend form, then verify the lint’s suggested output and behavior with representative cases.