feat: do not add new enum if it already exists

[winstxnhdw]

Summary

This PR introduces a check for the existence of another enum within the current scope, and if it exist, we skip add_enum_def.

Why?

Currently, when using the bool_to_enum assist more than once, it is possible to add multiple enum definitions. For example, the following snippet,

#[derive(PartialEq, Eq)]
enum Bool {
    True,
    False,
}

fn main() {
    let a = Bool::True;
    let b = true;
    println!("Hello, world!");
}

will be transformed into,

#[derive(PartialEq, Eq)]
enum Bool {
    True,
    False,
}

#[derive(PartialEq, Eq)]
enum Bool {
    True,
    False,
}

fn main() {
    let a = Bool::True;
    let b = Bool::True;
    println!("Hello, world!");
}

This can be annoying for users to clean up.

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit c4bcec2 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit c4bcec2 with merge 5577e4e...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 5577e4e to master...