Deduplicate imports

Hello!

I searched the issues and could not find a similar issue, if there is already one on that topic I missed feel free to close this one.

I find myself solving some merge conflicts lately that happen for use statements, sometimes ending up with more than one import of the same name e.g.:

use std::vec::{Vec, Vec};

And running rustfmt on that does not deduplicate the import.

In practice the use statements causing issues are much bigger and it can be a bit of pain to deduplicate imports manually. In addition, the duplicate import prevents compilation because of the name Vec being redefined

error[E0252]: the name Vec is defined multiple times

Rust playground:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0cd8ef07ca6cde1a2892c9b8e1697ff4

It feels like it should be possible as I've already seen rustfmt merging multiple imports from the same module into one use statement, so there is a mechanism to group names together and sort them, I'm guessing that switching the underlying container for the grouped names to some Set-like object should do the trick.

I haven't had time to check the source for that particular case, but I thought I might as well open the issue here if someone knew where the code for this is and how it could be changed to manage that case 🙂

Cheers!

Start with the Rust Playground reproducer linked in the issue and inspect rustfmt's import-merging logic for grouped names. Trace how duplicate names are represented and add coverage for use std::vec::{Vec, Vec};; done means rustfmt removes the duplicate and the formatted code no longer triggers E0252.