Add GAT related tests

[cowboy-bebug]

Hi there 👋 I wanted to tackle this task as a first time contributor.

Adding GAT-related tests for the following issues:

• rustfmt eating up my where clauses on generic associated types #4257
• where removed between type name and = #4322
• rustfmt removes GATs from associated type #4943
• Required lifetime bounds for GATs in where clauses get removed  #4954

Thanks!
Closes #4969.

[calebcartwright]

Thanks! Could you make sure none of those files already exist with _ in the filename instead of - (e.g. issue_4943.rs vs issue-4943.rs)? I feel like there's probably some overlap.

This is the correct behaviour right? Here's another similar example:

Yes indeed, the Rust Style Guide prescribes this formatting for where clauses very explicitly here.

[cowboy-bebug]

@calebcartwright Yep there aren't any duplicates. Do you want me to add a commit to mv all issue_XXXX.rs -> issue-XXXX.rs?

[calebcartwright]

Do you want me to add a commit to mv all issue_XXXX.rs -> issue-XXXX.rs?

Thank you for the offer but no let's not. That's because I actually want our test files to follow the same _ file name convention that we follow for our own source files and which in my experience has been the convention in the other rust-lang/* projects. A change like that would also be really noisy and would be better suited to its own PR.

@calebcartwright Yep there aren't any duplicates.

Oh I see, you've renamed the file for 4943. Please keep the _ as is with existing files, and update the names of the new files added here with _ as well. Apologies, I know we don't have this documented and currently have a mix with a greater portion of - but since we're already on the topic let's stick with the strategic target of _ 😄

[cowboy-bebug]

The files are all snake_cased now 😄

[calebcartwright]

Awesome, thank you!