Detect creation of new collection when it could be emptied

[samueltardieu]

Description

See uutils/coreutils#10695

When a variable holding a collection (such as Vec/VecDeque/HashMap/BTreeMap) is replaced by a new empty instance, it can be more efficient to instead clear the existing one to be able to reuse the allocated memory.

For example, x = Vec::new() can be replaced by x.clear(): x will be an empty vector, except that the storage (and capacity) will be inherited from the previous instance.

Version

Additional Labels

No response

[yash-kumarx]

Hello, I would like to work on this issue. May I work on it?

[yash-kumarx]

@rustbot claim

[yash-kumarx]

Sorry for the delay in submitting this PR. For first few days, I was understanding clippy's codebase. Gathered knowledge on lints, writing comprehensive UI tests, running them on local. Trying to ensure, I followed all the contribution guidelines properly.
This is my first contribution to Clippy, so I wanted to make sure I got it right. Happy to address any feedback!

[hriztam]

Hello, if the issue is still open I would I to work on it @samueltardieu

[yash-kumarx]

Hey @hriztam,

I have been working with this issue already, and also raised a pr a week ago. @samueltardieu has asked me to make the desired changes in the pr so I have already raised the updated pr with modified changes to resolve the issue completely.
The pr is in the process of being reviewed by him.

Thanks for your interest. There are many other issues in the clippy repo which you can focus on and resolve