Warn if user calls `.to_string()` multiple times on variable.

[Tanguille]

What it does

Warn that .to_string() is only required once. Even with pedantic clippy this doesn't give any warning.

Advantage

It can happen to anyone if you process a lot of code that you accidently call .to_string() more then once on a variable. It takes up useless characters in your file and looks sloppy. This looks like something clippy could fix automatically with the --fix argument.

Drawbacks

I can't think of any since I don't know any situation where it would make sense to call .to_string() twice in a variable.

Example

 let uri = req.uri().path().to_string().to_string();

Could be written as:

 let uri = req.uri().path().to_string();

[y21]

This lint exists: https://rust-lang.github.io/rust-clippy/master/index.html#/redundant_clone
With #![warn(clippy::redundant_clone)]:

warning: redundant clone
 --> src/main.rs:6:9
  |
6 |         .to_string()
  |         ^^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:4:5
  |
4 | /     x
5 | |         .to_string()
6 | |         .to_string()
  | |________^

The lint is currently opt-in because it has way too many false positives, see: https://github.com/rust-lang/rust-clippy/issues?q=is%3Aissue+redundant_clone

The PR #11364 reworks it.