New lint: to_string() on a temporary Cow<'_, str>

[Veetaha]

What it does

The lint is meant to detect calling .to_string() on a temporary Cow<'_, str>.
This is not a rare case when a beginner makes such a mistake.

Categories (optional)

• Kind: clippy::perf

What is the advantage of the recommended code over the original code:

• Avoid wasting a potentially owned string inside of the temporary Cow<'_, str>.

Drawbacks

None.

Example

let path_str: String = path.to_string_lossy().to_string();

Could be written as:

let path_str: String = path.to_string_lossy().into_owned();

[camsteffen]

Why only apply it to a temporary Cow?

Scanning the implementors of ToOwned, you could similarly lint on OsStr::to_os_string and Path::to_path_buf.

Another case is (*cow).to_owned(). But maybe that's too contrived?

[Kixunil]

How about into() or String::from()? One advantage of such conversion is that it keeps working when the type changes a bit. Also useful for Box<str>.

Also why only temporary? I think doing the same when the cow is no longer used even when it's bound should be doable. (I realize this is kinda reimplementing part of borrow checking, but I hope it's simple enough.)

In general any conversion of Cow<'_, Foo> into Foo::Owned when the original value is not used later should be linted.

[Veetaha]

This is a duplicate of #2387
I suggest proceeding with discussions there