New lint: Implement From instead of Into

[davidpdrsn]

What it does

Finds implementations of Into and suggests implementing From instead. That should always be possible since Rust 1.41 as per https://doc.rust-lang.org/stable/std/convert/trait.Into.html#implementing-into-for-conversions-to-external-types-in-old-versions-of-rust.

According the std docs implementing From is preferred since it gives you Into for free where the reverse isn't true:

It is important to understand that Into does not provide a From implementation (as From does with Into). Therefore, you should always try to implement From and then fall back to Into if From can't be implemented.

Categories (optional)

• Kind: Style.

Drawbacks

There might still be cases where one cannot implement From but implementing Into is possible. If such cases exist and are hard to detect this lint might not be a good idea. Could lead to false positives.

Example

struct StringWrapper(String);

impl Into<StringWrapper> for String {
    fn into(self) -> StringWrapper {
        StringWrapper(self)
    }
}

Could be written as:

struct StringWrapper(String);

impl From<String> for StringWrapper {
    fn from(s: String) -> StringWrapper {
        StringWrapper(s)
    }
}

[giraffate]

related issue: #1567

[1c3t3a]

I would like to work on this!