Suggest implementing `TryFrom<&str>` for types implementing `FromStr`

[Luro02]

It is not much work to implement TryFrom<&str> for types implementing FromStr, you can even generalize it;

impl<T> TryFrom<&str> for T
where
    T: FromStr
{
    type Error = T::Err;

    fn try_from(input: &str) -> Result<Self, Self::Error> {
        Self::from_str(input)
    }
}

I don't think there is much of a reason to not implement it.

You might even be able to provide a more efficient implementation:

struct Hello<'a>(Cow<'a, str>);

impl FromStr for Hello<'static> {
    fn from_str(input: &str) -> Result<Self, Self::Err> {
        Ok(Self(Cow::Owned(input.into())))
    }
}

impl<'a> TryFrom<&'a str> for Hello<'a> {
    fn try_from(input: &'a str) -> Result<Self, Self::Error> {
        Ok(Self(Cow::Borrowed(input)))
   }
}

[kangalio]

Funny, I was just about to open an issue about the exact opposite; warning about implementing TryFrom<&str> if FromStr is also implemented.

TryFrom is mainly used for conversions between types which are semantically the same. You can see that in the TryFrom implementations of the standard library:

• conversions from integer to integer (signed->unsigned, larger width->smaller width, number primitive->NonZero number)
• conversions from list to list (Vec->[T; N], smart pointer of [T]->[T; N])

Parsing a value out of a string doesn't fall under that category though.

Apart from this philosophical argument, it becomes messy to have two trait implementations for the exact same thing, particularly when only some crates do it.

In the end I suppose this is all subjective and there will be plenty of people disagreeing with me. Therefore I think it would be best to have two new lints in the style category, one or both of them disabled by default:

• clippy::no_try_from_str lints when there's a FromStr implementation but no TryFrom<&str>
• clippy::try_from_str lints when there's a FromStr implementation and a TryFrom<&str>

[emilk]

I've opened a separate ticket for the inverse:

• Suggest impl FromStr instead of TryFrom<&str> #14522