False positive on single_char_pattern with serde_json::Value.find(_)

[llogiq]

fn foo(v: &serde_json::Value) {
    let _ = v.find("x")...
}

will trigger the lint, however Value cannot find single characters for some reason. (Found in https://github.com/Thinkofname/steven-rust)

[oli-obk]

That's not really a false positive, but a hint that Value should be fixed. How about we lint on implementing a PATTERN_METHODS method on a type, but it has the wrong signature? serde already runs clippy, so we'd catch that there.

[llogiq]

Yes, I am aware that serde's Value implementation is to blame. However, the user likely isn't – and as long as we don't go around fixing incomplete PATTERN_METHODS implementations, to them this is a false positive, regardless if clippy is doing the right thing. So we should document that somehow.

Linting incomplete implementations would be a good way forward. I'm doing something similar with len_zero and len_without_is_empty.

[oli-obk]

can we also check if the method's generics are what we think they should be and not lint in that case?

Basically:

1. is a generic method?
2. has just one generic argument?
3. generic argument is also the type of the passed argument?
4. generic argument requires Pattern trait?
5. lint

[jugglerchris]

There's a similar false positive on foo.contains("X") where foo is a HashSet<String>. HashSet<T>::contains takes a T, not a Pattern.

[mcarton]

Yes, the current lint ignores the type of foo. I’ve looked at that, but finding the method generics at that point is tricky.

[shssoichiro]

When this was originally implemented, I had also suggested searching for Pattern trait as an argument. At that time it was deemed too difficult, so this lint was implemented with a list of method names to check on. Hence this issue.

I think improving this lint to search for Pattern arguments rather than a list of method names would be the safest/most complete fix, but the consensus seems to be that implementing that would be difficult. If I have free time maybe I can go back in and give this a second shot.