cmp_owned false positives - proc_macro Ident

[elichai]

Hi,
I noticed a false positive when comparing a string with ident.to_string()
clippy suggests you don't need to add to_string() and you can instead dereference the ident.

1. You can't dereference it because there's no reference involved.
2. there's no PartialEq implementation between &str/String and Ident.

Example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b6ae15acb40aee8fe582f1620ac569d7

Your example is:

extern crate proc_macro2;
use proc_macro2::*;

fn main() {
    let a1 = Ident::new("Hi", Span::call_site());
    if "Hi" == a1.to_string() {
        println!("yay");
    } else {
        println!(":(");
    }
}

proc-macro2 has impl<T: ?Sized> PartialEq<T> for Ident where T: AsRef<str> so you can write
a1 == "Hi" to avoid creating the string.

The reason for the bug is that the lint assumes that PartialEq is implemented symmetrically i.e. T: PartialEq<U> implies U: PartialEq<T>. The PartialEq docs actually require this so maybe proc-macro2 is missing an impl.

[elichai]

Hi, Nice catch :)
bad I'm not sure the solution you're proposing is even possible: https://play.rust-lang.org/?gist=75e6e15847489b8c6ee7c8af6507dd66