ptr_arg false positives

[clarfonthey]

Currently, impls on String and Vec will fire this lint, especially the ones in the standard library. This shouldn't fire if the methods are for &self, just when the type is used explicitly.

[oli-obk]

I'm assuming this will also trigger on impl Foo for String { fn bar(&self) {} }

needs a check whether the argument is the first argument of a method with self style (the latter part needs to be checked in the FnSig I think)

[JoshMcguigan]

@oli-obk I created a PR (#3131) after adding to the ptr_arg test file like you've described. I expected at that point to get a test failure, because the expectation was that this lint would fire (incorrectly) which wouldn't match the existing ptr_arg.stderr. But after adding the test case the tests still pass.

Can you provide additional guidance here? I am new to the clippy codebase and just want to make sure I'm not missing something.

Thanks!