use_self lint false positive in macros

[pavel-mukhanov]

Minimal example

macro_rules! impl_binary_form_scalar {
    ($type:tt, $read:ident) => {
        impl BinaryValue for $type {
            fn to_bytes(&self) -> Vec<u8> {
                vec![*self as u8]
            }
        }
    };
}

This macros is invoked for

impl_binary_form_scalar! { u8,  read_u8 }

and

impl_binary_form_scalar! { i8,  read_i8 }

Clippy suggests to replace u8 with Self vec![*self as Self] which in case with i8 is not possible.
Also, I can't figure out how to disable lint in this particular place because

#![cfg_attr(feature="cargo-clippy", allow(clippy::use_self))]
fn to_bytes(&self) -> Vec<u8> {

is not working.

cargo +nightly clippy -V
clippy 0.0.212 (016d92d6 2019-03-10)

[oli-obk]

we should just silence this lint inside macros

[mati865]

🤔 #2098

[oli-obk]

😆 ok, the arguments go both ways. I guess we just need to figure out how to disable the lint with an allow.

@pavel-mukhanov have you tried #[allow(clippy::use_self)] without the cfg-attr?

[pavel-mukhanov]

@oli-obk it works if applied to impl defenition 👍

#[allow(clippy::use_self)]
impl BinaryValue for $type {
     fn to_bytes(&self) -> Vec<u8> {
           vec![*self as u8]
     }
}

[mati865]

I was about to say you can't use _ as Self without triggering E0605 and lint should skip not trigger in that cases.
After deeper thought it's possible to write such code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=62c6d2c8d16f5e39702ccfc65242cbd6

[oli-obk]

Ah, we should have more fine grained lint attribute checks. We're probably using check_item for the Self and then a visitor to look into the methods. That visitor should not use cx.span_lint directly. Instead we might have to resort to https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/struct.TyCtxt.html#method.struct_span_lint_hir