identity_conversion false positive (macro + ?)

[realcr]

Hi, I think I'm having a false positive with the identity_conversion lint. Minimal example:

pub trait SafeSignedArithmetic: Copy {
    type Unsigned;

    fn checked_add_unsigned(self, u: Self::Unsigned) -> Option<Self>;
}

macro_rules! impl_safe_signed_arithmetic {
    ( $i:ty, $u:ty ) => {
        impl SafeSignedArithmetic for $i {
            type Unsigned = $u;

            fn checked_add_unsigned(self, u: $u) -> Option<$i> {
                let u_half = (u / 2) as $i;
                let u_rem = (u % 2) as $i;

                self.checked_add(u_half)?
                    .checked_add(u_half)?
                    .checked_add(u_rem)
            }
        }
    };
}

impl_safe_signed_arithmetic!(i8, u8);

fn main() {
    println!("Hello, world!");
}

Versions:

$ cargo clippy -V
clippy 0.0.212 (3710ec5 2019-05-11)

$ rustc --version
rustc 1.36.0-nightly (af98304b9 2019-05-11)

Clippy's output:

$ cargo clippy
    Checking check_identical_conversion v0.1.0 (/home/real/temp/check_identical_conversion)
warning: identical conversion
  --> src/main.rs:17:41
   |
17 |                 self.checked_add(u_half)?
   |                                         ^
...
25 | impl_safe_signed_arithmetic!(i8, u8);
   | ------------------------------------- in this macro invocation
   |
   = note: #[warn(clippy::identity_conversion)] on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_conversion

warning: identical conversion
  --> src/main.rs:18:41
   |
18 |                     .checked_add(u_half)?
   |                                         ^
...
25 | impl_safe_signed_arithmetic!(i8, u8);
   | ------------------------------------- in this macro invocation
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_conversion

    Finished dev [unoptimized + debuginfo] target(s) in 0.20s

Interestingly enough, if I try to implement this code without the macro, the clippy warnings disappear.

Thank you for developing clippy!

[flip1995]

This is already fixed in the latest release of Clippy: #4082 #4089

Playground

Try running rustup update nightly and try it again. If the issue still persists feel free to reopen.

[Manishearth]

Beta is coming soon, we should consider backports