`mismatched_target_os` false negative when using the `cfg` macro instead of the attribute.

[VZout]

Lint name: mismatched_target_os

I tried this code:

fn main() {
    // Clippy throws error
    //#[cfg(linux)]
    //println!("Hello, Linux!");

    // Clippy doesn't care
    if cfg!(linux) {
        println!("Hello, Linux!");
    }
}

I expected to see the error operating system used in target family position here like you would see if you used the attribute #[cfg(linux)]

Instead, clippy doesn't give any error.

Meta

Rust version (rustc -Vv):

rustc 1.52.1 (9bc8c42bb 2021-05-09)
binary: rustc
commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
commit-date: 2021-05-09
host: x86_64-pc-windows-msvc
release: 1.52.1
LLVM version: 12.0.0

[fanzeyi]

Agree, I came here to report the same issue. It would be great if clippy can detect this. A very easy mistake to make :)

[fanzeyi]

I tried to take this on and I have a working draft. I still need to tidy up -- reducing duplicated code and write tests, but I think I got it mostly working. I will try to open a PR tomorrow night. Getting late here. :)

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