Running `cargo clippy` after `cargo new` fails with `eq_op`

[whatisaphone]

(Issue originally mentioned here)

Running cargo clippy directly after cargo new fails. This is maybe just my opinion, but it seems a little unfortunate that the default project template doesn't pass the linter.

It fails on assert_eq!(2 + 2, 4). I'm pretty sure that used to be fine. I would expect clippy to flag assert_eq!(4, 4), but not assert_eq!(2 + 2, 4).

---

Lint name: eq_op

I tried this code:

cargo +nightly-2021-01-23 new --lib foo
cd foo
cargo +nightly-2021-01-23 clippy --all-targets

(This is the default `src/lib.rs`, for reference)

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}

I expected to see this happen:

no errors

Instead, this happened:

error: identical args used in this `assert_eq!` macro call
 --> src\lib.rs:5:20
  |
5 |         assert_eq!(2 + 2, 4);
  |                    ^^^^^^^^
  |
  = note: `#[deny(clippy::eq_op)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#eq_op

error: aborting due to previous error

error: could not compile `foo`

Meta

clippy 0.1.51 (22ddcd1a 2021-01-22)

rustc 1.51.0-nightly (22ddcd1a1 2021-01-22)
binary: rustc
commit-hash: 22ddcd1a13082b7be0fc99b720677efd2b733816
commit-date: 2021-01-22
host: x86_64-pc-windows-msvc
release: 1.51.0-nightly
LLVM version: 11.0.1

[rail-rain]

Linting cases of equality in terms of value instead of tokens like 2 + 2 == 4 is a feature of this lint from the beginning. Such cases in the test, and the original author spent time to write a fair amount of code to cover these cases. I think what made difference is #6167, which added assert_eq! and its variation to the lint. I agree with that this is "a little unfortunate", but I don't consider this is a false positive that needs to be fixed. Now this lint defines assert_eq!(2 + 2, 4); is problematic, and it would be if it was in production.

[rail-rain]

I had a second thought that this lint might need to ignore everthing in tests, and opened a new issue, which also solves this issue. I still think it is fine to lint assert_eq!(2 + 2, 4) if it's in somewhere other than tests though.

[DevinR528]

Is this something that can be fixed? I would be interested in working on it, something along the lines of

if expr_in_fn_with_name("it_works", expr) && in_file_named("lib", expr.span) { return; }
// or
if snippet(expr.span) == "assert_eq!(2+2, 4)" && in_file_named("lib", expr.span) { return; }

Basically just so there are no errors on a brand new project, the other option would be to change cargo new to do something more interesting?

[rail-rain]

Update: eq_op now ignores all test code (#7811), and therefore Clippy no longer emits an error on it_works. This issue can be closed.

[giraffate]

Thanks for letting us know!