Using mutagen breaks type inference in the existing program

[joshlf]

Given the following lib.rs:

fn foo() {}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
	let x = [0u8];
	assert_eq!((&x[..]).as_ref(), []);
    }
}

Running cargo check --tests succeeds. However, if we add an attribute, #[cfg_attr(test, mutagen::mutate)], to fn foo() {}, then cargo check --tests fails:

error[E0282]: type annotations needed
 --> src/lib.rs:9:2
  |
9 |     assert_eq!((&x[..]).as_ref(), []);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
  |
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

Version information

Using Mutagen pulled from this repository at 94321e5.

$ cargo version
cargo 1.43.0-nightly (bda50510d 2020-03-02)

[samuelpilz]

This is indeed unfortunate. I investigated this same issue some time ago: https://users.rust-lang.org/t/unexpected-compilation-error-with-procedural-macros/34310

The problem also appears when adding use json::parse; (when json is a dependency) instead of applying the #[mutate] procedural macro

[joshlf]

Ah so based on that issue, and from perusing the code, I suspect that just removing the dependency on json may do the trick. In particular:

• Mutagen itself never implements PartialEq for any foreign types
• Code generated by Mutagen doesn't import anything other than the mutagen crate

Thus, I can't think of where else the issue could be coming from. From a quick skim of the code, it looks like it should be pretty straightforward to replace the existing uses of the json crate with uses of serde_json instead.

[samuelpilz]

I thought serde_json used json internally. This is not the case.

One of the latest refactorings brought the json dependency into the core crate. I will move the only function that requires json into the runner crate (which is allowed to use the json dependency and it helps a lot)

Edit: This analysis is incorrect. I will try to substitute with serde_json instead

[samuelpilz]

serde_json seems to use the same PartialEq impls: https://github.com/serde-rs/json/blob/master/src/value/partial_eq.rs

adding use serde_json::from_str; to your example above produces the same error message