Use of RUSTFLAGS="--error-format=json" causes cargo mutants to fail

[TomMD]

$ RUSTFLAGS="--error-format=json" cargo mutants
...
error: Option 'error-format' given more than once

That error is repeated many times till termination with:

*** result: Failure

   1.281211253s ERROR cargo build failed in an unmutated tree, so no mutants were tested

[sourcefrog]

This seems to come from the fact that cargo build fails with that RUSTFLAGS set, regardless of anything in cargo-mutants:

; env RUSTFLAGS="--error-format=json" cargo build 
   Compiling unix_mode v0.1.4-pre (/home/mbp/src/unix_mode)
error: Option 'error-format' given more than once

error: could not compile `unix_mode` (lib)

I'm testing here on a very simple tree that has no build.rs, https://github.com/sourcefrog/unix_mode.

This looks a bit like rust-lang/cargo#6375 or rust-lang/cargo#11452 but I'm not sure it's exactly the same. It seems more like a cargo bug than a cargo-mutants bug.

It would be possible for cargo-mutants to parse RUSTFLAGS and strip out things that we might think won't work with cargo build, but I don't know if that really fits here. Maybe it's better to just not pass it.

I filed rust-lang/cargo#12486 and unless some new information emerges I don't think there's anything to do in cargo-mutants.

[sourcefrog]

As @ehuss commented there rust-lang/cargo#12486

If you want JSON output, the correct way to do that is cargo --message-format=json.