[cargo fix] `unit_arg` removes calls to functions with side-effects

[soruh]

When running cargo fix --clippy -Z unstable-options the following happens:

fn function_with_side_effects_but_unit_return() {
    print("This is a side-effect!");
}

This

fn some_function() -> Result<()> {
    // ...
    Ok(function_with_side_effects_but_unit_return())
}

will be turned into this

fn some_function() -> Result<()> {
    // ...
    Ok(())
}

but it should either be turned into this

fn some_function() -> Result<()> {
    // ...
    function_with_side_effects_but_unit_return();
    Ok(())
}

or not be changed at all.

This behavior results from the unit_arg Lint, so that probably should be changed to no longer being MachineApplicable or it's "fixing" logic should be adjusted to produce the above result.

I don't know if the "Known problems" section is used for problems when using cargo fix, but I think while this is being looked into it might make sense to add this there.

Only tested on this version:

$ cargo clippy -V
clippy 0.0.212 (e8d5a9e 2019-10-22)
$ cargo -V
cargo 1.40.0-nightly (3a9abe3f0 2019-10-15)
$ rustc -V
rustc 1.40.0-nightly (4a8c5b20c 2019-10-23)

[flip1995]

Fix is in #4455, but is awaiting review.

[soruh]

Ah sorry, I seem to have missed that.
Do you think It would make sense to note this under the "Known Problems" section of the doc, while your PR is being reviewed?

[flip1995]

Yeah, I think it would make sense to turn this into MaybeIncorrect and add a note to the documentation, until #4455 is reviewed. Also a note can be added to the lint, that the unit returning expression might need to be preserved.