error when compiling with --allow warnings with certain targets

[dakom]

When I set RUSTFLAGS="--allow warnings" and try to cargo build --target wasm32-unknown-unknown I get the following error:

error: output of --print=file-names has changed in the compiler, cannot parse
command was: `rustc - --crate-name ___ --print=file-names --allow warnings --target wasm32-unknown-unknown --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg`

rustc 1.43.0-nightly (564758c4c 2020-03-08)
cargo 1.43.0-nightly (bda5051 2020-03-02)

originally filed here: rust-lang/rust#70070

[ehuss]

Hm, that's a tricky one. The --allow warnings prevents these lines from being printed:

warning: dropping unsupported crate type `dylib` for target `wasm32-unknown-unknown`
warning: dropping unsupported crate type `proc-macro` for target `wasm32-unknown-unknown`

This is required by Cargo in order to know which crate types are valid (and what their output names are).

I don't have any good ideas on how to accommodate this. Some bad ones:

• Filter RUSTFLAGS to remove things like "--allow".
• Filter RUSTFLAGS with an allowed list of flags. (Occasionally certain rustc flags cause problems with this code that queries rustc with the --print flag.)
• Make a better interface in rustc for querying this information (maybe like a more structured format like JSON).
  • Or a similar option would be for rustc to print some kind of separator between the different groups of output.

[epage]

If we had #8424, I wonder if we could still show that warning.

[ia0]

This bug is quite annoying because I'm not sure how to work around it right now. Even when specifying the crate type I get the error:

RUSTFLAGS=--allow=warnings cargo rustc --crate-type=cdylib --target=wasm32-unknown-unknown

The best I found so far is to just ignore all the output:

cargo check --target=wasm32-unknown-unknown 2>/dev/null

[mo8it]

Is this really a Cargo issue? It looks like a rustc argument parsing issue to me.

If you point me in the right direction, I would be happy to try to fix it since it currently affects Rustlings: rust-lang/rustlings#2036

[ehuss]

As mentioned above, this isn't really easy to fix. I think we may need to transition to a JSON interface, and that isn't simple.

What is your use case for -Awarnings? I generally wouldn't recommend using that flag, since it could silence important information.

[mo8it]

My use case in Rustlings is that I run cargo clippy and capture its output. Then, I run cargo test and don't want its output of cargo check. I only want the output of the tests. Is there a way to achieve this otherwise?

[ia0]

My use-case can most probably be done better. The idea is that I have exercises with todo!()s that have to be completed. I want the Cargo.lock files of those exercises to be up-to-date when I change (possibly nested) dependencies somewhere else in the mono-repo. So part of my CI is to cargo check --target=wasm32-unknown-unknown 2> /dev/null to make sure the Cargo.lock is idempotent. The warnings I want to hide are mostly unused_variables, unused_imports, and dead_code, because I don't want the person doing the exercise to care about removing leading underscores on variable names, or adding missing imports, etc. And I also don't want to see irrelevant warnings in my CI which should run without warnings (because sometimes it's also hard to make sure everything runs with --deny=warnings). My work-around of ignoring all output seems good enough so far. It didn't hide anything important as far as I'm aware of. So I'm really just describing my usecase as a data point for possible decisions in this issue.