error when compiling with --allow warnings and --target wasm32-unknown-unknown

[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