Cargo metadata includes unused crates

[arlyon]

Problem

Hello!

I am working on arlyon/async-stripe which uses tarpaulin for coverage tracking. I recently introduced the tracing crate to clean up the logging story, whose MSRV is 1.49.0 (same at the library). However, the MSRV of a transitive dependency, valuable, is 1.51.0 because it uses the new resolver. Now, valuable is not actually included in the build (as evidenced by cargo tree -i valuable) because it is disabled by default, but cargo metadata fails:

❯ cargo metadata        
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
error: failed to download `valuable v0.1.0`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/arlyon/.cargo/registry/src/github.com-1ecc6299db9ec823/valuable-0.1.0/Cargo.toml`

Caused by:
  feature `resolver` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["resolver"]` to enable this feature

My question / bug report is:

• does it make sense to report metadata about crates that are not being built
• if it does, can we support a flag to filter out those we don't want to include?

It feels strange having to bump my MSRV because of an unused optional transitive dependency breaking CI.

Steps

1. Create a project with rust 1.49.0: cargo +1.49.0 new metadata-repro
2. Add tracing-subscriber to your project: cargo +1.49.0 add tracing-subscriber
3. Run: cargo metadata: cargo +1.49.0 metadata

Possible Solution(s)

Assuming omitting the data for unused features is a breaking change, consider providing a flag such that only the dependencies actually used by your program are emitted.

Notes

No response

Version

cargo 1.49.0 (d00d64df9 2020-12-05)
release: 1.49.0
commit-hash: d00d64df9f803bf5bba8714ca498d8f9159d07f6
commit-date: 2020-12-05

[ehuss]

Hm, I'm not quite seeing the issue with tracing. Can you say which version you have, or more detail on how to reproduce?

~/Temp> cargo +1.49.0 new foo
     Created binary (application) `foo` package
~/Temp> cd foo
~/Temp/foo> cargo add tracing
    Updating 'https://github.com/rust-lang/crates.io-index' index
      Adding tracing v0.1.31 to dependencies
~/Temp/foo> cargo +1.49.0 metadata
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
    Updating crates.io index
{"packages":…}

I also don't see valuable in tracing's dependency tree.

[arlyon]

Hi! Thanks for taking a look! :) Sorry, my bad, seems it is tracing-subscriber that triggers this issue, not tracing. I have updated the repro instructions.

[ehuss]

Ah, I see. The issue is that tracing-core has [target."cfg(tracing_unstable)".dependencies.valuable]. cargo metadata needs to download the package in order to display the package information in the "packages" array. It is normal behavior for cargo metadata to include all packages that could potentially be used.

This can be avoided by using the --filter-platform option to limit which targets are included. Otherwise, I can't think of a way to avoid this issue. The only thing I can think of is for cargo to have first-class support for "unstable" features that would avoid the need for using the cfg(tracing_unstable) expression.

[arlyon]

I will have a look at the possibility of having tarpaulin potentially use the filter-platform flag. Thank you for the insight :)

[epage]

Looking over this, I'm not seeing any remaining action items (or opportunities to avoid this confusion in the future), so I'm going to close this. If there is a reason we should re-open it, let us know!