Crates Universe: The optional-only dependency is not fetched even if needed

[golovasteek]

I’m trying to build a rust binary, that has some optional dependencies in the Cargo.toml, for my configuration I want to use these optional dependencies. But it looks like the optional dependencies are not even fetched, and not added to the “@crate_index” repository. That means that even if I specify them explicitly in the rust_binar binary:

rust_binary(
    name = "gui",
    srcs = glob(["src/**/*.rs"]),
    deps = all_crate_deps() + [
        "@crate_index//:astc-decode",
    ],
)

they can not be used:

ERROR: /Users/evgenypetrov/work/bazel_playground/rust/gui/BUILD.bazel:5:12: no such target '@crate_index//:astc-decode': target 'astc-decode' not declared in package '' defined by /private/var/tmp/_bazel_evgenypetrov/0e70737357a2a67453272d7916c8876e/external/crate_index/BUILD.bazel (Tip: use `query "@crate_index//:*"` to see all the targets in that package) and referenced by '//rust/gui:gui'
ERROR: Analysis of target '//rust/gui:gui' failed; build aborted: Analysis failed

Here is the repo, that reproduces the problem

https://github.com/golovasteek/bazel_playground/blob/6f2ade625c80a73a0fcdfc8f936cd46963674454/rust/gui/BUILD.bazel

[UebelAndre]

Hi, does this occur on 0.21.1?

[golovasteek]

Yes. It still occurs on 0.21.1. I've update the example workspace with new release

https://github.com/golovasteek/bazel_playground/tree/main

[golovasteek]

If add a dummy cargo crate, that depends on "astc-decode" unconditionally, then I can use it in the initial package as well.

Here is the example change

https://github.com/golovasteek/bazel_playground/pull/1/files

[gferon]

I think it works as designed. Without any changes, your gui crate, which has sdl2 marked as optional, is being processed with default features (similar to what cargo would do). If you want to fetch its dependencies with another feature, you need to use a crate.annotation in the WORKSPACE.

See https://github.com/golovasteek/bazel_playground/pull/2 for a working example.

@UebelAndre maybe we could try to explain this in the documentation somewhere? I don't exactly know what would be the best place for that.

EDIT: I suppose we could rethink the whole think to mimick what Cargo.lock does, which contains all of the possible usable crates, but that would require a lot of effort, and is not worth it IMO because we already don't support features the way cargo does (since Bazel doesn't have this concept).

[UebelAndre]

I think I could agree that optional dependencies for workspace members should be rendered. But maybe not added to the dependencies api? Or maybe that's expanded to support an optional parameter that would include them? Either way, sounds like a good change to make if you wanted to work on a PR. I'm happy to collaborate and help push the change through if someone wanted to drive it 😄

[golovasteek]

I've spent some learning the code. I've found where the dependencies are lost initially. I'm not sure if and how I can add the test. And how to check influence on dependencies_api.

[TroyKomodo]

I think it works as designed. Without any changes, your gui crate, which has sdl2 marked as optional, is being processed with default features (similar to what cargo would do). If you want to fetch its dependencies with another feature, you need to use a crate.annotation in the WORKSPACE.

See golovasteek/bazel_playground#2 for a working example.

@UebelAndre maybe we could try to explain this in the documentation somewhere? I don't exactly know what would be the best place for that.

EDIT: I suppose we could rethink the whole think to mimick what Cargo.lock does, which contains all of the possible usable crates, but that would require a lot of effort, and is not worth it IMO because we already don't support features the way cargo does (since Bazel doesn't have this concept).

Does this actually work?

crate.annotation(
    crate = "member",
    crate_features = ["serde"],
)

This doesnt seem to actually cause the feature to be included when doing dependency resolution.