dependencies foo?/bar that are disabled should not need to be available during dependency resolution

[decathorpe]

Problem

I'm trying to adapt the tooling that is used in Fedora Linux (and some other related distributions) to work with dep:foo and foo?/bar feature dependency syntax, but I have now hit a rather annoying roadblock:

It appears that conditionally enabled dependency features (accidentally?) are still required to be available during dependency resolution, even if they end up being completely unused (because they are not enabled). This might be an oversight from when this feature was implemented (for Rust 1.60.0), since other optional dependencies that are disabled aren't required to be available during dependency resolution either.

Needing to pull in additional dependencies (that end up being completely unused during the build) would be a real pain for Rust builds in Linux distributions, since it will increase the "breadth" of the dependency tree greatly for any crate that uses weak dependency features like this.

Steps

For example, in time v0.3.17, the "default" feature pulls in the following features as dependencies: {"default", "std", "alloc", "serde?/alloc"}. In this case, the dep:serde dependency isn't even included, and so the the serde?/alloc feature should be ignored. However, cargo still fails to build the crate if serde is not available: error: no matching package named serde found

Possible Solution(s)

Cargo should ignore optional dependencies that are only mentioned in disabled "weak dependency features" during dependency resolution (they are already ignored when building the crate), similar to other disabled optional dependencies.

Notes

Version

I have tested this with the cargo binary provided by Fedora Linux (because that's the only cargo that is available for our package builds):

cargo 1.67.0
release: 1.67.0
host: x86_64-unknown-linux-gnu
libgit2: 1.5.1 (sys:0.16.0 system)
libcurl: 7.87.0 (sys:0.4.59+curl-7.86.0 system ssl:OpenSSL/3.0.7)
os: Fedora 39.0.0 [64-bit]

However, no downstream patches are applied to cargo in Fedora Linux as far as I can tell, so this should be very close to cargo from an official Rust toolchain installed with rustup.

[epage]

I think the related issues to this are #11352 and #10801

[ehuss]

I'm going to close as a duplicate of #10801. For now, the resolver must assume that the dependency is used, since that information isn't available until after dependency resolution.

[c410-f3r]

Also dup of #11426