unused-crate-dependencies conflicts in lib and bin crates

[tae-soo-kim]

Problem

Cargo.toml

[package]
name = "hello"
version = "0.1.0"
edition = "2021"

[dependencies]
libc = "*"

src/lib.rs

#![deny(unused_crate_dependencies)]

src/main.rs

#![deny(unused_crate_dependencies)]

use hello as _;
use libc;

fn main() {
    let _: libc::size_t = 0;
}

In package hello, libc is needed by bin crate, not lib crate.

Steps

To reproduce, run cargo build. The build fails.

Possible Solution(s)

Because libc is needed by the bin crate, we have to list it as a package dependency. But then the lib crate complains.

One possible solution is to have each crate specify its own dependencies. The package-level dependencies would then be the union of all crate-level dependencies. But crate-level dependencies make more sense because users of a package may not want everything in the package. For example a user merely interested in using the library crate shouldn't be forced to pull dependencies required by binary crates.

Notes

No response

Version

No response

[weihanglo]

Thanks for the report. This seems like a duplicate of rust-lang/rust#95513 and rust-lang/rust#57274 and others alike. Close in favor of those.

[tae-soo-kim]

I don't really understand rust-lang/rust#57274, but rust-lang/rust#95513 and below links seem relevant.

• Allow dependencies that only apply to specific cargo targets (bin, example, etc.) #1982
• Allow specifying dependencies for individual artifacts rfcs#2887
• https://github.com/dmarcuse/rfcs/blob/master/text/0000-cargo-artifact-specific-dependencies.md
• https://github.com/apple/swift-evolution/blob/main/proposals/0226-package-manager-target-based-dep-resolution.md