Proposal: Adding a prune command/flag to the cargo CLI

[coillteoir]

Problem

Dependencies may be left orphaned or unused in a project as it grows and changes. This can increase build times and storage requirements for developer workstations. While it is up to developers to ensure their dependencies are in order, issuing a cargo command to remove these dependencies would be beneficial.

This seems like a common enough use case among developers to integrate it with the Cargo CLI.

Proposed Solution

Implementing a prune command cargo prune --[flags] to automatically mark unused dependencies. Some flags to consider would be a dry-run flag, dev dependencies only, and more.

Notes

Docker has a similar command docker image prune which will remove all unused images in a local registry. docs

Npm has a similar solution. docs

There is an existing solution to this problem. github

[epage]

Huh, I thought we had an issue for this but I can't find it.

Existing solutions

• https://crates.io/crates/cargo-machete
• https://crates.io/crates/cargo-udeps

The big challenge with this is combining reliability with being able to detect unused deps across all build configurations.

If we did this, I imagine this would work better to be a lint controlled by #12235 that would support cargo fix removing the dependency.

[ehuss]

I think the primary issue tracking this is rust-lang/rust#57274, which contains some commentary about why this is very hard and some ideas for solutions.

[Eh2406]

By the way, the link npm and docker commands are not removing the unused dependency requirement they are instead cleaning up the built artifacts. To translate cargo they are cleaning the target directory, not the cargo.toml. So the work on GC is relevant, at least based on links if not the description.

[coillteoir]

Ah okay, thank you. I knew docker's prune is just to remove images without running containers, I thought npm was tidying up unused dependencies.