The cargo team has been talking about this for some time now, and this is an issue I'd like to open to cc others and get some discussion about this.

The build-plan feature of Cargo is broken, and has always been broken. It has never actually accounted for all the build intricacies of crate graphs that Cargo supports. It was landed to unblock initial work with the understanding that it wouldn't be stabilized before it was fleshed out more, but the work to flesh it out more hasn't happened since then. The build plan does not support, for example:

• Transitively passing arguments from build scripts to later crates
• Encoding information about pipelining
• Requires implementors to have their own parsing of build script outputs and such

There is not a known great solution for these fundamental issues, and there has not been much effort to investigate this. Cargo has received very little maintenance of build plans and new features in Cargo typically don't consider keeping build plans working, leading to issues like #7604. Additionally build plans are only very lightly tested, so there's not a huge amount of protection against regressions.

Given all this the Cargo team is leaning towards deleting this feature. The existence of the feature seems to give the guise that it's supported and officially recommended, when in fact it's neither supported nor recommended. The feature, as implemented, seems dead in the water without significant refactorings that may require starting from scratch anyway.

The feature, however, currently has what we believe is enough users that we don't want to just outright delete it. We'd like to cc some folks who we believe are using this feature on this issue. If you're not using the feature or you're not interested, sorry for the cc but feel free to unsubscribe yourself!

The Cargo team is specifically interested in asking others if they are aware of the downsides of the build plan feature, and also if others are available to help fix this feature and implement a working system. The Cargo team does not have the bandwidth to either design a working build plan or implement it at this time, so both the design and implementation work is what we're interested in getting help for.

While it's useful to list out the bugs in the current build plan, the Cargo team is hesitant to land fixes such as #7604 at this time. These "small fixes" perpetrate the sub-par integration of build plans into Cargo today and makes the codebase harder to maintain. What we're particularly interested is figuring out a way to get build plans onto more solid footing such that they can actually be supported.

cc @edwin0cheng
cc @vlad20012
cc @matklad
cc @Manishearth