Tracking Issue for feature-metadata

[epage]

Summary

RFC: #3416
Implementation: #NNNN
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#feature-metadata

Extend the schema for feature activations to be a table, not just an array, so more fields can be added in the future.

Unresolved Issues

No response

Future Extensions

• RFC: Cargo feature descriptions rfcs#3485
• RFC: Cargo feature deprecation rfcs#3486
• RFC: Cargo feature visibility (private/public) rfcs#3487

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

[amab8901]

The docs link u posted (https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#feature-metadata) seems obsolete since there is no section called feature-metadata in that page

[weihanglo]

@amab8901 it is just a placeholder because it hasn't yet implemented

[amab8901]

I think I found the place in the cargo source code that needs to be changed to implement this feature:

The Vec<String> should probably be replaced with BTreeMap<String,String>. At least that's my first thought. But then I realize it will prob break the old syntax. So now I'm thinking maybe we need some kind of generic? Or perhaps some kind of match statement to deal with Vec vs BTreeMap separately?

[epage]

That section of code is an internal data structure to track cargo adds command-line. Cargo.tomls definition is in crates/cargo-util-schemas. An untagged enum would need to be used, much like we have "simple" and "detailed" dependencies. All of this would need to be behind an unstable feature as mentioned behind our contributor guide. We'd also need to make sure that people not using this syntax aren't switched to it on cargo publish so we don't force everyone to upgrade cargo.

With all that said, I'm unsure how much benefit there is on moving this forward until we have fields to take advantage of it.

[amab8901]

do u have an RFC for the fields that we are waiting for?

[weihanglo]

See future extensions in the PR description.

[amab8901]

My understanding is that you first need to solve this RFC before you can add those fields. Otherwise, when you try to add those fields, the compiler will complain and say "this is invalid syntax in Cargo.toml !"

[tgross35]

If you are looking to put up a PR, I think the thing that could be changed now is to add a new unstable feature that allows parsing the alternate syntax for feature dependencies:

[features]
foo = []
bar = ["foo"]
# This should be the same as the above
bar = { enables = ["foo"] }

This was accepted with RFC3416. The other attributes (e.g. doc) can't yet be added since they haven't yet been accepted, but it will be easy to expand to them once this groundwork is in.