RFC: Unblock Cargo feature metadata

text/3416-feature-metadata.md

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+- Feature Name: feature-metadata
+- Start Date: 2023-04-14
+- RFC PR: [rust-lang/rfcs#3416](https://github.com/rust-lang/rfcs/pull/3416)
+- Rust Issue:
+  [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+
+[summary]: #summary
+
+This RFC adds a "detailed" feature definition:
+```toml
+[features]
+# same as `foo = []`
+foo = { enables = [] }
+```
+
+This is to unblock the following RFCs:
+
+- [Cargo feature descriptions](https://github.com/rust-lang/rfcs/pull/3485)
+- [Cargo feature deprecation](https://github.com/rust-lang/rfcs/pull/3486)
+- [Cargo feature visibility](https://github.com/rust-lang/rfcs/pull/3487)
+
+# Motivation
+
+[motivation]: #motivation
+
+Features are widely used as a way to do things like reduce dependency count,
+gate `std` or `alloc`-dependent parts of code, or hide unstable API. Use is so
+common that many larger crates wind up with tens of feature gates, such as
+[`tokio`] with 24. Despite being a first class component of crate structure,
+there are some limitations that don't have elegant solutions:
+
+- Documentation is difficult, often requiring library authors to manually manage
+  a table of descriptions
+- There is no way to deprecate old features, as a way to help crates maintain
+  semvar compliance
+- Features cannot be hidden from use in any way
+
+This RFC proposes a plan that add that information to `Cargo.toml`, solving
+these problems.
+
+# Guide-level explanation
+
+[guide-level-explanation]: #guide-level-explanation
+
+Usage is simple: features will be able to be specified as a table, instead of
+just a dependency array. This sample section of `Cargo.toml` shows new
+possibilities:
+
+```toml
+[features]
+# Current configuration will continue to work
+foo = []
+# New configurations
+bar = { enables = ["foo"], doc = "simple docstring here"}
+baz = { enables = ["foo"], public = false}
+qux = { enables = [], deprecated = true }
+quux = { enables = [], deprecated = { since = "1.2.3", note = "don't use this!" } }
+
+# Features can also be full tables if descriptions are longer
+[features.corge]
+enables = ["bar", "baz"]
+doc = """
+# corge
+
+This could be a longer description of this feature
+"""
+```
+
+The `enables` key is synonymous with the existing array, describing what other
+features are enabled by a given feature. For example,
+`foo = ["dep:serde", "otherfeat"]` will be identical to
+`foo = { enables = ["dep:serde", "otherfeat"] }`
+
+All other keys are described in their individual RFCs.
+
+## General Implementation & Usage
+
+Use cases for these new keys will likely develop with time,
+but one of the simplest applications is for information output with `cargo
+add`:
+
+```text
+crab@rust foobar % cargo add regex
+    Updating crates.io index
+      Adding regex v1.7.3 to dependencies.
+             Features:
+             + perf             Enables all performance related features
+             + perf-dfa         Enables the use of a lazy DFA for matching
+             + perf-inline      Enables the use of aggressive inlining inside
+                                match routines
+             + perf-literal     Enables the use of literal optimizations for
+                                speeding up matches
+             + std              When enabled, this will cause regex to use the
+                                standard library
+             + unicode          Enables all Unicode features
+             - deprecated (D)   Not a real feature, but it could be
+
+    Updating crates.io index
+```
+
+Features like `aho-corasick`, `memchr`, or `use_std` would likely be
+`public = false` since they aren't listed on the crate landing page.
+
+# Reference-level explanation
+
+[reference-level-explanation]: #reference-level-explanation
+
+`enables` will take the place of the feature dependency array that currently
+exists. Semantics will remain unchanged.
+
+This is a required key. If there are no requirements, an empty list should be
+provided (`enables = []`). This content is already in the index.
+
+The availability of this new syntax should not require an MSRV bump.
+This means we need to make sure that if you use `feature_name = []` in your `Cargo.toml`,
+then the published `Cargo.toml` should as well.
+However, we leave it as an implementation detail whether using `feature_name = { enables =[] }`
+requires an MSRV bump for users of your published package as we have not been
+actively streamlining the workflow for maintaining separate development and
+published MSRVs.
+
+# Drawbacks
+
+[drawbacks]: #drawbacks
+
+- Added complexity to Cargo. Parsing is trivial, but exact implementation
+  details do add test surface area
+- Extending the `Cargo.toml` schema, particularly having a field support
+  additional types, is disruptive to third-party parsers
+
+# Rationale and alternatives
+
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+This RFC has no impact on the Index Summaries.
+Future RFCs will need to work with that.
+
+## Naming
+
+- `enables` reads better on the line than `enable`
+- `enables` is likely an easier word for non-native speakers than `activates`
+- `required` is used elsewhere to say "this should automatically be available if requirements are met"
+
+## Schema
+
+We could split the special feature syntax (`dep:`, etc) as distinct fields
+but we'd prefer trivial conversion from the "simple" schema to the "detailed" schema,
+like `dependencies`.
+However, we likely would want to prefer using new fields over adding more syntax,
+like with [disabling default features](https://github.com/rust-lang/cargo/issues/3126).
+
+# Prior art
+
+[prior-art]: #prior-art
+
+# Unresolved questions
+
+[unresolved-questions]: #unresolved-questions
+
+# Future possibilities
+
+[future-possibilities]: #future-possibilities
+
+- [Cargo feature descriptions](https://github.com/rust-lang/rfcs/pull/3485)
+- [Cargo feature deprecation](https://github.com/rust-lang/rfcs/pull/3486)
+- [Cargo feature visibility](https://github.com/rust-lang/rfcs/pull/3487)
+- [Cargo feature stability](https://github.com/rust-lang/cargo/issues/10881)
+
+[cargo #12335]: https://github.com/rust-lang/cargo/issues/12235
+[cargo #10882]: https://github.com/rust-lang/cargo/issues/10882
+[`cargo-info`]: https://github.com/rust-lang/cargo/issues/948
+[`deprecated`]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
+[`deprecated-suggestions`]: https://github.com/rust-lang/rust/issues/94785
+[discussion on since]: https://github.com/rust-lang/rfcs/pull/3416#discussion_r1172895497
+[`public_private_dependencies`]: https://rust-lang.github.io/rfcs/1977-public-private-dependencies.html
+[`rustdoc-cargo-configuration`]: https://github.com/rust-lang/rfcs/pull/3421
+[`tokio`]: https://docs.rs/crate/tokio/latest/features
+[visibility attribute]: https://ant.apache.org/ivy/history/latest-milestone/ivyfile/conf.html