RFC: Cargo feature descriptions

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+- Feature Name: feature-documentation
+- Start Date: 2023-09-09
+- RFC PR: [rust-lang/rfcs#3485](https://github.com/rust-lang/rfcs/pull/3485)
+- Rust Issue:
+  [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+
+[summary]: #summary
+
+This RFC describes a new key to under `features` in `Cargo.toml` for
+documentation. This will allow Cargo to display this information to the user and
+provide a way for `rustdoc` to eventually render this data (how this is rendered
+is outside the scope of this RFC).
+
+Please see the parent meta RFC for background information: [`feature-metadata`].
+
+# Motivation
+
+[motivation]: #motivation
+
+Cargo features have become extremely widely used, with many crates having at
+least some level of configuration and larger crates winding up with tens of
+gates. Desipte being a first class component of crate structure, they suffer
+from a documentation problem: users need to maintain documentation separate from
+feature definition, typically a manually-created table within API docs.
+
+This RFC proposes adding feature documentation to `Cargo.toml`, which will allow
+for keeping feature definitions and documentation together.
+
+# Guide-level explanation
+
+[guide-level-explanation]: #guide-level-explanation
+
+A new `doc` key will be allowed within a feature's table. This key provides a
+markdown docstring describing the feature. Like with `#[doc(...)]`, the first
+line will be treated as a summary.
+
+```toml
+[features]
+# Feature without documentation
+foo = []
+
+# Short documentation comment
+bar = { enables = ["foo"], doc = "simple docstring here"}
+
+# Tables are preferred for longer descriptions
+[features.corge]
+enables = ["bar", "baz"]
+doc = """
+# corge
+
+This could be a longer description of this feature
+"""
+```
+
+See [`feature-metadata`] for information about `enables`.
+
+# Reference-level explanation
+
+[reference-level-explanation]: #reference-level-explanation
+
+The new `doc` key accepts markdown-flavored text, and should be thought of as
+the equivalent to a `#[doc(...)]` attribute. Like doc comments, the first line
+should be treated as a summary. Intra-doc link support is not included in this
+RFC, so they should not be used.
+
+There is nothing in this RFC that cargo **must** do with this action, since it is
+mainly intended for the consumption of `rustdoc` or `docs.rs`. However, it can
+be used for general diagnostic information such as during `cargo add` or a
+possible `cargo info` command. A sample application 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
+
+    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)*
+
+Any tools that want the information in `doc` will require access to the
+manifest. Adding this information to the index was decided against due to
+concerns about bloat, but this is further discussed in
+[future possibilities][future-possibilities].
+
+# Drawbacks
+
+[drawbacks]: #drawbacks
+
+- Added complexity to Cargo.
+  - Exact implementation details do add test surface area
+  - A markdown parser is required to properly parse the `doc` field.
+- Docstrings can be lengthy, adding noise to `Cargo.toml`. This could
+  potentially be solved with the below mentioned `doc-file` key.
+- When rendering features in documentation, this RFC does not specify any way
+  for `rustdoc` to get the information it requires. This will require separate
+  design work.
+- Unlike with the [`document-features`](https://crates.io/crates/document-features)
+  crate there is no way to group features in into sections or have a
+  user-specified layout
+- Users cannot control features ordering in documentation since the TOML specification defines table keys as unordered.
+
+# Rationale and alternatives
+
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- To avoid increasing the size of the registry index, this does not add `doc`
+  to a package's index entry.  This means a `.crate` file must be downloaded
+  and extracted to access the features.
+- Feature descriptions could be specified somewhere in Rust source files. This
+  has the downside of creating multiple sources of truth on features.
+- Cargo could parse doc comments in `Cargo.toml`, like the `document-features`
+  crate (linked below).
+
+  ```toml
+  # RFC proposal
+  foo = { enables = [], doc = "foo feature" }
+
+  # Alternative equivalent using doc comments
+  ## foo feature
+  foo = []
+  ```
+
+  This was decided against as part of this RFC because it would mean that
+  TOML-compliant parsers (including anything `serde`-based) would be
+  insufficient to extract all information in the manifest, requiring custom
+  deserialization of the fields via a format-preserving parser. This differs
+  from documentation in Rust source as the doc-comment behavior is described
+  specified within the grammar with parsers supporting extracting those
+  elements.
+
+# Prior art
+
+[prior-art]: #prior-art
+
+- There is an existing crate that uses TOML comments to create a features table:
+  <https://docs.rs/document-features/latest/document_features/>
+- `docs.rs` displays a feature table, but it is fairly limited. If features
+  start with `_`, they are hidden from this table ([example](https://docs.rs/crate/regex/latest/features)).
+- `lib.rs` extracts feature documentation from `Cargo.toml` and source ([example](https://lib.rs/crates/regex/features))
+
+# Unresolved questions
+
+[unresolved-questions]: #unresolved-questions
+
+- Rather than being consistent with `rustdoc` and accepting markdown, should the
+  `doc` key be consistent with `package.description` and only support plain
+  text? This RFC proposes making this decision at time of implementation when the
+  challenges of supporting markdown are better understood.
+
+# Future possibilities
+
+[future-possibilities]: #future-possibilities
+
+- Rustdoc can build on this to show feature documentation.
+- At some point, the decision to not include `doc` in the index could be
+  reevaluated. Including only the first (summary) line of `doc` could be a
+  possibility.
+- `cargo add` can show the `doc` and `deprecated` summary with the listed
+  features.
+- [`cargo-info`] can use this information to provide feature descriptions.
+- crates-io could be updated to render feature documentation
+- Feature documentation could be allowed in a separate markdown file. For
+  convenience, markdown anchors could be used to specify a section, so multiple
+  features can share the same file. This could be a good option for features
+  requiring long descriptions.
+
+  ```toml
+  foo = { enables = [], doc-file = "features.md#foo" }
+  bar = { enables = [], doc-file = "features.md#bar" }
+  ```
+
+[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
+[`feature-metadata`]: https://github.com/rust-lang/rfcs/pull/3416