RFC: Remove implicit features in a new edition

text/3491-remove-implicit-features.md

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+- Feature Name: `remove-implicit-feature`
+- Start Date: 2023-09-18
+- RFC PR: [rust-lang/rfcs#3491](https://github.com/rust-lang/rfcs/pull/3491)
+- Tracking Issue: [rust-lang/cargo#12826](https://github.com/rust-lang/cargo/issues/12826)
+
+# Summary
+[summary]: #summary
+
+By default, cargo will treat any optional dependency as a
+[feature](https://doc.rust-lang.org/cargo/reference/features.html).
+As of cargo 1.60, these can be disabled by declaring a feature that activates
+the optional dependency as `dep:<name>`
+(see [RFC #3143](https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html)).
+
+On the next edition, cargo will stop exposing optional dependencies as features
+implicitly, requiring users to add `foo = ["dep:foo"]` if they still want it exposed.
+
+# Motivation
+[motivation]: #motivation
+
+While implicit features offer a low overhead way of defining features,
+- It is easy to overlook using `dep:` when the optional dependency is not intended to be exposed
+  - Making it easy for crate authors to use the wrong syntax and be met with errors ([rust-lang/cargo#10125](https://github.com/rust-lang/cargo/issues/10125))
+  - Potentially breaking people if they are later removed ([rust-lang/cargo#12687)](https://github.com/rust-lang/cargo/pull/12687))
+  - Leading to confusing choices when `cargo add` lists features that look the same (e.g. `cargo add serde` showing `derive` and `serde_derive`)
+  - Leading to confusing errors for callers when they reference the dependency, instead of the feature, and things don't work right
+- Tying feature names to dependency names is a code smell because it ties the API to the implementation
+- It requires people and tools to deal with this special case ([rust-lang/cargo#10543](https://github.com/rust-lang/cargo/issues/10543))
+  - Granted, anything having to deal with old editions will still have to deal with this
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+
+
+Updated documentation:
+
+## Optional Dependencies
+*(Replaces the [same section in the book](https://doc.rust-lang.org/cargo/reference/features.html#optional-dependencies))*
+
+Dependencies can be marked "optional", which means they will not be compiled
+by default.
+
+For example, let's say that our 2D image processing library uses
+an external package to handle GIF images. This can be expressed like this:
+
+```toml
+[features]
+gif = ["dep:giffy"]
+
+[dependencies]
+giffy = { version = "0.11.1", optional = true }
+```
+
+This means that this dependency will only be included if the `gif`
+feature is enabled.
+
+> **Note**: Prior to the 202X edition, features were implicitly created for
+> optional dependencies not referenced via `dep:`.
+
+> **Note**: Another way to optionally include a dependency is to use
+> [platform-specific dependencies]. Instead of using features, these are
+> conditional based on the target platform.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## Existing editions
+
+As is expected for edition changes, `cargo fix --edition` will add `foo = ["dep:foo"]` features as needed.
+Where undesired, users can remove these and switch their references to the
+dependency from `foo` to `dep:foo`,
+dealing with the [potential breaking changes](https://doc.rust-lang.org/cargo/reference/semver.html#cargo-remove-opt-dep).
+
+Ideally, this will be accomplished by `cargo` emitting an allow-by-default
+warning when parsing a workspace member's package when an optional dependency
+is not referenced via `dep:` in the features
+([rust-lang/cargo#9088](https://github.com/rust-lang/cargo/issues/9088))
+using the planned warning control system
+([rust-lang/cargo#12235](https://github.com/rust-lang/cargo/issues/12235)).
+The warning will be named something like `cargo::implicit_feature` and be part
+of the `cargo::rust-202X-compatibility` group.
+
+Suggested text:
+```
+implicit features for optional dependencies is deprecated and will be unavailable in the 202X edition.
+```
+This would be machine applicable with a suggestion to add `foo = ["dep:foo"]`.  `cargo fix` would then insert this feature.
+
+If that system is not ready in time, we can always hard code the change in `cargo fix`.
+
+## Next edition
+
+On the next edition, this warning will be a hard error.
+
+Suggested text:
+```
+unused optional dependency `foo`.  To use it, a feature must activate it with `dep:foo` syntax
+```
+This could be machine applicable with a suggestion to add `foo = ["dep:foo"]`.
+
+## Other
+
+To help users through this, `cargo add` will be updated so that `cargo add foo
+--optional` will create a `foo = ["dep:foo"]` if its not already referenced by
+another features
+([rust-lang/cargo#11010](https://github.com/rust-lang/cargo/issues/11010)).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Some boilerplate is needed for all features, rather than just a subset
+- Extra ecosystem churn on the next edition update
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Instead of a `cargo::rust-202X-compatibility` group, we could put this in
+`rust-202X-compatibility` so there is one less group to enable to prepare for
+the next edition at the risk of user confusion over a "rust" lint controlling cargo.
+
+Instead of an error, optional dependencies could be
+- Make optional dependencies private features ([RFC #3487](https://github.com/rust-lang/rfcs/pull/3487))
+  - Seems like the need for this would be relatively low and be less obvious
+  - We could still transition to this in the future
+- Allow access to the feature via `#[cfg(accessible)]` ([RFC #2523](https://rust-lang.github.io/rfcs/2523-cfg-path-version.html))
+
+# Prior art
+[prior-art]: #prior-art
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+# Future possibilities
+[future-possibilities]: #future-possibilities