2024: Document cargo changes

src/SUMMARY.md

@@ -44,4 +44,6 @@
   - [Disallow references to `static mut`](rust-2024/static-mut-reference.md)
   - [Public/private dependencies](rust-2024/public-private-dependencies.md)
   - [Cargo: Remove implicit features](rust-2024/cargo-remove-implicit-features.md)
+  - [Cargo: Table and key name consistency](rust-2024/cargo-table-key-names.md)
+  - [Cargo: Reject unused inherited default-features](rust-2024/cargo-inherited-default-features.md)
   - [Rustfmt: Combine all delimited exprs as last argument](rust-2024/rustfmt-overflow-delimited-expr.md)

src/rust-2024/cargo-inherited-default-features.md

@@ -0,0 +1,52 @@
+# Cargo: Reject unused inherited default-features
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+
+## Summary
+
+- `default-features = false` is no longer allowed in an inherited workspace dependency if the workspace dependency specifies `default-features = true` (or does not specify `default-features`).
+
+## Details
+
+[Workspace inheritance] allows you to specify dependencies in one place (the workspace), and then to refer to those workspace dependencies from within a package.
+There was an inadvertent interaction with how `default-features` is specified that is no longer allowed in the 2024 Edition.
+
+Unless the workspace specifies `default-features = false`, it is no longer allowed to specify `default-features = false` in an inherited package dependency.
+For example, with a workspace that specifies:
+
+```toml
+[workspace.dependencies]
+regex = "1.10.4"
+```
+
+The following is now an error:
+
+```toml
+[package]
+name = "foo"
+version = "1.0.0"
+edition = "2024"
+
+[dependencies]
+regex = { workspace = true, default-features = false }  # ERROR
+```
+
+The reason for this change is to avoid confusion when specifying `default-features = false` when the default feature is already enabled, since it has no effect.
+
+If you want the flexibility of deciding whether or not a dependency enables the default-features of a dependency, be sure to set `default-features = false` in the workspace definition.
+Just beware that if you build multiple workspace members at the same time, the features will be unified so that if one member sets `default-features = true` (which is the default if not explicitly set), the default-features will be enabled for all members using that dependency.
+
+## Migration
+
+When using `cargo fix --edition`, Cargo will automatically update your `Cargo.toml` file to remove `default-features = false` in this situation.
+
+If you would prefer to update your `Cargo.toml` manually, check for any warnings when running a build and remove the corresponding entries.
+Previous editions should display something like:
+
+```
+warning: /home/project/Cargo.toml: `default-features` is ignored for regex,
+since `default-features` was not specified for `workspace.dependencies.regex`,
+this could become a hard error in the future
+```
+
+[workspace inheritance]: ../../cargo/reference/specifying-dependencies.html#inheriting-a-dependency-from-a-workspace

src/rust-2024/cargo-remove-implicit-features.md

@@ -2,11 +2,46 @@
 
 🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
 
-This feature has not yet been implemented.
-More information may be found in the tracking issue at <https://github.com/rust-lang/cargo/issues/12826>.
-
 ## Summary
 
+- Optional dependencies must now be explicitly specified in the `[features]` table.
+
 ## Details
 
+In previous editions, when an [optional dependency] is specified, Cargo would automatically add an implicit [feature] of the same name as the dependency. For example:
+
+```toml
+[dependencies]
+jpeg-decoder = { version = "0.3.1", optional = true }
+```
+
+This would automatically add a feature `jpeg-decoder = ["dep:jpeg-decoder"]` to provide a way to enable the dependency.
+The `dep:` entries are specific syntax for referring to optional dependencies.
+This implicit feature is only added if `"dep:jpeg-decoder"` is not specified in any other feature.
+
+In the 2024 Edition, this implicit feature is no longer added, and you are required to explicitly specify the dependency in the `[features]` table.
+For example, instead of exposing the particular internal name of some dependency, you may consider using a more general term for the feature name:
+
+```toml
+[features]
+graphics = ["dep:jpeg-decoder"]
+```
+
+`cargo add --optional <NAME>` automatically adds a feature for the dependency to the `[features]` table if it isn't already there.
+
+### Motivation
+
+A reason for requiring this to be explicit is that it encourages a conscious decision about the public exposure of the feature name, and makes it clearer when reading the `[features]` table which features exist.
+This can help avoid tying the implementation details (the dependency names) to the public set of feature names.
+
+Also, removing features is a [SemVer incompatible change][semver], which may not be obvious when removing an optional dependency that you thought was private.
+
 ## Migration
+
+When using `cargo fix --edition`, Cargo will automatically update your `Cargo.toml` file to include the implicit features if necessary.
+
+If you would prefer to update your `Cargo.toml` manually, add a `foo = ["dep:foo"]` entry for each optional dependency named *foo* if `dep:foo` is not already specified anywhere in the `[features]` table.
+
+[optional dependency]: ../../cargo/reference/features.html#optional-dependencies
+[feature]: ../../cargo/reference/features.html
+[semver]: ../../cargo/reference/semver.html#cargo-feature-remove

src/rust-2024/cargo-table-key-names.md

@@ -0,0 +1,42 @@
+# Cargo: Table and key name consistency
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+
+## Summary
+
+- Several table and key names in `Cargo.toml` have been removed where there was previously two ways to specify it.
+    - Removed `[project]`, use `[package]` instead.
+    - Removed `default_features`, use `default-features` instead.
+    - Removed `crate_type`, use `crate-type` instead.
+    - Removed `dev_dependencies`, use `dev-dependencies` instead.
+    - Removed `build_dependencies`, use `build-dependencies` instead.
+
+## Details
+
+Several table and keys names are no longer allowed in the 2024 Edition.
+There were two ways to specify these tables or keys, and this helps ensure there is only one way to specify them.
+
+Some were due to a change in decisions over time, and some were inadvertent implementation artifacts.
+In order to avoid confusion, and to enforce a single style for specifying these tables and keys, only one variant is now allowed.
+
+For example:
+
+```toml
+[dev_dependencies]
+rand = { version = "0.8.5", default_features = false }
+```
+
+Should be changed to:
+
+```toml
+[dev-dependencies]
+rand = { version = "0.8.5", default-features = false }
+```
+
+Notice that the underscores were changed to dashes for `dev_dependencies` and `default_features`.
+
+## Migration
+
+When using `cargo fix --edition`, Cargo will automatically update your `Cargo.toml` file to use the preferred table and key names.
+
+If you would prefer to update your `Cargo.toml` manually, be sure to go through the list above and make sure only the new forms are used.