chore(lints): Add documentation to public lints

src/cargo/util/lints.rs

@@ -256,6 +256,7 @@ pub struct LintGroup {
     pub feature_gate: Option<&'static Feature>,
 }
 
+/// This lint group is only to be used for testing purposes
 const TEST_DUMMY_UNSTABLE: LintGroup = LintGroup {
     name: "test_dummy_unstable",
     desc: "test_dummy_unstable is meant to only be used in tests",
@@ -272,6 +273,10 @@ pub struct Lint {
     pub default_level: LintLevel,
     pub edition_lint_opts: Option<(Edition, LintLevel)>,
     pub feature_gate: Option<&'static Feature>,
+    /// This is a markdown formatted string that will be used when generating
+    /// the lint documentation. If docs is `None`, the lint will not be
+    /// documented.
+    pub docs: Option<&'static str>,
 }
 
 impl Lint {
@@ -420,13 +425,15 @@ fn level_priority(
     }
 }
 
+/// This lint is only to be used for testing purposes
 const IM_A_TEAPOT: Lint = Lint {
     name: "im_a_teapot",
     desc: "`im_a_teapot` is specified",
     groups: &[TEST_DUMMY_UNSTABLE],
     default_level: LintLevel::Allow,
     edition_lint_opts: None,
     feature_gate: Some(Feature::test_dummy_unstable()),
+    docs: None,
 };
 
 pub fn check_im_a_teapot(
@@ -476,26 +483,57 @@ pub fn check_im_a_teapot(
     Ok(())
 }
 
-/// By default, cargo will treat any optional dependency as a [feature]. As of
-/// cargo 1.60, these can be disabled by declaring a feature that activates the
-/// optional dependency as `dep:<name>` (see [RFC #3143]).
-///
-/// In the 2024 edition, `cargo` will stop exposing optional dependencies as
-/// features implicitly, requiring users to add `foo = ["dep:foo"]` if they
-/// still want it exposed.
-///
-/// For more information, see [RFC #3491]
-///
-/// [feature]: https://doc.rust-lang.org/cargo/reference/features.html
-/// [RFC #3143]: https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html
-/// [RFC #3491]: https://rust-lang.github.io/rfcs/3491-remove-implicit-features.html
 const IMPLICIT_FEATURES: Lint = Lint {
     name: "implicit_features",
     desc: "implicit features for optional dependencies is deprecated and will be unavailable in the 2024 edition",
     groups: &[],
     default_level: LintLevel::Allow,
     edition_lint_opts: None,
     feature_gate: None,
+    docs: Some(r#"
+Note: This only runs on edition 2021 and below
+
+### What it does
+Checks for implicit features for optional dependencies
+
+### Why it is bad
+By default, cargo will treat any optional dependency as a [feature]. As of
+cargo 1.60, these can be disabled by declaring a feature that activates the
+optional dependency as `dep:<name>` (see [RFC #3143]).
+
+In the 2024 edition, `cargo` will stop exposing optional dependencies as
+features implicitly, requiring users to add `foo = ["dep:foo"]` if they
+still want it exposed.
+
+For more information, see [RFC #3491]
+
+### Example
+```toml
+edition = "2021"
+
+[dependencies]
+bar = { version = "0.1.0", optional = true }
+
+[features]
+# No explicit feature activation for `bar`
+```
+
+Instead, the dependency should have an explicit feature:
+```toml
+edition = "2021"
+
+[dependencies]
+bar = { version = "0.1.0", optional = true }
+
+[features]
+bar = ["dep:bar"]
+```
+
+[feature]: https://doc.rust-lang.org/cargo/reference/features.html
+[RFC #3143]: https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html
+[RFC #3491]: https://rust-lang.github.io/rfcs/3491-remove-implicit-features.html
+"#
+    ),
 };
 
 pub fn check_implicit_features(
@@ -575,6 +613,24 @@ const UNKNOWN_LINTS: Lint = Lint {
     default_level: LintLevel::Warn,
     edition_lint_opts: None,
     feature_gate: None,
+    docs: Some(
+        r#"
+### What it does
+Checks for unknown lints in the `[lints.cargo]` table
+
+### Why it is bad
+- The lint name could be misspelled, leading to confusion as to why it is
+  not working as expected
+- The unknown lint could end up causing an error if `cargo` decides to make
+  a lint with the same name in the future
+
+### Example
+```toml
+[lints.cargo]
+this-lint-does-not-exist = "warn"
+```
+"#,
+    ),
 };
 
 fn output_unknown_lints(
@@ -684,6 +740,43 @@ const UNUSED_OPTIONAL_DEPENDENCY: Lint = Lint {
     default_level: LintLevel::Warn,
     edition_lint_opts: None,
     feature_gate: None,
+    docs: Some(
+        r#"
+### What it does
+Checks for optional dependencies that are not activated by any feature
+
+### Why it is bad
+Starting in the 2024 edition, `cargo` no longer implicitly creates features
+for optional dependencies (see [RFC #3491]). This means that any optional
+dependency not specified with `"dep:<name>"` in some feature is now unused.
+This change may be surprising to users who have been using the implicit
+features `cargo` has been creating for optional dependencies.
+
+### Example
+```toml
+edition = "2024"
+
+[dependencies]
+bar = { version = "0.1.0", optional = true }
+
+[features]
+# No explicit feature activation for `bar`
+```
+
+Instead, the dependency should be removed or activated in a feature:
+```toml
+edition = "2024"
+
+[dependencies]
+bar = { version = "0.1.0", optional = true }
+
+[features]
+bar = ["dep:bar"]
+```
+
+[RFC #3491]: https://rust-lang.github.io/rfcs/3491-remove-implicit-features.html
+"#,
+    ),
 };
 
 pub fn unused_dependencies(