feat: Add an xtask to generate lint documentation

Author: Muscraft

Diff for: .cargo/config.toml

@@ -2,6 +2,7 @@
 build-man = "run --package xtask-build-man --"
 stale-label = "run --package xtask-stale-label --"
 bump-check = "run --package xtask-bump-check --"
+lint-docs = "run --package xtask-lint-docs --"
 
 [env]
 # HACK: Until this is stabilized, `snapbox`s polyfill could get confused

Diff for: .github/workflows/main.yml

@@ -83,6 +83,13 @@ jobs:
       - run: rustup update stable && rustup default stable
       - run: cargo stale-label
 
+  lint-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup update stable && rustup default stable
+      - run: cargo lint-docs --check
+
   # Ensure Cargo.lock is up-to-date
   lockfile:
     runs-on: ubuntu-latest

Diff for: Cargo.lock

@@ -0,0 +1,14 @@
+[package]
+name = "xtask-lint-docs"
+version = "0.1.0"
+edition.workspace = true
+publish = false
+
+[dependencies]
+anyhow.workspace = true
+cargo.workspace = true
+clap.workspace = true
+itertools.workspace = true
+
+[lints]
+workspace = true

Diff for: crates/xtask-lint-docs/Cargo.toml

@@ -0,0 +1,108 @@
+use cargo::util::command_prelude::{flag, ArgMatchesExt};
+use cargo::util::lints::{Lint, LintLevel};
+use itertools::Itertools;
+use std::fmt::Write;
+use std::path::PathBuf;
+
+fn cli() -> clap::Command {
+    clap::Command::new("xtask-lint-docs").arg(flag("check", "Check that the docs are up-to-date"))
+}
+
+fn main() -> anyhow::Result<()> {
+    let args = cli().get_matches();
+    let check = args.flag("check");
+
+    let mut allow = Vec::new();
+    let mut warn = Vec::new();
+    let mut deny = Vec::new();
+    let mut forbid = Vec::new();
+
+    let mut lint_docs = String::new();
+    for lint in cargo::util::lints::LINTS
+        .iter()
+        .sorted_by_key(|lint| lint.name)
+    {
+        if lint.docs.is_some() {
+            let sectipn = match lint.default_level {
+                LintLevel::Allow => &mut allow,
+                LintLevel::Warn => &mut warn,
+                LintLevel::Deny => &mut deny,
+                LintLevel::Forbid => &mut forbid,
+            };
+            sectipn.push(lint.name);
+            add_lint(lint, &mut lint_docs)?;
+        }
+    }
+
+    let mut buf = String::new();
+    writeln!(buf, "# Lints\n")?;
+    writeln!(
+        buf,
+        "Note: [Cargo's linting system is unstable](unstable.md#lintscargo) and can only be used on nightly toolchains"
+    )?;
+    writeln!(buf)?;
+
+    if !allow.is_empty() {
+        add_level_section(LintLevel::Allow, &allow, &mut buf)?;
+    }
+    if !warn.is_empty() {
+        add_level_section(LintLevel::Warn, &warn, &mut buf)?;
+    }
+    if !deny.is_empty() {
+        add_level_section(LintLevel::Deny, &deny, &mut buf)?;
+    }
+    if !forbid.is_empty() {
+        add_level_section(LintLevel::Forbid, &forbid, &mut buf)?;
+    }
+
+    buf.push_str(&lint_docs);
+
+    if check {
+        let old = std::fs::read_to_string(lint_docs_path())?;
+        if old != buf {
+            anyhow::bail!(
+                "The lints documentation is out-of-date. Run `cargo lint-docs` to update it."
+            );
+        }
+    } else {
+        std::fs::write(lint_docs_path(), buf)?;
+    }
+    Ok(())
+}
+
+fn add_lint(lint: &Lint, buf: &mut String) -> std::fmt::Result {
+    writeln!(buf, "## `{}`", lint.name)?;
+    writeln!(buf, "Set to `{}` by default", lint.default_level)?;
+    writeln!(buf, "{}\n", lint.docs.as_ref().unwrap())
+}
+
+fn add_level_section(level: LintLevel, lint_names: &[&str], buf: &mut String) -> std::fmt::Result {
+    let title = match level {
+        LintLevel::Allow => "Allowed-by-default",
+        LintLevel::Warn => "Warn-by-default",
+        LintLevel::Deny => "Deny-by-default",
+        LintLevel::Forbid => "Forbid-by-default",
+    };
+    writeln!(buf, "## {title}\n")?;
+    writeln!(
+        buf,
+        "These lints are all set to the '{}' level by default.",
+        level
+    )?;
+
+    for name in lint_names {
+        writeln!(buf, "- [`{}`](#{})", name, name)?;
+    }
+    writeln!(buf)?;
+    Ok(())
+}
+
+fn lint_docs_path() -> PathBuf {
+    let pkg_root = env!("CARGO_MANIFEST_DIR");
+    let ws_root = PathBuf::from(format!("{pkg_root}/../.."));
+    let path = {
+        let path = ws_root.join("src/doc/src/reference/lints.md");
+        path.canonicalize().unwrap_or(path)
+    };
+    path
+}

Diff for: crates/xtask-lint-docs/src/main.rs

@@ -13,7 +13,7 @@ use std::path::Path;
 use toml_edit::ImDocument;
 
 const LINT_GROUPS: &[LintGroup] = &[TEST_DUMMY_UNSTABLE];
-const LINTS: &[Lint] = &[
+pub const LINTS: &[Lint] = &[
     IM_A_TEAPOT,
     IMPLICIT_FEATURES,
     UNKNOWN_LINTS,

Diff for: src/cargo/util/lints.rs

@@ -45,6 +45,7 @@
     * [SemVer Compatibility](reference/semver.md)
     * [Future incompat report](reference/future-incompat-report.md)
     * [Reporting build timings](reference/timings.md)
+    * [Lints](reference/lints.md)
     * [Unstable Features](reference/unstable.md)
 
 * [Cargo Commands](commands/index.md)

Diff for: src/doc/src/SUMMARY.md

@@ -23,4 +23,5 @@ The reference covers the details of various areas of Cargo.
 * [SemVer Compatibility](semver.md)
 * [Future incompat report](future-incompat-report.md)
 * [Reporting build timings](timings.md)
+* [Lints](lints.md)
 * [Unstable Features](unstable.md)

Diff for: src/doc/src/reference/index.md

@@ -0,0 +1,116 @@
+# Lints
+
+Note: [Cargo's linting system is unstable](unstable.md#lintscargo) and can only be used on nightly toolchains
+
+## Allowed-by-default
+
+These lints are all set to the 'allow' level by default.
+- [`implicit_features`](#implicit_features)
+
+## Warn-by-default
+
+These lints are all set to the 'warn' level by default.
+- [`unknown_lints`](#unknown_lints)
+- [`unused_optional_dependency`](#unused_optional_dependency)
+
+## `implicit_features`
+Set to `allow` by default
+
+### 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
+
+
+## `unknown_lints`
+Set to `warn` by default
+
+### 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"
+```
+
+
+## `unused_optional_dependency`
+Set to `warn` by default
+
+### 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
+
+