Add -Zcheck-target-cfgs and check-target-cfgs in preparation for linting

Author: Urgau

src/cargo/core/features.rs

@@ -579,6 +579,9 @@ features! {
 
     /// Allows use of multiple build scripts.
     (unstable, multiple_build_scripts, "", "reference/unstable.html#multiple-build-scripts"),
+
+    /// Allows checking target cfgs and configuring the `cargo.unexpected_cfgs` lint
+    (unstable, check_target_cfgs, "", "reference/unstable.html#check-target-cfgs"),
 }
 
 /// Status and metadata for a single unstable feature.
@@ -825,6 +828,7 @@ unstable_cli_options!(
     #[serde(deserialize_with = "deserialize_comma_separated_list")]
     build_std_features: Option<Vec<String>>  = ("Configure features enabled for the standard library itself when building the standard library"),
     cargo_lints: bool = ("Enable the `[lints.cargo]` table"),
+    check_target_cfgs: bool = ("Enable unexpected cfgs checking in `[target.'cfg(...)']` tables"),
     checksum_freshness: bool = ("Use a checksum to determine if output is fresh rather than filesystem mtime"),
     codegen_backend: bool = ("Enable the `codegen-backend` option in profiles in .cargo/config.toml file"),
     config_include: bool = ("Enable the `include` key in config files"),
@@ -1335,6 +1339,7 @@ impl CliUnstable {
             "build-std" => self.build_std = Some(parse_list(v)),
             "build-std-features" => self.build_std_features = Some(parse_list(v)),
             "cargo-lints" => self.cargo_lints = parse_empty(k, v)?,
+            "check-target-cfgs" => self.check_target_cfgs = parse_empty(k, v)?,
             "codegen-backend" => self.codegen_backend = parse_empty(k, v)?,
             "config-include" => self.config_include = parse_empty(k, v)?,
             "direct-minimal-versions" => self.direct_minimal_versions = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -129,6 +129,7 @@ Each new feature described below should explain how to use it.
     * [warnings](#warnings) --- controls warning behavior; options for allowing or denying warnings.
     * [Package message format](#package-message-format) --- Message format for `cargo package`.
     * [`fix-edition`](#fix-edition) --- A permanently unstable edition migration helper.
+    * [check-target-cfgs](#check-target-cfgs) --- Allows checking unexpected cfgs in `[target.'cfg(...)']`
 
 ## allow-features
 
@@ -1968,6 +1969,31 @@ For example:
 cargo +nightly fix -Zfix-edition=end=2024,future
 ```
 
+## check-target-cfgs
+
+* Tracking Issue: [#00000](https://github.com/rust-lang/cargo/issues/00000)
+
+This feature checks for unexpected cfgs in `[target.'cfg(...)']` entries, based
+on `rustc --print=check-cfg`.
+
+```sh
+cargo check -Zcheck-target-cfgs
+```
+
+It follows the lint Rust `unexpected_cfgs` lint configuration:
+
+```toml
+[target.'cfg(foo)'.dependencies]
+cfg-if = "1.0"
+
+[lints.cargo.unexpected_cfgs]
+level = "deny"
+
+[lints.rust.unexpected_cfgs]
+level = "warn"
+check-cfg = ['cfg(foo)']
+```
+
 # Stabilized and removed features
 
 ## Compile progress