Add special check-cfg config for the unexpected_cfgs lint

This permits things like this to be recognized and passed to
rustc/rustdoc.

```rust
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ["cfg(foo)"] }
```

Author: Urgau

src/cargo/core/compiler/mod.rs

@@ -1353,12 +1353,37 @@ fn check_cfg_args(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> Vec<OsStri
         // Cargo and docs.rs than rustc and docs.rs. In particular, all users of docs.rs use
         // Cargo, but not all users of rustc (like Rust-for-Linux) use docs.rs.
 
-        vec![
+        let mut args = vec![
             OsString::from("--check-cfg"),
             OsString::from("cfg(docsrs)"),
             OsString::from("--check-cfg"),
             arg_feature,
-        ]
+        ];
+
+        // Also include the custom arguments specified in `[lints.rust.unexpected_cfgs.check_cfg]`
+        if let Ok(Some(lints)) = unit.pkg.manifest().resolved_toml().resolved_lints() {
+            if let Some(rust_lints) = lints.get("rust") {
+                if let Some(unexpected_cfgs) = rust_lints.get("unexpected_cfgs") {
+                    if let Some(config) = unexpected_cfgs.config() {
+                        if let Some(check_cfg) = config.get("check-cfg") {
+                            if let Ok(check_cfgs) =
+                                toml::Value::try_into::<Vec<String>>(check_cfg.clone())
+                            {
+                                for check_cfg in check_cfgs {
+                                    args.push(OsString::from("--check-cfg"));
+                                    args.push(OsString::from(check_cfg));
+                                }
+                            // warn (if wise) about `check-cfg` not being a list-of-string
+                            } else if unit.show_warnings(&build_runner.bcx.gctx) {
+                                let _ = build_runner.bcx.gctx.shell().warn("`lints.rust.unexpected_cfgs.check-cfg` must be a list of string");
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        args
     } else {
         Vec::new()
     }

src/cargo/util/toml/mod.rs

@@ -2282,9 +2282,14 @@ supported tools: {}",
                 for config_name in config.keys() {
                     // manually report unused manifest key warning since we collect all the "extra"
                     // keys and values inside the config table
-                    let message =
-                        format!("unused manifest key: `lints.{tool}.{name}.{config_name}`");
-                    warnings.push(message);
+                    //
+                    // except for `rust.unexpected_cfgs.check-cfg` which is used by rustc/rustdoc
+                    if !(tool == "rust" && name == "unexpected_cfgs" && config_name == "check-cfg")
+                    {
+                        let message =
+                            format!("unused manifest key: `lints.{tool}.{name}.{config_name}`");
+                        warnings.push(message);
+                    }
                 }
             }
         }

tests/testsuite/check_cfg.rs

@@ -496,3 +496,28 @@ fn build_script_test() {
         .with_stdout_contains_n("test [..] ... ok", 3)
         .run();
 }
+
+#[cargo_test(>=1.79, reason = "--check-cfg was stabilized in Rust 1.79")]
+fn config_simple() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2015"
+
+                [lints.rust]
+                unexpected_cfgs = { level = "warn", check-cfg = ["cfg(has_foo)", "cfg(has_bar, values(\"yes\", \"no\"))"] }
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("check -v")
+        .with_stderr_contains(x!("rustc" => "cfg" of "has_foo"))
+        .with_stderr_contains(x!("rustc" => "cfg" of "has_bar" with "yes" "no"))
+        .with_stderr_does_not_contain("[..]unused manifest key[..]")
+        .run();
+}