Auto merge of #13913 - Urgau:check-cfg-lints-sub-config, r=<try>

Add special `check-cfg` lint config for the `unexpected_cfgs` lint

This PR adds a special `check-cfg` lint config for the `unexpected_cfgs` lint, as it was decided by T-cargo (in today's meeting).

The goal of this lint config is to provide a simple and cost-less alternative to the build-script `cargo::rustc-check-cfg` instruction.

```toml
[lints.rust]
unexpected_cfgs = { check-cfg = ["cfg(foo, values(\"bar\"))"] }
```

Regarding the implementation, everything is as straight forward as possible, nothing over-engineered; I added the possibility to omit the `level` field if a sub-config is specified instead, since it seems useful and was easy to implement.

There are many small-ish commit, I recommend reviewing them independently.

r? `@epage` (or `@weihanglo` maybe)

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -1503,6 +1503,13 @@ impl TomlLint {
             Self::Config(config) => config.priority,
         }
     }
+
+    pub fn config(&self) -> Option<&toml::Table> {
+        match self {
+            Self::Level(_) => None,
+            Self::Config(config) => Some(&config.config),
+        }
+    }
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone)]
@@ -1511,6 +1518,8 @@ pub struct TomlLintConfig {
     pub level: TomlLintLevel,
     #[serde(default)]
     pub priority: i8,
+    #[serde(flatten)]
+    pub config: toml::Table,
 }
 
 #[derive(Serialize, Deserialize, Debug, Copy, Clone, Eq, PartialEq)]

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

@@ -2265,7 +2265,7 @@ supported tools: {}",
         if tool == "cargo" && !gctx.cli_unstable().cargo_lints {
             warn_for_cargo_lint_feature(gctx, warnings);
         }
-        for name in lints.keys() {
+        for (name, config) in lints {
             if let Some((prefix, suffix)) = name.split_once("::") {
                 if tool == prefix {
                     anyhow::bail!(
@@ -2278,6 +2278,19 @@ supported tools: {}",
                 } else {
                     anyhow::bail!("`lints.{tool}.{name}` is not a valid lint name")
                 }
+            } else if let Some(config) = config.config() {
+                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
+                    //
+                    // 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);
+                    }
+                }
             }
         }
     }