Allow lint config to have extra custom configs

And report the unused manifest key warning for every key that we do not
use, which is currently every of them.

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/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 +2278,14 @@ 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
+                    let message =
+                        format!("unused manifest key: `lints.{tool}.{name}.{config_name}`");
+                    warnings.push(message);
+                }
             }
         }
     }

tests/testsuite/lints_table.rs

@@ -172,8 +172,8 @@ fn warn_on_unused_key() {
     foo.cargo("check")
         .with_stderr(
             "\
-[WARNING] [CWD]/Cargo.toml: unused manifest key: lints.rust.rust-2018-idioms.unused
-[WARNING] [CWD]/Cargo.toml: unused manifest key: workspace.lints.rust.rust-2018-idioms.unused
+[WARNING][..]unused manifest key: `lints.rust.rust-2018-idioms.unused`
+[WARNING][..]unused manifest key: `lints.rust.rust-2018-idioms.unused`
 [CHECKING] foo v0.0.1 ([CWD])
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [..]s
 ",