fix: mark clippy-preview unstable, remove cruft

Author: yaahc

src/bin/cargo/commands/clippy.rs

@@ -80,9 +80,17 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
             return Err(CliError::new(err, 101));
         }
     };
+
     let mode = CompileMode::Check { test };
     let compile_opts = args.compile_options(config, mode, Some(&ws))?;
 
+    if !config.cli_unstable().unstable_options {
+        return Err(failure::format_err!(
+            "`clippy-preview` is unstable, pass `-Z unstable-options` to enable it"
+        )
+        .into());
+    }
+
     ops::compile(&ws, &compile_opts)?;
     Ok(())
 }

src/cargo/util/config.rs

@@ -416,7 +416,6 @@ impl Config {
         match self.get_env(key)? {
             Some(v) => Ok(Some(v)),
             None => {
-                // println!("{:#?}", self);
                 let config_key = key.to_config();
                 let o_cv = self.get_cv(&config_key)?;
                 match o_cv {