Delay parsing of --cfg and --check-cfg options.

By storing the unparsed values in `Config` and then parsing them within
`run_compiler`, the parsing functions can use the main symbol interner,
and not create their own short-lived interners.

This change also eliminates the need for one `EarlyErrorHandler` in
rustdoc, because parsing errors can be reported by another, slightly
later `EarlyErrorHandler`.

Author: nnethercote

compiler/rustc_driver_impl/src/lib.rs

@@ -317,13 +317,11 @@ fn run_compiler(
         return Ok(());
     }
 
-    let cfg = interface::parse_cfg(&early_error_handler, matches.opt_strs("cfg"));
-    let check_cfg = interface::parse_check_cfg(&early_error_handler, matches.opt_strs("check-cfg"));
     let (odir, ofile) = make_output(&matches);
     let mut config = interface::Config {
         opts: sopts,
-        crate_cfg: cfg,
-        crate_check_cfg: check_cfg,
+        crate_cfg: matches.opt_strs("cfg"),
+        crate_check_cfg: matches.opt_strs("check-cfg"),
         input: Input::File(PathBuf::new()),
         output_file: ofile,
         output_dir: odir,