message: Refactor to use constant for arg name
  A static constant is used to define the arg name since this may change
  after review but the string is used as an ID in processing arguments.
diff: @@ -2,6 +2,8 @@ use crate::command_prelude::*;
  use cargo::ops;
  +const PRINT_CFG_ARG_NAME: &str = "print-cfg";
  +
  pub fn cli() -> App {
  subcommand("rustc")
  .setting(AppSettings::TrailingVarArg)
  @@ -32,7 +34,7 @@ pub fn cli() -> App {
  .arg_unit_graph()
  .arg(
  opt(
  - "--print-cfg",
  + PRINT_CFG_ARG_NAME,
  "Output the compiler configuration in JSON (unstable)",
  )
  .hidden(true),
  @@ -68,8 +70,8 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
  } else {
  Some(target_args)
  };
  - if args.is_present("--print-cfg") {
  - config.cli_unstable().fail_if_stable_opt("--print-cfg", 8923)?;
  + if args.is_present(PRINT_CFG_ARG_NAME) {
  + config.cli_unstable().fail_if_stable_opt(PRINT_CFG_ARG_NAME, 8923)?;
  ops::print_cfg(&ws, &compile_opts)?;
  } else {
  ops::compile(&ws, &compile_opts)?;
  