message: Change old flags to warnings instead of errors.
  Except --all which is ambiguous which flag it means.
diff: @@ -83,14 +83,21 @@ pub fn cli() -> App {
  }
  pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
  - if args.is_present("no-indent") {
  - return Err(format_err!("the --no-indent flag has been changed to --prefix=none").into());
  - }
  - if args.is_present("prefix-depth") {
  - return Err(
  - format_err!("the --prefix-depth flag has been changed to --prefix=depth").into(),
  - );
  - }
  + let prefix = if args.is_present("no-indent") {
  + config
  + .shell()
  + .warn("the --no-indent flag has been changed to --prefix=none")?;
  + "none"
  + } else if args.is_present("prefix-depth") {
  + config
  + .shell()
  + .warn("the --prefix-depth flag has been changed to --prefix=depth")?;
  + "depth"
  + } else {
  + args.value_of("prefix").unwrap()
  + };
  + let prefix = tree::Prefix::from_str(prefix).map_err(|e| anyhow::anyhow!("{}", e))?;
  +
  if args.is_present("all") {
  return Err(format_err!(
  "The `cargo tree` --all flag has been changed to --no-dedupe.\n\
  @@ -98,22 +105,30 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
  )
  .into());
  }
  - if args.is_present("all-targets") {
  - return Err(format_err!("the --all-targets flag has been changed to --target=all").into());
  - }
  - if args.is_present("no-dev-dependencies") {
  - return Err(format_err!(
  - "the --no-dev-dependencies flag has changed to --dep-kinds=no-dev"
  - )
  - .into());
  - }
  +
  + let target = if args.is_present("all-targets") {
  + config
  + .shell()
  + .warn("the --all-targets flag has been changed to --target=all")?;
  + Some("all")
  + } else {
  + args.value_of("target")
  + };
  + let target = tree::Target::from_cli(target);
  +
  + let dep_kinds = if args.is_present("no-dev-dependencies") {
  + config
  + .shell()
  + .warn("the --no-dev-dependencies flag has changed to --dep-kinds=no-dev")?;
  + Some("no-dev")
  + } else {
  + args.value_of("dep-kinds")
  + };
  + let dep_kinds = parse_dep_kinds(dep_kinds)?;
  +
  let ws = args.workspace(config)?;
  let charset = tree::Charset::from_str(args.value_of("charset").unwrap())
  .map_err(|e| anyhow::anyhow!("{}", e))?;
  - let prefix = tree::Prefix::from_str(args.value_of("prefix").unwrap())
  - .map_err(|e| anyhow::anyhow!("{}", e))?;
  - let target = tree::Target::from_cli(args.value_of("target"));
  - let dep_kinds = parse_dep_kinds(args.value_of("dep-kinds"))?;
  let opts = tree::TreeOptions {
  features: values(args, "features"),
  all_features: args.is_present("all-features"),
  