Module clap::_derive::_tutorial

Available on crate feature unstable-doc only.

Documentation: Derive Tutorial

1. Quick Start
2. Configuring the Parser
3. Adding Arguments
   1. Positionals
   2. Options
   3. Flags
   4. Subcommands
   5. Defaults
4. Validation
   1. Enumerated values
   2. Validated values
   3. Argument Relations
   4. Custom Validation
5. Testing

See also

• FAQ: When should I use the builder vs derive APIs?
• The cookbook for more application-focused examples

Quick Start

You can create an application declaratively with a struct and some attributes. This requires enabling the derive feature flag.

use std::path::PathBuf;

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// Optional name to operate on
    #[clap(value_parser)]
    name: Option<String>,

    /// Sets a custom config file
    #[clap(short, long, value_parser, value_name = "FILE")]
    config: Option<PathBuf>,

    /// Turn debugging information on
    #[clap(short, long, action = clap::ArgAction::Count)]
    debug: u8,

    #[clap(subcommand)]
    command: Option<Commands>,
}

#[derive(Subcommand)]
enum Commands {
    /// does testing things
    Test {
        /// lists test values
        #[clap(short, long, action)]
        list: bool,
    },
}

fn main() {
    let cli = Cli::parse();

    // You can check the value provided by positional arguments, or option arguments
    if let Some(name) = cli.name.as_deref() {
        println!("Value for name: {}", name);
    }

    if let Some(config_path) = cli.config.as_deref() {
        println!("Value for config: {}", config_path.display());
    }

    // You can see how many times a particular flag or argument occurred
    // Note, only flags can have multiple occurrences
    match cli.debug {
        0 => println!("Debug mode is off"),
        1 => println!("Debug mode is kind of on"),
        2 => println!("Debug mode is on"),
        _ => println!("Don't be crazy"),
    }

    // You can check for the existence of subcommands, and if found use their
    // matches just as you would the top level cmd
    match &cli.command {
        Some(Commands::Test { list }) => {
            if *list {
                println!("Printing testing lists...");
            } else {
                println!("Not printing testing lists...");
            }
        }
        None => {}
    }

    // Continued program logic goes here...
}

$ 01_quick_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    01_quick_derive[EXE] [OPTIONS] [NAME] [SUBCOMMAND]

ARGS:
    <NAME>    Optional name to operate on

OPTIONS:
    -c, --config <FILE>    Sets a custom config file
    -d, --debug            Turn debugging information on
    -h, --help             Print help information
    -V, --version          Print version information

SUBCOMMANDS:
    help    Print this message or the help of the given subcommand(s)
    test    does testing things

By default, the program does nothing:

$ 01_quick_derive
Debug mode is off

But you can mix and match the various features

$ 01_quick_derive -dd test
Debug mode is on
Not printing testing lists...

Configuring the Parser

You use derive Parser to start building a parser.

use clap::Parser;

#[derive(Parser)]
#[clap(name = "MyApp")]
#[clap(author = "Kevin K. <kbknapp@gmail.com>")]
#[clap(version = "1.0")]
#[clap(about = "Does awesome things", long_about = None)]
struct Cli {
    #[clap(long, value_parser)]
    two: String,
    #[clap(long, value_parser)]
    one: String,
}

fn main() {
    let cli = Cli::parse();

    println!("two: {:?}", cli.two);
    println!("one: {:?}", cli.one);
}

$ 02_apps_derive --help
MyApp 1.0
Kevin K. <kbknapp@gmail.com>
Does awesome things

USAGE:
    02_apps_derive[EXE] --two <TWO> --one <ONE>

OPTIONS:
    -h, --help         Print help information
        --one <ONE>    
        --two <TWO>    
    -V, --version      Print version information

$ 02_apps_derive --version
MyApp 1.0

You can use #[clap(author, version, about)] attribute defaults to fill these fields in from your Cargo.toml file.

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)] // Read from `Cargo.toml`
struct Cli {
    #[clap(long, value_parser)]
    two: String,
    #[clap(long, value_parser)]
    one: String,
}

fn main() {
    let cli = Cli::parse();

    println!("two: {:?}", cli.two);
    println!("one: {:?}", cli.one);
}

$ 02_crate_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    02_crate_derive[EXE] --two <TWO> --one <ONE>

OPTIONS:
    -h, --help         Print help information
        --one <ONE>    
        --two <TWO>    
    -V, --version      Print version information

$ 02_crate_derive --version
clap [..]

You can use attributes to change the application level behavior of clap. Any Command] builder function can be used as an attribute.

use clap::{AppSettings, Parser};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
#[clap(allow_negative_numbers = true)]
#[clap(global_setting(AppSettings::DeriveDisplayOrder))]
struct Cli {
    #[clap(long, value_parser)]
    two: String,
    #[clap(long, value_parser)]
    one: String,
}

fn main() {
    let cli = Cli::parse();

    println!("two: {:?}", cli.two);
    println!("one: {:?}", cli.one);
}

$ 02_app_settings_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    02_app_settings_derive[EXE] --two <TWO> --one <ONE>

OPTIONS:
        --two <TWO>    
        --one <ONE>    
    -h, --help         Print help information
    -V, --version      Print version information

$ 02_app_settings_derive --one -1 --one -3 --two 10
two: "10"
one: "-3"

Adding Arguments

Positionals

You can have users specify values by their position on the command-line:

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(value_parser)]
    name: Option<String>,
}

fn main() {
    let cli = Cli::parse();

    println!("name: {:?}", cli.name.as_deref());
}

$ 03_03_positional_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_03_positional_derive[EXE] [NAME]

ARGS:
    <NAME>    

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_03_positional_derive
name: None

$ 03_03_positional_derive bob
name: Some("bob")

Options

You can name your arguments with a flag:

• Order doesn’t matter
• They can be optional
• Intent is clearer

The #[clap(short = 'n')] and #[clap(long = "name")] attributes that define the flags are Arg methods that are derived from the field name when no value is specified (#[clap(short)] and #[clap(long)]).

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(short, long, value_parser)]
    name: Option<String>,
}

fn main() {
    let cli = Cli::parse();

    println!("name: {:?}", cli.name.as_deref());
}

$ 03_02_option_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_02_option_derive[EXE] [OPTIONS]

OPTIONS:
    -h, --help           Print help information
    -n, --name <NAME>    
    -V, --version        Print version information

$ 03_02_option_derive
name: None

$ 03_02_option_derive --name bob
name: Some("bob")

$ 03_02_option_derive --name=bob
name: Some("bob")

$ 03_02_option_derive -n bob
name: Some("bob")

$ 03_02_option_derive -n=bob
name: Some("bob")

$ 03_02_option_derive -nbob
name: Some("bob")

Flags

Flags can also be switches that can be on/off. This is enabled via the #[clap(action = ArgAction::SetTrue)] attribute though this is implied when the field is a bool.

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(short, long, action)]
    verbose: bool,
}

fn main() {
    let cli = Cli::parse();

    println!("verbose: {:?}", cli.verbose);
}

$ 03_01_flag_bool_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_01_flag_bool_derive[EXE] [OPTIONS]

OPTIONS:
    -h, --help       Print help information
    -v, --verbose    
    -V, --version    Print version information

$ 03_01_flag_bool_derive
verbose: false

$ 03_01_flag_bool_derive --verbose
verbose: true

$ 03_01_flag_bool_derive --verbose --verbose
verbose: true

Or counted with #[clap(action = clap::ArgAction::Count)]:

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(short, long, action = clap::ArgAction::Count)]
    verbose: u8,
}

fn main() {
    let cli = Cli::parse();

    println!("verbose: {:?}", cli.verbose);
}

$ 03_01_flag_count_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_01_flag_count_derive[EXE] [OPTIONS]

OPTIONS:
    -h, --help       Print help information
    -v, --verbose    
    -V, --version    Print version information

$ 03_01_flag_count_derive
verbose: 0

$ 03_01_flag_count_derive --verbose
verbose: 1

$ 03_01_flag_count_derive --verbose --verbose
verbose: 2

Subcommands

Subcommands are derived with #[derive(Subcommand)] and be added via #[clap(subcommand)] attribute. Each instance of a Subcommand can have its own version, author(s), Args, and even its own subcommands.

use clap::{Parser, Subcommand};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
#[clap(propagate_version = true)]
struct Cli {
    #[clap(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    /// Adds files to myapp
    Add {
        #[clap(value_parser)]
        name: Option<String>,
    },
}

fn main() {
    let cli = Cli::parse();

    // You can check for the existence of subcommands, and if found use their
    // matches just as you would the top level cmd
    match &cli.command {
        Commands::Add { name } => {
            println!("'myapp add' was used, name is: {:?}", name)
        }
    }
}

We used a struct-variant to define the add subcommand. Alternatively, you can use a struct for your subcommand’s arguments:

use clap::{Args, Parser, Subcommand};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
#[clap(propagate_version = true)]
struct Cli {
    #[clap(subcommand)]
    command: Commands,
}

#[derive(Subcommand)]
enum Commands {
    /// Adds files to myapp
    Add(Add),
}

#[derive(Args)]
struct Add {
    #[clap(value_parser)]
    name: Option<String>,
}

fn main() {
    let cli = Cli::parse();

    // You can check for the existence of subcommands, and if found use their
    // matches just as you would the top level cmd
    match &cli.command {
        Commands::Add(name) => {
            println!("'myapp add' was used, name is: {:?}", name.name)
        }
    }
}

$ 03_04_subcommands_derive help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_04_subcommands_derive[EXE] <SUBCOMMAND>

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

SUBCOMMANDS:
    add     Adds files to myapp
    help    Print this message or the help of the given subcommand(s)

$ 03_04_subcommands_derive help add
03_04_subcommands_derive[EXE]-add [..]
Adds files to myapp

USAGE:
    03_04_subcommands_derive[EXE] add [NAME]

ARGS:
    <NAME>    

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_04_subcommands_derive add bob
'myapp add' was used, name is: Some("bob")

Because we used command: Commands instead of command: Option<Commands>:

$ 03_04_subcommands_derive
? failed
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_04_subcommands_derive[EXE] <SUBCOMMAND>

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

SUBCOMMANDS:
    add     Adds files to myapp
    help    Print this message or the help of the given subcommand(s)

Because we added #[clap(propagate_version = true)]:

$ 03_04_subcommands_derive --version
clap [..]

$ 03_04_subcommands_derive add --version
03_04_subcommands_derive[EXE]-add [..]

Defaults

We’ve previously showed that arguments can be required or optional. When optional, you work with a Option and can unwrap_or. Alternatively, you can set #[clap(default_value_t)].

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(default_value_t = String::from("alice"), value_parser)]
    name: String,
}

fn main() {
    let cli = Cli::parse();

    println!("name: {:?}", cli.name);
}

$ 03_05_default_values_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    03_05_default_values_derive[EXE] [NAME]

ARGS:
    <NAME>    [default: alice]

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 03_05_default_values_derive
name: "alice"

$ 03_05_default_values_derive bob
name: "bob"

Validation

Enumerated values

If you have arguments of specific values you want to test for, you can derive ValueEnum.

This allows you specify the valid values for that argument. If the user does not use one of those specific values, they will receive a graceful exit with error message informing them of the mistake, and what the possible valid values are

use clap::{Parser, ValueEnum};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// What mode to run the program in
    #[clap(arg_enum, value_parser)]
    mode: Mode,
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum)]
enum Mode {
    Fast,
    Slow,
}

fn main() {
    let cli = Cli::parse();

    match cli.mode {
        Mode::Fast => {
            println!("Hare");
        }
        Mode::Slow => {
            println!("Tortoise");
        }
    }
}

$ 04_01_enum_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_01_enum_derive[EXE] <MODE>

ARGS:
    <MODE>    What mode to run the program in [possible values: fast, slow]

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_01_enum_derive fast
Hare

$ 04_01_enum_derive slow
Tortoise

$ 04_01_enum_derive medium
? failed
error: "medium" isn't a valid value for '<MODE>'
	[possible values: fast, slow]

For more information try --help

Validated values

More generally, you can validate and parse into any data type.

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// Network port to use
    #[clap(value_parser = clap::value_parser!(u16).range(1..))]
    port: u16,
}

fn main() {
    let cli = Cli::parse();

    println!("PORT = {}", cli.port);
}

$ 04_02_parse_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_02_parse_derive[EXE] <PORT>

ARGS:
    <PORT>    Network port to use

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_02_parse_derive 22
PORT = 22

$ 04_02_parse_derive foobar
? failed
error: Invalid value "foobar" for '<PORT>': invalid digit found in string

For more information try --help

$ 04_02_parse_derive 0
? failed
error: Invalid value "0" for '<PORT>': 0 is not in 1..=65535

For more information try --help

A custom parser can be used to improve the error messages or provide additional validation:

use std::ops::RangeInclusive;

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// Network port to use
    #[clap(value_parser = port_in_range)]
    port: u16,
}

fn main() {
    let cli = Cli::parse();

    println!("PORT = {}", cli.port);
}

const PORT_RANGE: RangeInclusive<usize> = 1..=65535;

fn port_in_range(s: &str) -> Result<u16, String> {
    let port: usize = s
        .parse()
        .map_err(|_| format!("`{}` isn't a port number", s))?;
    if PORT_RANGE.contains(&port) {
        Ok(port as u16)
    } else {
        Err(format!(
            "Port not in range {}-{}",
            PORT_RANGE.start(),
            PORT_RANGE.end()
        ))
    }
}

$ 04_02_validate_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_02_validate_derive[EXE] <PORT>

ARGS:
    <PORT>    Network port to use

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

$ 04_02_validate_derive 22
PORT = 22

$ 04_02_validate_derive foobar
? failed
error: Invalid value "foobar" for '<PORT>': `foobar` isn't a port number

For more information try --help

$ 04_02_validate_derive 0
? failed
error: Invalid value "0" for '<PORT>': Port not in range 1-65535

For more information try --help

Argument Relations

You can declare dependencies or conflicts between Args or even ArgGroups.

ArgGroups make it easier to declare relations instead of having to list each individually, or when you want a rule to apply “any but not all” arguments.

Perhaps the most common use of ArgGroups is to require one and only one argument to be present out of a given set. Imagine that you had multiple arguments, and you want one of them to be required, but making all of them required isn’t feasible because perhaps they conflict with each other.

use clap::{ArgGroup, Parser};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
#[clap(group(
            ArgGroup::new("vers")
                .required(true)
                .args(&["set-ver", "major", "minor", "patch"]),
        ))]
struct Cli {
    /// set version manually
    #[clap(long, value_name = "VER", value_parser)]
    set_ver: Option<String>,

    /// auto inc major
    #[clap(long, action)]
    major: bool,

    /// auto inc minor
    #[clap(long, action)]
    minor: bool,

    /// auto inc patch
    #[clap(long, action)]
    patch: bool,

    /// some regular input
    #[clap(group = "input", value_parser)]
    input_file: Option<String>,

    /// some special input argument
    #[clap(long, group = "input", value_parser)]
    spec_in: Option<String>,

    #[clap(short, requires = "input", value_parser)]
    config: Option<String>,
}

fn main() {
    let cli = Cli::parse();

    // Let's assume the old version 1.2.3
    let mut major = 1;
    let mut minor = 2;
    let mut patch = 3;

    // See if --set_ver was used to set the version manually
    let version = if let Some(ver) = cli.set_ver.as_deref() {
        ver.to_string()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (cli.major, cli.minor, cli.patch);
        match (maj, min, pat) {
            (true, _, _) => major += 1,
            (_, true, _) => minor += 1,
            (_, _, true) => patch += 1,
            _ => unreachable!(),
        };
        format!("{}.{}.{}", major, minor, patch)
    };

    println!("Version: {}", version);

    // Check for usage of -c
    if let Some(config) = cli.config.as_deref() {
        let input = cli
            .input_file
            .as_deref()
            .unwrap_or_else(|| cli.spec_in.as_deref().unwrap());
        println!("Doing work using input {} and config {}", input, config);
    }
}

$ 04_03_relations_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_03_relations_derive[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

ARGS:
    <INPUT_FILE>    some regular input

OPTIONS:
    -c <CONFIG>                
    -h, --help                 Print help information
        --major                auto inc major
        --minor                auto inc minor
        --patch                auto inc patch
        --set-ver <VER>        set version manually
        --spec-in <SPEC_IN>    some special input argument
    -V, --version              Print version information

$ 04_03_relations_derive
? failed
error: The following required arguments were not provided:
    <--set-ver <VER>|--major|--minor|--patch>

USAGE:
    04_03_relations_derive[EXE] [OPTIONS] <--set-ver <VER>|--major|--minor|--patch> [INPUT_FILE]

For more information try --help

$ 04_03_relations_derive --major
Version: 2.2.3

$ 04_03_relations_derive --major --minor
? failed
error: The argument '--major' cannot be used with '--minor'

USAGE:
    04_03_relations_derive[EXE] <--set-ver <VER>|--major|--minor|--patch>

For more information try --help

$ 04_03_relations_derive --major -c config.toml
? failed
error: The following required arguments were not provided:
    <INPUT_FILE|--spec-in <SPEC_IN>>

USAGE:
    04_03_relations_derive[EXE] -c <CONFIG> <--set-ver <VER>|--major|--minor|--patch> <INPUT_FILE|--spec-in <SPEC_IN>>

For more information try --help

$ 04_03_relations_derive --major -c config.toml --spec-in input.txt
Version: 2.2.3
Doing work using input input.txt and config config.toml

Custom Validation

As a last resort, you can create custom errors with the basics of clap’s formatting.

use clap::{CommandFactory, ErrorKind, Parser};

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// set version manually
    #[clap(long, value_name = "VER", value_parser)]
    set_ver: Option<String>,

    /// auto inc major
    #[clap(long, action)]
    major: bool,

    /// auto inc minor
    #[clap(long, action)]
    minor: bool,

    /// auto inc patch
    #[clap(long, action)]
    patch: bool,

    /// some regular input
    #[clap(value_parser)]
    input_file: Option<String>,

    /// some special input argument
    #[clap(long, value_parser)]
    spec_in: Option<String>,

    #[clap(short, value_parser)]
    config: Option<String>,
}

fn main() {
    let cli = Cli::parse();

    // Let's assume the old version 1.2.3
    let mut major = 1;
    let mut minor = 2;
    let mut patch = 3;

    // See if --set-ver was used to set the version manually
    let version = if let Some(ver) = cli.set_ver.as_deref() {
        if cli.major || cli.minor || cli.patch {
            let mut cmd = Cli::command();
            cmd.error(
                ErrorKind::ArgumentConflict,
                "Can't do relative and absolute version change",
            )
            .exit();
        }
        ver.to_string()
    } else {
        // Increment the one requested (in a real program, we'd reset the lower numbers)
        let (maj, min, pat) = (cli.major, cli.minor, cli.patch);
        match (maj, min, pat) {
            (true, false, false) => major += 1,
            (false, true, false) => minor += 1,
            (false, false, true) => patch += 1,
            _ => {
                let mut cmd = Cli::command();
                cmd.error(
                    ErrorKind::ArgumentConflict,
                    "Can only modify one version field",
                )
                .exit();
            }
        };
        format!("{}.{}.{}", major, minor, patch)
    };

    println!("Version: {}", version);

    // Check for usage of -c
    if let Some(config) = cli.config.as_deref() {
        // todo: remove `#[allow(clippy::or_fun_call)]` lint when MSRV is bumped.
        #[allow(clippy::or_fun_call)]
        let input = cli
            .input_file
            .as_deref()
            // 'or' is preferred to 'or_else' here since `Option::as_deref` is 'const'
            .or(cli.spec_in.as_deref())
            .unwrap_or_else(|| {
                let mut cmd = Cli::command();
                cmd.error(
                    ErrorKind::MissingRequiredArgument,
                    "INPUT_FILE or --spec-in is required when using --config",
                )
                .exit()
            });
        println!("Doing work using input {} and config {}", input, config);
    }
}

$ 04_04_custom_derive --help
clap [..]
A simple to use, efficient, and full-featured Command Line Argument Parser

USAGE:
    04_04_custom_derive[EXE] [OPTIONS] [INPUT_FILE]

ARGS:
    <INPUT_FILE>    some regular input

OPTIONS:
    -c <CONFIG>                
    -h, --help                 Print help information
        --major                auto inc major
        --minor                auto inc minor
        --patch                auto inc patch
        --set-ver <VER>        set version manually
        --spec-in <SPEC_IN>    some special input argument
    -V, --version              Print version information

$ 04_04_custom_derive
? failed
error: Can only modify one version field

USAGE:
    clap [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom_derive --major
Version: 2.2.3

$ 04_04_custom_derive --major --minor
? failed
error: Can only modify one version field

USAGE:
    clap [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom_derive --major -c config.toml
? failed
Version: 2.2.3
error: INPUT_FILE or --spec-in is required when using --config

USAGE:
    clap [OPTIONS] [INPUT_FILE]

For more information try --help

$ 04_04_custom_derive --major -c config.toml --spec-in input.txt
Version: 2.2.3
Doing work using input input.txt and config config.toml

Testing

clap reports most development errors as debug_assert!s. Rather than checking every subcommand, you should have a test that calls Command::debug_assert:

use clap::Parser;

#[derive(Parser)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    /// Network port to use
    #[clap(value_parser)]
    port: u16,
}

fn main() {
    let cli = Cli::parse();

    println!("PORT = {}", cli.port);
}

#[test]
fn verify_cli() {
    use clap::CommandFactory;
    Cli::command().debug_assert()
}