structopt 0.3.26

Parse command line argument by defining a struct.
Documentation 
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use structopt::StructOpt;

#[test]
fn generic_struct_flatten() {
    #[derive(StructOpt, PartialEq, Debug)]
    struct Inner {
        pub answer: isize,
    }

    #[derive(StructOpt, PartialEq, Debug)]
    struct Outer<T: StructOpt> {
        #[structopt(flatten)]
        pub inner: T,
    }

    assert_eq!(
        Outer {
            inner: Inner { answer: 42 }
        },
        Outer::from_iter(&["--answer", "42"])
    )
}

#[test]
fn generic_struct_flatten_w_where_clause() {
    #[derive(StructOpt, PartialEq, Debug)]
    struct Inner {
        pub answer: isize,
    }

    #[derive(StructOpt, PartialEq, Debug)]
    struct Outer<T>
    where
        T: StructOpt,
    {
        #[structopt(flatten)]
        pub inner: T,
    }

    assert_eq!(
        Outer {
            inner: Inner { answer: 42 }
        },
        Outer::from_iter(&["--answer", "42"])
    )
}

#[test]
fn generic_enum() {
    #[derive(StructOpt, PartialEq, Debug)]
    struct Inner {
        pub answer: isize,
    }

    #[derive(StructOpt, PartialEq, Debug)]
    enum GenericEnum<T: StructOpt> {
        Start(T),
        Stop,
    }

    assert_eq!(
        GenericEnum::Start(Inner { answer: 42 }),
        GenericEnum::from_iter(&["test", "start", "42"])
    )
}

#[test]
fn generic_enum_w_where_clause() {
    #[derive(StructOpt, PartialEq, Debug)]
    struct Inner {
        pub answer: isize,
    }

    #[derive(StructOpt, PartialEq, Debug)]
    enum GenericEnum<T>
    where
        T: StructOpt,
    {
        Start(T),
        Stop,
    }

    assert_eq!(
        GenericEnum::Start(Inner { answer: 42 }),
        GenericEnum::from_iter(&["test", "start", "42"])
    )
}

#[test]
fn generic_w_fromstr_trait_bound() {
    use std::{fmt, str::FromStr};

    #[derive(StructOpt, PartialEq, Debug)]
    struct Opt<T>
    where
        T: FromStr,
        <T as FromStr>::Err: fmt::Debug + fmt::Display,
    {
        answer: T,
    }

    assert_eq!(
        Opt::<isize> { answer: 42 },
        Opt::<isize>::from_iter(&["--answer", "42"])
    )
}

#[test]
fn generic_wo_trait_bound() {
    use std::time::Duration;

    #[derive(StructOpt, PartialEq, Debug)]
    struct Opt<T> {
        answer: isize,
        #[structopt(skip)]
        took: Option<T>,
    }

    assert_eq!(
        Opt::<Duration> {
            answer: 42,
            took: None
        },
        Opt::<Duration>::from_iter(&["--answer", "42"])
    )
}

#[test]
fn generic_where_clause_w_trailing_comma() {
    use std::{fmt, str::FromStr};

    #[derive(StructOpt, PartialEq, Debug)]
    struct Opt<T>
    where
        T: FromStr,
        <T as FromStr>::Err: fmt::Debug + fmt::Display,
    {
        pub answer: T,
    }

    assert_eq!(
        Opt::<isize> { answer: 42 },
        Opt::<isize>::from_iter(&["--answer", "42"])
    )
}