Multiple occurences with multiple values

[emfax]

I'm trying to make an argument that can have multiple occurrences with possible multiple values.

Something like this,

command --copy /from --copy /from /to

Here's my try at a derived parser,

#[derive(Parser, Debug)]
struct Args {
    #[arg(short, long, value_delimiter = ' ', num_args = 1..=2)]
    pub copy: Vec<Vec<PathBuf>>,
}

At the moment however the compiler complains about this,

the method `value_parser` exists for reference `&&&&&&_infer_ValueParser_for<Vec<PathBuf>>`, but its trait bounds were not satisfied
the following trait bounds were not satisfied:
`Vec<PathBuf>: ValueEnum`
which is required by `&&&&&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_ValueEnum`
`Vec<PathBuf>: ValueParserFactory`
which is required by `&&&&&&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_ValueParserFactory`
`Vec<PathBuf>: From<OsString>`
which is required by `&&&&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_From_OsString`
`Vec<PathBuf>: From<&'s std::ffi::OsStr>`
which is required by `&&&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_From_OsStr`
`Vec<PathBuf>: From<std::string::String>`
which is required by `&&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_From_String`
`Vec<PathBuf>: From<&'s str>`
which is required by `&_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_From_str`
`Vec<PathBuf>: FromStr`
which is required by `_infer_ValueParser_for<Vec<PathBuf>>: clap::builder::impl_prelude::_impls_FromStr`

Is it possible to have such an argument?

Using clap = { version = "4.5.21", features = ["derive"] }

[epage]

tl;dr run cargo add clap -F unstable-v5 but keep in mind the other breaking changes with that feature and that we may introduce more until we release clap v5.0.0.

Whats happening is the derive sees Vec<T> and treats that as multiple occurrences of T but clap doesn't know (and an attribute wasn't specified) to say how to parse Vec<PathBuf> out of a single value.

Multiple values per occurrence is generally handled as a flat structure (Vec<T>). We've slowly been adding support for grouping values per occurrence. Supporting this in the derive would technically be a breaking change (#4626) which won't be available until clap v5. We have implemented support for this behind the unstable-v5 feature flag

[emfax]

Thanks for the answer and the great explanation.

Also, well done on v5. Looks like it's gonna be great.