v0.3 support for Option<Vec<T>>

[ryankurte]

hey thanks for making a super useful crate!

i've just updated from v0.2.x to v0.3.x and am running into a problem with parsing vectors that i'm not sure how to resolve.

In v0.2.x the following is valid, and passing an option -d something ends up with a vector containing something.

#[derive(Debug, Clone, PartialEq, StructOpt)]
pub struct Options {
    #[structopt(short = "d", long = "data", parse(from_str = "data_from_str"))]
    /// Data as a string
    pub data: Option<Vec<u8>>,
}

pub fn data_from_str(s: &str) -> Vec<u8> {
    s.as_bytes().to_vec()
}

In v0.3.x this results in an error from structopt derive

#[derive(Debug, Clone, PartialEq, StructOpt)]
pub struct Options {
    #[structopt(short = "d", long = "data", parse(from_str = data_from_str))]
    /// Data as a string
    pub data: Option<Vec<u8>>,
}

pub fn data_from_str(s: &str) -> Vec<u8> {
    s.as_bytes().to_vec()
}

error[E0308]: mismatched types
 --> src/main.rs:8:15
  |
8 |     pub data: Option<Vec<u8>>,
  |               ^^^^^^ expected u8, found struct `std::vec::Vec`
  |
  = note: expected type `std::vec::Vec<u8>`
             found type `std::vec::Vec<std::vec::Vec<u8>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `current`.

To learn more, run the command again with --verbose.

is it still possible to achieve this in v0.3.x?

[CreepySkeleton]

This is because Option<Vec<T>> is a special case now, it's different from Option<T>.

To suppress this special casing you can use a type alias, as described in changelog

type Something = Vec<u8>;

#[derive(StructOpt)]
struct Opt {
    #[structopt(short = "d", long = "data", parse(from_str = data_from_str))]
    /// Data as a string
    pub data: Option<Something>,
}

structopt doesn't know anything about Something so it'll be counted as Option<T>, just like in structopt v0.2. Alternatively, you can address Vec as std::vec::Vec, structopt doesn't know anything about it pretty much for the same reason.

Also, pay attention that you don't need #[structopt(short = "d")] anymore, use #[structopt(short)] instead, the option's name will be deduced from the field. The same applies to long. Take a look at the docs.

[ryankurte]

Ahh that's the one, thanks!