Lifetimes of App, Arg, ArgGroup: removing or relaxing

[Rupsbant]

Affected Version of clap

All 2.x. Breaking API change

Expected Behavior Summary

I want to decouple groups of options to several structs. I want to have a pair functions: one that adds arguments to an App with generated names, one that generates a structure with the added arguments.

Actual Behavior Summary

Uncompilable: the types don't allow to create String in a function and return a borrow.

Sample code

struct A {
...
}
impl A {
  fn arguments<'a, 'b>(prefix: &str) -> Vec<Arg<'a, 'b>> {
    let name = format!("{}.name", prefix);
    ...
  }
}

Proposed solution

Since clap is dependent on its speed it's probably not an option to force 'String' instead of '&str'. However I think Cow should give enough options.

Alternative for me

Create a struct AParse to contain the owned strings, split the function arguments in two parts. A can generate an AParsestruct before creating the App object.

[kbknapp]

This is a focus of 3.x so stay tuned. I've been able to get some work done in spurts during lulls in my day job (which have been few and far between this year). Watch the 3x-master branch for details 😉

[kbknapp]

Personally, I'd like to move away from storing &strs inside the Args as the name. This would solve lifetime issues as I want 3.x to be able to remove the Arg|App<'a, 'b> lifetimes. This could be solved by moving to owned Strings which takes a slight performance hit.

History

clap has two lifetimes for both Arg and App which reference the name, or key of the struct and data related to displaying of the help message or errors. clap uses name and key interchangeably.

clap used to only accept &'static strs because that's what 99% of users use. However, some users do dynamically create args and such, so we moved to &'key str and &'data str to refer to the name/key and display data. There reason these two are separate is because tying them to the same lifetime was cumbersome when one wanted to use 'static for one, but dynamically mess with the other.

Why not use String?

clap originally could have used an owned String to hold both the name and data, however for performance reasons we elected not to. I'm considering moving to String for the name/key as these are typically small, but am reluctant to use String for the data piece as this could be huge and I have yet to see someone who uses anything other than &'static str for such data.

Moving Forward

I want to get away from using &str for the name/key as this is preventing several key features like the ability to create no-* args on the fly.

We could move to Cow<'static, str> for data piece, but so far all attempts to do this have been a hassle. However, I'm OK with it being a hassle if the end result for users is better.

I'm open to ideas.