facet-args 0.41.0

Type-safe command-line argument parsing powered by Facet reflection
Documentation 
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Provides CLI argument parsing (WIP).

```rust
use facet_pretty::FacetPretty;
use facet::Facet;
use facet_args as args;

#[derive(Facet)]
struct Args {
    #[facet(args::positional)]
    path: String,

    #[facet(args::named, args::short = 'v')]
    verbose: bool,

    #[facet(args::named, args::short = 'j')]
    concurrency: usize,
}

# fn main() -> Result<(), Box<dyn std::error::Error>> {
let args: Args = facet_args::from_slice(&["--verbose", "-j", "14", "example.rs"])?;
eprintln!("args: {}", args.pretty());
Ok(())
# }
```

## Behavior

The behavior of facet-args is still in flux, but here are the broad strokes:

  * We're always parsing to a struct (not an enum, vec etc.)
  * The struct we're parsing to is always owned — no borrowing happening here, it
    gets too complicated with `&'slice [&'text str]`
  * Arguments are either `positional` or `named` — fields lacking either annotation are ignored
  * Accepted syntaxes for short flags are: `args::short = 'v'` and `args::short = "v"` (where v can be any letter)
  * `positional` args of type `Vec` (or anything that has a `Def::List`) will soak up all the positional
    arguments — if followed by `positional` arguments of type `String` for example, those will never
    get filled
  * After parsing every available argument, uninitialized struct fields are filled with their default value
    if they have `facet(default)` set: this includes `Vec`.