help-rust/README.md

Intermediate problem:

```rust
pub struct Equals<'a> {
  field: &'a str,
  value: Value
}

impl Equals<'_> {
  pub fn from_args(mut args: ModuleArgs) -> Equals {
    Equals {
      field: match args.remove("field") {
        Some(Value::Str(s)) => s,
        _ => panic!("The Equals filter needs a field to filter in “field”")
      },
      value: args.remove("value").expect("The Equals filter needs a reference value in “value”")
    }
  }
}
```

Error on `Some(Value::Str(s)) => s` even though `args` is mutable and trashed at the end of the function.

<hr>

## setup

In module `modules`, I have:

```rust
pub trait Module {
  fn run(&self, record: &mut Record) -> Result<bool, ()>;
}
```

In module `filters`, I have:

```rust
pub struct Equals<'a> { … }

impl Equals<'_> {
  pub fn from_args(mut args: ModuleArgs) -> Equals {
    Equals { … }
  }
}

impl Module for Equals<'_> {
  fn run(&self, record: &mut Record) -> Result<bool, ()> { … }
}
```

## goal, “naive-style”

What I really want (for a start) is that, in module `modules`:

```rust
struct Available {
  name: &'static str,
  cons: fn(ModuleArgs) -> dyn Module
}

const available: (Available) = (
  Available{ name: "equals", cons: move |a| filters::Equals::from_args(a) }
);
```

with `available` being a tuple of available modules, each having a name and a constructor.

## problem

The above code gives me two errors:

1. one I somewhat understand:

   ```
   error[E0277]: the size for values of type `(dyn modules::Module + 'static)` cannot be known at compilation time
     --> src/modules.rs:10:45
      |
   10 |   Available{ name: "equals", cons: move |a| filters::Equals::from_args(a) }
      |                                             ^ doesn't have a size known at compile-time
   ```

2. and one I don’t understand:

   ```
   error[E0308]: mismatched types
     --> src/modules.rs:10:45
      |
   10 |   Available{ name: "equals", cons: move |a| filters::Equals::from_args(a) }
      |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait modules::Module, found struct `filters::Equals`
   ```