implement into and into_iter

[OmarTawfik]

Thank you for the awesome crate!
Here is a feature suggestion, that is sorely missed in our projects. It has also been requested twice already (#43 and #62).

#[new(into)]

To make type conversion easier, #[new(into)] attribute changes the parameter type to impl Into<T>, and populates the field with value.into(). This is useful for containers like strings, accepting a String or a &str:

#[derive(new)]
struct Foo {
    #[new(into)]
    x: String,
}

let _ = Foo::new("Hello".to_string());
let _ = Foo::new(format!("Hello"));
let _ = Foo::new("Hello");

It is also useful for wrapping types like Box<T> and Rc<T>, without having to wrap at every callsite:

#[derive(new)]
struct Foo {
    #[new(into)]
    x: Rc<Bar>,
}

let _ = Foo::new(Rc::new(bar));
let _ = Foo::new(bar.into());
let _ = Foo::new(bar);

#[new(into_iter = "T")]

For iterators/collections, #[new(into_iter = "T")] attribute changes the parameter type to impl IntoIterator<Item = T>, and populates the field with value.into_iter().collect(). This allows initializing with a variety of types, like [T], Option<T>, Vec<T>, sets, maps, and even other iterators:

#[derive(new)]
struct Foo {
    #[new(into_iter = "bool")]
    x: Vec<bool>,
}

let _ = Foo::new(vec![true, false]);
let _ = Foo::new([true, false]);
let _ = Foo::new(Some(true));

About this PR

I split this into three parts to make reviewing easier:

• 08e5390 fixes broken integration tests (I suspect CI is out of date).
• 3e310d9 fixes formatting for a couple of files (using the default cargo fmt).
• 8f60880 implements the feature, adding tests, and documentation to both README.md and lib.rs.

Please let me know if I can do anything else.

[OmarTawfik]

cc @nrc

Closes #43
Closes #62

[DenisGorbachev]

@nrc Is it possible to merge this PR please? This would be a time-saver for a lot of projects

[polarathene]

@nrc friendly ping

[nrc]

Thanks! This is a great improvement, sorry it took so long to review

[nrc]

Merged manually to resolve conflicts