feat: #[constructor(into)] syntax for automatic Into generics

[gavincrawford]

Resolves #541

Synopsis

The following pattern is common in many Rust projects/libraries as a way to expand what types constructors accept:

pub struct UserID(String);

impl UserID {
    pub fn new<S: Into<String>>(value: S) -> Self {
        Self(value.into())
    }
}

It would be convenient to be able to have a derived constructor that is capable of replicating this behavior for types that implement Into<T>.

Solution

This solution uses a new attribute, #[constructor(into)] (very subject to change) that allows users to designate specific fields to accept generic inputs. For example:

#[derive(Constructor)]
struct UserID {
    #[constructor(into)]
    id: String
}

will have a corresponding new implementation that accepts id: impl Into<String>.

One caveat of this feature is that Into::into() is non-constant, which means the new, derived constructor function can't be either. In this implementation, I chose to handle this conditionally-- if Into<T> fields are present, new is not constant; otherwise, it still is.

I haven't added docs/tests yet, because I think it'd be worth talking about exactly what we want this syntax to look like, and if this is a feature that is within the scope of what this crate is meant to provide.

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)