Method delegation with less boilerplate

This crate removes some boilerplate for structs that simply delegate some of their methods to one or more of their fields.

It gives you the delegate! macro, which delegates method calls to selected expressions (usually inner fields).

Example:

A Stack data structure implemented using an inner Vec via delegation.

use delegate::delegate;

#[derive(Clone, Debug)]
struct Stack<T> {
    inner: Vec<T>,
}
impl<T> Stack<T> {
    pub fn new() -> Self<T> {
        Self { inner: vec![] }
    }

    delegate! {
        to self.inner {
            pub fn is_empty(&self) -> bool;
            pub fn push(&mut self, value: T);
            pub fn pop(&mut self) -> Option<T>;
            pub fn clear(&mut self);

            #[call(len)]
            pub fn size(&self) -> usize;

            #[call(last)]
            pub fn peek(&self) -> Option<&T>;

        }
    }
}

Features:

Delegate to a method with a different name

struct Stack { inner: Vec<u32> }
impl Stack {
    delegate! {
        to self.inner {
            #[call(push)]
            pub fn add(&mut self, value: u32);
        }
    }
}

Use an arbitrary inner field expression

struct Wrapper { inner: Rc<RefCell<Vec<u32>>> }
impl Wrapper {
    delegate! {
        to self.inner.deref().borrow_mut() {
            pub fn push(&mut self, val: u32);
        }
    }
}

Delegate to enum variants

use delegate::delegate;

enum Enum {
    A(A),
    B(B),
    C { v: C },
}

struct A {
    val: usize,
}

impl A {
    fn dbg_inner(&self) -> usize {
        dbg!(self.val);
        1
    }
}
struct B {
    val_a: String,
}

impl B {
    fn dbg_inner(&self) -> usize {
        dbg!(self.val_a.clone());
        2
    }
}

struct C {
    val_c: f64,
}

impl C {
    fn dbg_inner(&self) -> usize {
        dbg!(self.val_c);
        3
    }
}

impl Enum {
    delegate! {
        // transformed to
        //
        // ```rust
        // match self {
        //     Enum::A(a) => a.dbg_inner(),
        //     Enum::B(b) => { println!("i am b"); b }.dbg_inner(),
        //     Enum::C { v: c } => { c }.dbg_inner(),
        // }
        // ```
        to match self {
            Enum::A(a) => a,
            Enum::B(b) => { println!("i am b"); b },
            Enum::C { v: c } => { c },
        } {
            fn dbg_inner(&self) -> usize;
        }
    }
}

Change the return type of the delegated method using a From or TryFrom impl or omit it altogether

struct Inner;
impl Inner {
    pub fn method(&self, num: u32) -> u32 { num }
}
struct Wrapper { inner: Inner }
impl Wrapper {
    delegate! {
        to self.inner {
            // calls method, converts result to u64 using `From`
            #[into]
            pub fn method(&self, num: u32) -> u64;

            // calls method, returns ()
            #[call(method)]
            pub fn method_noreturn(&self, num: u32);

            // calls method, converts result to i6 using `TryFrom`
            #[try_into]
            #[call(method)]
            pub fn method2(&self, num: u32) -> Result<u16, std::num::TryFromIntError>;

            // calls method, converts result to i6 using `TryFrom`, unwrap the result
            #[try_into(unwrap)]
            #[call(method)]
            pub fn method3(&self, num: u32) -> u16;
        }
    }
}

Call await on async functions

struct Inner;
impl Inner {
    pub async fn method(&self, num: u32) -> u32 { num }
}
struct Wrapper { inner: Inner }
impl Wrapper {
    delegate! {
        to self.inner {
            // calls method(num).await, returns impl Future<Output = u32>
            pub async fn method(&self, num: u32) -> u32;
            // calls method(num).await.into(), returns impl Future<Output = u64>
            #[into]
            #[call(method)]
            pub async fn method_into(&self, num: u32) -> u64;
        }
    }
}

You can use the #[await(true/false)] attribute on delegated methods to specify if .await should be generated after the delegated expression. It will be generated by default if the delegated method is async.

Delegate to multiple fields

struct MultiStack {
    left: Vec<u32>,
    right: Vec<u32>,
}
impl MultiStack {
    delegate! {
        to self.left {
            /// Push an item to the top of the left stack
            #[call(push)]
            pub fn push_left(&mut self, value: u32);
        }
        to self.right {
            /// Push an item to the top of the right stack
            #[call(push)]
            pub fn push_right(&mut self, value: u32);
        }
    }
}

Inserts #[inline(always)] automatically (unless you specify #[inline] manually on the method)
Specify expressions in the signature that will be used as delegated arguments

use delegate::delegate;
struct Inner;
impl Inner {
    pub fn polynomial(&self, a: i32, x: i32, b: i32, y: i32, c: i32) -> i32 {
        a + x * x + b * y + c
    }
}
struct Wrapper { inner: Inner, a: i32, b: i32, c: i32 }
impl Wrapper {
    delegate! {
        to self.inner {
            // Calls `polynomial` on `inner` with `self.a`, `self.b` and
            // `self.c` passed as arguments `a`, `b`, and `c`, effectively
            // calling `polynomial(self.a, x, self.b, y, self.c)`.
            pub fn polynomial(&self, [ self.a ], x: i32, [ self.b ], y: i32, [ self.c ]) -> i32 ;
            // Calls `polynomial` on `inner` with `0`s passed for arguments
            // `a` and `x`, and `self.b` and `self.c` for `b` and `c`,
            // effectively calling `polynomial(0, 0, self.b, y, self.c)`.
            #[call(polynomial)]
            pub fn linear(&self, [ 0 ], [ 0 ], [ self.b ], y: i32, [ self.c ]) -> i32 ;
        }
    }
}

Modify how will an input parameter be passed to the delegated method with parameter attribute modifiers. Currently, the following modifiers are supported:
- #[into]: Calls .into() on the parameter passed to the delegated method.
- #[as_ref]: Calls .as_ref() on the parameter passed to the delegated method.

use delegate::delegate;
//!
struct InnerType {}
impl InnerType {
    fn foo(&self, other: Self) {}
}
//!
impl From<Wrapper> for InnerType {
    fn from(wrapper: Wrapper) -> Self {
        wrapper.0
    }
}
//!
struct Wrapper(InnerType);
impl Wrapper {
    delegate! {
        to self.0 {
            // Calls `self.0.foo(other.into());`
            pub fn foo(&self, #[into] other: Self);
        }
    }
}

License

Licensed under either of

Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Conduct