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;
          }
      }
  }

• Use modifiers that alter the generated method body

  use delegate::delegate;
  struct Inner;
  impl Inner {
      pub fn method(&self, num: u32) -> u32 { num }
      pub fn method_res(&self, num: u32) -> Result<u32, ()> { Ok(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_res, unwraps the result
              #[unwrap]
              pub fn method_res(&self, num: u32) -> u32;
  
              // calls method_res, unwraps the result, then calls into
              #[unwrap]
              #[into]
              #[call(method_res)]
              pub fn method_res_into(&self, num: u32) -> u64;
  
              // specify explicit type for into
              #[into(u64)]
              #[call(method)]
              pub fn method_into_explicit(&self, num: u32) -> u64;
          }
      }
  }

• Add additional arguments to method

  struct Inner(u32);
  impl Inner {
      pub fn new(m: u32) -> Self {
          // some "very complex" constructing work
          Self(m)
      }
      pub fn method(&self, n: u32) -> u32 {
          self.0 + n
      }
  }
  
  struct Wrapper {
      inner: OnceCell<Inner>,
  }
  
  impl Wrapper {
      pub fn new() -> Self {
          Self {
              inner: OnceCell::new(),
          }
      }
      fn content(&self, val: u32) -> &Inner {
          self.inner.get_or_init(|| Inner(val))
      }
      delegate! {
          to |k: u32| self.content(k) {
              // `wrapper.method(k, num)` will call `self.content(k).method(num)`
              pub fn method(&self, num: u32) -> u32;
          }
      }
  }

• 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)

• You can use an attribute on a whole segment to automatically apply it to all methods in that segment:

  struct Wrapper { inner: Inner }
  
  impl Wrapper {
   delegate! {
     #[unwrap]
     to self.inner {
       fn foo(&self) -> u32; // calls self.inner.foo().unwrap()
       fn bar(&self) -> u32; // calls self.inner.bar().unwrap()
     }
   }
  }

• 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.
  • #[newtype]: Accesses the first tuple element (.0) of 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);
              // Calls `self.0.bar(other.0);`
              pub fn bar(&self, #[newtype] other: Self);
          }
      }
  }

• Delegate associated functions

  use delegate::delegate;
  
  struct A {}
  impl A {
      fn foo(a: u32) -> u32 {
          a + 1
      }
  }
  
  struct B;
  
  impl B {
      delegate! {
          to A {
              fn foo(a: u32) -> u32;
          }
      }
  }
  
  assert_eq!(B::foo(1), 2);

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

Please follow the Rust Code of Conduct. For escalation or moderation issues please contact the crate author(s) listed in Cargo.toml.