Crate auto_enums

A library for to allow multiple return types by automatically generated enum.

This crate is a procedural macro implementation of the features discussions in rust-lang/rfcs#2414. This idea is also known as “Anonymous sum types”.

This library provides the following attribute macros:

• #[auto_enum]

  Parses syntax, creates the enum, inserts variants, and passes specified traits to #[enum_derive].

• #[enum_derive]

  Implements specified traits to the enum.

#[auto_enum]

#[auto_enum]’s basic feature is to wrap the value returned by the obvious branches (match, if, return, etc..) by an enum that implemented the specified traits.

use auto_enums::auto_enum;

#[auto_enum(Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    match x {
        0 => 1..10,
        _ => vec![5, 10].into_iter(),
    }
}

#[auto_enum] generates code in two stages.

First, #[auto_enum] will do the following.

• parses syntax
• creates the enum
• inserts variants

Code like this will be generated:

fn foo(x: i32) -> impl Iterator<Item = i32> {
    #[::auto_enums::enum_derive(Iterator)]
    enum __Enum1<__T1, __T2> {
        __T1(__T1),
        __T2(__T2),
    }

    match x {
        0 => __Enum1::__T1(1..10),
        _ => __Enum1::__T2(vec![5, 10].into_iter()),
    }
}

Next, #[enum_derive] implements the specified traits.

Code like this will be generated:

fn foo(x: i32) -> impl Iterator<Item = i32> {
    enum __Enum1<__T1, __T2> {
        __T1(__T1),
        __T2(__T2),
    }

    impl<__T1, __T2> ::core::iter::Iterator for __Enum1<__T1, __T2>
    where
        __T1: ::core::iter::Iterator,
        __T2: ::core::iter::Iterator<Item = <__T1 as ::core::iter::Iterator>::Item>,
    {
        type Item = <__T1 as ::core::iter::Iterator>::Item;
        #[inline]
        fn next(&mut self) -> ::core::option::Option<Self::Item> {
            match self {
                __Enum1::__T1(x) => x.next(),
                __Enum1::__T2(x) => x.next(),
            }
        }
        #[inline]
        fn size_hint(&self) -> (usize, ::core::option::Option<usize>) {
            match self {
                __Enum1::__T1(x) => x.size_hint(),
                __Enum1::__T2(x) => x.size_hint(),
            }
        }
    }

    match x {
        0 => __Enum1::__T1(1..10),
        _ => __Enum1::__T2(vec![5, 10].into_iter()),
    }
}

Nested arms/branches

#[auto_enum] can also parse nested arms/branches by using the #[nested] attribute.

use auto_enums::auto_enum;

#[auto_enum(Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    match x {
        0 => 1..10,
        #[nested]
        _ => match x {
            1 => vec![5, 10].into_iter(),
            _ => 0..=x,
        },
    }
}

#[nested] can be used basically in the same place as #[auto_enum], except that #[nested] cannot be used in functions.

Recursion

If an error due to recursion occurs, you need to box branches where recursion occurs.

use auto_enums::auto_enum;

struct Type {
    child: Vec<Type>,
}

impl Type {
    #[auto_enum(Iterator)]
    fn method(&self) -> impl Iterator<Item = ()> + '_ {
        if self.child.is_empty() {
            Some(()).into_iter()
        } else {
            // Boxing is only needed on branches where recursion occurs.
            Box::new(self.child.iter().flat_map(|c| c.method())) as Box<dyn Iterator<Item = _>>
        }
    }
}

Positions where #[auto_enum] can be used.

#[auto_enum] can be used in the following three places. However, since stmt_expr_attributes and proc_macro_hygiene are not stabilized, you need to use empty #[auto_enum] for functions except nightly.

• functions

  use auto_enums::auto_enum;
  
  #[auto_enum(Iterator)]
  fn func(x: i32) -> impl Iterator<Item=i32> {
      if x == 0 {
          Some(0).into_iter()
      } else {
          0..x
      }
  }

• expressions

  use auto_enums::auto_enum;
  
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn expr(x: i32) -> impl Iterator<Item=i32> {
      #[auto_enum(Iterator)]
      match x {
          0 => Some(0).into_iter(),
          _ => 0..x,
      }
  }

• let binding

  use auto_enums::auto_enum;
  
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn let_binding(x: i32) -> impl Iterator<Item=i32> {
      #[auto_enum(Iterator)]
      let iter = match x {
          0 => Some(0).into_iter(),
          _ => 0..x,
      };
      iter
  }

Supported syntax

• if and match

  Wrap each branch with a variant.

  use auto_enums::auto_enum;
  
  // if
  #[auto_enum(Iterator)]
  fn expr_if(x: i32) -> impl Iterator<Item=i32> {
      if x == 0 {
          Some(0).into_iter()
      } else {
          0..x
      }
  }
  
  // match
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn expr_match(x: i32) -> impl Iterator<Item=i32> {
      #[auto_enum(Iterator)]
      let iter = match x {
          0 => Some(0).into_iter(),
          _ => 0..x,
      };
      iter
  }

• loop

  Wrap each break with a variant. Nested loops and labeled break are also supported.

  use auto_enums::auto_enum;
  
  #[auto_enum(Iterator)]
  fn expr_loop(mut x: i32) -> impl Iterator<Item = i32> {
      loop {
          if x < 0 {
              break x..0;
          } else if x % 5 == 0 {
              break 0..=x;
          }
          x -= 1;
      }
  }

• return (in functions)

  #[auto_enum] can parse the return in the scope.

  This analysis is valid only when the return type is impl Trait.

  use auto_enums::auto_enum;
  
  // return (in functions)
  #[auto_enum(Iterator)]
  fn func(x: i32) -> impl Iterator<Item=i32> {
      if x == 0 {
          return Some(0).into_iter();
      }
  
      if x > 0 {
          0..x
      } else {
          x..=0
      }
  }

• return (in closures)

  #[auto_enum] can parse the return in the scope.

  This analysis is valid only when the following two conditions are satisfied.

  • #[auto_enum] must be used directly for that closure (or the let binding of the closure).
  • ? operator not used in the scope.

  use auto_enums::auto_enum;
  
  // return (in closures)
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn closure() -> impl Iterator<Item=i32> {
      #[auto_enum(Iterator)]
      let f = |x| {
          if x == 0 {
              return Some(0).into_iter();
          }
  
          if x > 0 {
              0..x
          } else {
              x..=0
          }
      };
      f(1)
  }

• ? operator (in functions)

  #[auto_enum] can parse the ? operator in the scope.

  This analysis is valid only when the return type is Result<T, impl Trait>.

  use auto_enums::auto_enum;
  use std::fmt::{Debug, Display};
  
  // `?` operator (in functions)
  #[auto_enum(Debug, Display)]
  fn func(x: i32) -> Result<i32, impl Debug + Display> {
      if x == 0 {
          Err("`x` is zero")?;
      }
  
      // The last branch of the function is not parsed.
      if x < 0 {
          Err(x)?
      } else {
          Ok(x + 1)
      }
  }

  ? operator is expanded as follows:

  match expr {
      Ok(val) => val,
      Err(err) => return Err(Enum::Variant(err)),
  }

• ? operator (in closures)

  #[auto_enum] can parse the ? operator in the scope.

  However, #[auto_enum] must be used directly for that closure (or the let binding of the closure).

  use auto_enums::auto_enum;
  use std::fmt::{Debug, Display};
  
  // `?` operator (in closures)
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn closure() -> Result<i32, impl Debug + Display> {
      #[auto_enum(Debug, Display)]
      let f = |x| {
          if x == 0 {
              Err("`x` is zero")?
          }
  
          // The last branch of the function is not interpreted as a branch.
          if x < 0 {
              Err(x)?
          } else {
              Ok(x + 1)
          }
      };
      f(1)
  }

• Block, unsafe block, method call, parentheses, and type ascription

  The following expressions are recursively searched until an if, match, loop or unsupported expression is found.

  • blocks
  • unsafe blocks
  • method calls
  • parentheses
  • type ascriptions

  use auto_enums::auto_enum;
  
  // block
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn expr_block(x: i32) -> impl Iterator<Item=i32> {
      #[auto_enum(Iterator)]
      {
          if x == 0 {
              Some(0).into_iter()
          } else {
              0..x
          }
      }
  }
  
  // method call
  #[auto_enum] // Nightly does not need an empty attribute to the function.
  fn expr_method(x: i32) -> impl Iterator<Item=i32> {
     #[auto_enum(Iterator)]
      match x {
          0 => Some(0).into_iter(),
          _ => 0..x,
      }.map(|y| y + 1)
  }
  
  // parentheses
  #[auto_enum(Iterator)]
  fn expr_parentheses(x: i32) -> impl Iterator<Item=i32> {
      (if x == 0 { Some(0).into_iter() } else { 0..x })
  }

Expression that no value will be returned

If the last expression of a branch is one of the following, it is interpreted that no value will be returned (variant assignment is skipped).

• panic!(..)
• unreachable!(..)
• return
• break
• continue
• None?
• Err(..)?
• Expression level marker (marker! macro).
• An item definition.

Also, if the branch contains #[nested], it is interpreted as returning an anonymous enum generated by #[auto_enum], not a value.

use auto_enums::auto_enum;

#[auto_enum(Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    match x {
        0 => 1..10,
        1 => panic!(), // variant assignment is skipped
        _ => vec![5, 10].into_iter(),
    }
}

You can also skip that branch explicitly by #[never] attribute.

use auto_enums::auto_enum;

#[auto_enum(Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    match x {
        0 => 1..10,
        #[never]
        1 => loop {
            panic!()
        },
        _ => vec![5, 10].into_iter(),
    }
}

Expression level marker (marker! macro)

#[auto_enum] replaces marker! macros with variants. If values of two or more are specified by marker! macros, #[auto_enum] can be used for unsupported expressions and statements.

use auto_enums::auto_enum;

#[auto_enum(Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    if x < 0 {
        return x..=0;
    }
    marker!(1..10)
}

The default name of the macro is "marker", but you can change it by marker option.

use auto_enums::auto_enum;

#[auto_enum(marker = bar, Iterator)]
fn foo(x: i32) -> impl Iterator<Item = i32> {
    if x < 0 {
        return x..=0;
    }
    bar!(1..10)
}

Rust Nightly

When using #[auto_enum] for expressions and statements, #[auto_enum] for function is unnecessary.

// Add this to your crate root:
#![feature(proc_macro_hygiene, stmt_expr_attributes)]

use auto_enums::auto_enum;

fn foo(x: i32) -> i32 {
    #[auto_enum(Iterator)]
    let iter = match x {
        0 => 1..10,
        _ => vec![5, 10].into_iter(),
    };

    iter.fold(0, |sum, x| sum + x)
}

You can also return closures.

// Add this to your crate root:
#![feature(fn_traits, unboxed_closures)]

use auto_enums::auto_enum;

#[auto_enum(Fn)]
fn foo(x: bool) -> impl Fn(i32) -> i32 {
    if x {
        |y| y + 1
    } else {
        |z| z - 1
    }
}

#[enum_derive]

#[enum_derive] implements the supported traits and passes unsupported traits to #[derive].

If you want to use traits that are not supported by #[enum_derive], you can use another crate that provides derives macros, or you can define derives macros yourself (derive_utils probably can help it).

Basic usage of #[enum_derive]

use auto_enums::enum_derive;

// `#[enum_derive]` implements `Iterator`, and `#[derive]` implements `Clone`.
#[enum_derive(Iterator, Clone)]
enum Foo<A, B> {
    A(A),
    B(B),
}

#[enum_derive] adds the dependency of the specified trait if it is not specified.

use auto_enums::enum_derive;

// `#[enum_derive]` implements `Iterator` and `ExactSizeIterator`.
#[enum_derive(ExactSizeIterator)]
enum Foo<A, B> {
    A(A),
    B(B),
}

Supported traits

Some traits support is disabled by default. Note that some traits have aliases.

When using features that depend on unstable APIs, the unstable feature must be explicitly enabled

The standard library (std, core)

[std|core]::iter

• Iterator - example | generated code
• DoubleEndedIterator - example | generated code
• ExactSizeIterator - example | generated code
• FusedIterator - example | generated code
• Extend - example | generated code
• TrustedLen (requires "trusted_len" and "unstable" crate features) - example | generated code

See also iter-enum crate.

[std|core]::future

• Future - example | generated code

std::io (requires "std" crate feature)

• Read (alias: io::Read) - example | generated code
• BufRead (alias: io::BufRead) - example | generated code
• Write (alias: io::Write) - example | generated code
• Seek (alias: io::Seek) - example | generated code

See also io-enum crate.

[std|core]::ops

• Deref (requires "ops" crate feature)
• DerefMut (requires "ops" crate feature)
• Index (requires "ops" crate feature)
• IndexMut (requires "ops" crate feature)
• RangeBounds (requires "ops" crate feature)
• Fn (requires "fn_traits" and "unstable" crate features)
• FnMut (requires "fn_traits" and "unstable" crate features)
• FnOnce (requires "fn_traits" and "unstable" crate features)
• Coroutine (requires "coroutine_trait" and "unstable" crate features)

[std|core]::convert

• AsRef (requires "convert" crate feature)
• AsMut (requires "convert" crate feature)

[std|core]::fmt

• Debug (alias: fmt::Debug) - example | generated code
• Display (alias: fmt::Display)
• fmt::Binary (requires "fmt" crate feature)
• fmt::LowerExp (requires "fmt" crate feature)
• fmt::LowerHex (requires "fmt" crate feature)
• fmt::Octal (requires "fmt" crate feature)
• fmt::Pointer (requires "fmt" crate feature)
• fmt::UpperExp (requires "fmt" crate feature)
• fmt::UpperHex (requires "fmt" crate feature)
• fmt::Write

std::error (requires "std" crate feature)

• Error - example | generated code

External libraries

You can use support for external library traits by activating each crate feature.

To use support for external library traits, you need to use the path starting with the feature name. For example:

use auto_enums::auto_enum;
use rayon::prelude::*;

#[auto_enum(rayon::ParallelIterator)] // Note that this is not `#[auto_enum(ParallelIterator)]`
fn func(x: i32) -> impl ParallelIterator {
    match x {
        0 => (1..10).into_par_iter(),
        _ => vec![5, 10].into_par_iter(),
    }
}

futures v0.3 (requires "futures03" crate feature)

• futures03::Stream - example | generated code
• futures03::Sink - example | generated code
• futures03::AsyncRead - example | generated code
• futures03::AsyncWrite - example | generated code
• futures03::AsyncSeek - example | generated code
• futures03::AsyncBufRead - example | generated code

futures v0.1 (requires "futures01" crate feature)

• futures01::Future
• futures01::Stream
• futures01::Sink

rayon (requires "rayon" crate feature)

• rayon::ParallelIterator - example | generated code
• rayon::IndexedParallelIterator - example | generated code
• rayon::ParallelExtend - example | generated code

serde (requires "serde" crate feature)

• serde::Serialize - example | generated code

tokio v1 (requires "tokio1" crate feature)

• tokio1::AsyncRead - example | generated code
• tokio1::AsyncWrite - example | generated code
• tokio1::AsyncSeek - example | generated code
• tokio1::AsyncBufRead - example | generated code

tokio v0.3 (requires "tokio03" crate feature)

• tokio03::AsyncRead
• tokio03::AsyncWrite
• tokio03::AsyncSeek
• tokio03::AsyncBufRead

tokio v0.2 (requires "tokio02" crate feature)

• tokio02::AsyncRead
• tokio02::AsyncWrite
• tokio02::AsyncSeek
• tokio02::AsyncBufRead

tokio v0.1 (requires "tokio01" crate feature)

• tokio01::AsyncRead
• tokio01::AsyncWrite

http_body v1 (requires "http_body1" crate feature)

• http_body1::Body - example | generated code

Inherent methods

These don’t derive traits, but derive inherent methods instead.

• Transpose (requires "transpose_methods" crate feature) - this derives the following conversion methods.

  • transpose — convert from enum<Option<T1>,..> to Option<enum<T1,..>>

  • transpose — convert from enum<Result<T1, E1>,..> to Result<enum<T1,..>, enum<E1,..>>

  • transpose_ok — convert from enum<Result<T1, E>,..> to Option<enum<T1,..>, E>

    Examples:

    use auto_enums::auto_enum;
    use std::{fs::File, io, path::Path};
    
    #[auto_enum(Transpose, Write)]
    fn output_stream(file: Option<&Path>) -> io::Result<impl io::Write> {
        match file {
            Some(f) => File::create(f),
            None => Ok(io::stdout()),
        }.transpose_ok()
    }

  • transpose_err — convert from enum<Result<T, E1>,..> to Result<T, enum<E1,..>>

Optional features

• std (enabled by default)
  • Enable to use std library’s traits.
• ops
  • Enable to use [std|core]::ops’s Deref, DerefMut, Index, IndexMut, and RangeBounds traits.
• convert
  • Enable to use [std|core]::convert’s AsRef and AsMut traits.
• fmt
  • Enable to use [std|core]::fmt’s traits other than Debug, Display and Write.
• transpose_methods
  • Enable to use transpose* methods.
• futures03
  • Enable to use futures v0.3 traits.
• futures01
  • Enable to use futures v0.1 traits.
• rayon
  • Enable to use rayon traits.
• serde
  • Enable to use serde traits.
• tokio1
  • Enable to use tokio v1 traits.
• tokio03
  • Enable to use tokio v0.3 traits.
• tokio02
  • Enable to use tokio v0.2 traits.
• tokio01
  • Enable to use tokio v0.1 traits.
• http_body1
  • Enable to use http_body v1 traits.
• coroutine_trait
  • Enable to use [std|core]::ops::Coroutine trait.
  • Note that this feature is unstable and may cause incompatible changes between patch versions.
• fn_traits
  • Enable to use [std|core]::ops’s Fn, FnMut, and FnOnce traits.
  • Note that this feature is unstable and may cause incompatible changes between patch versions.
• trusted_len
  • Enable to use [std|core]::iter::TrustedLen trait.
  • Note that this feature is unstable and may cause incompatible changes between patch versions.

type_analysis feature

Analyze return type of function and let binding.

Note that this feature is still experimental.

Examples:

use auto_enums::auto_enum;

#[auto_enum] // there is no need to specify std library's traits
fn func1(x: i32) -> impl Iterator<Item = i32> {
    match x {
        0 => 1..10,
        _ => vec![5, 10].into_iter(),
    }
}

#[auto_enum]
fn func2(x: i32) {
    // Unlike `feature(impl_trait_in_bindings)`, this works on stable compilers.
    #[auto_enum]
    let iter: impl Iterator<Item = i32> = match x {
        0 => Some(0).into_iter(),
        _ => 0..x,
    };
}

Please be careful if you return another traits with the same name.

Known limitations

• There needs to explicitly specify the trait to be implemented (type_analysis crate feature reduces this limitation).
• There needs to be marker macros for unsupported expressions.

Related Projects

• derive_utils: A procedural macro helper for easily writing derives macros for enums.
• io-enum: #[derive(Read, Write, Seek, BufRead)] for enums.
• iter-enum: #[derive(Iterator, DoubleEndedIterator, ExactSizeIterator, Extend)] for enums.

Attribute Macros

• auto_enum
  An attribute macro for to allow multiple return types by automatically generated enum.
• enum_derive
  An attribute macro like a wrapper of #[derive], implementing the supported traits and passing unsupported traits to #[derive].