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0.1.26 Apr 18, 2023
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#132 in Procedural macros

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penum is a procedural macro that is used for enum conformity and static dispatch. This is done by specifying a declarative pattern that expresses how we should interpret the enum. It's a tool for asserting how enums should look and behave through simple expressive rust grammar.

  • Patterns — can be thought of as a toy shape sorter that sorts through enum variants and makes sure they fit. So each variant has a certain shape that must satisfy the patterns we've specified. There are 3 shapes to choose from, tuples (), structs {} and units.

  • Predicates — are used in combination with patterns to assert what the matched variants field types should implement. They can be expressed like a regular where clause, e.g where T: Trait<Type>. The generic parameters needs to be introduced inside a pattern fragment.

  • Smart dispatch — lets us express how an enum should behave in respect to its variants. The symbol that is used to express this is ^ and should be put in front of the trait you wish to be dispatched.


This crate is available on crates.io and can be used by adding the following to your project's Cargo.toml:

penum = "0.1.26"

Or run this command in your cargo project:

$ cargo add penum


A Penum expression can look like this:

                      Dispatch symbol.
#[penum( (T) where T: ^Trait )]
         ^^^       ^^^^^^^^^
         |         |
         |         Predicate bound.
         Pattern fragment.

A Penum expression without specifying a pattern:

#[penum( impl Trait for Type )]

Shorthand syntax for _ where Type: ^Trait

More details

Important to include ^ for traits that you want to dispatch.

#[penum( impl Type: ^Trait )]

Note that in a penum impl for expression, no ^ is needed.

#[penum( impl Trait for Type )]

In Rust 1.68.0, From<bool> for {f32,f64} has stabilized. That means you can do this.

#[penum( impl From<bool> for {f32,f64} )]

Trivial example

Use Penum to automatically implement a trait for the enum.

#[penum(impl String: ^AsRef<str>)]
enum Store {
    V2(String, i32),
    V3(i32, usize, String),
    V4(i32, String, usize),
    V5 { age: usize, name: String },
  • Will turn into this:
impl AsRef<str> for Store {
    fn as_ref(&self) -> &str {
        match self {
            Store::V2(val, ..) => val.as_ref(),
            Store::V3(_, _, val) => val.as_ref(),
            Store::V4(_, val, ..) => val.as_ref(),
            Store::V5 { name, .. } => name.as_ref(),
            _ => "",

There is also support for user defined traits, but make sure that they are tagged before the enum.

trait Trait {
    fn method(&self, text: &str) -> &Option<&str>;

Supported std traits

Any, Borrow, BorrowMut, Eq, AsMut, AsRef, From, Into, TryFrom, TryInto, Default, Binary, Debug, Display, LowerExp, LowerHex, Octal, Pointer, UpperExp, UpperHex, Future, IntoFuture, FromIterator, FusedIterator, IntoIterator, Product, Sum, Sized, ToSocketAddrs, Add, AddAssign, BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Deref, DerefMut, Div, DivAssign, Drop, Index, IndexMut, Mul, MulAssign, MultiMethod, Neg, Not, Rem, RemAssign, Shl, ShlAssign, Shr, ShrAssign, Sub, SubAssign, Termination, SliceIndex, FromStr, ToString

Penum is smart enough to infer certain return types for non-matching variants. e.g Option<T>, &Option<T>, String, &str. It can even handle &String, referenced non-const types. The goal is to support any type, which we could potentially do by checking for types implementing the Default trait.

Note, when dispatching traits with associated types, it's important to declare them. e.g Add<i32, Output = i32>.


Used penum to force every variant to be a tuple with one field that must implement Trait.

#[penum( (T, ..) where T: Trait )]
enum Guard {
    // ERROR: `String` doesn't implement `Trait`

    // ERROR: `Option<String>` doesn't implement `Trait`

    // ERROR: `Vec<String>` doesn't implement `Trait`

    // ERROR: `Byr()` doesn't match pattern `(T)`

    Bxr { name: usize }, 
    // ERROR: `{ nname: usize }` doesn't match pattern `(T)`

    // ERROR: `Brr` doesn't match pattern `(T)`

    Bir(i32, String), // Works!
    Beer(i32)         // Works!

If you don't care about the actual pattern matching, then you could use _ to automatically infer every shape and field. Combine this with concrete dispatch types, and you got yourself a auto dispatcher.

Under development

For non-std types we rely on the Default trait, which means, if we can prove that a type implements Default we can automatically add them as return types for non-matching variants,

#[penum( _ where Ce: ^Special, Be: ^AsInner<i32> )]
enum Foo {
    V2(i32, Be),
    V4 { name: String, age: Be },

// Will create these implementations
impl Special for Foo {
    fn ret(&self) -> Option<&String> {
        match self {
            Foo::V3(val) => val.ret(),
            _ => None,

impl AsInner<i32> for Foo {
    fn as_inner(&self) -> &i32 {
        match self {
            Foo::V2(_, val) => val.as_inner(),
            Foo::V4 { age, .. } => age.as_inner(),
            _ => &0,
  • It's identical to this:
#[penum(impl Ce: ^Special, Be: ^AsInner<i32>)]

More details

  • Impls — can be seen as a shorthand for a concrete type that implements this trait, and are primarily used as a substitute for regular generic trait bound expressions. They look something like this, (impl Copy, impl Copy) | {name: impl Clone}

  • Placeholders — are single unbounded wildcards, or if you are familiar with rust, it's the underscore _ identifier and usually means that something is ignored, which means that they will satisfy any type (_, _) | {num: _}.

  • Variadic — are similar to placeholders, but instead of only being able to substitute one type, variadics can be substituted by 0 or more types. Like placeholders, they are a way to express that we don't care about the rest of the parameters in a pattern. The look something like this(T, U, ..) | {num: T, ..}.


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