Crate zoet

Adds #[zoet] macro to reduce boilerplate when implementing common traits.

If you are sick of writing impl Deref for Bar etc. and it didn't compile because you confused it with AsRef, had a hard-to-debug problem because you implemented PartialOrd and mistakenly thought that deriving Ord would do the sane thing, and/or you would rather just implement these core traits as regular functions in your impl Bar like lesser languages, this crate is for you!

It is superficially similar to the various derive macros such as derive_more, except that rather than generating traits based on the contents of a struct, it generates them based on individual functions. An example works better than a textual description ever would:

use core::{
    cmp::Ordering,
    hash::{Hash, Hasher},
};
use zoet::zoet;

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
struct Length(usize);
#[zoet]
impl Length {
    #[zoet(Default)] // generates `impl Default for Length`
    pub fn new() -> Self {
        Self(0)
    }

    #[zoet(From)] // generates `From<usize> for Length`
    fn from_usize(value: usize) -> Self {
        Self(value)
    }

    #[zoet(From)] // generates `From<Length> for usize`
    fn to_usize(self) -> usize {
        self.0
    }

    #[zoet(AsRef, Borrow, Deref)] // generates all of those.
    fn as_usize(&self) -> &usize {
        &self.0
    }

    #[zoet(Hash)] // see note below about traits with generic functions
    fn hash(&self, state: &mut impl Hasher) {
        self.0.hash(state)
    }

    #[zoet(Add, AddAssign)] // generates `impl Add for Length` and `impl AddAssign for Length`
    fn add_assign(&mut self, rhs: Self) {
        self.0 += rhs.0;
    }

    #[zoet(Ord, PartialOrd)] // you get the idea by now
    fn ord(&self, other: &Self) -> Ordering {
        self.0.cmp(&other.0)
    }
}

let mut v = Length::default();
v += Length(1);
assert_eq!(v + Length(2), Length(3));
v += Length(4);
assert_eq!(v, Length(5));
assert_eq!(Length::from(v), Length(5));

Supported traits

Transformations for most traits in the standard library are provided. Omitted are those which are just marker traits (there's no code to generate), those which require multiple functions, and some which don't quite seem worth it. The current list is as follows:

• core::borrow: Borrow, BorrowMut.
• core::clone: Clone.
• core::cmp: Ord, PartialEq, PartialOrd.
• core::convert: AsMut, AsRef, From, Into, TryFrom, TryInto.
• core::default: Default.
• core::fmt: Binary Debug Display LowerExp LowerHex Octal Pointer UpperExp UpperHex, Write (implements write_str).
• core::future: Future.
• core::hash: Hash (implements hash).
• core::iterator: IntoIterator, Iterator (implements next).
• core::ops: Deref, DerefMut, Drop, Index, IndexMut, plus all arithmetic and bit ops and assignment variants such as Add and AddAssign.
• core::str: FromStr.

The alloc feature (which is enabled by default) also adds these:

• alloc::borrow: ToOwned.
• alloc::string: ToString.

Most of the generated traits normally just include the trait boilerplate and forward the arguments to your method. There are a few useful extra special cases:

• PartialOrd can also be applied to an Ord-shaped function, in which case it wraps the result with Some() to make it fit. This allows you to do #zoet[(Ord, PartialOrd)] to implement both with the same function and avoid ordering-related bugs.

• Add etc. can be applied to an AddAssign-shaped function, in which case it generates a trivial implementation which mutates its mut self and returns it.

What it generates

A suitable impl is emitted which proxies to your function, such as this:

 #[automatically_derived]
 #[allow(unused_qualifications)]
 impl ::core::default::Default for Length {
     #[inline]
     fn default() -> Self {
         <Length>::new()
     }
 }

Gotchas

Due to limitations in macro processing, you must add #[zoet] to your struct's impl block so that the self type of its associated functions can be determined. This is obviously not necessary (or possible) for free functions as they don't have a self type.

Generic parameters on the function and/or its inherent impl are all just accumulated and added to the trait impl's generic parameters, which does the right thing for the vast majority of traits. However, where a trait's function is itself generic, zoet isn't (yet) smart enough to figure out which of the generic parameter is for the function. As a perfectly good workaround, use an impl Trait parameter instead. So while Hash defines its single method as fn hash<H: Hasher>(&self, state: &mut H), your function needs to be something like fn hash(&self, state: &mut impl Hasher).

Because macros run before type checking, they only knows the names of the types, and not the actual types. zoet prefers to be liberal and pass through types rather than attempt to parse them, but we need to unpick the result type used by some traits such as TryInto into the success and error types, or rather, the names of the success and error types. As such, it expects the result type to be called (or be a path ending in) Result or Fallible, and if the second parameter is missing, the identifier Error is used. Idiomatic Rust code shouldn't have a problem with this, but if you have unusual error-handling, you may trip over this.

While this macro makes it easy to stamp out loads of core traits, don't go crazy but consider each trait you add and whether there is a more suitable macro to do the job, or indeed whether that trait should be added. The example above generates Default based on new(), but since that function returns 0 which is the default value anyway, it'd be better to #derive(Default) and implement new() in terms of that. Similarly, its Add and AddAssign trait implementations just delegating to its field's Add and AddAssign traits, and the can be completely eliminated by using derive_more and deriving Add and AddAssign on the struct. If your struct doesn't satisfy Borrow's invariants, you shouldn't unthinkingly do #[zoet(AsRef, Borrow, Deref)].

When things don't go quite as you expect, you are also reminded that cargo-expand exists and can be used to inspect the expanded text.

Attribute Macros

zoet	The #[zoet] macro.