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2363-arbitrary-enum-discriminant.md

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Summary

This RFC gives users a way to control the discriminants of variants of all enumerations, not just the ones that are shaped like C-like enums (i.e. where all the variants have no fields).

The change is minimal: allow any variant to be adorned with an explicit discriminant value, whether or not that variant has any field.

Motivation

Stylo, the style system of Servo, represents CSS properties with a large enumeration PropertyDeclaration where each variant has only one field which represents the value of a given CSS property. Here is a subset of it:

#[repr(u16)]
enum PropertyDeclaration {
    Color(Color),
    Height(Length),
    InlineSize(Length),
    TransformOrigin(TransformOrigin),
}

For various book-keeping reasons, Servo also generates a LonghandId enumeration with the same variants as PropertyDeclaration but without the fields, thus making LonghandId a C-like enumeration:

#[derive(Clone, Copy)]
#[repr(u16)]
enum LonghandId {
    Color,
    Height,
    InlineSize,
    TransformOrigin,
}

Given that rustc guarantees that #[repr(u16)] enumerations start with their discriminant stored as a u16, going from &PropertyDeclaration to LonghandId is then just a matter of unsafely coercing &self as a &LonghandId:

impl PropertyDeclaration {
    fn id(&self) -> LonghandId {
        unsafe { *(self as *const Self as *const LonghandId) }
    }
}

This works great, but doesn't scale if we want to replicate this behaviour for an enumeration that is a subset of PropertyDeclaration, for example an enumeration AnimationValue that is limited to animatable properties:

#[repr(u16)]
enum AnimationValue {
    Color(Color),
    Height(Length),
    TransformOrigin(TransformOrigin),
}

impl AnimationValue {
    fn id(&self) -> LonghandId {
        // We can't just unsafely read `&self` as a `&LonghandId` because
        // the discriminant of `AnimationValue::TransformOrigin` isn't equal
        // to `LonghandId::TransformOrigin` anymore.
        match *self {
            AnimationValue::Color(_) => LonghandId::Color,
            AnimationValue::Height(_) => LonghandId::Height,
            AnimationValue::TransformOrigin(_) => LonghandId::TransformOrigin,
        }
    }
}

This is not sustainable, as the jump table generated by rustc to compile this huge match expression is larger than 4KB in the final Gecko binary, when this operation could be a trivial u16 copy. This is worked around in Servo by generating spurious Void variants for the non-animatable properties in AnimationValue:

enum Void {}

#[repr(u16)]
enum AnimationValue {
    Color(Color),
    Height(Length),
    InlineSize(Void),
    TransformOrigin(TransformOrigin),
}

impl AnimationValue {
    fn id(&self) -> LonghandId 
        // We can use the unsafe trick again.
        unsafe { *(self as *const Self as *const LonghandId) }
    }
}

This is unfortunately quite painful to use, given now all methods matching against AnimationValue need to have dummy arms for all of these variants:

impl AnimationValue {
    fn do_something(&self) {
        match *self {
            AnimationValue::Color(ref color) => {
                do_something_with_color(color)
            }
            AnimationValue::Height(ref height) => {
                do_something_with_height(height)
            }
            // This shouldn't be needed.
            AnimationValue::InlineSize(ref void) => {
                match *void {}
            }
            AnimationValue::TransformOrigin(ref origin) => {
                do_something_with_transform_origin(origin)
            }
        }
    }
}

We suggest generalising the explicit discriminant notation to all enums, regardless of whether their variants have fields or not:

#[repr(u16)]
enum AnimationValue {
    Color(Color) = LonghandId::Color as u16,
    Height(Length) = LonghandId::Height as u16,
    TransformOrigin(TransformOrigin) = LonghandId::TransformOrigin as u16,
}

impl AnimationValue {
    fn id(&self) -> LonghandId 
        // We can use the unsafe trick again.
        unsafe { *(self as *const Self as *const LonghandId) }
    }

    fn do_something(&self) {
        // No spurious variant anymore.
        match *self {
            AnimationValue::Color(ref color) => {
                do_something_with_color(color)
            }
            AnimationValue::Height(ref height) => {
                do_something_with_height(height)
            }
            AnimationValue::TransformOrigin(ref origin) => {
                do_something_with_transform_origin(origin)
            }
        }
    }
}

Guide-level explanation

An enumeration with only field-less variants can currently have explicit discriminant values:

enum ForceFromage {
    Emmental = 0,
    Camembert = 1,
    Roquefort = 2,
}

With this RFC, users are allowed to put explicit discriminant values on any variant of any enumeration, not just the ones where all variants are field-less:

enum ParisianSandwichIngredient {
    Bread(BreadKind) = 0,
    Ham(HamKind) = 1,
    Butter(ButterKind) = 2,
}

Reference-level explanation

Grammar

The production for enumeration items becomes:

EnumItem :
    OuterAttribute*
    IDENTIFIER ( EnumItemTuple | EnumItemStruct)? EnumItemDiscriminant?

Semantics

The limitation that only field-less enumerations can have explicit discriminant values is lifted, and no other change is made to their semantics:

  • enumerations with fields still can't be casted to numeric types with the as operator;
  • if the first variant doesn't have an explicit discriminant, it is set to zero;
  • any unspecified discriminant is set to one higher than the one from the previous variant;
  • under the default representation, the specified discriminants are interpreted as isize;
  • two variants cannot share the same discriminant.

Drawbacks

This introduces one more knob to the representation of enumerations.

Rationale and alternatives

Reusing the current syntax and semantics for explicit discriminants of field-less enumerations means that the changes to the grammar and semantics of the language are minimal. There are a few possible alternatives nonetheless.

Discriminant values in attributes

We could specify the discriminant values in variant attributes, but this would be at odds with the syntax for field-less enumerations.

enum ParisianSandwichIngredient {
    #[discriminant = 0]
    Bread(BreadKind),
    #[discriminant = 1]
    Ham(HamKind),
    #[discriminant = 2]
    Butter(ButterKind),
}

Use discriminants of a separate field-less enumeration

We could tell rustc to tie the discriminants of the enumeration to the variants of a separate field-less enumeration.

#[discriminant(IngredientKind)]
enum ParisianSandwichIngredient {
    Bread(BreadKind),
    Ham(HamKind),
    Butter(ButterKind),
}

enum IngredientKind {
    Bread,
    Ham,
    Butter,
}

This isn't applicable if such a separate field-less enumeration doesn't exist, and this can easily be done as a procedural macro using the feature described in this RFC. It also looks way more like spooky action at a distance.

Prior art

No prior art.

Unresolved questions

Should discriminants of enumerations with fields be specified as variant attributes?

Should they?

Should this apply only to enumerations with an explicit representation?

Should it?

Thanks

Thanks to Mazdak Farrokhzad (@Centril) and Simon Sapin (@SimonSapin) for the reviews, and my local bakery for their delicious baguettes. 🥖