scale-info ·

A library to describe Rust types, geared towards providing info about the structure of SCALE encodable types.

The definitions provide third party tools (e.g. a UI client) with information about how they are able to decode types agnostic of language.

At its core is the TypeInfo trait:

pub trait TypeInfo {
    fn type_info() -> Type;
}

Types implementing this trait build up and return a Type struct:

pub struct Type<F: Form = MetaForm> {
	/// The unique path to the type. Can be empty for built-in types
	path: Path<F>,
	/// The generic type parameters of the type in use. Empty for non generic types
	type_params: Vec<F::TypeId>,
	/// The actual type definition
	type_def: TypeDef<F>,
}

Types are defined as one of the following variants:

pub enum TypeDef<F: Form = MetaForm> {
	/// A composite type (e.g. a struct or a tuple)
	Composite(TypeDefComposite<F>),
	/// A variant type (e.g. an enum)
	Variant(TypeDefVariant<F>),
	/// A sequence type with runtime known length.
	Sequence(TypeDefSequence<F>),
	/// An array type with compile-time known length.
	Array(TypeDefArray<F>),
	/// A tuple type.
	Tuple(TypeDefTuple<F>),
	/// A Rust primitive type.
	Primitive(TypeDefPrimitive),
}

Built-in Type Definitions

The following "built-in" types have predefined TypeInfo definitions:

• Primitives: bool, char, str, u8, u16, u32, u64, u128, i8, i16, i32, i64 , i128.

• Sequence: Variable size sequence of elements of T, where T implements TypeInfo. e.g. [T], &[T], &mut [T], Vec<T>

• Array: Fixed size [T: $n] for any T which implements TypeInfo, where $n is one of the predefined sizes.

• Tuple: Tuples consisting of up to 10 fields with types implementing TypeInfo.

User-defined Types

There are two kinds of user-defined types: Composite and Variant.

Both make use of the Path and Field types in their definition:

Fields

A fundamental building block to represent user defined types is the Field struct which defines the Type of a field together with its optional name. Builders for the user defined types enforce the invariant that either all fields have a name (e.g. structs) or all fields are unnamed (e.g. tuples).

Path

The path of a type is a unique sequence of identifiers. Rust types typically construct a path from the namespace and the identifier e.g. foo::bar::Baz is converted to the path ["foo", "bar ", "Baz"].

Composite

Composite data types are composed of a set of Fields.

Structs are represented by a set of named fields, enforced during construction:

struct Foo<T> {
    bar: T,
    data: u64,
}

impl<T> TypeInfo for Foo<T>
where
    T: TypeInfo + 'static,
{
    fn type_info() -> Type {
        Type::builder()
            .path(Path::new("Foo", module_path!()))
            .type_params(vec![MetaType::new::<T>()])
            .composite(Fields::named()
                .field_of::<T>("bar")
                .field_of::<u64>("data")
            )
    }
}

Tuples are represented by a set of unnamed fields, enforced during construction:

struct Foo(u32, bool);

impl TypeInfo for Foo {
    fn type_info() -> Type {
        Type::builder()
            .path(Path::new("Foo", module_path!()))
            .composite(Fields::unnamed()
                .field_of::<u32>()
                .field_of::<bool>()
            )
    }
}

Variant

Variant types aka enums or tagged unions are composed of a set of variants. Variants can have unnamed fields, named fields or no fields at all:

enum Foo<T>{
    A(T),
    B { f: u32 },
    C,
}

impl<T> TypeInfo for Foo<T>
where
    T: TypeInfo + 'static,
{
    fn type_info() -> Type {
        Type::builder()
            .path(Path::new("Foo", module_path!()))
            .type_params(vec![MetaType::new::<T>()])
            .variant(
                Variants::with_fields()
                    .variant("A", Fields::unnamed().field_of::<T>())
                    .variant("B", Fields::named().field_of::<u32>("f"))
                    .variant("C", Fields::unit())
            )
    }
}

If all variants contain no fields then the discriminant can be set explicitly, enforced by the builder during construction:

enum Foo {
	A,
	B,
	C = 33,
}

impl TypeInfo for Foo {
    fn type_info() -> Type {
        Type::builder()
            .path(Path::new("Foo", module_path!()))
            .variant(
                Variants::fieldless()
                    .variant("A", 1)
                    .variant("B", 2)
                    .variant("C", 33)
            )
    }
}

The Registry

Information about types is provided within the so-called type registry (Registry). Type definitions are registered there and are associated with unique IDs that the outside can use to refer to them providing a lightweight way to decrease overhead instead of using type identifiers.

All concrete TypeInfo structures have two forms: One meta form (MetaType) that acts as a bridge to other forms and a compact form that is later to be serialized. The IntoCompact trait is implemented by them in order to compact a type definition using an instance of a type registry.

After compactification all type definitions are stored in the type registry. Note that the type registry should be serialized as part of the metadata structure where the registered types are utilized to allow consumers to resolve the types.

As a minor additional compaction step non-documentation strings are also compacted by the same mechanics.

Serialization

Currently the only supported serialization format is JSON, an example of which can be found here.

Future support for binary formats is planned, either SCALE itself or a more compressed format where the monomorphization of Rust generic types could potentially result in very large files.

Resources

• See usage for describing types for ink! smart contracts metadata.
• Original design draft (outdated)