Derive Macro borsh::BorshSchema

#[derive(BorshSchema)]
{
    // Attributes available to this derive:
    #[borsh]
}

Derive macro available if borsh is built with features = ["unstable__schema"].

derive proc-macro for borsh::BorshSchema trait

Bounds

Generally, BorshSchema adds borsh::BorshSchema bound to any type parameter found in item’s fields.

/// impl<U, V> borsh::BorshSchema for A<U, V>
/// where
///     U: borsh::BorshSchema,
///     V: borsh::BorshSchema,
#[derive(BorshSchema)]
struct A<U, V> {
    x: U,
    y: V,
}

/// impl<U, V> borsh::BorshSchema for A<U, V>
/// where
///     U: borsh::BorshSchema,
#[derive(BorshSchema)]
struct A<U, V> {
    x: U,
    #[borsh(skip)]
    y: V,
}

Attributes

1. #[borsh(crate = "path::to::borsh")] (item level attribute)

syntax

Attribute takes literal string value, which is the Path to borsh crate used.

usage

Attribute is optional.

1. If the attribute is not provided, crate_name is used to find a version of borsh in [dependencies] of the relevant Cargo.toml. If there is no match, a compilation error, similar to the following, is raised:

 1  error: proc-macro derive panicked
   --> path/to/file.rs:27:10
    |
 27 | #[derive(BorshSchema, BorshSerialize)]
    |          ^^^^^^^^^^^
    |
    = help: message: called `Result::unwrap()` on an `Err` value: CrateNotFound { crate_name: "borsh", path: "/path/to/Cargo.toml" }

2. If the attribute is provided, the check for borsh in [dependencies] of the relevant Cargo.toml is skipped.

Examples of usage:

use reexporter::borsh::BorshSchema;

// specifying the attribute removes need for a direct import of `borsh` into `[dependencies]`
#[derive(BorshSchema)]
#[borsh(crate = "reexporter::borsh")]
struct B {
    x: u64,
    y: i32,
    c: String,
}

use reexporter::borsh::{self, BorshSchema};

// specifying the attribute removes need for a direct import of `borsh` into `[dependencies]`
#[derive(BorshSchema)]
#[borsh(crate = "borsh")]
struct B {
    x: u64,
    y: i32,
    c: String,
}

2. borsh(use_discriminant=<bool>) (item level attribute)

This attribute is only applicable to enums. use_discriminant allows to override the default behavior of serialization of enums with explicit discriminant. use_discriminant is false behaves like version of borsh of 0.10.3. You must specify use_discriminant for all enums with explicit discriminants in your project.

This is equivalent of borsh version 0.10.3 (explicit discriminant is ignored and this enum is equivalent to A without explicit discriminant):

#[derive(BorshSchema)]
#[borsh(use_discriminant = false)]
enum A {
    A
    B = 10,
}

To have explicit discriminant value serialized as is, you must specify borsh(use_discriminant=true) for enum.

#[derive(BorshSchema)]
#[borsh(use_discriminant = true)]
enum B {
    A
    B = 10,
}

borsh, expressions, evaluating to isize, as discriminant

This case is not supported:

const fn discrim() -> isize {
    0x14
}

#[derive(BorshSchema)]
#[borsh(use_discriminant = true)]
enum X {
    A,
    B = discrim(), // expressions, evaluating to `isize`, which are allowed outside of `borsh` context
    C,
    D,
    E = 10,
    F,
}

borsh explicit discriminant does not support literal values outside of u8 range

This is not supported:

#[derive(BorshSchema)]
#[borsh(use_discriminant = true)]
enum X {
    A,
    B = 0x100, // literal values outside of `u8` range
    C,
    D,
    E = 10,
    F,
}

3. #[borsh(skip)] (field level attribute)

#[borsh(skip)] makes derive skip including schema from annotated field into schema’s implementation.

#[borsh(skip)] makes derive skip adding any type parameters, present in the field, to parameters bound by borsh::BorshSchema.

#[derive(BorshSchema)]
struct A {
    x: u64,
    #[borsh(skip)]
    y: f32,
}

4. #[borsh(schema(params = ...))] (field level attribute)

syntax

Attribute takes literal string value, which is a comma-separated list of ParameterOverride-s, which may be empty.

usage

It may be used in order to:

1. fix complex cases, when derive hasn’t figured out the right bounds on type parameters and declaration parameters automatically.
2. remove parameters, which do not take part in serialization/deserialization, from bounded ones and from declaration parameters.

ParameterOverride describes an entry like order_param => override_type,

e.g. K => <K as TraitName>::Associated.

Such an entry instructs BorshSchema derive to:

1. add override_type to types, bounded by borsh::BorshSchema in implementation block.
2. add <override_type>::declaration() to parameters vector in fn declaration() method of BorshSchema trait that is being derived.
3. the order_param is required to establish the same order in parameters vector (2.) as that of type parameters in generics of type, that BorshSchema is derived for.
4. entries, specified for a field, together replace whatever would’ve been derived automatically for 1. and 2. .

// derive here figures the bound erroneously as `T: borsh::BorshSchema` .
// attribute replaces it with <T as TraitName>::Associated: borsh::BorshSchema`
#[derive(BorshSchema)]
struct A<V, T>
where
    T: TraitName,
{
    #[borsh(schema(params = "T => <T as TraitName>::Associated"))]
    field: <T as TraitName>::Associated,
    another: V,
}

// K in PrimaryMap isn't stored during serialization / read during deserialization.
// thus, it's not a parameter, relevant for `BorshSchema`
// ...
// impl<K: EntityRef, V> borsh::BorshSchema for A<K, V>
// where
//     V: borsh::BorshSchema,
#[derive(BorshSchema)]
struct A<K: EntityRef, V> {
    #[borsh(
        schema(
            params = "V => V"
        )
    )]
    x: PrimaryMap<K, V>,
    y: String,
}

#[derive(BorshSchema)]
pub struct PrimaryMap<K, V>
where
    K: EntityRef,
{
    elems: Vec<V>,
    unused: PhantomData<K>,
}

interaction with #[borsh(skip)]

#[borsh(schema(params = ...))] is not allowed to be used simultaneously with #[borsh(skip)].

5. #[borsh(schema(with_funcs(declaration = ..., definitions = ...)))] (field level attribute)

syntax

Each of declaration and definitions nested sub-attributes takes literal string value, which is a syn’s ExprPath.

Currently both declaration and definitions are required to be specifed at the same time.

usage

Attribute adds possibility to specify full path of 2 functions, optionally qualified with generics, with which to generate borsh schema for annotated field.

It may be used when BorshSchema cannot be implemented for field’s type, if it’s from foreign crate.

It may be used to override the implementation of schema for some other reason.

use indexmap::IndexMap;

mod index_map_impl {
    pub mod schema {
        use std::collections::BTreeMap;

        use borsh::{
            schema::{Declaration, Definition},
            BorshSchema,
        };

        pub fn declaration<K: borsh::BorshSchema, V: borsh::BorshSchema>() -> Declaration {
            let params = vec![<K>::declaration(), <V>::declaration()];
            format!(r#"{}<{}>"#, "IndexMap", params.join(", "))
        }

        pub fn add_definitions_recursively<K: borsh::BorshSchema, V: borsh::BorshSchema>(
            definitions: &mut BTreeMap<Declaration, Definition>,
        ) {
            let definition = Definition::Sequence {
                elements: <(K, V)>::declaration(),
            };
            let no_recursion_flag = definitions.get(&declaration::<K, V>()).is_none();
            <() as BorshSchema>::add_definition(declaration::<K, V>(), definition, definitions);
            if no_recursion_flag {
                <(K, V)>::add_definitions_recursively(definitions);
            }
        }
    }
}

#[derive(BorshSchema)]
struct B<K, V> {
    #[borsh(
        schema(
            with_funcs(
                declaration = "index_map_impl::schema::declaration::<K, V>",
                definitions = "index_map_impl::schema::add_definitions_recursively::<K, V>"
            ),
        )
    )]
    x: IndexMap<K, V>,
    y: String,
}

interaction with #[borsh(skip)]

#[borsh(schema(with_funcs(declaration = ..., definitions = ...)))] is not allowed to be used simultaneously with #[borsh(skip)].