Derive macro available if borsh is built with `features = ["unstable__schema"]`.
# derive proc-macro for [`BorshSchema`] trait
## Bounds
Generally, `BorshSchema` adds `borsh::BorshSchema` bound to any type parameter
found in item's fields.
```rust
use borsh::BorshSchema;
/// 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,
}
```
```rust
use borsh::BorshSchema;
/// 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 syn's [Path](https://docs.rs/syn/2.0.92/syn/struct.Path.html) to `borsh` crate used.
###### usage
Attribute is optional.
1. If the attribute is not provided, [crate_name](https://docs.rs/proc-macro-crate/3.2.0/proc_macro_crate/fn.crate_name.html) 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:
```bash
1 error: proc-macro derive panicked
--> path/to/file.rs:27:10
|
|
= 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:
(example is not tested, as there's usually no `reexporter` crate during doc build)
```rust,ignore
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,
}
```
```rust,ignore
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):
```rust
use borsh::BorshSchema;
#[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.
```rust
use borsh::BorshSchema;
#[derive(BorshSchema)]
#[borsh(use_discriminant = true)]
enum B {
A,
B = 10,
}
```
###### borsh, expressions, evaluating to `isize`, as discriminant
This case is not supported:
```rust,compile_fail
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:
```rust,compile_fail
#[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`.
```rust
use 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. .
```rust
use borsh::BorshSchema;
trait TraitName {
type Associated;
fn method(&self);
}
// 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,
}
```
```rust
use borsh::BorshSchema;
use core::marker::PhantomData;
trait EntityRef {
fn key_property(&self) -> u64;
}
// 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](https://docs.rs/syn/latest/syn/struct.ExprPath.html).
Currently both `declaration` and `definitions` are required to be specified 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.
```rust
use borsh::BorshSchema;
use indexmap::IndexMap;
/// this a stub module, representing a 3rd party crate `indexmap`
mod indexmap {
/// this a stub struct, representing a 3rd party `indexmap::IndexMap`
/// or some local type we want to override trait implementation for
pub struct IndexMap<K, V> {
pub(crate) tuples: Vec<(K, V)>,
}
}
mod index_map_impl {
pub mod schema {
use std::collections::BTreeMap;
use borsh::{
schema::{Declaration, Definition, self},
BorshSchema,
};
pub fn declaration<K: BorshSchema, V: BorshSchema>() -> Declaration {
let params = vec![<K>::declaration(), <V>::declaration()];
format!(r#"{}<{}>"#, "IndexMap", params.join(", "))
}
pub fn add_definitions_recursively<K: BorshSchema, V: BorshSchema>(
definitions: &mut BTreeMap<Declaration, Definition>,
) {
let definition = Definition::Sequence {
elements: <(K, V)>::declaration(),
length_width: Definition::DEFAULT_LENGTH_WIDTH,
length_range: Definition::DEFAULT_LENGTH_RANGE,
};
let no_recursion_flag = definitions.get(&declaration::<K, V>()).is_none();
schema::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,
}
# fn main() {
# }
```
###### interaction with `#[borsh(skip)]`
`#[borsh(schema(with_funcs(declaration = ..., definitions = ...)))]` is not allowed to be used simultaneously with `#[borsh(skip)]`.