Derive macro available if borsh is built with `features = ["derive"]`.
# derive proc-macro for [`BorshDeserialize`] trait
## Bounds
Generally, `BorshDeserialize` adds `borsh::de::BorshDeserialize` bound to any type parameter
found in item's fields and `core::default::Default` bound to any type parameter found
in item's skipped fields.
```rust
use borsh::BorshDeserialize;
/// impl<U, V> borsh::de::BorshDeserialize for A<U, V>
/// where
/// U: borsh::de::BorshDeserialize,
/// V: borsh::de::BorshDeserialize,
#[derive(BorshDeserialize)]
struct A<U, V> {
x: U,
y: V,
}
```
```rust
use borsh::BorshDeserialize;
/// impl<U, V> borsh::de::BorshDeserialize for A<U, V>
/// where
/// U: borsh::de::BorshDeserialize,
/// V: core::default::Default,
#[derive(BorshDeserialize)]
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::BorshDeserialize;
// specifying the attribute removes need for a direct import of `borsh` into `[dependencies]`
#[derive(BorshDeserialize)]
#[borsh(crate = "reexporter::borsh")]
struct B {
x: u64,
y: i32,
c: String,
}
```
```rust,ignore
use reexporter::borsh::{self, BorshDeserialize};
// specifying the attribute removes need for a direct import of `borsh` into `[dependencies]`
#[derive(BorshDeserialize)]
#[borsh(crate = "borsh")]
struct B {
x: u64,
y: i32,
c: String,
}
```
### 2. `#[borsh(init=...)]` (item level attribute)
###### syntax
Attribute's value is syn's [Path](https://docs.rs/syn/2.0.92/syn/struct.Path.html)-s, passed to borsh top level meta attribute as value of `init` argument.
###### usage
`#[borsh(init=...)]` allows to automatically run an initialization function right after deserialization.
This adds a lot of convenience for objects that are architectured to be used as strictly immutable.
```rust
type CryptoHash = String;
use borsh::BorshDeserialize;
#[derive(BorshDeserialize)]
#[borsh(init=init)]
struct Message {
message: String,
timestamp: u64,
hash: CryptoHash,
}
impl Message {
pub fn init(&mut self) {
self.hash = {
let mut hash = CryptoHash::new();
hash.push_str(&self.message);
hash.push_str(&format!("{}", self.timestamp));
hash
};
}
}
```
### 3. `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.
It's useful for backward compatibility and you can set this value to `false` to deserialise data serialised by older version of `borsh`.
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::BorshDeserialize;
#[derive(BorshDeserialize)]
#[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::BorshDeserialize;
#[derive(BorshDeserialize)]
#[borsh(use_discriminant = true)]
enum B {
A,
B = 10,
}
```
###### borsh, expressions, evaluating to `isize`, as discriminant
This case is not supported:
```rust,compile_fail
use borsh::BorshDeserialize;
const fn discrim() -> isize {
0x14
}
#[derive(BorshDeserialize)]
#[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(BorshDeserialize)]
#[borsh(use_discriminant = true)]
enum X {
A,
B = 0x100, // literal values outside of `u8` range
C,
D,
E = 10,
F,
}
```
### 4. `#[borsh(skip)]` (field level attribute)
`#[borsh(skip)]` makes derive skip deserializing annotated field.
`#[borsh(skip)]` makes derive skip adding any type parameters, present in the field, to parameters bound by `borsh::de::BorshDeserialize`.
It adds `core::default::Default` bound to any
parameters encountered in annotated field.
```rust
use borsh::BorshDeserialize;
#[derive(BorshDeserialize)]
struct A {
x: u64,
#[borsh(skip)]
y: f32,
}
```
### 5. `#[borsh(bound(deserialize = ...))]` (field level attribute)
###### syntax
Attribute takes literal string value, which is a comma-separated list of syn's [WherePredicate](https://docs.rs/syn/latest/syn/enum.WherePredicate.html)-s, which may be empty.
###### usage
Attribute adds possibility to override bounds for `BorshDeserialize` in order to enable:
1. removal of bounds on type parameters from struct/enum definition itself and moving them to the trait's implementation block.
2. fixing complex cases, when derive hasn't figured out the right bounds on type parameters automatically.
```rust
use borsh::BorshDeserialize;
#[cfg(feature = "hashbrown")]
use hashbrown::HashMap;
#[cfg(feature = "std")]
use std::collections::HashMap;
use core::hash::Hash;
/// additional bounds `T: Ord + Hash + Eq` (required by `HashMap`) are injected into
/// derived trait implementation via attribute to avoid adding the bounds on the struct itself
#[cfg(any(feature = "hashbrown", feature = "std"))]
#[derive(BorshDeserialize)]
struct A<T, U> {
a: String,
#[borsh(bound(
deserialize =
"T: Ord + Hash + Eq + borsh::de::BorshDeserialize,
U: borsh::de::BorshDeserialize"
))]
b: HashMap<T, U>,
}
```
```rust
use borsh::BorshDeserialize;
trait TraitName {
type Associated;
fn method(&self);
}
// derive here figures the bound erroneously as `T: borsh::de::BorshDeserialize,`
#[derive(BorshDeserialize)]
struct A<T, V>
where
T: TraitName,
{
#[borsh(bound(deserialize = "<T as TraitName>::Associated: borsh::de::BorshDeserialize"))]
field: <T as TraitName>::Associated,
another: V,
}
```
###### interaction with `#[borsh(skip)]`
`#[borsh(bound(deserialize = ...))]` replaces bounds, which are derived automatically,
irrelevant of whether `#[borsh(skip)]` attribute is present.
```rust
use borsh::BorshDeserialize;
#[cfg(feature = "hashbrown")]
use hashbrown::HashMap;
#[cfg(feature = "std")]
use std::collections::HashMap;
/// implicit derived `core::default::Default` bounds on `K` and `V` type parameters are removed by
/// empty bound specified, as `HashMap` has its own `Default` implementation
#[cfg(any(feature = "hashbrown", feature = "std"))]
#[derive(BorshDeserialize)]
struct A<K, V, U>(
#[borsh(skip, bound(deserialize = ""))]
HashMap<K, V>,
U,
);
```
### 6. `#[borsh(deserialize_with = ...)]` (field level attribute)
###### syntax
Attribute takes literal string value, which is a syn's [ExprPath](https://docs.rs/syn/latest/syn/struct.ExprPath.html).
###### usage
Attribute adds possibility to specify full path of function, optionally qualified with generics,
with which to deserialize the annotated field.
It may be used when `BorshDeserialize` cannot be implemented for field's type, if it's from foreign crate.
It may be used to override the implementation of deserialization for some other reason.
```rust
use borsh::BorshDeserialize;
use indexmap::IndexMap;
use core::hash::Hash;
/// 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 {
use super::IndexMap;
use core::hash::Hash;
pub fn deserialize_index_map<
R: borsh::io::Read,
K: borsh::de::BorshDeserialize + Hash + Eq,
V: borsh::de::BorshDeserialize,
>(
reader: &mut R,
) -> ::core::result::Result<IndexMap<K, V>, borsh::io::Error> {
let vec: Vec<(K, V)> = borsh::BorshDeserialize::deserialize_reader(reader)?;
// the line of implementation for type from real `indexmap` crate
// let result: IndexMap<K, V> = vec.into_iter().collect();
let result = IndexMap {
tuples: vec,
};
Ok(result)
}
}
#[derive(BorshDeserialize)]
struct B<K: Hash + Eq, V> {
#[borsh(
deserialize_with = "index_map_impl::deserialize_index_map",
)]
x: IndexMap<K, V>,
y: String,
}
# fn main() {
# }
```
###### usage (comprehensive example)
[borsh/examples/serde_json_value.rs](https://github.com/near/borsh-rs/blob/master/borsh/examples/serde_json_value.rs) is
a more complex example of how the attribute may be used.
###### interaction with `#[borsh(skip)]`
`#[borsh(deserialize_with = ...)]` is not allowed to be used simultaneously with `#[borsh(skip)]`.