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# synstructure
[](https://crates.io/crates/synstructure)
[](https://travis-ci.org/mystor/synstructure)
[](https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html)
> NOTE: What follows is an exerpt from the module level documentation. For full
This crate provides helper types for matching against enum variants, and
extracting bindings to each of the fields in the deriving Struct or Enum in
a generic way.
If you are writing a `#[derive]` which needs to perform some operation on
every field, then you have come to the right place!
# Example: `WalkFields`
### Trait Implementation
```rust
pub trait WalkFields: std::any::Any {
fn walk_fields(&self, walk: &mut FnMut(&WalkFields));
}
impl WalkFields for i32 {
fn walk_fields(&self, _walk: &mut FnMut(&WalkFields)) {}
}
```
### Custom Derive
```rust
#[macro_use]
extern crate synstructure;
#[macro_use]
extern crate quote;
extern crate proc_macro2;
fn walkfields_derive(s: synstructure::Structure) -> proc_macro2::TokenStream {
let body = s.each(|bi| quote!{
walk(#bi)
});
s.bound_impl(quote!(example_traits::WalkFields), quote!{
fn walk_fields(&self, walk: &mut FnMut(&example_traits::WalkFields)) {
match *self { #body }
}
})
}
decl_derive!([WalkFields] => walkfields_derive);
/*
* Test Case
*/
fn main() {
test_derive! {
walkfields_derive {
enum A<T> {
B(i32, T),
C(i32),
}
}
expands to {
#[allow(non_upper_case_globals)]
const _DERIVE_example_traits_WalkFields_FOR_A: () = {
extern crate example_traits;
impl<T> example_traits::WalkFields for A<T>
where T: example_traits::WalkFields
{
fn walk_fields(&self, walk: &mut FnMut(&example_traits::WalkFields)) {
match *self {
A::B(ref __binding_0, ref __binding_1,) => {
{ walk(__binding_0) }
{ walk(__binding_1) }
}
A::C(ref __binding_0,) => {
{ walk(__binding_0) }
}
}
}
}
};
}
}
}
```
# Example: `Interest`
### Trait Implementation
```rust
pub trait Interest {
fn interesting(&self) -> bool;
}
impl Interest for i32 {
fn interesting(&self) -> bool { *self > 0 }
}
```
### Custom Derive
```rust
#[macro_use]
extern crate synstructure;
#[macro_use]
extern crate quote;
extern crate proc_macro2;
fn interest_derive(mut s: synstructure::Structure) -> proc_macro2::TokenStream {
let body = s.fold(false, |acc, bi| quote!{
#acc || example_traits::Interest::interesting(#bi)
});
s.bound_impl(quote!(example_traits::Interest), quote!{
fn interesting(&self) -> bool {
match *self {
#body
}
}
})
}
decl_derive!([Interest] => interest_derive);
/*
* Test Case
*/
fn main() {
test_derive!{
interest_derive {
enum A<T> {
B(i32, T),
C(i32),
}
}
expands to {
#[allow(non_upper_case_globals)]
const _DERIVE_example_traits_Interest_FOR_A: () = {
extern crate example_traits;
impl<T> example_traits::Interest for A<T>
where T: example_traits::Interest
{
fn interesting(&self) -> bool {
match *self {
A::B(ref __binding_0, ref __binding_1,) => {
false ||
example_traits::Interest::interesting(__binding_0) ||
example_traits::Interest::interesting(__binding_1)
}
A::C(ref __binding_0,) => {
false ||
example_traits::Interest::interesting(__binding_0)
}
}
}
}
};
}
}
}
```
For more example usage, consider investigating the `abomonation_derive` crate,
which makes use of this crate, and is fairly simple.