# type_hash
Generate a hash for a Rust type.
The primary use-case for this crate is for detecting differences in message
types between versions of a crate.
The `TypeHash` trait is implemented for most built-in types and a derive macro
is provided, for implementing it for your own types.
## Examples
```rust
use type_hash::TypeHash;
#[derive(TypeHash)]
pub enum Message {
LaunchMissiles { destination: String },
CancelMissiles,
}
fn main() {
let hash = Message::type_hash();
// this will only change if the type definition changes
assert_eq!(hash, 11652809455620829461);
}
```
## Customising derived TypeHash implementations
### `#[type_hash(foreign_type)]`
If a struct field has a foreign type that does not implement `TypeHash`, you can
mark it as a foreign type and the derive `TypeHash` implementation will use the
name of the type in the hash instead. You need to be a little bit careful here
because a change in the third party crate could change your type in an undetectable
way.
```rust
#[derive(TypeHash)]
pub struct MyStruct {
#[type_hash(foreign_type)]
data: ArrayVec<[u16; 7]>
}
```
### `#[type_hash(skip)]`
Skip a field, so it is not part of the hash.
```
#[derive(TypeHash)]
pub struct MyStruct {
#[type_hash(skip)]
not_important: Vec<i64>,
}
```
### `#[type_hash(as = "...")]`
Hash a field as if it had a different type. This allows you to change the
type of a field to a different type that is still compatible for your
application, without affecting the hash.
```
#[derive(TypeHash)]
pub struct MyStruct {
#[type_hash(as = "HashSet<i64>")]
numbers: BTreeSet<i64>,
}
```