# Property attributes
This section calls out a few attributes you can use to change the way properties are captured. See [the crate docs](https://docs.rs/emit/1.0.0/emit/index.html#attributes) for a complete list of attributes defined by `emit`.
## `#[cfg]`
You can add the standard `#[cfg]` attribute to properties in templates. If the `#[cfg]` evaluates to `false` then the entire hole will be omitted from the template.
```rust
# extern crate emit;
emit::emit!("Hello, {#[cfg(disabled)] user}");
```
```text
Event {
mdl: "my_app",
tpl: "Hello, ",
extent: Some(
"2024-10-02T22:01:01.431485400Z",
),
props: {},
}
```
## `#[key]`
The [`#[key`](https://docs.rs/emit/1.0.0/emit/attr.key.html) attribute can be used to set the name of a captured property. This can be used to give a property a name that isn't a valid Rust identifier:
```rust
# extern crate emit;
# let user = "Rust";
emit::emit!("Hello, {user}", #[emit::key("user.name")] user);
```
```text
Event {
mdl: "my_app",
tpl: "Hello, {user.name}",
extent: Some(
"2024-10-02T22:01:24.321035400Z",
),
props: {
"user.name": "Rust",
},
}
```
## `#[fmt]`
The [`#[fmt]`](https://docs.rs/emit/1.0.0/emit/attr.fmt.html) attribute applies a formatter to a property value when rendering it in the template. The accepted syntax is the same as Rust's [`std::fmt`](https://doc.rust-lang.org/std/fmt/index.html):
```rust
# extern crate emit;
emit::emit!("pi is {pi}", #[emit::fmt(".3")] pi: 3.1415927);
```
```text
Event {
mdl: "my_app",
tpl: "pi is {pi}",
extent: Some(
"2024-10-02T22:01:58.842629700Z",
),
props: {
"pi": 3.1415927,
},
}
```
When rendered, the template will produce:
```text
pi is 3.142
```
## `#[as_debug]`
The [`#[as_debug]`](https://docs.rs/emit/1.0.0/emit/attr.as_debug.html) attribute captures a property value using its [`Debug`](https://doc.rust-lang.org/std/fmt/trait.Debug.html) implementation, instead of the default `Display + 'static`:
```rust
# extern crate emit;
#[derive(Debug)]
struct User<'a> {
name: &'a str,
}
emit::emit!(
"Hello, {user}",
#[emit::as_debug]
user: User {
name: "Rust",
}
);
```
```text
Event {
mdl: "my_app",
tpl: "Hello, {user}",
extent: Some(
"2024-10-02T22:03:23.588049400Z",
),
props: {
"user": User {
name: "Rust",
},
},
}
```
Note that the structure of the captured value is lost. It'll be treated as a string like `"User { name: \"Rust\" }"` when serialized:
```json
{
"mdl": "my_app",
"tpl": "Hello, {user}",
"ts": "2024-10-02T22:03:23.588049400Z",
"user": "User { name: \"Rust\" }"
}
```
See [Property capturing](./property-capturing.md) for more details.
## `#[as_serde]`
The [`#[as_serde]`](https://docs.rs/emit/1.0.0/emit/attr.as_serde.html) attribute captures a property value using its [`Serialize`](https://docs.rs/serde/latest/serde/trait.Serialize.html) implementation, instead of the default `Display + 'static`:
```rust
# extern crate emit;
# #[macro_use] extern crate serde;
#[derive(Serialize)]
struct User<'a> {
name: &'a str,
}
emit::emit!(
"Hello, {user}",
#[emit::as_serde]
user: User {
name: "Rust",
}
);
```
```text
Event {
mdl: "my_app",
tpl: "Hello, {user}",
extent: Some(
"2024-10-02T22:05:05.258099900Z",
),
props: {
"user": User {
name: "Rust",
},
},
}
```
The structure of properties captured this way is fully preserved:
```json
{
"mdl": "my_app",
"tpl": "Hello, {user}",
"ts": "2024-10-02T22:05:05.258099900Z",
"user": {
"name": "Rust"
}
}
```
See [Property capturing](./property-capturing.md) for more details.