pub trait FromData<'r>: Sized {
    type Error: Send + Debug;

    fn from_data<'life0, 'async_trait>(
        req: &'r Request<'life0>,
        data: Data<'r>
    ) -> Pin<Box<dyn Future<Output = Outcome<'r, Self>> + Send + 'async_trait>>
        'r: 'async_trait,
        'life0: 'async_trait,
        Self: 'async_trait
; }
Expand description

Trait implemented by data guards to derive a value from request body data.

Data Guards

A data guard is a guard that operates on a request’s body data. Data guards validate and parse request body data via implementations of FromData. In other words, a type is a data guard iff it implements FromData.

Data guards are the target of the data route attribute parameter:

#[post("/submit", data = "<var>")]
fn submit(var: DataGuard) { /* ... */ }

A route can have at most one data guard. Above, var is used as the argument name for the data guard type DataGuard. When the submit route matches, Rocket will call the FromData implementation for the type T. The handler will only be called if the guard returns successfully.

Async Trait

FromData is an async trait. Implementations of FromData must be decorated with an attribute of #[rocket::async_trait]:

use rocket::request::Request;
use rocket::data::{self, Data, FromData};

impl<'r> FromData<'r> for MyType {
    type Error = MyError;

    async fn from_data(req: &'r Request<'_>, data: Data<'r>) -> data::Outcome<'r, Self> {
        /* .. */


Say that you have a custom type, Person:

struct Person<'r> {
    name: &'r str,
    age: u16

Person has a custom serialization format, so the built-in Json type doesn’t suffice. The format is <name>:<age> with Content-Type: application/x-person. You’d like to use Person as a data guard, so that you can retrieve it directly from a client’s request body:

#[post("/person", data = "<person>")]
fn person(person: Person<'_>) -> &'static str {
    "Saved the new person to the database!"

A FromData implementation for such a type might look like:

use rocket::request::{self, Request};
use rocket::data::{self, Data, FromData, ToByteUnit};
use rocket::http::{Status, ContentType};

enum Error {

impl<'r> FromData<'r> for Person<'r> {
    type Error = Error;

    async fn from_data(req: &'r Request<'_>, data: Data<'r>) -> data::Outcome<'r, Self> {
        use Error::*;
        use rocket::outcome::Outcome::*;

        // Ensure the content type is correct before opening the data.
        let person_ct = ContentType::new("application", "x-person");
        if req.content_type() != Some(&person_ct) {
            return Forward(data);

        // Use a configured limit with name 'person' or fallback to default.
        let limit = req.limits().get("person").unwrap_or(256.bytes());

        // Read the data into a string.
        let string = match data.open(limit).into_string().await {
            Ok(string) if string.is_complete() => string.into_inner(),
            Ok(_) => return Failure((Status::PayloadTooLarge, TooLarge)),
            Err(e) => return Failure((Status::InternalServerError, Io(e))),

        // We store `string` in request-local cache for long-lived borrows.
        let string = request::local_cache!(req, string);

        // Split the string into two pieces at ':'.
        let (name, age) = match string.find(':') {
            Some(i) => (&string[..i], &string[(i + 1)..]),
            None => return Failure((Status::UnprocessableEntity, NoColon)),

        // Parse the age.
        let age: u16 = match age.parse() {
            Ok(age) => age,
            Err(_) => return Failure((Status::UnprocessableEntity, InvalidAge)),

        Success(Person { name, age })

// The following routes now typecheck...

#[post("/person", data = "<person>")]
fn person(person: Person<'_>) { /* .. */ }

#[post("/person", data = "<person>")]
fn person2(person: Result<Person<'_>, Error>) { /* .. */ }

#[post("/person", data = "<person>")]
fn person3(person: Option<Person<'_>>) { /* .. */ }

#[post("/person", data = "<person>")]
fn person4(person: Person<'_>) -> &str {
    // Note that this is only possible because the data in `person` live
    // as long as the request through request-local cache.

Required Associated Types

The associated error to be returned when the guard fails.

Required Methods

Asynchronously validates, parses, and converts an instance of Self from the incoming request body data.

If validation and parsing succeeds, an outcome of Success is returned. If the data is not appropriate given the type of Self, Forward is returned. If parsing fails, Failure is returned.

Implementations on Foreign Types