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Paul van Tilburg c1b14084f9 Fix a few minor mistakes in the guide.

2021-06-17 17:21:58 -07:00

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Requests

Together, a route attribute and function signature specify what must be true about a request in order for the route's handler to be called. You've already seen an example of this in action:

# #[macro_use] extern crate rocket;
# fn main() {}

#[get("/world")]
fn handler() { /* .. */ }

This route indicates that it only matches against GET requests to the /world route. Rocket ensures that this is the case before handler is called. Of course, you can do much more than specify the method and path of a request. Among other things, you can ask Rocket to automatically validate:

The type of a dynamic path segment.
The type of several dynamic path segments.
The type of incoming body data.
The types of query strings, forms, and form values.
The expected incoming or outgoing format of a request.
Any arbitrary, user-defined security or validation policies.

The route attribute and function signature work in tandem to describe these validations. Rocket's code generation takes care of actually validating the properties. This section describes how to ask Rocket to validate against all of these properties and more.

Methods

A Rocket route attribute can be any one of get, put, post, delete, head, patch, or options, each corresponding to the HTTP method to match against. For example, the following attribute will match against POST requests to the root path:

# #[macro_use] extern crate rocket;
# fn main() {}

#[post("/")]
# fn handler() {}

The grammar for these attributes is defined formally in the route API docs.

HEAD Requests

Rocket handles HEAD requests automatically when there exists a GET route that would otherwise match. It does this by stripping the body from the response, if there is one. You can also specialize the handling of a HEAD request by declaring a route for it; Rocket won't interfere with HEAD requests your application explicitly handles.

Reinterpreting

Because web browsers only support submitting HTML forms as GET or POST requests, Rocket reinterprets request methods under certain conditions. If a POST request contains a body of Content-Type: application/x-www-form-urlencoded and the form's first field has the name _method and a valid HTTP method name as its value (such as "PUT"), that field's value is used as the method for the incoming request. This allows Rocket applications to submit non-POST forms. The todo example makes use of this feature to submit PUT and DELETE requests from a web form.

Dynamic Paths

You can declare path segments as dynamic by using angle brackets around variable names in a route's path. For example, if we want to say Hello! to anything, not just the world, we can declare a route like so:

# #[macro_use] extern crate rocket;
# fn main() {}

#[get("/hello/<name>")]
fn hello(name: &str) -> String {
    format!("Hello, {}!", name)
}

If we were to mount the path at the root (.mount("/", routes![hello])), then any request to a path with two non-empty segments, where the first segment is hello, will be dispatched to the hello route. For example, if we were to visit /hello/John, the application would respond with Hello, John!.

Any number of dynamic path segments are allowed. A path segment can be of any type, including your own, as long as the type implements the FromParam trait. We call these types parameter guards. Rocket implements FromParam for many of the standard library types, as well as a few special Rocket types. For the full list of provided implementations, see the FromParam API docs. Here's a more complete route to illustrate varied usage:

# #[macro_use] extern crate rocket;
# fn main() {}

#[get("/hello/<name>/<age>/<cool>")]
fn hello(name: &str, age: u8, cool: bool) -> String {
    if cool {
        format!("You're a cool {} year old, {}!", age, name)
    } else {
        format!("{}, we need to talk about your coolness.", name)
    }
}

Multiple Segments

You can also match against multiple segments by using <param..> in a route path. The type of such parameters, known as segments guards, must implement FromSegments. A segments guard must be the final component of a path: any text after a segments guard will result in a compile-time error.

As an example, the following route matches against all paths that begin with /page:

# use rocket::get;
use std::path::PathBuf;

#[get("/page/<path..>")]
fn get_page(path: PathBuf) { /* ... */ }

The path after /page/ will be available in the path parameter, which may be empty for paths that are simply /page, /page/, /page//, and so on. The FromSegments implementation for PathBuf ensures that path cannot lead to path traversal attacks. With this, a safe and secure static file server can be implemented in just 4 lines:

# #[macro_use] extern crate rocket;
# fn main() {}

use std::path::{Path, PathBuf};
use rocket::fs::NamedFile;

#[get("/<file..>")]
async fn files(file: PathBuf) -> Option<NamedFile> {
    NamedFile::open(Path::new("static/").join(file)).await.ok()
}

! tip: Rocket makes it even easier to serve static files!

If you need to serve static files from your Rocket application, consider using FileServer, which makes it as simple as:

rocket.mount("/public", FileServer::from("static/"))

Ignored Segments

A component of a route can be fully ignored by using <_>, and multiple components can be ignored by using <_..>. In other words, the wildcard name _ is a dynamic parameter name that ignores that dynamic parameter. An ignored parameter must not appear in the function argument list. A segment declared as <_> matches anything in a single segment while segments declared as <_..> match any number of segments with no conditions.

As an example, the foo_bar route below matches any GET request with a 3-segment URI that starts with /foo/ and ends with /bar. The everything route below matches every GET request.

# #[macro_use] extern crate rocket;

#[get("/foo/<_>/bar")]
fn foo_bar() -> &'static str {
    "Foo _____ bar!"
}

#[get("/<_..>")]
fn everything() -> &'static str {
    "Hey, you're here."
}

# // Ensure there are no collisions.
# rocket_guide_tests::client(routes![foo_bar, everything]);

Forwarding

Let's take a closer look at this route attribute and signature pair from a previous example:

# #[macro_use] extern crate rocket;
# fn main() {}

#[get("/hello/<name>/<age>/<cool>")]
fn hello(name: &str, age: u8, cool: bool) { /* ... */ }

What if cool isn't a bool? Or, what if age isn't a u8? When a parameter type mismatch occurs, Rocket forwards the request to the next matching route, if there is any. This continues until a route doesn't forward the request or there are no remaining routes to try. When there are no remaining routes, a customizable 404 error is returned.

Routes are attempted in increasing rank order. Rocket chooses a default ranking from -12 to -1, detailed in the next section, but a route's rank can also be manually set with the rank attribute. To illustrate, consider the following routes:

# #[macro_use] extern crate rocket;

#[get("/user/<id>")]
fn user(id: usize) { /* ... */ }

#[get("/user/<id>", rank = 2)]
fn user_int(id: isize) { /* ... */ }

#[get("/user/<id>", rank = 3)]
fn user_str(id: &str) { /* ... */ }

#[launch]
fn rocket() -> _ {
    rocket::build().mount("/", routes![user, user_int, user_str])
}

Notice the rank parameters in user_int and user_str. If we run this application with the routes mounted at the root path, as is done in rocket() above, requests to /user/<id> (such as /user/123, /user/Bob, and so on) will be routed as follows:

The user route matches first. If the string at the <id> position is an unsigned integer, then the user handler is called. If it is not, then the request is forwarded to the next matching route: user_int.
The user_int route matches next. If <id> is a signed integer, user_int is called. Otherwise, the request is forwarded.
The user_str route matches last. Since <id> is always a string, the route always matches. The user_str handler is called.

! note: A route's rank appears in [brackets] during launch.

You'll also find a route's rank logged in brackets during application launch: GET /user/<id> [3] (user_str).

Forwards can be caught by using a Result or Option type. For example, if the type of id in the user function was Result<usize, &str>, then user would never forward. An Ok variant would indicate that <id> was a valid usize, while an Err would indicate that <id> was not a usize. The Err's value would contain the string that failed to parse as a usize.

! tip: It's not just forwards that can be caught!

In general, when any guard fails for any reason, including parameter guards, you can use an Option or Result type in its place to catch the failure.

By the way, if you were to omit the rank parameter in the user_str or user_int routes, Rocket would emit an error and abort launch, indicating that the routes collide, or can match against similar incoming requests. The rank parameter resolves this collision.

Default Ranking

If a rank is not explicitly specified, Rocket assigns a default rank. The default rank prefers static segments over dynamic segments in both paths and queries: the more static a route's path and query are, the higher its precedence.

There are three "colors" to paths and queries:

static, meaning all components are static
partial, meaning at least one component is dynamic
wild, meaning all components are dynamic

Static paths carry more weight than static queries. The same is true for partial and wild paths. This results in the following default ranking table:

path color	query color	default rank
static	static	-12
static	partial	-11
static	wild	-10
static	none	-9
partial	static	-8
partial	partial	-7
partial	wild	-6
partial	none	-5
wild	static	-4
wild	partial	-3
wild	wild	-2
wild	none	-1

Recall that lower ranks have higher precedence. As an example, consider this application from before:

# #[macro_use] extern crate rocket;

#[get("/foo/<_>/bar")]
fn foo_bar() { }

#[get("/<_..>")]
fn everything() { }

# // Ensure there are no collisions.
# rocket_guide_tests::client(routes![foo_bar, everything]);

Default ranking ensures that foo_bar, with a "partial" path color, has higher precedence than everything with a "wild" path color. This default ranking prevents what would have otherwise been a routing collision.

Request Guards

Request guards are one of Rocket's most powerful instruments. As the name might imply, a request guard protects a handler from being called erroneously based on information contained in an incoming request. More specifically, a request guard is a type that represents an arbitrary validation policy. The validation policy is implemented through the FromRequest trait. Every type that implements FromRequest is a request guard.

Request guards appear as inputs to handlers. An arbitrary number of request guards can appear as arguments in a route handler. Rocket will automatically invoke the FromRequest implementation for request guards before calling the handler. Rocket only dispatches requests to a handler when all of its guards pass.

For instance, the following dummy handler makes use of three request guards, A, B, and C. An input can be identified as a request guard if it is not named in the route attribute.

# #[macro_use] extern crate rocket;
# fn main() {}

# type A = rocket::http::Method;
# type B = A;
# type C = A;

#[get("/<param>")]
fn index(param: isize, a: A, b: B, c: C) { /* ... */ }

Request guards always fire in left-to-right declaration order. In the example above, the order will be A followed by B followed by C. Failure is short-circuiting; if one guard fails, the remaining are not attempted. To learn more about request guards and implementing them, see the FromRequest documentation.

Custom Guards

You can implement FromRequest for your own types. For instance, to protect a sensitive route from running unless an ApiKey is present in the request headers, you might create an ApiKey type that implements FromRequest and then use it as a request guard:

# #[macro_use] extern crate rocket;
# fn main() {}
# type ApiKey = rocket::http::Method;

#[get("/sensitive")]
fn sensitive(key: ApiKey) { /* .. */ }

You might also implement FromRequest for an AdminUser type that authenticates an administrator using incoming cookies. Then, any handler with an AdminUser or ApiKey type in its argument list is assured to only be invoked if the appropriate conditions are met. Request guards centralize policies, resulting in a simpler, safer, and more secure applications.

Guard Transparency

When a request guard type can only be created through its FromRequest implementation, and the type is not Copy, the existence of a request guard value provides a type-level proof that the current request has been validated against an arbitrary policy. This provides powerful means of protecting your application against access-control violations by requiring data accessing methods to witness a proof of authorization via a request guard. We call the notion of using a request guard as a witness guard transparency.

As a concrete example, the following application has a function, health_records, that returns all of the health records in a database. Because health records are sensitive information, they should only be accessible by super users. The SuperUser request guard authenticates and authorizes a super user, and its FromRequest implementation is the only means by which a SuperUser can be constructed. By declaring the health_records function as follows, access control violations against health records are guaranteed to be prevented at compile-time:

# type Records = ();
# type SuperUser = ();
fn health_records(user: &SuperUser) -> Records { /* ... */ }

The reasoning is as follows:

The health_records function requires an &SuperUser type.
The only constructor for a SuperUser type is FromRequest.
Only Rocket can provide an active &Request to construct via FromRequest.
Thus, there must be a Request authorizing a SuperUser to call health_records.

! note

At the expense of a lifetime parameter in the guard type, guarantees can be made even stronger by tying the lifetime of the Request passed to FromRequest to the request guard, ensuring that the guard value always corresponds to an active request.

We recommend leveraging request guard transparency for all data accesses.

Forwarding Guards

Request guards and forwarding are a powerful combination for enforcing policies. To illustrate, we consider how a simple authorization system might be implemented using these mechanisms.

We start with two request guards:

User: A regular, authenticated user.

The FromRequest implementation for User checks that a cookie identifies a user and returns a User value if so. If no user can be authenticated, the guard forwards.
AdminUser: A user authenticated as an administrator.

The FromRequest implementation for AdminUser checks that a cookie identifies an administrative user and returns an AdminUser value if so. If no user can be authenticated, the guard forwards.

We now use these two guards in combination with forwarding to implement the following three routes, each leading to an administrative control panel at /admin:

# #[macro_use] extern crate rocket;
# fn main() {}

# type Template = ();
# type AdminUser = rocket::http::Method;
# type User = rocket::http::Method;

use rocket::response::Redirect;

#[get("/login")]
fn login() -> Template { /* .. */ }

#[get("/admin")]
fn admin_panel(admin: AdminUser) -> &'static str {
    "Hello, administrator. This is the admin panel!"
}

#[get("/admin", rank = 2)]
fn admin_panel_user(user: User) -> &'static str {
    "Sorry, you must be an administrator to access this page."
}

#[get("/admin", rank = 3)]
fn admin_panel_redirect() -> Redirect {
    Redirect::to(uri!(login))
}

The three routes above encode authentication and authorization. The admin_panel route only succeeds if an administrator is logged in. Only then is the admin panel displayed. If the user is not an admin, the AdminUser guard will forward. Since the admin_panel_user route is ranked next highest, it is attempted next. This route succeeds if there is any user signed in, and an authorization failure message is displayed. Finally, if a user isn't signed in, the admin_panel_redirect route is attempted. Since this route has no guards, it always succeeds. The user is redirected to a log in page.

Cookies

A reference to a CookieJar is an important, built-in request guard: it allows you to get, set, and remove cookies. Because &CookieJar is a request guard, an argument of its type can simply be added to a handler:

# #[macro_use] extern crate rocket;
# fn main() {}
use rocket::http::CookieJar;

#[get("/")]
fn index(cookies: &CookieJar<'_>) -> Option<String> {
    cookies.get("message").map(|crumb| format!("Message: {}", crumb.value()))
}

This results in the incoming request's cookies being accessible from the handler. The example above retrieves a cookie named message. Cookies can also be set and removed using the CookieJar guard. The cookies example on GitHub illustrates further use of the CookieJar type to get and set cookies, while the CookieJar documentation contains complete usage information.

Private Cookies

Cookies added via the CookieJar::add() method are set in the clear. In other words, the value set is visible to the client. For sensitive data, Rocket provides private cookies. Private cookies are similar to regular cookies except that they are encrypted using authenticated encryption, a form of encryption which simultaneously provides confidentiality, integrity, and authenticity. Thus, private cookies cannot be inspected, tampered with, or manufactured by clients. If you prefer, you can think of private cookies as being signed and encrypted.

Support for private cookies must be manually enabled via the secrets crate feature:

## in Cargo.toml
rocket = { version = "0.5.0-rc.1", features = ["secrets"] }

The API for retrieving, adding, and removing private cookies is identical except methods are suffixed with _private. These methods are: get_private, get_private_pending, add_private, and remove_private. An example of their usage is below:

# #[macro_use] extern crate rocket;
# fn main() {}

use rocket::http::{Cookie, CookieJar};
use rocket::response::{Flash, Redirect};

/// Retrieve the user's ID, if any.
#[get("/user_id")]
fn user_id(cookies: &CookieJar<'_>) -> Option<String> {
    cookies.get_private("user_id")
        .map(|crumb| format!("User ID: {}", crumb.value()))
}

/// Remove the `user_id` cookie.
#[post("/logout")]
fn logout(cookies: &CookieJar<'_>) -> Flash<Redirect> {
    cookies.remove_private(Cookie::named("user_id"));
    Flash::success(Redirect::to("/"), "Successfully logged out.")
}

Secret Key

To encrypt private cookies, Rocket uses the 256-bit key specified in the secret_key configuration parameter. When compiled in debug mode, a fresh key is generated automatically. In release mode, Rocket requires you to set a secret key if the secrets feature is enabled. Failure to do so results in a hard error at launch time. The value of the parameter may either be a 256-bit base64 or hex string or a 32-byte slice.

Generating a string suitable for use as a secret_key configuration value is usually done through tools like openssl. Using openssl, a 256-bit base64 key can be generated with the command openssl rand -base64 32.

For more information on configuration, see the Configuration section of the guide.

Format

A route can specify the data format it is willing to accept or respond with by using the format route parameter. The value of the parameter is a string identifying an HTTP media type or a shorthand variant. For instance, for JSON data, the string application/json or simply json can be used.

When a route indicates a payload-supporting method (PUT, POST, DELETE, and PATCH), the format route parameter instructs Rocket to check against the Content-Type header of the incoming request. Only requests where the Content-Type header matches the format parameter will match to the route.

As an example, consider the following route:

# #[macro_use] extern crate rocket;
# fn main() {}

# type User = String;

#[post("/user", format = "application/json", data = "<user>")]
fn new_user(user: User) { /* ... */ }

The format parameter in the post attribute declares that only incoming requests with Content-Type: application/json will match new_user. (The data parameter is described in the next section.) Shorthand is also supported for the most common format arguments. Instead of using the full Content-Type, format = "application/json", you can also write shorthands like format = "json". For a full list of available shorthands, see the ContentType::parse_flexible() documentation.

When a route indicates a non-payload-supporting method (GET, HEAD, OPTIONS) the format route parameter instructs Rocket to check against the Accept header of the incoming request. Only requests where the preferred media type in the Accept header matches the format parameter will match to the route.

As an example, consider the following route:

# #[macro_use] extern crate rocket;
# fn main() {}
# type User = ();

#[get("/user/<id>", format = "json")]
fn user(id: usize) -> User { /* .. */ }

The format parameter in the get attribute declares that only incoming requests with application/json as the preferred media type in the Accept header will match user. If instead the route had been declared as post, Rocket would match the format against the Content-Type header of the incoming response.

Body Data

Body data processing, like much of Rocket, is type directed. To indicate that a handler expects body data, annotate it with data = "<param>", where param is an argument in the handler. The argument's type must implement the FromData trait. It looks like this, where T is assumed to implement FromData:

# #[macro_use] extern crate rocket;

# type T = String;

#[post("/", data = "<input>")]
fn new(input: T) { /* .. */ }

Any type that implements FromData is also known as a data guard.

JSON

The Json<T> guard deserialzies body data as JSON. The only condition is that the generic type T implements the Deserialize trait from serde.

# #[macro_use] extern crate rocket;

use rocket::serde::{Deserialize, json::Json};

#[derive(Deserialize)]
struct Task<'r> {
    description: &'r str,
    complete: bool
}

#[post("/todo", data = "<task>")]
fn new(task: Json<Task<'_>>) { /* .. */ }

See the JSON example on GitHub for a complete example.

Temporary Files

The TempFile data guard streams data directly to a temporary file which can then be persisted. It makes accepting file uploads trivial:

# #[macro_use] extern crate rocket;

use rocket::fs::TempFile;

#[post("/upload", format = "plain", data = "<file>")]
async fn upload(mut file: TempFile<'_>) -> std::io::Result<()> {
    # let permanent_location = "/tmp/perm.txt";
    file.persist_to(permanent_location).await
}

Streaming

Sometimes you just want to handle incoming data directly. For example, you might want to stream the incoming data to some sink. Rocket makes this as simple as possible via the Data type:

# #[macro_use] extern crate rocket;

use rocket::tokio;

use rocket::data::{Data, ToByteUnit};

#[post("/debug", data = "<data>")]
async fn debug(data: Data<'_>) -> std::io::Result<()> {
    // Stream at most 512KiB all of the body data to stdout.
    data.open(512.kibibytes())
        .stream_to(tokio::io::stdout())
        .await?;

    Ok(())
}

The route above accepts any POST request to the /debug path. At most 512KiB of the incoming is streamed out to stdout. If the upload fails, an error response is returned. The handler above is complete. It really is that simple!

! note: Rocket requires setting limits when reading incoming data.

To aid in preventing DoS attacks, Rocket requires you to specify, as a ByteUnit, the amount of data you're willing to accept from the client when opening a data stream. The ToByteUnit trait makes specifying such a value as idiomatic as 128.kibibytes().

Forms

Forms are one of the most common types of data handled in web applications, and Rocket makes handling them easy. Rocket supports both multipart and x-www-form-urlencoded forms out of the box, enabled by the Form data guard and derivable FromForm trait.

Say your application is processing a form submission for a new todo Task. The form contains two fields: complete, a checkbox, and type, a text field. You can easily handle the form request in Rocket as follows:

# #[macro_use] extern crate rocket;

use rocket::form::Form;

#[derive(FromForm)]
struct Task<'r> {
    complete: bool,
    r#type: &'r str,
}

#[post("/todo", data = "<task>")]
fn new(task: Form<Task<'_>>) { /* .. */ }

The Form type implements the FromData trait as long as its generic parameter implements the FromForm trait. In the example, we've derived the FromForm trait automatically for the Task structure. FromForm can be derived for any structure whose fields implement FromForm, or equivalently, FromFormField. If a POST /todo request arrives, the form data will automatically be parsed into the Task structure. If the data that arrives isn't of the correct Content-Type, the request is forwarded. If the data doesn't parse or is simply invalid, a customizable error is returned. As before, a forward or failure can be caught by using the Option and Result types:

# use rocket::{post, form::Form};
# type Task<'r> = &'r str;

#[post("/todo", data = "<task>")]
fn new(task: Option<Form<Task<'_>>>) { /* .. */ }

Parsing Strategy

Rocket's FromForm parsing is lenient by default: a Form<T> will parse successfully from an incoming form even if it contains extra, duplicate, or missing fields. Extras or duplicates are ignored -- no validation or parsing of the fields occurs -- and missing fields are filled with defaults when available. To change this behavior and make form parsing strict, use the Form<Strict<T>> data type, which emits errors if there are any extra or missing fields, irrespective of defaults.

You can use a Form<Strict<T>> anywhere you'd use a Form<T>. Its generic parameter is also required to implement FromForm. For instance, we can simply replace Form<T> with Form<Strict<T>> above to get strict parsing:

# #[macro_use] extern crate rocket;

use rocket::form::{Form, Strict};

# #[derive(FromForm)] struct Task<'r> { complete: bool, description: &'r str, }

#[post("/todo", data = "<task>")]
fn new(task: Form<Strict<Task<'_>>>) { /* .. */ }

Strict can also be used to make individual fields strict while keeping the overall structure and remaining fields lenient:

# #[macro_use] extern crate rocket;
# use rocket::form::{Form, Strict};

#[derive(FromForm)]
struct Input {
    required: Strict<bool>,
    uses_default: bool
}

#[post("/", data = "<input>")]
fn new(input: Form<Input>) { /* .. */ }

Lenient is the lenient analog to Strict, which forces parsing to be lenient. Form is lenient by default, so a Form<Lenient<T>> is redundant, but Lenient can be used to overwrite a strict parse as lenient: Option<Lenient<T>>.

Defaults

A form guard may specify a default value to use when a field is missing. The default value is used only when parsing is lenient. When strict, all errors, including missing fields, are propagated directly.

Some types with defaults include bool, which defaults to false, useful for checkboxes, Option<T>, which defaults to None, and form::Result, which defaults to Err(Missing) or otherwise collects errors in an Err of Errors<'_>. Defaulting guards can be used just like any other form guard:

# use rocket::form::FromForm;
use rocket::form::{self, Errors};

#[derive(FromForm)]
struct MyForm<'v> {
    maybe_string: Option<&'v str>,
    ok_or_error: form::Result<'v, Vec<&'v str>>,
    here_or_false: bool,
}

# rocket_guide_tests::assert_form_parses_ok!(MyForm, "");

The default can be overridden or unset using the #[field(default = expr) field attribute. If expr is not literally None, the parameter sets the default value of the field to be expr.into(). If expr is None, the parameter unsets the default value of the field, if any.

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    // Set the default value to be `"hello"`.
    //
    // Note how an `&str` is automatically converted into a `String`.
    #[field(default = "hello")]
    greeting: String,
    // Remove the default value of `false`, requiring all parses of `MyForm`
    // to contain an `is_friendly` field.
    #[field(default = None)]
    is_friendly: bool,
}

See the FromForm derive documentation for full details on the default attribute parameter as well documentation on the more expressive default_with parameter option.

Field Renaming

By default, Rocket matches the name of an incoming form field to the name of a structure field. While this behavior is typical, it may also be desired to use different names for form fields and struct fields while still parsing as expected. You can ask Rocket to look for a different form field for a given structure field by using one or more #[field(name = "name")] or #[field(name = uncased("name")] field annotation. The uncased variant case-insensitively matches field names.

As an example, say that you're writing an application that receives data from an external service. The external service POSTs a form with a field named first-Name which you'd like to write as first_name in Rust. Such a form structure can be written as:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct External<'r> {
    #[field(name = "first-Name")]
    first_name: &'r str
}

If you want to accept both firstName case-insensitively as well as first_name case-sensitively, you'll need to use two annotations:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct External<'r> {
    #[field(name = uncased("firstName"))]
    #[field(name = "first_name")]
    first_name: &'r str
}

This will match any casing of firstName including FirstName, firstname, FIRSTname, and so on, but only match exactly on first_name.

If instead you wanted to match any of first-name, first_name or firstName, in each instance case-insensitively, you would write:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct External<'r> {
    #[field(name = uncased("first-name"))]
    #[field(name = uncased("first_name"))]
    #[field(name = uncased("firstname"))]
    first_name: &'r str
}

Cased and uncased renamings can be mixed and matched, and any number of renamings is allowed. Rocket will emit an error at compile-time if field names conflict, preventing ambiguous parsing at runtime.

Ad-Hoc Validation

Fields of forms can be easily ad-hoc validated via the #[field(validate)] attribute. As an example, consider a form field age: u16 which we'd like to ensure is greater than 21. The following structure accomplishes this:

# #[macro_use] extern crate rocket;

#[derive(FromForm)]
struct Person {
    #[field(validate = range(21..))]
    age: u16
}

The expression range(21..) is a call to form::validate::range. Rocket passes a borrow of the attributed field, here self.age, as the first parameter to the function call. The rest of the fields are pass as written in the expression.

Any function in the form::validate module can be called, and other fields of the form can be passed in by using self.$field where $field is the name of the field in the structure. You can also apply more than one validation to a field by using multiple attributes. For example, the following form validates that the value of the field confirm is equal to the value of the field value and that it doesn't contain no:

# #[macro_use] extern crate rocket;

#[derive(FromForm)]
struct Password<'r> {
    #[field(name = "password")]
    value: &'r str,
    #[field(validate = eq(self.value))]
    #[field(validate = omits("no"))]
    confirm: &'r str,
}

In reality, the expression after validate = can be any expression as long as it evaluates to a value of type Result<(), Errors<'_>> (aliased by form::Result), where an Ok value means that validation was successful while an Err of Errors<'_> indicates the error(s) that occurred. For instance, if you wanted to implement an ad-hoc Luhn validator for credit-card-like numbers, you might write:

# #[macro_use] extern crate rocket;
extern crate time;

use rocket::form::{self, Error};

#[derive(FromForm)]
struct CreditCard {
    #[field(validate = luhn(self.cvv, &self.expiration))]
    number: u64,
    #[field(validate = range(..9999))]
    cvv: u16,
    expiration: time::Date,
}

fn luhn<'v>(number: &u64, cvv: u16, exp: &time::Date) -> form::Result<'v, ()> {
    # let valid = false;
    if !valid {
        Err(Error::validation("invalid credit card number"))?;
    }

    Ok(())
}

If a field's validation doesn't depend on other fields (validation is local), it is validated prior to those fields that do. For CreditCard, cvv and expiration will be validated prior to number.

Collections

Rocket's form support allows your application to express any structure with any level of nesting and collection, eclipsing the expressivity offered by any other web framework. To parse into these structures, Rocket separates a field's name into "keys" by the delimiters . and [], each of which in turn is separated into "indices" by :. In other words, a name has keys and a key has indices, each a strict subset of its parent. This is depicted in the example below with two form fields:

food.bart[bar:foo].blam[0_0][1000]=some-value&another_field=another_val
|-------------------------------|   name
|--| |--| |-----|  |--| |-|  |--|   keys
|--| |--| |-| |-|  |--| |-|  |--|   indices

Rocket pushes form fields to FromForm types as they arrive. The type then operates on one key (and all of its indices) at a time and shifts to the next key, from left-to-right, before invoking any other FromForm types with the rest of the field. A shift encodes a nested structure while indices allows for structures that need more than one value to allow indexing.

! note: A . after a [] is optional.

The form field name a[b]c is exactly equivalent to a[b].c. Likewise, the form field name .a is equivalent to a.

Nesting

Form structs can be nested:

use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm<'r> {
    owner: Person<'r>,
    pet: Pet<'r>,
}

#[derive(FromForm)]
struct Person<'r> {
    name: &'r str
}

#[derive(FromForm)]
struct Pet<'r> {
    name: &'r str,
    #[field(validate = eq(true))]
    good_pet: bool,
}

To parse into a MyForm, a form with the following fields must be submitted:

owner.name - string
pet.name - string
pet.good_pet - boolean

Such a form, URL-encoded, may look like:

# use rocket::form::FromForm;
# use rocket_guide_tests::{assert_form_parses, assert_not_form_parses};
# #[derive(FromForm, Debug, PartialEq)] struct MyForm { owner: Person, pet: Pet, }
# #[derive(FromForm, Debug, PartialEq)] struct Person { name: String }
# #[derive(FromForm, Debug, PartialEq)] struct Pet { name: String, good_pet: bool, }

# assert_form_parses! { MyForm,
"owner.name=Bob&pet.name=Sally&pet.good_pet=on",
# "owner.name=Bob&pet.name=Sally&pet.good_pet=yes",
# "owner.name=Bob&pet.name=Sally&pet.good_pet=on",
# "pet.name=Sally&owner.name=Bob&pet.good_pet=on",
# "pet.name=Sally&pet.good_pet=on&owner.name=Bob",
# =>

// ...which parses as this struct.
MyForm {
    owner: Person {
        name: "Bob".into()
    },
    pet: Pet {
        name: "Sally".into(),
        good_pet: true,
    }
}
# };

Note that . is used to separate each field. Identically, [] can be used in place of or in addition to .:

# use rocket::form::FromForm;
# use rocket_guide_tests::{assert_form_parses, assert_not_form_parses};
# #[derive(FromForm, Debug, PartialEq)] struct MyForm { owner: Person, pet: Pet, }
# #[derive(FromForm, Debug, PartialEq)] struct Person { name: String }
# #[derive(FromForm, Debug, PartialEq)] struct Pet { name: String, good_pet: bool, }

// All of these are identical to the previous...
# assert_form_parses! { MyForm,
"owner[name]=Bob&pet[name]=Sally&pet[good_pet]=on",
"owner[name]=Bob&pet[name]=Sally&pet.good_pet=on",
"owner.name=Bob&pet[name]=Sally&pet.good_pet=on",
"pet[name]=Sally&owner.name=Bob&pet.good_pet=on",
# =>

// ...and thus parse as this struct.
MyForm {
    owner: Person {
        name: "Bob".into()
    },
    pet: Pet {
        name: "Sally".into(),
        good_pet: true,
    }
}
# };

Any level of nesting is allowed.

Vectors

A form can also contain sequences:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    numbers: Vec<usize>,
}

To parse into a MyForm, a form with the following fields must be submitted:

numbers[$k] - usize (or equivalently, numbers.$k)

...where $k is the "key" used to determine whether to push the rest of the field to the last element in the vector or create a new one. If the key is the same as the previous key seen by the vector, then the field's value is pushed to the last element. Otherwise, a new element is created. The actual value of $k is irrelevant: it is only used for comparison, has no semantic meaning, and is not remembered by Vec. The special blank key is never equal to any other key.

Consider the following examples.

# use rocket::form::FromForm;
# use rocket_guide_tests::{assert_form_parses, assert_not_form_parses};
# #[derive(FromForm, PartialEq, Debug)] struct MyForm { numbers: Vec<usize>, }
// These form strings...
# assert_form_parses! { MyForm,
"numbers[]=1&numbers[]=2&numbers[]=3",
"numbers[a]=1&numbers[b]=2&numbers[c]=3",
"numbers[a]=1&numbers[b]=2&numbers[a]=3",
"numbers[]=1&numbers[b]=2&numbers[c]=3",
"numbers.0=1&numbers.1=2&numbers[c]=3",
"numbers=1&numbers=2&numbers=3",
# =>

// ...parse as this struct:
MyForm {
    numbers: vec![1 ,2, 3]
}
# };

// These, on the other hand...
# assert_form_parses! { MyForm,
"numbers[0]=1&numbers[0]=2&numbers[]=3",
"numbers[]=1&numbers[b]=3&numbers[b]=2",
# =>

// ...parse as this struct:
MyForm {
    numbers: vec![1, 3]
}
# };

You might be surprised to see the last example, "numbers=1&numbers=2&numbers=3", in the first list. This is equivalent to the previous examples as the "key" seen by the Vec (everything after numbers) is empty. Thus, Vec pushes to a new usize for every field. usize, like all types that implement FromFormField, discard duplicate and extra fields when parsed leniently, keeping only the first field.

Nesting in Vectors

Any FromForm type can appear in a sequence:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    name: String,
    pets: Vec<Pet>,
}

#[derive(FromForm)]
struct Pet {
    name: String,
    #[field(validate = eq(true))]
    good_pet: bool,
}

To parse into a MyForm, a form with the following fields must be submitted:

name - string
pets[$k].name - string
pets[$k].good_pet - boolean

Examples include:

# use rocket::form::FromForm;
# use rocket_guide_tests::{assert_form_parses, assert_not_form_parses};
# #[derive(FromForm, Debug, PartialEq)] struct MyForm { name: String, pets: Vec<Pet>, }
# #[derive(FromForm, Debug, PartialEq)] struct Pet { name: String, good_pet: bool, }
// These form strings...
assert_form_parses! { MyForm,
"name=Bob&pets[0].name=Sally&pets[0].good_pet=on",
"name=Bob&pets[sally].name=Sally&pets[sally].good_pet=yes",
# =>

// ...parse as this struct:
MyForm {
    name: "Bob".into(),
    pets: vec![Pet { name: "Sally".into(), good_pet: true }],
}
# };

// These, on the other hand, fail to parse:
# assert_not_form_parses! { MyForm,
"name=Bob&pets[0].name=Sally&pets[1].good_pet=on",
"name=Bob&pets[].name=Sally&pets[].good_pet=on",
# };

Nested Vectors

Since vectors are FromForm themselves, they can appear inside of vectors:

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    v: Vec<Vec<usize>>,
}

The rules are exactly the same.

# use rocket::form::FromForm;
# use rocket_guide_tests::assert_form_parses;
# #[derive(FromForm, Debug, PartialEq)] struct MyForm { v: Vec<Vec<usize>>, }
# assert_form_parses! { MyForm,
"v=1&v=2&v=3" => MyForm { v: vec![vec![1], vec![2], vec![3]] },
"v[][]=1&v[][]=2&v[][]=3" => MyForm { v: vec![vec![1], vec![2], vec![3]] },
"v[0][]=1&v[0][]=2&v[][]=3" => MyForm { v: vec![vec![1, 2], vec![3]] },
"v[][]=1&v[0][]=2&v[0][]=3" => MyForm { v: vec![vec![1], vec![2, 3]] },
"v[0][]=1&v[0][]=2&v[0][]=3" => MyForm { v: vec![vec![1, 2, 3]] },
"v[0][0]=1&v[0][0]=2&v[0][]=3" => MyForm { v: vec![vec![1, 3]] },
"v[0][0]=1&v[0][0]=2&v[0][0]=3" => MyForm { v: vec![vec![1]] },
# };

Maps

A form can also contain maps:

# use rocket::form::FromForm;
use std::collections::HashMap;

#[derive(FromForm)]
struct MyForm {
    ids: HashMap<String, usize>,
}

To parse into a MyForm, a form with the following fields must be submitted:

ids[$string] - usize (or equivalently, ids.$string)

...where $string is the "key" used to determine which value in the map to push the rest of the field to. Unlike with vectors, the key does have a semantic meaning and is remembered, so ordering of fields is inconsequential: a given string $string always maps to the same element.

As an example, the following are equivalent and all parse to { "a" => 1, "b" => 2 }:

# use std::collections::HashMap;
#
# use rocket::form::FromForm;
# use rocket_guide_tests::{map, assert_form_parses};
#
# #[derive(Debug, PartialEq, FromForm)]
# struct MyForm {
#     ids: HashMap<String, usize>,
# }
// These form strings...
# assert_form_parses! { MyForm,
"ids[a]=1&ids[b]=2",
"ids[b]=2&ids[a]=1",
"ids[a]=1&ids[a]=2&ids[b]=2",
"ids.a=1&ids.b=2",
# =>

// ...parse as this struct:
MyForm {
    ids: map! {
        "a" => 1usize,
        "b" => 2usize,
    }
}
# };

Both the key and value of a HashMap can be any type that implements FromForm. Consider a value representing another structure:

# use std::collections::HashMap;

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    ids: HashMap<usize, Person>,
}

#[derive(FromForm)]
struct Person {
    name: String,
    age: usize
}

To parse into a MyForm, a form with the following fields must be submitted:

ids[$usize].name - string
ids[$usize].age - usize

Examples include:

# use std::collections::HashMap;
#
# use rocket::form::FromForm;
# use rocket_guide_tests::{map, assert_form_parses};
#

# #[derive(FromForm, Debug, PartialEq)] struct MyForm { ids: HashMap<usize, Person>, }
# #[derive(FromForm, Debug, PartialEq)] struct Person { name: String, age: usize }

// These form strings...
# assert_form_parses! { MyForm,
"ids[0]name=Bob&ids[0]age=3&ids[1]name=Sally&ids[1]age=10",
"ids[0]name=Bob&ids[1]age=10&ids[1]name=Sally&ids[0]age=3",
"ids[0]name=Bob&ids[1]name=Sally&ids[0]age=3&ids[1]age=10",
# =>

// ...which parse as this struct:
MyForm {
    ids: map! {
        0usize => Person { name: "Bob".into(), age: 3 },
        1usize => Person { name: "Sally".into(), age: 10 },
    }
}
# };

Now consider the following structure where both the key and value represent structures:

# use std::collections::HashMap;

# use rocket::form::FromForm;

#[derive(FromForm)]
struct MyForm {
    m: HashMap<Person, Pet>,
}

#[derive(FromForm, PartialEq, Eq, Hash)]
struct Person {
    name: String,
    age: usize
}

#[derive(FromForm)]
struct Pet {
    wags: bool
}

! warning: The HashMap key type, here Person, must implement Eq + Hash.

Since the key is a collection, here Person, it must be built up from multiple fields. This requires being able to specify via the form field name that the field's value corresponds to a key in the map. The is done with the syntax k:$key which indicates that the field corresponds to the key named $key. Thus, to parse into a MyForm, a form with the following fields must be submitted:

m[k:$key].name - string
m[k:$key].age - usize
m[$key].wags or m[v:$key].wags - boolean

! note: The syntax v:$key also exists.

The shorthand m[$key] is equivalent to m[v:$key].

Note that $key can be anything: it is simply a symbolic identifier for a key/value pair in the map and has no bearing on the actual values that will be parsed into the map.

Examples include:

# use std::collections::HashMap;
#
# use rocket::form::FromForm;
# use rocket_guide_tests::{map, assert_form_parses};
#

# #[derive(FromForm, Debug, PartialEq)] struct MyForm { m: HashMap<Person, Pet>, }
# #[derive(FromForm, Debug, PartialEq, Eq, Hash)] struct Person { name: String, age: usize }
# #[derive(FromForm, Debug, PartialEq)] struct Pet { wags: bool }

// These form strings...
# assert_form_parses! { MyForm,
"m[k:alice]name=Alice&m[k:alice]age=30&m[v:alice].wags=no",
"m[k:alice]name=Alice&m[k:alice]age=30&m[alice].wags=no",
"m[k:123]name=Alice&m[k:123]age=30&m[123].wags=no",
# =>

// ...which parse as this struct:
MyForm {
    m: map! {
        Person { name: "Alice".into(), age: 30 } => Pet { wags: false }
    }
}
# };

// While this longer form string...
# assert_form_parses! { MyForm,
"m[k:a]name=Alice&m[k:a]age=40&m[a].wags=no&\
m[k:b]name=Bob&m[k:b]age=72&m[b]wags=yes&\
m[k:cat]name=Katie&m[k:cat]age=12&m[cat]wags=yes",
# =>

// ...parses as this struct:
MyForm {
    m: map! {
        Person { name: "Alice".into(), age: 40 } => Pet { wags: false },
        Person { name: "Bob".into(), age: 72 } => Pet { wags: true },
        Person { name: "Katie".into(), age: 12 } => Pet { wags: true },
    }
}
# };

Arbitrary Collections

Any collection can be expressed with any level of arbitrary nesting, maps, and sequences. Consider the extravagently contrived type:

use std::collections::{BTreeMap, HashMap};
# use rocket::form::FromForm;

#[derive(FromForm, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
struct Person {
    name: String,
    age: usize
}

# type Foo =
HashMap<Vec<BTreeMap<Person, usize>>, HashMap<usize, Person>>
# ;
# /*
|-[k:$k1]-----------|------|------| |-[$k1]-----------------|
     |---[$i]-------|------|------|         |-[k:$j]*|
           |-[k:$k2]|------|                 ~~[$j]~~|name*|
                    |-name*|                 ~~[$j]~~|age-*|
                    |-age*-|
           |~~~~~~~~~~~~~~~|v:$k2*|
# */

! warning: The BTreeMap key type, here Person, must implement Ord.

As illustrated above with * marking terminals, we need the following form fields for this structure:

[k:$k1][$i][k:$k2]name - string
[k:$k1][$i][k:$k2]age - usize
[k:$k1][$i][$k2] - usize
[$k1][k:$j] - usize
[$k1][$j]name - string
[$k1][$j]age - string

Where we have the following symbolic keys:

$k1: symbolic name of the top-level key
$i: symbolic name of the vector index
$k2: symbolic name of the sub-level (BTreeMap) key
$j: symbolic name and/or value top-level value's key

# use std::collections::BTreeMap;
# use std::collections::HashMap;
#
# use rocket::form::FromForm;
# use rocket_guide_tests::{map, bmap, assert_form_parses};
# #[derive(FromForm, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
# struct Person { name: String, age: usize }

type Foo = HashMap<Vec<BTreeMap<Person, usize>>, HashMap<usize, Person>>;

// This (long, contrived) form string...
# assert_form_parses! { Foo,
"[k:top_key][i][k:sub_key]name=Bobert&\
[k:top_key][i][k:sub_key]age=22&\
[k:top_key][i][sub_key]=1337&\
[top_key][7]name=Builder&\
[top_key][7]age=99",

// We could also set the top-level value's key explicitly:
// [top_key][k:7]=7
# "[k:top_key][i][k:sub_key]name=Bobert&\
# [k:top_key][i][k:sub_key]age=22&\
# [top_key][k:7]=7&\
# [k:top_key][i][sub_key]=1337&\
# [top_key][7]name=Builder&\
# [top_key][7]age=99",
# =>

// ...parses as this (long, contrived) map:
map! {
    vec![bmap! {
        Person { name: "Bobert".into(), age: 22 } => 1337usize,
    }]
    =>
    map! {
        7usize => Person { name: "Builder".into(), age: 99 }
    }
}
# };

Context

The Contextual form guard acts as a proxy for any other form guard, recording all submitted form values and produced errors and associating them with their corresponding field name. Contextual is particularly useful for rendering forms with previously submitted values and errors associated with form input.

To retrieve the context for a form, use Form<Contextual<'_, T>> as a data guard, where T implements FromForm. The context field contains the form's Context:

# use rocket::post;
# type T = String;

use rocket::form::{Form, Contextual};

#[post("/submit", data = "<form>")]
fn submit(form: Form<Contextual<'_, T>>) {
    if let Some(ref value) = form.value {
        // The form parsed successfully. `value` is the `T`.
    }

    // We can retrieve raw field values and errors.
    let raw_id_value = form.context.field_value("id");
    let id_errors = form.context.field_errors("id");
}

Context is nesting-aware for errors. When Context is queried for errors for a field named foo.bar, it returns errors for fields that are a prefix of foo.bar, namely foo and foo.bar. Similarly, if queried for errors for a field named foo.bar.baz, errors for field foo, foo.bar, and foo.bar.baz will be returned.

Context serializes as a map, so it can be rendered in templates that require Serialize types. See Context for details about its serialization format. The forms example, too, makes use of form contexts, as well as every other forms feature.

Query Strings

Query strings are URL-encoded forms that appear in the URL of a request. Query parameters are declared like path parameters but otherwise handled like regular URL-encoded form fields. The table below summarizes the analogy:

Path Synax	Query Syntax	Path Type Bound	Query Type Bound
`<param>`	`<param>`	`FromParam`	`FromForm`
`<param..>`	`<param..>`	`FromSegments`	`FromForm`
`static`	`static`	N/A	N/A

Because dynamic parameters are form types, they can be single values, collections, nested collections, or anything in between, just like any other form field.

Static Parameters

A request matches a route iff its query string contains all of the static parameters in the route's query string. A route with a static parameter param (any UTF-8 text string) in a query will only match requests with that exact path segment in its query string.

! note: This is truly an iff!

Only the static parameters in query route string affect routing. Dynamic parameters are allowed to be missing by default.

For example, the route below will match requests with path / and at least the query segments hello and cat=♥:

# #[macro_use] extern crate rocket;

#[get("/?hello&cat=♥")]
fn cats() -> &'static str {
    "Hello, kittens!"
}

// The following GET requests match `cats`. `%E2%99%A5` is encoded `♥`.
# let status = rocket_guide_tests::client(routes![cats]).get(
"/?cat=%E2%99%A5&hello"
# ).dispatch().status();
# assert_eq!(status, rocket::http::Status::Ok);
# let status = rocket_guide_tests::client(routes![cats]).get(
"/?hello&cat=%E2%99%A5"
# ).dispatch().status();
# assert_eq!(status, rocket::http::Status::Ok);
# let status = rocket_guide_tests::client(routes![cats]).get(
"/?dogs=amazing&hello&there&cat=%E2%99%A5"
# ).dispatch().status();
# assert_eq!(status, rocket::http::Status::Ok);

Dynamic Parameters

A single dynamic parameter of <param> acts identically to a form field declared as param. In particular, Rocket will expect the query form to contain a field with key param and push the shifted field to the param type. As with forms, default values are used when parsing fails. The example below illustrates this with a single value name, a collection color, a nested form person, and an other value that will default to None:

# #[macro_use] extern crate rocket;

#[derive(Debug, PartialEq, FromFormField)]
enum Color {
    Red,
    Blue,
    Green
}

#[derive(Debug, PartialEq, FromForm)]
struct Pet<'r> {
  name: &'r str,
  age: usize,
}

#[derive(Debug, PartialEq, FromForm)]
struct Person<'r> {
  pet: Pet<'r>,
}

#[get("/?<name>&<color>&<person>&<other>")]
fn hello(name: &str, color: Vec<Color>, person: Person<'_>, other: Option<usize>) {
    assert_eq!(name, "George");
    assert_eq!(color, [Color::Red, Color::Green, Color::Green, Color::Blue]);
    assert_eq!(other, None);
    assert_eq!(person, Person {
      pet: Pet { name: "Fi Fo Alex", age: 1 }
    });
}

// A request with these query segments matches as above.
# rocket_guide_tests::client(routes![hello]).get("/?\
color=red&\
color=green&\
person.pet.name=Fi+Fo+Alex&\
color=green&\
person.pet.age=1\
color=blue&\
extra=yes\
# ").dispatch();

Note that, like forms, parsing is field-ordering insensitive and lenient by default.

Trailing Parameter

A trailing dynamic parameter of <param..> collects all of the query segments that don't otherwise match a declared static or dynamic parameter. In other words, the otherwise unmatched segments are pushed, unshifted, to the <param..> type:

# #[macro_use] extern crate rocket;

use rocket::form::Form;

#[derive(FromForm)]
struct User<'r> {
    name: &'r str,
    active: bool,
}

#[get("/?hello&<id>&<user..>")]
fn user(id: usize, user: User<'_>) {
    assert_eq!(id, 1337);
    assert_eq!(user.name, "Bob Smith");
    assert_eq!(user.active, true);
}

// A request with these query segments matches as above.
# rocket_guide_tests::client(routes![user]).get("/?\
name=Bob+Smith&\
id=1337\
active=yes\
# ").dispatch();

Error Catchers

Application processing is fallible. Errors arise from the following sources:

A failing guard.
A failing responder.
A routing failure.

If any of these occur, Rocket returns an error to the client. To generate the error, Rocket invokes the catcher corresponding to the error's status code and scope. Catchers are similar to routes except in that:

Catchers are only invoked on error conditions.
Catchers are declared with the catch attribute.
Catchers are registered with register() instead of mount().
Any modifications to cookies are cleared before a catcher is invoked.
Error catchers cannot invoke guards.
Error catchers should not fail to produce a response.
Catchers are scoped to a path prefix.

To declare a catcher for a given status code, use the catch attribute, which takes a single integer corresponding to the HTTP status code to catch. For instance, to declare a catcher for 404 Not Found errors, you'd write:

# #[macro_use] extern crate rocket;
# fn main() {}

use rocket::Request;

#[catch(404)]
fn not_found(req: &Request) { /* .. */ }

Catchers may take zero, one, or two arguments. If the catcher takes one argument, it must be of type &Request. It it takes two, they must be of type Status and &Request, in that order. As with routes, the return type must implement Responder. A concrete implementation may look like:

# #[macro_use] extern crate rocket;
# fn main() {}

# use rocket::Request;

#[catch(404)]
fn not_found(req: &Request) -> String {
    format!("Sorry, '{}' is not a valid path.", req.uri())
}

Also as with routes, Rocket needs to know about a catcher before it is used to handle errors. The process, known as "registering" a catcher, is similar to mounting a route: call the register() method with a list of catchers via the catchers! macro. The invocation to add the 404 catcher declared above looks like:

# #[macro_use] extern crate rocket;

# use rocket::Request;
# #[catch(404)] fn not_found(req: &Request) { /* .. */ }

fn main() {
    rocket::build().register("/", catchers![not_found]);
}

Scoping

The first argument to register() is a path to scope the catcher under called the catcher's base. A catcher's base determines which requests it will handle errors for. Specifically, a catcher's base must be a prefix of the erroring request for it to be invoked. When multiple catchers can be invoked, the catcher with the longest base takes precedence.

As an example, consider the following application:

# #[macro_use] extern crate rocket;

#[catch(404)]
fn general_not_found() -> &'static str {
    "General 404"
}

#[catch(404)]
fn foo_not_found() -> &'static str {
    "Foo 404"
}

#[launch]
fn rocket() -> _ {
    rocket::build()
        .register("/", catchers![general_not_found])
        .register("/foo", catchers![foo_not_found])
}

# let client = rocket::local::blocking::Client::debug(rocket()).unwrap();
#
# let response = client.get("/").dispatch();
# assert_eq!(response.into_string().unwrap(), "General 404");
#
# let response = client.get("/bar").dispatch();
# assert_eq!(response.into_string().unwrap(), "General 404");
#
# let response = client.get("/bar/baz").dispatch();
# assert_eq!(response.into_string().unwrap(), "General 404");
#
# let response = client.get("/foo").dispatch();
# assert_eq!(response.into_string().unwrap(), "Foo 404");
#
# let response = client.get("/foo/bar").dispatch();
# assert_eq!(response.into_string().unwrap(), "Foo 404");

Since there are no mounted routes, all requests will 404. Any request whose path begins with /foo (i.e, GET /foo, GET /foo/bar, etc) will be handled by the foo_not_found catcher while all other requests will be handled by the general_not_found catcher.

Default Catchers

A default catcher is a catcher that handles all status codes. They are invoked as a fallback if no status-specific catcher is registered for a given error. Declaring a default catcher is done with #[catch(default)] and must similarly be registered with register():

# #[macro_use] extern crate rocket;

use rocket::Request;
use rocket::http::Status;

#[catch(default)]
fn default_catcher(status: Status, request: &Request) { /* .. */ }

#[launch]
fn rocket() -> _ {
    rocket::build().register("/", catchers![default_catcher])
}

Catchers with longer bases are preferred, even when there is a status-specific catcher. In other words, a default catcher with a longer matching base than a status-specific catcher takes precedence.

Built-In Catcher

Rocket provides a built-in default catcher. It produces HTML or JSON, depending on the value of the Accept header. As such, custom catchers only need to be registered for custom error handling.

The error handling example illustrates catcher use in full, while the Catcher API documentation provides further details.

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Requests

Methods

HEAD Requests

Reinterpreting

Dynamic Paths

Multiple Segments

Ignored Segments

Forwarding

Default Ranking

Request Guards

Custom Guards

Guard Transparency

Forwarding Guards

Cookies

Private Cookies

Secret Key

Format

Body Data

JSON

Temporary Files

Streaming

Forms

Parsing Strategy

Defaults

Field Renaming

Ad-Hoc Validation

Collections

Nesting

Vectors

Nesting in Vectors

Nested Vectors

Maps

Arbitrary Collections

Context

Query Strings

Static Parameters

Dynamic Parameters

Trailing Parameter

Error Catchers

Scoping

Default Catchers

Built-In Catcher

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