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use FormItems;
/// Trait to create an instance of some type from an HTTP form.
/// [`Form`](::request::Form) requires its generic type to implement this trait.
///
/// # Deriving
///
/// This trait can be automatically derived. When deriving `FromForm`, every
/// field in the structure must implement
/// [`FromFormValue`](::request::FromFormValue). Rocket validates each field in
/// the structure by calling its `FromFormValue` implementation. You may wish to
/// implement `FromFormValue` for your own types for custom, automatic
/// validation.
///
/// ```rust
/// # #![feature(proc_macro_hygiene, decl_macro)]
/// # #![allow(deprecated, dead_code, unused_attributes)]
/// # #[macro_use] extern crate rocket;
/// #[derive(FromForm)]
/// struct TodoTask {
/// description: String,
/// completed: bool
/// }
/// # fn main() { }
/// ```
///
/// # Data Guard
///
/// Types that implement `FromForm` can be parsed directly from incoming form
/// data via the `data` parameter and `Form` type.
///
/// ```rust
/// # #![feature(proc_macro_hygiene, decl_macro)]
/// # #![allow(deprecated, dead_code, unused_attributes)]
/// # #[macro_use] extern crate rocket;
/// # use rocket::request::Form;
/// # #[derive(FromForm)]
/// # struct TodoTask { description: String, completed: bool }
/// #[post("/submit", data = "<task>")]
/// fn submit_task(task: Form<TodoTask>) -> String {
/// format!("New task: {}", task.description)
/// }
/// # fn main() { }
/// ```
///
/// # Implementing
///
/// Implementing `FromForm` should be a rare occurrence. Prefer instead to use
/// Rocket's built-in derivation.
///
/// When implementing `FromForm`, use the [`FormItems`] iterator to iterate
/// through the raw form key/value pairs. Be aware that form fields that are
/// typically hidden from your application, such as `_method`, will be present
/// while iterating. Ensure that you adhere to the properties of the `strict`
/// parameter, as detailed in the documentation below.
///
/// ## Example
///
/// Consider the following scenario: we have a struct `Item` with field name
/// `field`. We'd like to parse any form that has a field named either `balloon`
/// _or_ `space`, and we'd like that field's value to be the value for our
/// structure's `field`. The following snippet shows how this would be
/// implemented:
///
/// ```rust
/// use rocket::request::{FromForm, FormItems};
///
/// struct Item {
/// field: String
/// }
///
/// impl<'f> FromForm<'f> for Item {
/// // In practice, we'd use a more descriptive error type.
/// type Error = ();
///
/// fn from_form(items: &mut FormItems<'f>, strict: bool) -> Result<Item, ()> {
/// let mut field = None;
///
/// for item in items {
/// match item.key.as_str() {
/// "balloon" | "space" if field.is_none() => {
/// let decoded = item.value.url_decode().map_err(|_| ())?;
/// field = Some(decoded);
/// }
/// _ if strict => return Err(()),
/// _ => { /* allow extra value when not strict */ }
/// }
/// }
///
/// field.map(|field| Item { field }).ok_or(())
/// }
/// }
/// ```