Conversion trait for parameters used in uri! invocations.

Overview

In addition to implementing UriDisplay, to use a custom type in a uri! expression, the FromUriParam trait must be implemented. The UriDisplay derive automatically generates identity implementations of FromUriParam, so in the majority of cases, as with UriDisplay, this trait is never implemented manually.

In the rare case that UriDisplay is implemented manually, this trait, too, must be implemented explicitly. In the majority of cases, implementation can be automated. Rocket provides the impl_from_uri_param_identity macro to generate the identity implementations automatically. For a type T, these are:

• impl<P: UriPart> FromUriParam<P, T> for T
• impl<'x, P: UriPart> FromUriParam<P, &'x T> for T
• impl<'x, P: UriPart> FromUriParam<P, &'x mut T> for T

See impl_from_uri_param_identity for usage details.

Code Generation

This trait is invoked once per expression passed into a uri! invocation. In particular, for a route URI parameter of type T and a user-supplied expression of type S, <T as FromUriParam<S>>::from_uri_param is invoked. The returned value is used in place of the user’s value and rendered using its UriDisplay implementation.

This trait allows types that differ from the route URI parameter’s types to be used in their place at no cost. For instance, the following implementation, provided by Rocket, allows an &str to be used in a uri! invocation for route URI parameters declared as String:

impl<'a, P: UriPart> FromUriParam<P, &'a str> for String {
    type Target = &'a str;
}

Because the FromUriParam::Target type is the same as the input type, the conversion is a no-op and free of cost, allowing an &str to be used in place of a String without penalty. A similar no-op conversion exists for &RawStr:

impl<'a, 'b, P: UriPart> FromUriParam<P, &'a str> for &'b RawStr {
    type Target = &'a str;
}

Provided Implementations

The following types have identity implementations:

• String, i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, f32, f64, bool, IpAddr, Ipv4Addr, Ipv6Addr, &str, &RawStr, Cow<str>

The following conversions are implemented, allowing a value of the type on the left to be used when a type on the right is expected by a route:

• &str to String
• &str to RawStr
• String to &str
• String to RawStr
• T to Option<T>
• T to Result<T, E>
• T to Form<T>

The following types have identity implementations only in Path:

• &Path, PathBuf

The following conversions are implemented only in Path:

• &str to &Path
• &str to PathBuf
• PathBuf to &Path

See Foreign Impls for all provided implementations.

Implementing

This trait should only be implemented when you’d like to allow a type different from the route’s declared type to be used in its place in a uri! invocation. For instance, if the route has a type of T and you’d like to use a type of S in a uri! invocation, you’d implement FromUriParam<P, T> for S where P is Path for conversions valid in the path part of a URI, Uri for conversions valid in the query part of a URI, or P: UriPart when a conversion is valid in either case.

This is typically only warranted for owned-value types with corresponding reference types: String and &str, for instance. In this case, it’s desirable to allow an &str to be used in place of a String.

When implementing FromUriParam, be aware that Rocket will use the UriDisplay implementation of FromUriParam::Target, not of the source type. Incorrect implementations can result in creating unsafe URIs.

Example

The following example implements FromUriParam<Query, (&str, &str)> for a User type. The implementation allows an (&str, &str) type to be used in a uri! invocation where a User type is expected in the query part of the URI.

use std::fmt;

use rocket::http::RawStr;
use rocket::http::uri::{Formatter, UriDisplay, FromUriParam, Query};

#[derive(FromForm)]
struct User<'a> {
    name: &'a RawStr,
    nickname: String,
}

impl<'a> UriDisplay<Query> for User<'a> {
    fn fmt(&self, f: &mut Formatter<Query>) -> fmt::Result {
        f.write_named_value("name", &self.name)?;
        f.write_named_value("nickname", &self.nickname)
    }
}

impl<'a, 'b> FromUriParam<Query, (&'a str, &'b str)> for User<'a> {
    type Target = User<'a>;

    fn from_uri_param((name, nickname): (&'a str, &'b str)) -> User<'a> {
        User { name: name.into(), nickname: nickname.to_string() }
    }
}

With these implementations, the following typechecks:

use rocket::http::RawStr;
use rocket::request::Form;

#[post("/<name>?<user..>")]
fn some_route(name: &RawStr, user: Form<User>)  { /* .. */ }

let uri = uri!(some_route: name = "hey", user = ("Robert Mike", "Bob"));
assert_eq!(uri.path(), "/hey");
assert_eq!(uri.query(), Some("name=Robert%20Mike&nickname=Bob"));