Extract named query parameter

[niklasf]

Feature Request

Extract named parameters from query strings (?param1=foo&param2=bar).

Detailed Description

tide supports query extraction (#14) using UrlQuery. For example:

async fn hello_world(UrlQuery(q): UrlQuery<String>) -> String {
    q // "param1=foo&param2=bar" (how to extract the value of param1?)
}

UrlQuery<T> parses T using FromStr, but if we want to parse the query and extract individual parameters we're on our own.

Context

Working with query parameters is common in all kinds of applications.

Possible Implementation

Looking at tide itself, it could be implemented similarly to named segments. Alternatives from other frameworks:

• Rocket: Derivable FromForm trait. Function parameter is marked as query string in route declaration.

  #[get("/hello?<q>")]
  fn hello_world(q: &MyTypeThatImplsFromForm) -> /* ... */

• actix-web: Parameter Query<T: Deserialize> newtype using serde_urlencoded.
• tower-web: Derivable Extract trait. Function parameter must be named query_string.

#[get("/query-string")]
fn hello_world(&self, query_string: Option<Foo>) -> /* ... */

The actix-web approach might be best for Tide, because no macro magic is involved. (Well ... except that in Serde).

[yoshuawuyts]

Related: #108

[aturon]

This should be easier to explore now that #156 has landed!

[secretfader]

#175 addresses this feature.