Struct update rest pattern syntax support for named bindings

[jplatte]

As an extension to #199, it would be nice if SQLx supported the following for databases compatible with named bindings:

query!("INSERT INTO foo (id, a, b, c) VALUES (:id, :a, :b, :c)", id=new_id, ..foo)

// shorthand for
query!(
    "INSERT INTO foo (id, a, b, c) VALUES (:id, :a, :b, :c)",
    id=new_id,
    a=foo.a,
    b=foo.b,
    c=foo.c,
)

[malobre]

I love this suggestion !

In the context of web APIs, I often end up with a struct containing all the request parameters, something like this:

struct NewUserParameters {
  email: String,
  password: String,
  // and many more fields
}

Followed by some queries such as:

sqlx::query_as(concat!(
    "INSERT INTO users",
    " (email, password, etc..)",
    " VALUES ($1, $2, etc..)",
    " RETURNING *"
))
.bind(new_user_parameters.email)
.bind(new_user_parameters.password)
// etc

The suggested syntax would greatly reduce repetitions:

sqlx::query_as!(concat!(
    "INSERT INTO users",
    " (email, password, etc..)",
    " VALUES (:email, :password, etc..)",
    " RETURNING *"
), ..new_user_parameters); // Done !