A better panic message for assert_body_matches

[scrabsha]

The following code snippet shows is an obvious incorrect call to assert_body_matches:

use restest::assert_body_matches;

struct User {
    first_name: String,
    last_name: String,
}

#[test]
fn basic_message() {
    assert_body_matches! {
        User {
            first_name: "Grace".to_string(),
            last_name: "Hopper".to_string(),
        },
        User {
            first_name: _,
            last_name: "Lovelace",
        },
    };
}

As of 24fc843, it generates the following panic message:

thread 'basic_message' panicked at 'Matching failed', tests/error_message.rs:10:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is... less than ideal.

What could be done

What i'd love to see is a way to see is something like this:

thread 'basic_message' panicked at 'Failed to match the request output with the provided pattern:
               Value returned:           |           Pattern expected:
                                         |
   |   {                                 |  {
   |       first_name: "Grace",          |      first_name: _,
!! |       last_name: "Hopper",          |      last_name: "Lovelace",
   |   }                                 |  }
', tests/error_message.rs:5:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note that this output JSON-ifies the pattern. We could Rust-ify the value instead:

               Value returned:           |           Pattern expected:
                                         |
   |   User {                            |  User {
   |       first_name: "Grace",          |      first_name: _,
!! |       last_name: "Hopper",          |      last_name: "Lovelace",
   |   }                                 |  }

Implementation details

We can gather a set of "constraints" of the value returned by the server by using a custom Serializer, which could, from the previous example, return us the following constraints:

.first_name == "Grace"
.last_name == "Hopper"

We could gather the constraints from the pattern by defining a custom [Visitor], which could extract similar constraints from the pattern:

.first_name: _
.last_name: "Lovelace"

We could then visit each constraint from the pattern, get the corresponding constraint in the value-side, compare them (write the !! if required), then pretty-print both on each side.