Improved compiler errors when feature flags are missing

[ranger-ross]

This PR adds improved compiler errors when features flags are not enabled.

For example:
Given this following struct,

#[derive(Validate)]
struct User<'a> {
    #[garde(email)]
    email: &'a str,
}

without the email feature flag enabled, the following error message is output.

error: Validation rule `email` not found. Did you forget to add the `email` feature flag?
  --> src/main.rs:23:13
   |
23 |     #[garde(email)]
   |             ^^^^^

---

See original issue here #9

[jprochazk]

There's also the question of how to test this. It doesn't seem like trybuild exposes the ability to change the feature set used to compile a specific file. Do you have any ideas?

[ranger-ross]

There's also the question of how to test this. It doesn't seem like trybuild exposes the ability to change the feature set used to compile a specific file. Do you have any ideas?

I also have this problem in actix-web-validation and I have not found a nice solution 😅

I think one way would be to use trybuild in combination with #[cfg_attr(not(feature = "foo"), ignore)] added to the tests to enable/disable tests depending on the enabled features flags. But this requires running cargo test multiple times with different --feature <flag> combinations, which I don't love 🙁