Ignore fields with validation instruction with nest_all_fields

[LeMarwin]

Problem:

• nest_all_fields adds #[validate(nested)] for all fields, making the macro unusable in a situation where some fields have specific validation instruction.
• adding #[validate(skip)] along with other #[validate] attrs skips field validation entirely

Solution: only add #[validate(nested)] to fields that don't have other validation attributes.

Essentially this makes nest_all_fields semantically closer to validate_all_fields but inserts #[validate(nested)] if no validation instruction is present. But renaming it is a breaking change

Concrete example of what I wanted to enable:

#[derive(Validate)]
#[validate(nest_all_fields)]
struct Test {
    #[validate(skip)]
    _a: Nested,
    b: NestedValidated,
    #[validate(range(min = 10))]
    c: u8,
}

It's obvious that this should work: we gave explicit validation instruction, so there's no need to insert #[validate(nested)]

Moreover, this PR enables something like this:

#[derive(Validate)]
#[validate(nest_all_fields)]
struct Test {
    #[validate(skip)]
    _a: Nested,
    b: NestedValidated,
    #[validate(custom(function = "validate_c"))]
    c: NestedValidated,
}

which follows the same logic as the previous example, and is quite handy in general

[Keats]

How about things like that https://github.com/Keats/validator/blob/master/validator_derive_tests/tests/nested.rs#L273-L336 eg containers where you have length validation but also want to validate the values

[LeMarwin]

How about things like that https://github.com/Keats/validator/blob/master/validator_derive_tests/tests/nested.rs#L273-L336 eg containers where you have length validation but also want to validate the values

Okay, I missed that usecase since tests didn't break.

I guess that makes this PR a breaking change and thus undesirable, since if someone relied on #[validate(nest_all_fields)] to insert #[validate(nested)] for containers that would no longer be the case and this change would be silent.

How about adding validate_all_fields as a container-level attribute? So that the compiler actually checks that all fields have some kind of validation instruction and adds #[validate(nested)] if none is present?