Support for const args in custom validators

[Samuel-B-D]

I feel like it would be useful to have the ability to have const args for the validator.
Example scenario / use:

use validator::{Validate, ValidateArgs, ValidationError};

fn vec_string_max_len(value: &Vec<String>, const_arg: u64) -> Result<(), ValidationError> {
    for s in value.iter() {
        if s.length() > arg {
            return Err(ValidationError::new("String too long"));
        }
    }
    Ok(())
}

#[derive(Debug, Validate)]
struct TestStruct {
    #[validate(length(max = 10), custom(function = "vec_string_max_len", const_arg = "64"))]
    many_strings: Vec<String>,
}

let test_struct: TestStruct = [...]
test_struct.validate().is_ok();
// many_strings contain at most 10 strings, and each string is at most 64 characters long

That way, such validation can be defined in the model and don't need to be passed around each time you validate.
As of now, unless I am missing something, I don't really see a way to make such a validator in a way which doesn't require either passing along data in validate_args(...) or writing a validate function specific to that field only.

The suggested syntax above is to mirror the syntax of the current validate-time args, I don't really like it but that's a start 😅

[prk3]

You can achieve this now by using const generics. It will work only with ints and bools though.

use validator::{Validate, ValidateArgs, ValidationError};

fn vec_string_max_len<const N: usize>(value: &Vec<String>) -> Result<(), ValidationError> {
    for s in value.iter() {
        if s.len() > N {
            return Err(ValidationError::new("String too long"));
        }
    }
    Ok(())
}

#[derive(Debug, Validate)]
struct TestStruct {
    #[validate(length(max = 10), custom(function = "vec_string_max_len::<64>"))]
    many_strings: Vec<String>,
}

fn main() {
    let test_struct = TestStruct {
        many_strings: vec!["hello".into(), "world".into()],
    };
    test_struct.validate().is_ok();
}