Field value is passed to a custom function by value instead of by reference, contradicting with `README.md`.

[your-diary]

According to README.md (emphasis mine):

custom

Calls one of your functions to perform a custom validation. The field reference will be given as a parameter to the function, which should return a Result<(), ValidationError>.

However, as far as I tested:

• For String field, a custom function receives &String, which is as expected.

• However, for usize field, a custom function receives usize instead of &usize, contradicting with the snippet above.

Minimal Working Example

use validator::{Validate, ValidationError};

//compile error (changing `&usize` to `usize` fixes the error)
fn validate_usize(_: &usize) -> Result<(), ValidationError> {
    Ok(())
}

//OK (inconsistent with the function above)
fn validate_string(_: &str) -> Result<(), ValidationError> {
    Ok(())
}

#[derive(Debug, Validate)]
struct S {
    #[validate(custom(function = "validate_usize"))]
    i: usize,
    #[validate(custom(function = "validate_string"))]
    j: String,
}

fn main() {
    let s = S {
        i: 10,
        j: "hello".to_string(),
    };
    assert!(s.validate().is_ok());
}

   Compiling a v0.1.0 (/Users/user/a)
error[E0308]: mismatched types
  --> src/main.rs:13:17
   |
13 | #[derive(Debug, Validate)]
   |                 ^^^^^^^^ expected `&usize`, found `usize`
14 | struct S {
15 |     #[validate(custom(function = "validate_usize"))]
   |                                  ---------------- arguments to this function are incorrect
   |
note: function defined here
  --> src/main.rs:4:4
   |
4  | fn validate_usize(_: &usize) -> Result<(), ValidationError> {
   |    ^^^^^^^^^^^^^^ ---------
   = note: this error originates in the derive macro `Validate` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0308`.
error: could not compile `a` (bin "a") due to 1 previous error

[Keats]

The docs should be updated: fields that can be copied cheaply (essentially all numbers) are passed by values, the rest by reference.