Rust Validator

How `ValidationError`, `ValidationErrorsKind`, and `ValidationErrors` work together

ValidationError: This represents a single validation error.
```
pub struct ValidationError {
    pub code: Cow<'static, str>,
    pub message: Option<Cow<'static, str>>,
    pub params: HashMap<Cow<'static, str>, Value>,
}
```
- code: A short identifier for the type of error.
- message: An optional human-readable description.
- params: Additional context or data about the error.
The Display implementation for ValidationError shows that:
- If a message is provided, it’s used directly.
- Otherwise, it falls back to displaying the code and params.
ValidationErrorsKind: This enum represents different types of validation errors:
```
pub enum ValidationErrorsKind {
    Field(Vec<ValidationError>),
    Struct(Box<ValidationErrors>),
    List(BTreeMap<usize, Box<ValidationErrors>>),
}
```
- Field: For errors on a single field.
- Struct: For errors in a nested struct.
- List: For errors in a vector of nested structs.
ValidationErrors: This is a collection of all errors found during validation.
```
pub struct ValidationErrors(pub HashMap<&'static str, ValidationErrorsKind>);
```
It maps field names to their respective ValidationErrorsKind.

How they work together:

Field-level validation: When validating a single field, you create a ValidationError. If multiple errors occur on a field, they’re collected into a Vec<ValidationError> and stored as ValidationErrorsKind::Field.
Struct-level validation: For a struct, you create a ValidationErrors instance. Each field’s errors are added to this instance, with the field name as the key and the appropriate ValidationErrorsKind as the value.
Nested struct validation: When validating a nested struct, its errors are stored as ValidationErrorsKind::Struct, which contains a boxed ValidationErrors.
List validation: For a vector of structs, errors are stored as hValidationErrorsKind::List, mapping the index to a boxed ValidationErrors for each invalid item.
Error Display: The display_errors function shows how these types work together to create a readable error message:
- It recursively traverses the error structure, building a path to show where each error
See also validator/src/display_impl.rs for more insights on how errors are displayed.