try_create/

lib.rs

#![deny(rustdoc::broken_intra_doc_links)]

//! This crate provides a set of traits for standardizing object creation
//! patterns in Rust, covering infallible, fallible, conditional, and
//! policy-based validated construction.
//!
//! # Core Traits
//!
//! - [`IntoInner`]: A trait for types that can be converted into an "inner"
//!   value. This is often a prerequisite for construction traits.
//! - [`TryNew`]: For fallible construction that returns a [`Result`]. Use this
//!   when creating an instance might fail due to validation or other reasons.
//! - [`New`]: For infallible construction. If creation cannot logically fail
//!   (or if failure should deterministically panic), use this trait.
//!
//! # Advanced Construction Patterns
//!
//! - [`ConditionallyCreate`]: Provides a `create_conditionally` method that
//!   behaves differently in debug and release builds. In debug, it uses
//!   `TryNew::try_new().expect()`, panicking on failure. In release, it uses
//!   `New::new()`. This is useful for enforcing stricter checks during
//!   development.
//! - [`ValidationPolicy`]: Defines a contract for validation logic. A policy
//!   specifies how a value should be validated and what error type is
//!   returned upon failure. This allows for reusable validation strategies.
//! - [`TryNewValidated`]: Extends [`TryNew`] by associating a specific
//!   [`ValidationPolicy`] with the type. The [`TryNewValidated::try_new_validated`] method
//!   first applies the policy and then, if successful, proceeds with the
//!   underlying [`TryNew`] construction logic. This enables a two-phase
//!   construction process: external validation followed by internal creation.
//!
//! # Features
//!
//! - `std`: (Enabled by default) When enabled, the `Error` associated type
//!   for [`TryNew`] and [`ValidationPolicy`] is bound by `std::error::Error`.
//! - If `std` is not enabled, their `Error` types are bound by `core::fmt::Debug`.
//!
//! # Examples
//!
//! See the documentation for individual traits for specific examples.

// Re-export IntoInner for convenience, as it's a supertrait of TryNew.
pub use into_inner::IntoInner;

use duplicate::duplicate_item;
use num::Complex;
use std::convert::Infallible;

/// A trait for creating new instances of a type with fallible validation.
///
/// This trait provides a standardized way to create new instances of a type from an
/// "inner" value, where the construction process might fail (e.g., due to validation rules).
/// Implementors must also implement [`IntoInner`], which defines the `InnerType` used
/// for construction.
///
/// # Associated Types
///
/// - `InnerType`: (From the [`IntoInner`] supertrait) The type of the input value used to
///   attempt creation of a new instance of `Self`.
/// - `Error`: The error type that is returned if `try_new` fails. This error type
///   must implement `std::error::Error` if the `std` feature is enabled.
///   If the `std` feature is not enabled, it must implement `core::fmt::Debug`.
///
/// # Examples
///
/// ```rust
/// use try_create::{TryNew, IntoInner};
/// # #[cfg(feature = "std")]
/// # use std::error::Error;
/// # #[cfg(feature = "std")]
/// # use core::fmt;
///
/// // Define a custom error type
/// #[derive(Debug, PartialEq)]
/// struct NotPositiveError;
///
/// // Implement Display and Error for the custom error (required if using std::error::Error)
/// # #[cfg(feature = "std")]
/// impl fmt::Display for NotPositiveError {
///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
///         write!(f, "Value must be positive")
///     }
/// }
/// # #[cfg(feature = "std")]
/// impl Error for NotPositiveError {}
/// # #[cfg(not(feature = "std"))]
/// # trait Error: core::fmt::Debug {} // Minimal Error trait for no_std
/// # #[cfg(not(feature = "std"))]
/// # impl Error for NotPositiveError {}
///
///
/// // A struct that wraps an i32, ensuring it's positive.
/// #[derive(Debug, PartialEq)]
/// struct PositiveInteger {
///     value: i32,
/// }
///
/// // Implement IntoInner to define what `InnerType` is.
/// impl IntoInner for PositiveInteger {
///     type InnerType = i32;
///
///     fn into_inner(self) -> Self::InnerType {
///         self.value
///     }
/// }
///
/// // Implement TryNew for PositiveInteger.
/// impl TryNew for PositiveInteger {
///     type Error = NotPositiveError;
///     // `InnerType` is `i32`, inherited from `IntoInner`.
///
///     fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
///         if value > 0 {
///             Ok(PositiveInteger { value })
///         } else {
///             Err(NotPositiveError)
///         }
///     }
/// }
///
/// // Usage
/// assert_eq!(PositiveInteger::try_new(10), Ok(PositiveInteger { value: 10 }));
/// assert_eq!(PositiveInteger::try_new(0), Err(NotPositiveError));
/// assert_eq!(PositiveInteger::try_new(-5), Err(NotPositiveError));
///
/// let positive_num = PositiveInteger::try_new(42).unwrap();
/// assert_eq!(positive_num.into_inner(), 42);
/// ```
pub trait TryNew: Sized + IntoInner {
    /// The error type that can be returned by the `try_new` method.
    #[cfg(feature = "std")]
    type Error: std::error::Error;
    #[cfg(not(feature = "std"))]
    type Error: core::fmt::Debug; // Added for no_std consistency

    /// Attempts to create a new instance of `Self` from `value`.
    ///
    /// # Parameters
    ///
    /// - `value`: The `InnerType` value from which to create the instance.
    ///
    /// # Returns
    ///
    /// - `Ok(Self)` if the instance is created successfully.
    /// - `Err(Self::Error)` if creation fails (e.g., due to invalid input).
    fn try_new(value: Self::InnerType) -> Result<Self, Self::Error>;
}

/// A trait for creating new instances of a type infallibly.
///
/// This trait provides a method for creating new instances of a type from `Self::InnerType`
/// (defined by the `IntoInner` supertrait).
///
/// Implementors should ensure that the [`New`] method cannot fail in a way that would
/// typically return a `Result`. If invalid input could lead to an unrecoverable state
/// or violate invariants, [`New`] should panic. For fallible construction that returns
/// a `Result`, use the [[`TryNew`]] trait.
///
/// # Examples
///
/// ```rust
/// use try_create::{New, IntoInner, TryNew}; // Assuming TryNew is used for context or comparison
/// # #[cfg(feature = "std")]
/// # use std::error::Error;
/// # #[cfg(feature = "std")]
/// # use std::fmt;
///
/// // Example struct
/// #[derive(Debug, PartialEq)]
/// struct MyType(i32);
///
/// // Define a custom error for MyType for the TryNew example
/// #[derive(Debug, PartialEq)]
/// struct MyTypeError(String);
///
/// // Implement Display and Error for MyTypeError
/// # #[cfg(feature = "std")]
/// impl fmt::Display for MyTypeError {
///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
///         write!(f, "MyTypeError: {}", self.0)
///     }
/// }
///
/// # #[cfg(feature = "std")]
/// impl Error for MyTypeError {}
/// // For no_std, MyTypeError already derives Debug, which is sufficient.
///
/// impl IntoInner for MyType {
///     type InnerType = i32;
///     fn into_inner(self) -> Self::InnerType { self.0 }
/// }
///
/// // Example TryNew (optional here, but good for context)
/// impl TryNew for MyType {
///     type Error = MyTypeError; // Use the custom error type
///     fn try_new(value: i32) -> Result<Self, Self::Error> {
///         if value < 0 { Err(MyTypeError("Value cannot be negative".to_string())) }
///         else { Ok(MyType(value)) }
///     }
/// }
///
/// impl New for MyType {
///     fn new(value: i32) -> Self {
///         if value < 0 {
///             panic!("MyType::new: Value cannot be negative");
///         }
///         MyType(value)
///     }
/// }
///
/// assert_eq!(MyType::new(10), MyType(10));
/// // The following would panic:
/// // MyType::new(-5);
/// ```
pub trait New: Sized + IntoInner {
    /// Creates a new instance of `Self` from `value`.
    ///
    /// This method is expected to be infallible in terms of returning a `Result`.
    /// If the provided `value` cannot produce a valid instance of `Self`
    /// according to the type's invariants, this method should panic.
    ///
    /// # Parameters
    ///
    /// - `value`: The `Self::InnerType` value from which to create the instance.
    fn new(value: Self::InnerType) -> Self;
}

// It's necessary to import Debug for the bound on TryNew's error type.
// This is implicitly handled if `std::error::Error` is used (as it requires Debug)
// or if `core::fmt::Debug` is directly used for no_std.
// For clarity, if you were in a module that didn't automatically have `std::fmt::Debug`
// or `core::fmt::Debug` in scope, you might need:
// use core::fmt::Debug; // or std::fmt::Debug if std is assumed

/// A trait for conditionally creating objects.
///
/// In debug mode, it uses `Self::try_new().expect()` to create an instance,
/// panicking if `try_new` returns an error.
/// In release mode, it uses `Self::new()` to create an instance.
///
/// This trait requires the type to also implement [`TryNew`] and [`New`].
/// The error associated with [`TryNew`] (`<Self as TryNew>::Error`) must implement [`Debug`].
///
/// # Example
/// ```rust
/// use try_create::{ConditionallyCreate, TryNew, New, IntoInner};
/// # // Copied from TryNew example for self-containment, adjust if shared
/// # #[cfg(feature = "std")]
/// # use std::error::Error;
/// # #[cfg(feature = "std")]
/// # use std::fmt;
///
/// #[derive(Debug, PartialEq)]
/// struct NotPositiveError;
///
/// # #[cfg(feature = "std")]
/// impl fmt::Display for NotPositiveError {
///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
///         write!(f, "Value must be positive")
///     }
/// }
/// # #[cfg(feature = "std")]
/// impl Error for NotPositiveError {}
/// # #[cfg(not(feature = "std"))]
/// # trait Error: core::fmt::Debug {} // Minimal Error trait for no_std
/// # #[cfg(not(feature = "std"))]
/// # impl Error for NotPositiveError {}
///
///
/// #[derive(Debug, PartialEq)]
/// struct PositiveInteger { value: i32 }
///
/// impl IntoInner for PositiveInteger {
///     type InnerType = i32;
///     fn into_inner(self) -> Self::InnerType { self.value }
/// }
///
/// impl TryNew for PositiveInteger {
///     type Error = NotPositiveError;
///     fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
///         if value > 0 { Ok(PositiveInteger { value }) }
///         else { Err(NotPositiveError) }
///     }
/// }
///
/// // To use ConditionallyCreate, PositiveInteger must also implement New.
/// impl New for PositiveInteger {
///     fn new(value: Self::InnerType) -> Self {
///         match Self::try_new(value) {
///             Ok(instance) => instance,
///             Err(e) => panic!("PositiveInteger::new failed for a non-positive value. Error: {:?}", e),
///         }
///     }
/// }
///
/// // Now you can call:
/// # fn example_usage() { // Encapsulate in a function for the doctest
/// let val_ok = 10;
/// let _p1 = PositiveInteger::create_conditionally(val_ok);
/// assert_eq!(_p1, PositiveInteger { value: 10 });
///
/// // In debug, this would panic (difficult to test directly in doctest without specific harness):
/// // let val_err = -5;
/// // let _p2 = PositiveInteger::create_conditionally(val_err);
/// # }
/// # example_usage();
/// ```
pub trait ConditionallyCreate: Sized + TryNew + New
where
    <Self as TryNew>::Error: core::fmt::Debug,
{
    /// Conditionally creates an instance of `Self`.
    ///
    /// # Parameters
    ///
    /// - `value`: The `Self::InnerType` value (defined by `IntoInner`)
    ///   from which to create the instance.
    ///
    /// # Panics
    ///
    /// In debug mode, this method will panic if `Self::try_new(value)`
    /// returns `Err`. The panic message will include the error's description.
    /// In release mode, the panic behavior depends on the implementation
    /// of `Self::new(value)`.
    fn create_conditionally(value: Self::InnerType) -> Self {
        #[cfg(debug_assertions)]
        {
            // In debug mode, use try_new and panic on error.
            Self::try_new(value).expect("ConditionallyCreate: try_new() failed in debug mode")
        }
        #[cfg(not(debug_assertions))]
        {
            // In release mode, use new (which is assumed to be infallible
            // or to handle failure internally, e.g., by panicking).
            Self::new(value)
        }
    }
}

// Blanket implementation of `ConditionallyCreate` for all types `T`
// that implement [`TryNew`] and [`New`], and whose `TryNew::Error` type
// implements `Debug`.
impl<T> ConditionallyCreate for T
where
    T: TryNew + New,
    <T as TryNew>::Error: core::fmt::Debug, // This bound is inherited from the trait definition
{
    // The `create_conditionally` method already has a default implementation in the trait,
    // so it doesn't need to be redefined here.
}

/// Defines a contract for validation policies.
///
/// A validation policy specifies how a value of a certain type (`Value`)
/// should be validated and what error type (`Error`) is returned upon failure.
///
/// # Associated Types
///
/// - `Value`: The type of the value to be validated.
/// - `Error`: The type of the error returned if validation fails. This error type
///   must implement [`std::error::Error`] if the `std` feature is enabled.
///   If the `std` feature is not enabled, it must implement [`core::fmt::Debug`].
pub trait ValidationPolicy {
    /// The type of the value to be validated.
    type Value;

    /// The type of the error returned if validation fails.
    #[cfg(feature = "std")]
    type Error: std::error::Error;
    #[cfg(not(feature = "std"))]
    type Error: core::fmt::Debug; // For no_std consistency

    /// Validates a value by consuming it.
    ///
    /// If validation is successful, the original value is returned.
    /// Otherwise, an error is returned.
    /// This default implementation calls [`Self::validate_ref()]`.
    fn validate(v: Self::Value) -> Result<Self::Value, Self::Error> {
        Self::validate_ref(&v)?;
        Ok(v)
    }

    /// Validates a value by reference.
    ///
    /// Returns `Ok(())` if validation is successful, otherwise an error is returned.
    /// This method must be implemented by concrete policies.
    fn validate_ref(v: &Self::Value) -> Result<(), Self::Error>;
}

/// A trait for creating new instances of a type with fallible validation,
/// where the validation logic is defined by an associated `ValidationPolicy`.
///
/// This trait extends [`TryNew`] by associating a specific [`ValidationPolicy`]
/// with the type. The [`Self::try_new_validated()`] method first uses this policy to validate
/// the input value. If validation succeeds, it then typically calls the underlying
/// `try_new` method (or a similar construction logic) to create the instance.
///
/// # Supertraits
///
/// - [`TryNew`]: Implementors of `TryNewValidated` must also implement `TryNew`.
///   This means they must define an `InnerType` (via the [`IntoInner`] supertrait of `TryNew`),
///   an `Error` type, and a `try_new` method.
pub trait TryNewValidated: TryNew {
    /// An implementor of [`ValidationPolicy`].
    /// - The `Value` associated type of this `Policy` must be the same as the
    ///   `InnerType` of `Self` (i.e., `Self::Policy::Value == <Self as IntoInner>::InnerType`).
    /// - The `Error` associated type of this `Policy` must be convertible into
    ///   `Self::Error` (the error type defined by the `TryNew` trait for `Self`).
    ///   This is ensured by the `From` bound on `Self::Error`.
    type Policy: ValidationPolicy<Value = <Self as IntoInner>::InnerType>;

    /// Attempts to create a new instance of `Self` from `value`.
    /// This method should first validate `value` using `Self::Policy::validate_ref(&value)`
    /// (or `Self::Policy::validate(value)` if consuming the value is intended by the policy).
    /// If validation is successful, it proceeds to construct the `Self` instance,
    /// typically by calling `Self::try_new(value)`.
    /// If validation fails, the error from the policy is converted into `Self::Error` and returned.
    /// If construction after successful validation fails, the error from `Self::try_new` is returned.
    fn try_new_validated(
        value: <Self as IntoInner>::InnerType,
    ) -> Result<Self, Self::Error>
    where
        Self: Sized;
}

#[duplicate_item(
    T;
    [f32];
    [f64];
    [i8];
    [i16];
    [i32];
    [i64];
    [i128];
    [u8];
    [u16];
    [u32];
    [u64];
    [u128];
    [isize];
    [usize];
    [bool];
    [char];
    [Complex<f32>];
    [Complex<f64>];
)]
impl New for T {
    /// Creates a new instance of the fundamental type from the given value.
    ///
    /// This implementation is infallible and simply returns the input value.
    fn new(value: Self::InnerType) -> Self {
        value
    }
}

#[duplicate_item(
    T;
    [f32];
    [f64];
    [i8];
    [i16];
    [i32];
    [i64];
    [i128];
    [u8];
    [u16];
    [u32];
    [u64];
    [u128];
    [isize];
    [usize];
    [bool];
    [char];
    [Complex<f32>];
    [Complex<f64>];
)]
impl TryNew for T {
    type Error = Infallible; // No error type needed for fundamental types

    /// Creates a new instance of the fundamental type from the given value.
    ///
    /// This implementation is infallible and simply returns the input value.
    fn try_new(value: Self::InnerType) -> Result<Self, Infallible> {
        Ok(value)
    }
}

#[cfg(test)]
mod tests {
    use super::*; // Import traits from the parent module (your library)

    // --- Test setup for PositiveInteger ---
    #[derive(Debug, PartialEq)]
    struct TestNotPositiveError;

    // Implement Display and Error for TestNotPositiveError for std builds
    #[cfg(feature = "std")]
    impl std::fmt::Display for TestNotPositiveError {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "Value must be positive (Test Error)")
        }
    }

    #[cfg(feature = "std")]
    impl std::error::Error for TestNotPositiveError {}

    // For no_std, deriving Debug is sufficient as per TryNew's Error bound.

    #[derive(Debug, PartialEq)]
    struct TestPositiveInteger {
        value: i32,
    }

    impl IntoInner for TestPositiveInteger {
        type InnerType = i32;
        fn into_inner(self) -> Self::InnerType {
            self.value
        }
    }

    impl TryNew for TestPositiveInteger {
        type Error = TestNotPositiveError;
        fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
            if value > 0 {
                Ok(TestPositiveInteger { value })
            } else {
                Err(TestNotPositiveError)
            }
        }
    }

    impl New for TestPositiveInteger {
        fn new(value: Self::InnerType) -> Self {
            // Consistent with the example: panic if try_new would fail
            match Self::try_new(value) {
                Ok(instance) => instance,
                Err(_) => panic!("TestPositiveInteger::new: Value must be positive."),
            }
        }
    }

    mod try_new {
        use super::*; // Import traits from the parent module (your library)

        // --- Tests for PositiveInteger ---
        #[test]
        fn test_positive_integer_try_new_ok() {
            assert_eq!(
                TestPositiveInteger::try_new(10),
                Ok(TestPositiveInteger { value: 10 })
            );
        }

        #[test]
        fn test_positive_integer_try_new_err_zero() {
            assert_eq!(TestPositiveInteger::try_new(0), Err(TestNotPositiveError));
        }

        #[test]
        fn test_positive_integer_try_new_err_negative() {
            assert_eq!(TestPositiveInteger::try_new(-5), Err(TestNotPositiveError));
        }

        // --- Tests for MyType ---
        #[test]
        fn test_my_type_try_new_ok() {
            assert_eq!(TestMyType::try_new(5), Ok(TestMyType(5)));
        }

        #[test]
        fn test_my_type_try_new_err() {
            assert_eq!(
                TestMyType::try_new(-1),
                Err(TestMyTypeError("Value cannot be negative".to_string()))
            );
        }
    } // mod try_new

    mod new {
        use super::*; // Import traits from the parent module (your library)

        #[test]
        fn test_positive_integer_new_ok() {
            assert_eq!(
                TestPositiveInteger::new(10),
                TestPositiveInteger { value: 10 }
            );
        }

        #[test]
        #[should_panic(expected = "TestPositiveInteger::new: Value must be positive.")]
        fn test_positive_integer_new_panic_zero() {
            TestPositiveInteger::new(0);
        }

        #[test]
        #[should_panic(expected = "TestPositiveInteger::new: Value must be positive.")]
        fn test_positive_integer_new_panic_negative() {
            TestPositiveInteger::new(-10);
        }

        #[test]
        fn test_my_type_new_ok() {
            assert_eq!(TestMyType::new(100), TestMyType(100));
        }

        #[test]
        #[should_panic(expected = "TestMyType::new: Value cannot be negative")]
        fn test_my_type_new_panic() {
            TestMyType::new(-1);
        }
    } // mod new

    mod into_inner {
        use super::*;

        #[test]
        fn test_positive_integer_into_inner() {
            let pi = TestPositiveInteger::new(42);
            assert_eq!(pi.into_inner(), 42);
        }

        #[test]
        fn test_my_type_into_inner() {
            let mt = TestMyType::new(7);
            assert_eq!(mt.into_inner(), 7);
        }
    }

    // --- Test setup for MyType (from New example) ---
    #[derive(Debug, PartialEq)]
    struct TestMyTypeError(String);

    #[cfg(feature = "std")]
    impl std::fmt::Display for TestMyTypeError {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "TestMyTypeError: {}", self.0)
        }
    }

    #[cfg(feature = "std")]
    impl std::error::Error for TestMyTypeError {}

    #[derive(Debug, PartialEq)]
    struct TestMyType(i32);

    impl IntoInner for TestMyType {
        type InnerType = i32;
        fn into_inner(self) -> Self::InnerType {
            self.0
        }
    }

    impl TryNew for TestMyType {
        type Error = TestMyTypeError;
        fn try_new(value: i32) -> Result<Self, Self::Error> {
            if value < 0 {
                Err(TestMyTypeError("Value cannot be negative".to_string()))
            } else {
                Ok(TestMyType(value))
            }
        }
    }

    impl New for TestMyType {
        fn new(value: i32) -> Self {
            if value < 0 {
                panic!("TestMyType::new: Value cannot be negative");
            }
            TestMyType(value)
        }
    }

    mod conditionally_create {
        use super::*;

        #[test]
        #[cfg(debug_assertions)] // This test specifically targets the debug behavior
        fn test_positive_integer_conditionally_create_debug_ok() {
            assert_eq!(
                TestPositiveInteger::create_conditionally(10),
                TestPositiveInteger { value: 10 }
            );
        }

        #[test]
        #[cfg(debug_assertions)] // This test specifically targets the debug behavior
        #[should_panic(expected = "ConditionallyCreate: try_new() failed in debug mode")]
        fn test_positive_integer_conditionally_create_debug_panic() {
            TestPositiveInteger::create_conditionally(-5);
        }

        #[test]
        #[cfg(not(debug_assertions))] // This test specifically targets the release behavior
        fn test_positive_integer_conditionally_create_release_ok() {
            assert_eq!(
                TestPositiveInteger::create_conditionally(10),
                TestPositiveInteger { value: 10 }
            );
        }

        #[test]
        #[cfg(not(debug_assertions))] // This test specifically targets the release behavior
        #[should_panic(expected = "TestPositiveInteger::new: Value must be positive.")]
        fn test_positive_integer_conditionally_create_release_panic() {
            // In release, create_conditionally calls new(), which for TestPositiveInteger panics.
            TestPositiveInteger::create_conditionally(-5);
        }

        #[test]
        #[cfg(debug_assertions)]
        fn test_my_type_conditionally_create_debug_ok() {
            assert_eq!(TestMyType::create_conditionally(20), TestMyType(20));
        }

        #[test]
        #[cfg(debug_assertions)]
        #[should_panic(expected = "ConditionallyCreate: try_new() failed in debug mode")]
        fn test_my_type_conditionally_create_debug_panic() {
            TestMyType::create_conditionally(-3);
        }

        #[test]
        #[cfg(not(debug_assertions))]
        fn test_my_type_conditionally_create_release_ok() {
            assert_eq!(TestMyType::create_conditionally(20), TestMyType(20));
        }

        #[test]
        #[cfg(not(debug_assertions))]
        #[should_panic(expected = "TestMyType::new: Value cannot be negative")]
        fn test_my_type_conditionally_create_release_panic() {
            TestMyType::create_conditionally(-3);
        }
    }

    mod try_new_validated {
        use super::*;

        // --- Test setup for ValidationPolicy and TryNewValidated ---

        // 1. Define a concrete ValidationPolicy
        #[derive(Debug, PartialEq)]
        struct TestPolicyError(String);

        #[cfg(feature = "std")]
        impl std::fmt::Display for TestPolicyError {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                write!(f, "TestPolicyError: {}", self.0)
            }
        }

        #[cfg(feature = "std")]
        impl std::error::Error for TestPolicyError {}

        struct MinValuePolicy {
            min_value: i32,
        }

        impl ValidationPolicy for MinValuePolicy {
            type Value = i32;
            type Error = TestPolicyError;

            fn validate_ref(v: &Self::Value) -> Result<(), Self::Error> {
                // For this test, let's imagine a global or static policy.
                // A real policy might take its configuration (e.g., min_value) at construction.
                // For simplicity, we'll hardcode a value or use a fixed one.
                // Let's make it simple: value must be >= 0 for this policy.
                if *v >= 0 {
                    Ok(())
                } else {
                    Err(TestPolicyError("Value must be non-negative.".to_string()))
                }
            }
            // `validate` method uses the default implementation.
        }

        // 2. Define a concrete type that implements TryNewValidated

        #[derive(Debug, PartialEq)]
        enum TestValidatedTypeError {
            Policy(TestPolicyError),
            Construction(String),
        }

        #[cfg(feature = "std")]
        impl std::fmt::Display for TestValidatedTypeError {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                match self {
                    TestValidatedTypeError::Policy(e) => write!(f, "Policy error: {}", e),
                    TestValidatedTypeError::Construction(s) => {
                        write!(f, "Construction error: {}", s)
                    }
                }
            }
        }

        #[cfg(feature = "std")]
        impl std::error::Error for TestValidatedTypeError {
            fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                match self {
                    TestValidatedTypeError::Policy(e) => Some(e),
                    TestValidatedTypeError::Construction(_) => None,
                }
            }
        }
        // No-std: Debug is derived.

        impl From<TestPolicyError> for TestValidatedTypeError {
            fn from(e: TestPolicyError) -> Self {
                TestValidatedTypeError::Policy(e)
            }
        }

        #[derive(Debug, PartialEq)]
        struct MyValidatedType {
            value: i32,
        }

        impl IntoInner for MyValidatedType {
            type InnerType = i32;
            fn into_inner(self) -> Self::InnerType {
                self.value
            }
        }

        impl TryNew for MyValidatedType {
            type Error = TestValidatedTypeError;

            fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
                // Inner construction logic: value must be > 10
                if value > 10 {
                    Ok(MyValidatedType { value })
                } else {
                    Err(TestValidatedTypeError::Construction(
                        "Value must be greater than 10 for construction.".to_string(),
                    ))
                }
            }
        }

        impl TryNewValidated for MyValidatedType {
            type Policy = MinValuePolicy; // Using the policy defined above

            fn try_new_validated(value: <Self as IntoInner>::InnerType) -> Result<Self, Self::Error>
            where
                Self: Sized,
                Self::Error: From<<Self::Policy as ValidationPolicy>::Error>,
            {
                // 1. Validate using the policy. The `?` handles error conversion.
                Self::Policy::validate_ref(&value)?;

                // 2. If policy validation passes, proceed with `try_new`.
                Self::try_new(value)
            }
        }

        // --- Tests for ValidationPolicy (using MinValuePolicy) ---
        #[test]
        fn validation_policy_validate_ref_ok() {
            assert_eq!(MinValuePolicy::validate_ref(&5), Ok(()));
            assert_eq!(MinValuePolicy::validate_ref(&0), Ok(()));
        }

        #[test]
        fn validation_policy_validate_ref_err() {
            assert_eq!(
                MinValuePolicy::validate_ref(&-5),
                Err(TestPolicyError("Value must be non-negative.".to_string()))
            );
        }

        #[test]
        fn validation_policy_validate_consuming_ok() {
            // Tests the default `validate` method
            assert_eq!(MinValuePolicy::validate(5), Ok(5));
            assert_eq!(MinValuePolicy::validate(0), Ok(0));
        }

        #[test]
        fn validation_policy_validate_consuming_err() {
            // Tests the default `validate` method
            assert_eq!(
                MinValuePolicy::validate(-5),
                Err(TestPolicyError("Value must be non-negative.".to_string()))
            );
        }

        // --- Tests for TryNewValidated (using MyValidatedType) ---
        #[test]
        fn try_new_validated_success() {
            // Policy (>=0) passes for 15. try_new (>10) passes for 15.
            assert_eq!(
                MyValidatedType::try_new_validated(15),
                Ok(MyValidatedType { value: 15 })
            );
        }

        #[test]
        fn try_new_validated_policy_fail() {
            // Policy (>=0) fails for -5.
            let result = MyValidatedType::try_new_validated(-5);
            assert!(matches!(result, Err(TestValidatedTypeError::Policy(..))));
        }

        #[test]
        fn try_new_validated_construction_fail_after_policy_pass() {
            // Policy (>=0) passes for 5. try_new (>10) fails for 5.
            let result = MyValidatedType::try_new_validated(5);
            assert!(matches!(
                result,
                Err(TestValidatedTypeError::Construction(..))
            ));
        }

        #[test]
        fn try_new_validated_policy_pass_construction_pass_edge() {
            // Policy (>=0) passes for 11. try_new (>10) passes for 11.
            assert_eq!(
                MyValidatedType::try_new_validated(11),
                Ok(MyValidatedType { value: 11 })
            );
        }

        #[test]
        fn try_new_validated_policy_pass_construction_fail_edge() {
            // Policy (>=0) passes for 10. try_new (>10) fails for 10.
            let result = MyValidatedType::try_new_validated(10);
            assert!(matches!(
                result,
                Err(TestValidatedTypeError::Construction(..))
            ));
        }
    }
}