structural-typing 0.1.9

Type-level field presence tracking for Rust structs, inspired by TypeScript
Documentation 
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//! Runtime field access for structural types.
use core::marker::PhantomData;

use crate::presence::InferPresence;

/// Uniform field access for Present, Optional, or Absent.
pub trait Access<T>: InferPresence<T> {
    /// Compile-time constant indicating if this field is absent.
    const IS_ABSENT: bool;

    /// Remainder type when extracting Self from Source.
    type RemainderFrom<Source: Access<T>>: Access<T>;

    /// Get field reference if present.
    fn get(&self) -> Option<&T>;
    /// Get mutable field reference if present.
    fn get_mut(&mut self) -> Option<&mut T>;
    /// Consume and convert to Option.
    fn into_option(self) -> Option<T>;

    /// Construct from a value.
    fn from_value(value: T) -> (Self, Self::RemainderFrom<T>)
    where
        Self: Sized;

    /// Try to construct from an option.
    ///
    /// # Errors
    ///
    /// Returns `Err` with the original `Option<T>` if the conversion cannot be performed.
    #[allow(clippy::type_complexity)]
    fn try_from_option(
        value: Option<T>,
    ) -> Result<(Self, Self::RemainderFrom<Option<T>>), Option<T>>
    where
        Self: Sized;
}

/// Helper function to check if a field is absent (used by serde).
pub fn is_absent<A: Access<T>, T>(_value: &A) -> bool {
    A::IS_ABSENT
}

impl<T> Access<T> for PhantomData<T> {
    const IS_ABSENT: bool = true;

    type RemainderFrom<Source: Access<T>> = Source;

    #[inline]
    fn get(&self) -> Option<&T> {
        None
    }
    #[inline]
    fn get_mut(&mut self) -> Option<&mut T> {
        None
    }
    #[inline]
    fn into_option(self) -> Option<T> {
        None
    }

    #[inline]
    fn from_value(value: T) -> (Self, Self::RemainderFrom<T>) {
        (PhantomData, value)
    }

    #[inline]
    fn try_from_option(
        value: Option<T>,
    ) -> Result<(Self, Self::RemainderFrom<Option<T>>), Option<T>> {
        Ok((PhantomData, value))
    }
}

impl<T: Sized> Access<T> for T {
    const IS_ABSENT: bool = false;

    type RemainderFrom<Source: Access<T>> = PhantomData<T>;

    #[inline]
    fn get(&self) -> Option<&T> {
        Some(self)
    }
    #[inline]
    fn get_mut(&mut self) -> Option<&mut T> {
        Some(self)
    }
    #[inline]
    fn into_option(self) -> Option<T> {
        Some(self)
    }

    #[inline]
    fn from_value(value: T) -> (Self, Self::RemainderFrom<T>) {
        (value, PhantomData)
    }

    #[inline]
    fn try_from_option(
        value: Option<T>,
    ) -> Result<(Self, Self::RemainderFrom<Option<T>>), Option<T>> {
        match value {
            Some(v) => Ok((v, PhantomData)),
            None => Err(None),
        }
    }
}

impl<T> Access<T> for Option<T> {
    const IS_ABSENT: bool = false;

    type RemainderFrom<Source: Access<T>> = PhantomData<T>;

    #[inline]
    fn get(&self) -> Option<&T> {
        self.as_ref()
    }
    #[inline]
    fn get_mut(&mut self) -> Option<&mut T> {
        self.as_mut()
    }
    #[inline]
    fn into_option(self) -> Option<T> {
        self
    }

    #[inline]
    fn from_value(value: T) -> (Self, Self::RemainderFrom<T>) {
        (Some(value), PhantomData)
    }

    #[inline]
    fn try_from_option(
        value: Option<T>,
    ) -> Result<(Self, Self::RemainderFrom<Option<T>>), Option<T>> {
        Ok((value, PhantomData))
    }
}