serde_cursor 0.4.0

use core::marker::PhantomData;

use crate::const_str::ConstStr;

/// A single path segment of the type-level path constructed by [`Cursor!`](macro@crate::Cursor).
///
/// See the [crate-level](crate) documentation for more info.
pub enum PathSegment {
    /// Path segment representing the field of a map.
    ///
    /// ```txt
    /// Cursor!(field[0])
    ///         ^^^^^
    /// ```
    Field(&'static str),
    /// Path segment representing an index into a sequence.
    ///
    /// ```txt
    /// Cursor!(field[0])
    ///               ^
    /// ```
    Index(usize),
}

/// Equivalent to `const PATH_SEGMENT: PathSegment`, except it works on stable Rust.
pub trait ConstPathSegment {
    const VALUE: PathSegment;
}

/// Accessing a specific field of a map.
/// Represents the `package` in `Cursor!(package[0])`.
///
/// ```txt
/// Cursor!(field[0])
///         ^^^^^
/// ```
///
/// # The `Z` const-generic
///
/// It is always `false`.
///
/// This const-generic only exists so we can create an arbitrary amount of string literals
/// whose spans we associate with the user's input when they are writing the Cursor! macro
///
/// e.g. we want to syntax highlight the "dev-dependencies" as a single string:
///
/// ```txt
/// Cursor!(dev-dependencies)
/// ```
///
/// But that is made up of 3 tokens, and each of their `Span`s needs to associate with
/// a concrete token.
///
/// ```txt
/// Field<..., { ["", "", ""]; false }>
///               ^^ "dev"
///                   ^^ "-"
///                       ^^ "dependencies"
/// ```
pub struct Field<S: ConstStr, const Z: bool>(PhantomData<S>);

/// Access a specific element of a sequence.
/// Represents the `0` in `Cursor!(package[].dependencies[0])`.
///
/// ```txt
/// Cursor!(field[0])
///               ^
/// ```
pub struct Index<const N: usize>;

impl<S: ConstStr> ConstPathSegment for Field<S, false> {
    const VALUE: PathSegment = PathSegment::Field(S::VALUE);
}

impl<const N: usize> ConstPathSegment for Index<N> {
    const VALUE: PathSegment = PathSegment::Index(N);
}