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use ::{FromMeta, Result};
use proc_macro2::Span;
use std::ops::{Deref, DerefMut};
use syn::spanned::Spanned;
use syn::Meta;

/// A value and an associated position in source code. The main use case for this is
/// to preserve position information to emit warnings from proc macros. You can use
/// a `SpannedValue<T>` as a field in any struct that implements or derives any of
/// `darling`'s core traits.
///
/// To access the underlying value, use the struct's `Deref` implementation.
///
/// # Defaulting
/// This type is meant to be used in conjunction with attribute-extracted options,
/// but the user may not always explicitly set those options in their source code.
/// In this case, using `Default::default()` will create an instance which points
/// to `Span::call_site()`.
#[derive(Debug, Clone)]
pub struct SpannedValue<T> {
    value: T,
    span: Span,
}

impl<T> SpannedValue<T> {
    pub fn new(value: T, span: Span) -> Self {
        SpannedValue { value, span }
    }

    /// Get a reference to the source code location described by this struct.
    pub fn span(&self) -> &Span {
        &self.span
    }
}

impl<T: Default> Default for SpannedValue<T> {
    fn default() -> Self {
        SpannedValue::new(Default::default(), Span::call_site())
    }
}

impl<T> Deref for SpannedValue<T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.value
    }
}

impl<T> DerefMut for SpannedValue<T> {
    fn deref_mut(&mut self) -> &mut T {
        &mut self.value
    }
}

impl<T> AsRef<T> for SpannedValue<T> {
    fn as_ref(&self) -> &T {
        &self.value
    }
}

impl<T: FromMeta> FromMeta for SpannedValue<T> {
    fn from_meta(item: &Meta) -> Result<Self> {
        Ok(SpannedValue::new(
            FromMeta::from_meta(item).map_err(|e| e.with_span(item))?,
            item.span(),
        ))
    }
}

impl<T: Spanned> From<T> for SpannedValue<T> {
    fn from(value: T) -> Self {
        let span = value.span();
        SpannedValue::new(value, span)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use proc_macro2::Span;

    /// Make sure that `SpannedValue` can be seamlessly used as its underlying type.
    #[test]
    fn deref() {
        let test = SpannedValue::new("hello", Span::call_site());
        assert_eq!("hello", test.trim());
    }
}