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use syn::{parse_quote, punctuated::Punctuated, Attribute, DeriveInput, Meta, NestedMeta, Token};

use crate::{meta_list_contains, nested_meta_to_ident};

/// Functions to make it ergonomic to work with `struct` ASTs.
pub trait DeriveInputDeriveExt {
    /// Appends derives to the list of derives.
    ///
    /// **Note:** This can only be used with [*attribute*] macros, and not [*derive*] macros.
    ///
    /// * If the `derive` attribute does not exist, one will be created.
    /// * If the `derive` attribute exists, and there are existing `derive`s that overlap with the
    ///   derives to append, this macro will panic with the overlapping derives.
    /// * If the `derive` attribute exists, and there are no overlapping `derive`s, then they will
    ///   be combined.
    ///
    /// # Panics
    ///
    /// Panics if there are existing `derive`s that overlap with the derives to append.
    ///
    /// [*attribute*]: <https://doc.rust-lang.org/reference/procedural-macros.html#attribute-macros>
    /// [*derive*]: <https://doc.rust-lang.org/reference/procedural-macros.html#derive-mode-macros>
    fn append_derives(&mut self, derives: Punctuated<NestedMeta, Token![,]>);
}

impl DeriveInputDeriveExt for DeriveInput {
    fn append_derives(&mut self, derives_to_append: Punctuated<NestedMeta, Token![,]>) {
        let attr_derives_existing = self
            .attrs
            .iter_mut()
            .filter(|attr| attr.path.is_ident("derive"))
            .filter_map(|attr| match attr.parse_meta() {
                Ok(Meta::List(meta_list)) => Some((attr, meta_list)),
                _ => None,
            })
            .next();

        if let Some((attr, mut derives_existing)) = attr_derives_existing {
            // Emit warning if the user derives any of the existing derives, as we do that for them.
            let superfluous = derives_to_append
                .iter()
                .filter(|derive_to_append| meta_list_contains(&derives_existing, derive_to_append))
                .filter_map(nested_meta_to_ident)
                .map(|ident| format!("{}", ident))
                .collect::<Vec<_>>();
            if !superfluous.is_empty() {
                // TODO: Emit warning, pending <https://github.com/rust-lang/rust/issues/54140>
                // derives_existing
                //     .span()
                //     .warning(
                //         "The following are automatically derived by this proc macro attribute.",
                //     )
                //     .emit();
                panic!(
                    "The following are automatically derived when this attribute is used:\n\
                     {:?}",
                    superfluous
                );
            } else {
                // derives_existing.nested.push_punct(<Token![,]>::default());
                derives_existing.nested.extend(derives_to_append);

                // Replace the existing `Attribute`.
                //
                // `attr.parse_meta()` returns a `Meta`, which is not referenced by the
                // `DeriveInput`, so we have to replace `attr` itself.
                *attr = parse_quote!(#[#derives_existing]);
            }
        } else {
            // Add a new `#[derive(..)]` attribute with all the derives.
            let derive_attribute: Attribute = parse_quote!(#[derive(#derives_to_append)]);
            self.attrs.push(derive_attribute);
        }
    }
}

#[cfg(test)]
mod tests {
    use pretty_assertions::assert_eq;
    use syn::{parse_quote, DeriveInput};

    use super::DeriveInputDeriveExt;

    #[test]
    fn append_derives_creates_attr_when_attr_does_not_exist() {
        let mut ast: DeriveInput = parse_quote!(
            struct Struct;
        );
        let derives = parse_quote!(Clone, Copy);

        ast.append_derives(derives);

        let ast_expected: DeriveInput = parse_quote! {
            #[derive(Clone, Copy)]
            struct Struct;
        };
        assert_eq!(ast_expected, ast);
    }

    #[test]
    fn append_derives_appends_to_attr_when_attr_exists() {
        let mut ast: DeriveInput = parse_quote!(
            #[derive(Debug)]
            struct Struct;
        );
        let derives = parse_quote!(Clone, Copy);

        ast.append_derives(derives);

        let ast_expected: DeriveInput = parse_quote! {
            #[derive(Debug, Clone, Copy)]
            struct Struct;
        };
        assert_eq!(ast_expected, ast);
    }

    #[test]
    #[should_panic(
        expected = "The following are automatically derived when this attribute is used:\n\
                    [\"Clone\", \"Copy\"]"
    )]
    fn append_derives_panics_when_derives_exist() {
        let mut ast: DeriveInput = parse_quote!(
            #[derive(Clone, Copy, Debug)]
            struct Struct;
        );
        let derives = parse_quote!(Clone, Copy, Default);

        ast.append_derives(derives);
    }
}