educe 0.8.1

This crate offers procedural macros designed to facilitate the swift implementation of Rust's built-in traits.
Documentation 
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use syn::{Data, DeriveInput, Meta, Type, visit_mut::VisitMut};

use super::{
    TraitHandlerMultiple,
    models::{FieldAttribute, FieldAttributeBuilder, TypeAttributeBuilder},
};
use crate::{
    Trait,
    common::{ident_index::IdentOrIndex, quote_mixed},
    trait_handlers::TraitHandlerContext,
};

/// Generates the `Into` implementation for a struct.
pub(crate) struct IntoStructHandler;

impl TraitHandlerMultiple for IntoStructHandler {
    #[inline]
    fn trait_meta_handler<'a>(
        ast: &'a DeriveInput,
        _ctx: &mut TraitHandlerContext<'a>,
        token_stream: &mut proc_macro2::TokenStream,
        traits: &[Trait],
        meta: &[Meta],
    ) -> syn::Result<()> {
        let generated_impl_attributes =
            crate::common::attributes::generated_impl_attributes(&ast.attrs);

        let type_attribute = TypeAttributeBuilder {
            enable_types: true
        }
        .build_from_into_meta(meta)?;

        if let Data::Struct(data) = &ast.data {
            let fields = &data.fields;

            let field_attributes: Vec<FieldAttribute> = {
                let mut field_attributes = Vec::with_capacity(fields.len());

                for field in fields.iter() {
                    let field_attribute = FieldAttributeBuilder {
                        enable_types: true
                    }
                    .build_from_attributes(&field.attrs, traits)?;

                    for ty in field_attribute.types.keys() {
                        if !type_attribute.types.contains_key(ty) {
                            return Err(super::panic::no_into_impl(ty));
                        }
                    }

                    field_attributes.push(field_attribute);
                }

                field_attributes
            };

            for (target_key, target) in type_attribute.types {
                let target_ty = &target.ty;
                let target_matcher = super::common::TargetMatcher::new(target_ty);
                // By default a `From` impl is generated because it provides `Into` for free; the `into` flag asks for a direct `Into` impl instead.
                let generate_from = !target.force_into;

                // The conversion body takes the source value from `self` in an `Into` impl and from the `value` parameter in a `From` impl.
                let source = if generate_from { quote_mixed!(value) } else { quote_mixed!(self) };

                let bound = target.bound;

                let mut into_types: Vec<&Type> = Vec::new();

                let mut into_token_stream = proc_macro2::TokenStream::new();

                let (index, field, method) = super::common::select_field(
                    &data.fields,
                    &field_attributes,
                    &target_key,
                    &target_matcher,
                )?;

                let field_name = IdentOrIndex::from_ident_with_index(field.ident.as_ref(), index);

                if let Some(method) = method {
                    let mut method = method.clone();
                    crate::common::generics::ReplaceSelf::new(ast).visit_expr_path_mut(&mut method);
                    into_token_stream.extend(quote_mixed!( #method(#source.#field_name) ));
                } else {
                    let ty = &field.ty;

                    if target_matcher.matches(ty) {
                        into_token_stream.extend(quote_mixed!( #source.#field_name ));
                    } else {
                        into_types.push(ty);

                        into_token_stream.extend(
                            quote_mixed!( ::core::convert::Into::into(#source.#field_name) ),
                        );
                    }
                }

                let ident = &ast.ident;

                let bound = bound.into_where_predicates_by_generic_parameters_check_types_shallow(
                    &ast.generics.params,
                    &syn::parse2(quote_mixed!(::core::convert::Into<#target_ty>)).unwrap(),
                    &into_types,
                );

                // The generics are cloned so that this target does not affect the other `Into` implementations.
                let mut generics = ast.generics.clone();
                if generate_from {
                    crate::common::generics::ReplaceSelf::new(ast)
                        .visit_generics_mut(&mut generics);
                }

                let generics = crate::common::generics::with_predicates(generics, bound);

                let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();

                token_stream.extend(if generate_from {
                    quote_mixed! {
                        #generated_impl_attributes
                        impl #impl_generics ::core::convert::From<#ident #ty_generics> for #target_ty #where_clause {
                            #[inline]
                            fn from(value: #ident #ty_generics) -> Self {
                                #into_token_stream
                            }
                        }
                    }
                } else {
                    quote_mixed! {
                        #generated_impl_attributes
                        impl #impl_generics ::core::convert::Into<#target_ty> for #ident #ty_generics #where_clause {
                            #[inline]
                            fn into(self) -> #target_ty {
                                #into_token_stream
                            }
                        }
                    }
                });
            }
        }

        Ok(())
    }
}