gecko-dev/servo/components/style_derive/compute_squared_distance.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

use crate::animate::{AnimationFieldAttrs, AnimationInputAttrs, AnimationVariantAttrs};
use crate::cg;
use quote::Tokens;
use syn::{DeriveInput, Path};
use synstructure;

pub fn derive(mut input: DeriveInput) -> Tokens {
    let animation_input_attrs = cg::parse_input_attrs::<AnimationInputAttrs>(&input);
    let no_bound = animation_input_attrs.no_bound.unwrap_or_default();
    let mut where_clause = input.generics.where_clause.take();
    for param in input.generics.type_params() {
        if !no_bound.contains(&param.ident) {
            cg::add_predicate(
                &mut where_clause,
                parse_quote!(#param: crate::values::distance::ComputeSquaredDistance),
            );
        }
    }

    let (mut match_body, append_error_clause) = {
        let s = synstructure::Structure::new(&input);
        let mut append_error_clause = s.variants().len() > 1;

        let match_body = s.variants().iter().fold(quote!(), |body, variant| {
            let attrs = cg::parse_variant_attrs_from_ast::<AnimationVariantAttrs>(&variant.ast());
            if attrs.error {
                append_error_clause = true;
                return body;
            }

            let (this_pattern, this_info) = cg::ref_pattern(&variant, "this");
            let (other_pattern, other_info) = cg::ref_pattern(&variant, "other");
            let sum = if this_info.is_empty() {
                quote! { crate::values::distance::SquaredDistance::from_sqrt(0.) }
            } else {
                let mut sum = quote!();
                sum.append_separated(this_info.iter().zip(&other_info).map(|(this, other)| {
                    let field_attrs = cg::parse_field_attrs::<DistanceFieldAttrs>(&this.ast());
                    if field_attrs.field_bound {
                        let ty = &this.ast().ty;
                        cg::add_predicate(
                            &mut where_clause,
                            parse_quote!(#ty: crate::values::distance::ComputeSquaredDistance),
                        );
                    }

                    let animation_field_attrs =
                        cg::parse_field_attrs::<AnimationFieldAttrs>(&this.ast());

                    if animation_field_attrs.constant {
                        quote! {
                            {
                                if #this != #other {
                                    return Err(());
                                }
                                crate::values::distance::SquaredDistance::from_sqrt(0.)
                            }
                        }
                    } else {
                        quote! {
                            crate::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
                        }
                    }
                }), quote!(+));
                sum
            };
            quote! {
                #body
                (&#this_pattern, &#other_pattern) => {
                    Ok(#sum)
                }
            }
        });

        (match_body, append_error_clause)
    };
    input.generics.where_clause = where_clause;

    if append_error_clause {
        let input_attrs = cg::parse_input_attrs::<DistanceInputAttrs>(&input);
        if let Some(fallback) = input_attrs.fallback {
            match_body.append_all(quote! {
                (this, other) => #fallback(this, other)
            });
        } else {
            match_body.append_all(quote! { _ => Err(()) });
        }
    }

    let name = &input.ident;
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

    quote! {
        impl #impl_generics crate::values::distance::ComputeSquaredDistance for #name #ty_generics #where_clause {
            #[allow(unused_variables, unused_imports)]
            #[inline]
            fn compute_squared_distance(
                &self,
                other: &Self,
            ) -> Result<crate::values::distance::SquaredDistance, ()> {
                match (self, other) {
                    #match_body
                }
            }
        }
    }
}

#[darling(attributes(distance), default)]
#[derive(Default, FromDeriveInput)]
struct DistanceInputAttrs {
    fallback: Option<Path>,
}

#[darling(attributes(distance), default)]
#[derive(Default, FromField)]
struct DistanceFieldAttrs {
    field_bound: bool,
}