transformer.rs

use std::{
  marker::PhantomData,
  ops::{Index, IndexMut},
};

use lightningcss::{
  media_query::MediaFeatureValue,
  properties::{
    custom::{Token, TokenList, TokenOrValue},
    Property,
  },
  rules::{CssRule, CssRuleList},
  stylesheet::ParserOptions,
  traits::ParseWithOptions,
  values::{
    ident::Ident,
    length::{Length, LengthValue},
    string::CowArcStr,
  },
  visitor::{Visit, VisitTypes, Visitor},
};
use lightningcss::{stylesheet::StyleSheet, traits::IntoOwned};
use napi::{Env, JsFunction, JsObject, JsUnknown, Ref, ValueType};
use serde::{Deserialize, Serialize};
use smallvec::SmallVec;

use crate::at_rule_parser::AtRule;

pub struct JsVisitor {
  env: Env,
  visit_stylesheet: VisitorsRef,
  visit_rule: VisitorsRef,
  rule_map: VisitorsRef,
  property_map: VisitorsRef,
  visit_declaration: VisitorsRef,
  visit_length: Option<Ref<()>>,
  visit_angle: Option<Ref<()>>,
  visit_ratio: Option<Ref<()>>,
  visit_resolution: Option<Ref<()>>,
  visit_time: Option<Ref<()>>,
  visit_color: Option<Ref<()>>,
  visit_image: VisitorsRef,
  visit_url: Option<Ref<()>>,
  visit_media_query: VisitorsRef,
  visit_supports_condition: VisitorsRef,
  visit_custom_ident: Option<Ref<()>>,
  visit_dashed_ident: Option<Ref<()>>,
  visit_selector: Option<Ref<()>>,
  visit_token: VisitorsRef,
  token_map: VisitorsRef,
  visit_function: VisitorsRef,
  function_map: VisitorsRef,
  visit_variable: VisitorsRef,
  visit_env: VisitorsRef,
  env_map: VisitorsRef,
  types: VisitTypes,
}

// This is so that the visitor can work with bundleAsync.
// We ensure that we only call JsVisitor from the main JS thread.
unsafe impl Send for JsVisitor {}

#[derive(PartialEq, Eq, Clone, Copy)]
enum VisitStage {
  Enter,
  Exit,
}

type VisitorsRef = Visitors<Ref<()>>;

struct Visitors<T> {
  enter: Option<T>,
  exit: Option<T>,
}

impl<T> Visitors<T> {
  fn new(enter: Option<T>, exit: Option<T>) -> Self {
    Self { enter, exit }
  }

  fn for_stage(&self, stage: VisitStage) -> Option<&T> {
    match stage {
      VisitStage::Enter => self.enter.as_ref(),
      VisitStage::Exit => self.exit.as_ref(),
    }
  }
}

impl Visitors<Ref<()>> {
  fn get<U: napi::NapiValue>(&self, env: &Env) -> Visitors<U> {
    Visitors {
      enter: self.enter.as_ref().and_then(|p| env.get_reference_value_unchecked(p).ok()),
      exit: self.exit.as_ref().and_then(|p| env.get_reference_value_unchecked(p).ok()),
    }
  }
}

impl Visitors<JsObject> {
  fn named(&self, stage: VisitStage, name: &str) -> Option<JsFunction> {
    self
      .for_stage(stage)
      .and_then(|m| m.get_named_property::<JsFunction>(name).ok())
  }

  fn custom(&self, stage: VisitStage, obj: &str, name: &str) -> Option<JsFunction> {
    self
      .for_stage(stage)
      .and_then(|m| m.get_named_property::<JsUnknown>(obj).ok())
      .and_then(|v| {
        match v.get_type() {
          Ok(ValueType::Function) => return v.try_into().ok(),
          Ok(ValueType::Object) => {
            let o: napi::Result<JsObject> = v.try_into();
            if let Ok(o) = o {
              return o.get_named_property::<JsFunction>(name).ok();
            }
          }
          _ => {}
        }

        None
      })
  }
}

impl Drop for JsVisitor {
  fn drop(&mut self) {
    macro_rules! drop {
      ($id: ident) => {
        if let Some(v) = &mut self.$id {
          drop(v.unref(self.env));
        }
      };
    }

    macro_rules! drop_tuple {
      ($id: ident) => {
        if let Some(v) = &mut self.$id.enter {
          drop(v.unref(self.env));
        }
        if let Some(v) = &mut self.$id.exit {
          drop(v.unref(self.env));
        }
      };
    }

    drop_tuple!(visit_stylesheet);
    drop_tuple!(visit_rule);
    drop_tuple!(rule_map);
    drop_tuple!(visit_declaration);
    drop_tuple!(property_map);
    drop!(visit_length);
    drop!(visit_angle);
    drop!(visit_ratio);
    drop!(visit_resolution);
    drop!(visit_time);
    drop!(visit_color);
    drop_tuple!(visit_image);
    drop!(visit_url);
    drop_tuple!(visit_media_query);
    drop_tuple!(visit_supports_condition);
    drop_tuple!(visit_variable);
    drop_tuple!(visit_env);
    drop_tuple!(env_map);
    drop!(visit_custom_ident);
    drop!(visit_dashed_ident);
    drop_tuple!(visit_function);
    drop_tuple!(function_map);
    drop!(visit_selector);
    drop_tuple!(visit_token);
    drop_tuple!(token_map);
  }
}

impl JsVisitor {
  pub fn new(env: Env, visitor: JsObject) -> Self {
    let mut types = VisitTypes::empty();
    macro_rules! get {
      ($name: literal, $( $t: ident )|+) => {{
        let res: Option<JsFunction> = visitor.get_named_property($name).ok();
        if res.is_some() {
          types |= $( VisitTypes::$t )|+;
        }

        // We must create a reference so that the garbage collector doesn't destroy
        // the function before we try to call it (in the async bundle case).
        res.and_then(|res| env.create_reference(res).ok())
      }};
    }

    macro_rules! map {
      ($name: literal, $( $t: ident )|+) => {{
        if let Ok(obj) = visitor.get_named_property::<JsObject>($name) {
          types |= $( VisitTypes::$t )|+;
          env.create_reference(obj).ok()
        } else {
          None
        }
      }};
    }

    Self {
      env,
      visit_stylesheet: VisitorsRef::new(get!("StyleSheet", RULES), get!("StyleSheetExit", RULES)),
      visit_rule: VisitorsRef::new(get!("Rule", RULES), get!("RuleExit", RULES)),
      rule_map: VisitorsRef::new(map!("Rule", RULES), get!("RuleExit", RULES)),
      visit_declaration: VisitorsRef::new(get!("Declaration", PROPERTIES), get!("DeclarationExit", PROPERTIES)),
      property_map: VisitorsRef::new(map!("Declaration", PROPERTIES), map!("DeclarationExit", PROPERTIES)),
      visit_length: get!("Length", LENGTHS),
      visit_angle: get!("Angle", ANGLES),
      visit_ratio: get!("Ratio", RATIOS),
      visit_resolution: get!("Resolution", RESOLUTIONS),
      visit_time: get!("Time", TIMES),
      visit_color: get!("Color", COLORS),
      visit_image: VisitorsRef::new(get!("Image", IMAGES), get!("ImageExit", IMAGES)),
      visit_url: get!("Url", URLS),
      visit_media_query: VisitorsRef::new(
        get!("MediaQuery", MEDIA_QUERIES),
        get!("MediaQueryExit", MEDIA_QUERIES),
      ),
      visit_supports_condition: VisitorsRef::new(
        get!("SupportsCondition", SUPPORTS_CONDITIONS),
        get!("SupportsConditionExit", SUPPORTS_CONDITIONS),
      ),
      visit_variable: VisitorsRef::new(get!("Variable", TOKENS), get!("VariableExit", TOKENS)),
      visit_env: VisitorsRef::new(
        get!("EnvironmentVariable", TOKENS | MEDIA_QUERIES | ENVIRONMENT_VARIABLES),
        get!(
          "EnvironmentVariableExit",
          TOKENS | MEDIA_QUERIES | ENVIRONMENT_VARIABLES
        ),
      ),
      env_map: VisitorsRef::new(
        map!("EnvironmentVariable", TOKENS | MEDIA_QUERIES | ENVIRONMENT_VARIABLES),
        map!(
          "EnvironmentVariableExit",
          TOKENS | MEDIA_QUERIES | ENVIRONMENT_VARIABLES
        ),
      ),
      visit_custom_ident: get!("CustomIdent", CUSTOM_IDENTS),
      visit_dashed_ident: get!("DashedIdent", DASHED_IDENTS),
      visit_function: VisitorsRef::new(get!("Function", TOKENS), get!("FunctionExit", TOKENS)),
      function_map: VisitorsRef::new(map!("Function", TOKENS), map!("FunctionExit", TOKENS)),
      visit_selector: get!("Selector", SELECTORS),
      visit_token: VisitorsRef::new(get!("Token", TOKENS), None),
      token_map: VisitorsRef::new(map!("Token", TOKENS), None),
      types,
    }
  }
}

impl<'i> Visitor<'i, AtRule<'i>> for JsVisitor {
  type Error = napi::Error;

  fn visit_types(&self) -> VisitTypes {
    self.types
  }

  fn visit_stylesheet<'o>(&mut self, stylesheet: &mut StyleSheet<'i, 'o, AtRule<'i>>) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::RULES) {
      let env = self.env;
      let visit_stylesheet = self.visit_stylesheet.get::<JsFunction>(&env);
      if let Some(visit) = visit_stylesheet.for_stage(VisitStage::Enter) {
        call_visitor(&env, stylesheet, visit)?
      }

      stylesheet.visit_children(self)?;

      if let Some(visit) = visit_stylesheet.for_stage(VisitStage::Exit) {
        call_visitor(&env, stylesheet, visit)?
      }

      Ok(())
    } else {
      stylesheet.visit_children(self)
    }
  }

  fn visit_rule_list(
    &mut self,
    rules: &mut lightningcss::rules::CssRuleList<'i, AtRule<'i>>,
  ) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::RULES) {
      let env = self.env;
      let rule_map = self.rule_map.get::<JsObject>(&env);
      let visit_rule = self.visit_rule.get::<JsFunction>(&env);

      visit_list(
        rules,
        |value, stage| {
          // Use a more specific visitor function if available, but fall back to visit_rule.
          let name = match value {
            CssRule::Media(..) => "media",
            CssRule::Import(..) => "import",
            CssRule::Style(..) => "style",
            CssRule::Keyframes(..) => "keyframes",
            CssRule::FontFace(..) => "font-face",
            CssRule::FontPaletteValues(..) => "font-palette-values",
            CssRule::Page(..) => "page",
            CssRule::Supports(..) => "supports",
            CssRule::CounterStyle(..) => "counter-style",
            CssRule::Namespace(..) => "namespace",
            CssRule::CustomMedia(..) => "custom-media",
            CssRule::LayerBlock(..) => "layer-block",
            CssRule::LayerStatement(..) => "layer-statement",
            CssRule::Property(..) => "property",
            CssRule::Container(..) => "container",
            CssRule::Scope(..) => "scope",
            CssRule::MozDocument(..) => "moz-document",
            CssRule::Nesting(..) => "nesting",
            CssRule::Viewport(..) => "viewport",
            CssRule::StartingStyle(..) => "starting-style",
            CssRule::Unknown(v) => {
              let name = v.name.as_ref();
              if let Some(visit) = rule_map.custom(stage, "unknown", name) {
                let js_value = env.to_js_value(v)?;
                let res = visit.call(None, &[js_value])?;
                return env.from_js_value(res).map(serde_detach::detach);
              } else {
                "unknown"
              }
            }
            CssRule::Custom(c) => {
              let name = c.name.as_ref();
              if let Some(visit) = rule_map.custom(stage, "custom", name) {
                let js_value = env.to_js_value(c)?;
                let res = visit.call(None, &[js_value])?;
                return env.from_js_value(res).map(serde_detach::detach);
              } else {
                "custom"
              }
            }
            CssRule::Ignored => return Ok(None),
          };

          if let Some(visit) = rule_map.named(stage, name).as_ref().or(visit_rule.for_stage(stage)) {
            let js_value = env.to_js_value(value)?;
            let res = visit.call(None, &[js_value])?;
            env.from_js_value(res).map(serde_detach::detach)
          } else {
            Ok(None)
          }
        },
        |rule| rule.visit_children(self),
      )?;

      Ok(())
    } else {
      rules.visit_children(self)
    }
  }

  fn visit_declaration_block(
    &mut self,
    decls: &mut lightningcss::declaration::DeclarationBlock<'i>,
  ) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::PROPERTIES) {
      let env = self.env;
      let property_map = self.property_map.get::<JsObject>(&env);
      let visit_declaration = self.visit_declaration.get::<JsFunction>(&env);
      visit_declaration_list(
        &env,
        &mut decls.important_declarations,
        &visit_declaration,
        &property_map,
        |property| property.visit_children(self),
      )?;
      visit_declaration_list(
        &env,
        &mut decls.declarations,
        &visit_declaration,
        &property_map,
        |property| property.visit_children(self),
      )?;
      Ok(())
    } else {
      decls.visit_children(self)
    }
  }

  fn visit_length(&mut self, length: &mut LengthValue) -> Result<(), Self::Error> {
    visit(&self.env, length, &self.visit_length)
  }

  fn visit_angle(&mut self, angle: &mut lightningcss::values::angle::Angle) -> Result<(), Self::Error> {
    visit(&self.env, angle, &self.visit_angle)
  }

  fn visit_ratio(&mut self, ratio: &mut lightningcss::values::ratio::Ratio) -> Result<(), Self::Error> {
    visit(&self.env, ratio, &self.visit_ratio)
  }

  fn visit_resolution(
    &mut self,
    resolution: &mut lightningcss::values::resolution::Resolution,
  ) -> Result<(), Self::Error> {
    visit(&self.env, resolution, &self.visit_resolution)
  }

  fn visit_time(&mut self, time: &mut lightningcss::values::time::Time) -> Result<(), Self::Error> {
    visit(&self.env, time, &self.visit_time)
  }

  fn visit_color(&mut self, color: &mut lightningcss::values::color::CssColor) -> Result<(), Self::Error> {
    visit(&self.env, color, &self.visit_color)
  }

  fn visit_image(&mut self, image: &mut lightningcss::values::image::Image<'i>) -> Result<(), Self::Error> {
    visit(&self.env, image, &self.visit_image.enter)?;
    image.visit_children(self)?;
    visit(&self.env, image, &self.visit_image.exit)
  }

  fn visit_url(&mut self, url: &mut lightningcss::values::url::Url<'i>) -> Result<(), Self::Error> {
    visit(&self.env, url, &self.visit_url)
  }

  fn visit_media_list(&mut self, media: &mut lightningcss::media_query::MediaList<'i>) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::MEDIA_QUERIES) {
      let env = self.env;
      let visit_media_query = self.visit_media_query.get::<JsFunction>(&env);
      visit_list(
        &mut media.media_queries,
        |value, stage| {
          if let Some(visit) = visit_media_query.for_stage(stage) {
            let js_value = env.to_js_value(value)?;
            let res = visit.call(None, &[js_value])?;
            env.from_js_value(res).map(serde_detach::detach)
          } else {
            Ok(None)
          }
        },
        |q| q.visit_children(self),
      )?;
      Ok(())
    } else {
      media.visit_children(self)
    }
  }

  fn visit_media_feature_value(&mut self, value: &mut MediaFeatureValue<'i>) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::ENVIRONMENT_VARIABLES) && matches!(value, MediaFeatureValue::Env(_)) {
      let env_map = self.env_map.get::<JsObject>(&self.env);
      let visit_env = self.visit_env.get::<JsFunction>(&self.env);
      let call = |stage: VisitStage, value: &mut MediaFeatureValue, env: &Env| -> napi::Result<()> {
        let env_var = if let MediaFeatureValue::Env(env) = value {
          env
        } else {
          return Ok(());
        };
        let visit_type = env_map.named(stage, env_var.name.name());
        let visit = visit_env.for_stage(stage);
        let new_value: Option<TokenOrValue> = if let Some(visit) = visit_type.as_ref().or(visit) {
          let js_value = env.to_js_value(env_var)?;
          let res = visit.call(None, &[js_value])?;
          env.from_js_value(res).map(serde_detach::detach)?
        } else {
          None
        };

        match new_value {
          None => return Ok(()),
          Some(TokenOrValue::Length(l)) => *value = MediaFeatureValue::Length(Length::Value(l)),
          Some(TokenOrValue::Resolution(r)) => *value = MediaFeatureValue::Resolution(r),
          Some(TokenOrValue::Token(Token::Number { value: n, .. })) => *value = MediaFeatureValue::Number(n),
          Some(TokenOrValue::Token(Token::Ident(ident))) => *value = MediaFeatureValue::Ident(Ident(ident)),
          // TODO: ratio
          _ => {
            return Err(napi::Error::new(
              napi::Status::InvalidArg,
              format!("invalid environment value in media query: {:?}", new_value),
            ))
          }
        }

        Ok(())
      };

      call(VisitStage::Enter, value, &self.env)?;
      value.visit_children(self)?;
      call(VisitStage::Exit, value, &self.env)?;
      return Ok(());
    }

    value.visit_children(self)
  }

  fn visit_supports_condition(
    &mut self,
    condition: &mut lightningcss::rules::supports::SupportsCondition<'i>,
  ) -> Result<(), Self::Error> {
    visit(&self.env, condition, &self.visit_supports_condition.enter)?;
    condition.visit_children(self)?;
    visit(&self.env, condition, &self.visit_supports_condition.exit)
  }

  fn visit_custom_ident(
    &mut self,
    ident: &mut lightningcss::values::ident::CustomIdent,
  ) -> Result<(), Self::Error> {
    visit(&self.env, ident, &self.visit_custom_ident)
  }

  fn visit_dashed_ident(
    &mut self,
    ident: &mut lightningcss::values::ident::DashedIdent,
  ) -> Result<(), Self::Error> {
    visit(&self.env, ident, &self.visit_dashed_ident)
  }

  fn visit_selector_list(
    &mut self,
    selectors: &mut lightningcss::selector::SelectorList<'i>,
  ) -> Result<(), Self::Error> {
    if let Some(visit) = self
      .visit_selector
      .as_ref()
      .and_then(|v| self.env.get_reference_value_unchecked::<JsFunction>(v).ok())
    {
      map::<_, _, _, true>(&mut selectors.0, |value| {
        let js_value = self.env.to_js_value(value)?;
        let res = visit.call(None, &[js_value])?;
        self.env.from_js_value(res).map(serde_detach::detach)
      })?;
    }

    Ok(())
  }

  fn visit_token_list(
    &mut self,
    tokens: &mut lightningcss::properties::custom::TokenList<'i>,
  ) -> Result<(), Self::Error> {
    if self.types.contains(VisitTypes::TOKENS) {
      let env = self.env;
      let visit_token = self.visit_token.get::<JsFunction>(&env);
      let token_map = self.token_map.get::<JsObject>(&env);
      let visit_function = self.visit_function.get::<JsFunction>(&env);
      let function_map = self.function_map.get::<JsObject>(&env);
      let visit_variable = self.visit_variable.get::<JsFunction>(&env);
      let visit_env = self.visit_env.get::<JsFunction>(&env);
      let env_map = self.env_map.get::<JsObject>(&env);

      visit_list(
        &mut tokens.0,
        |value, stage| {
          let (visit_type, visit) = match value {
            TokenOrValue::Function(f) => (
              function_map.named(stage, f.name.0.as_ref()),
              visit_function.for_stage(stage),
            ),
            TokenOrValue::Var(_) => (None, visit_variable.for_stage(stage)),
            TokenOrValue::Env(e) => (env_map.named(stage, e.name.name()), visit_env.for_stage(stage)),
            TokenOrValue::Token(t) => {
              let name = match t {
                Token::Ident(_) => Some("ident"),
                Token::AtKeyword(_) => Some("at-keyword"),
                Token::Hash(_) => Some("hash"),
                Token::IDHash(_) => Some("id-hash"),
                Token::String(_) => Some("string"),
                Token::Number { .. } => Some("number"),
                Token::Percentage { .. } => Some("percentage"),
                Token::Dimension { .. } => Some("dimension"),
                _ => None,
              };
              let visit = if let Some(name) = name {
                token_map.named(stage, name)
              } else {
                None
              };
              (visit, visit_token.for_stage(stage))
            }
            _ => return Ok(None),
          };

          if let Some(visit) = visit_type.as_ref().or(visit) {
            let js_value = match value {
              TokenOrValue::Function(f) => env.to_js_value(f)?,
              TokenOrValue::Var(v) => env.to_js_value(v)?,
              TokenOrValue::Env(v) => env.to_js_value(v)?,
              TokenOrValue::Token(t) => env.to_js_value(t)?,
              _ => unreachable!(),
            };

            let res = visit.call(None, &[js_value])?;
            let res: Option<TokensOrRaw> = env.from_js_value(res).map(serde_detach::detach)?;
            Ok(res.map(|r| r.0))
          } else {
            Ok(None)
          }
        },
        |value| value.visit_children(self),
      )?;

      Ok(())
    } else {
      tokens.visit_children(self)
    }
  }
}

fn visit<V: Serialize + Deserialize<'static>>(
  env: &Env,
  value: &mut V,
  visit: &Option<Ref<()>>,
) -> napi::Result<()> {
  if let Some(visit) = visit
    .as_ref()
    .and_then(|v| env.get_reference_value_unchecked::<JsFunction>(v).ok())
  {
    call_visitor(env, value, &visit)?;
  }

  Ok(())
}

fn call_visitor<V: Serialize + Deserialize<'static>>(
  env: &Env,
  value: &mut V,
  visit: &JsFunction,
) -> napi::Result<()> {
  let js_value = env.to_js_value(value)?;
  let res = visit.call(None, &[js_value])?;
  let new_value: Option<V> = env.from_js_value(res).map(serde_detach::detach)?;
  match new_value {
    Some(new_value) => *value = new_value,
    None => {}
  }

  Ok(())
}

fn visit_declaration_list<'i, C: FnMut(&mut Property<'i>) -> napi::Result<()>>(
  env: &Env,
  list: &mut Vec<Property<'i>>,
  visit_declaration: &Visitors<JsFunction>,
  property_map: &Visitors<JsObject>,
  visit_children: C,
) -> napi::Result<()> {
  visit_list(
    list,
    |value, stage| {
      // Use a specific property visitor if available, or fall back to Property visitor.
      let visit = match value {
        Property::Custom(v) => {
          if let Some(visit) = property_map.custom(stage, "custom", v.name.as_ref()) {
            let js_value = env.to_js_value(v)?;
            let res = visit.call(None, &[js_value])?;
            return env.from_js_value(res).map(serde_detach::detach);
          } else {
            None
          }
        }
        _ => property_map.named(stage, value.property_id().name()),
      };

      if let Some(visit) = visit.as_ref().or(visit_declaration.for_stage(stage)) {
        let js_value = env.to_js_value(value)?;
        let res = visit.call(None, &[js_value])?;
        env.from_js_value(res).map(serde_detach::detach)
      } else {
        Ok(None)
      }
    },
    visit_children,
  )
}

fn visit_list<
  V,
  L: List<V>,
  F: Fn(&mut V, VisitStage) -> napi::Result<Option<ValueOrVec<V>>>,
  C: FnMut(&mut V) -> napi::Result<()>,
>(
  list: &mut L,
  visit: F,
  mut visit_children: C,
) -> napi::Result<()> {
  map(list, |value| {
    let mut new_value: Option<ValueOrVec<V>> = visit(value, VisitStage::Enter)?;

    match &mut new_value {
      Some(ValueOrVec::Value(v)) => {
        visit_children(v)?;

        if let Some(val) = visit(v, VisitStage::Exit)? {
          new_value = Some(val);
        }
      }
      Some(ValueOrVec::Vec(v)) => {
        map(v, |value| {
          visit_children(value)?;
          visit(value, VisitStage::Exit)
        })?;
      }
      None => {
        visit_children(value)?;
        if let Some(val) = visit(value, VisitStage::Exit)? {
          new_value = Some(val);
        }
      }
    }

    Ok(new_value)
  })
}

fn map<V, L: List<V>, F: FnMut(&mut V) -> napi::Result<Option<ValueOrVec<V, IS_VEC>>>, const IS_VEC: bool>(
  list: &mut L,
  mut f: F,
) -> napi::Result<()> {
  let mut i = 0;
  while i < list.len() {
    let value = &mut list[i];
    let new_value = f(value)?;
    match new_value {
      Some(ValueOrVec::Value(v)) => {
        list[i] = v;
        i += 1;
      }
      Some(ValueOrVec::Vec(vec)) => {
        if vec.is_empty() {
          list.remove(i);
        } else {
          let len = vec.len();
          list.replace(i, vec);
          i += len;
        }
      }
      None => {
        i += 1;
      }
    }
  }
  Ok(())
}

#[derive(serde::Serialize)]
#[serde(untagged)]
enum ValueOrVec<V, const IS_VEC: bool = false> {
  Value(V),
  Vec(Vec<V>),
}

// Manually implemented deserialize for better error messages.
// https://github.com/serde-rs/serde/issues/773
impl<'de, V: serde::Deserialize<'de>, const IS_VEC: bool> serde::Deserialize<'de> for ValueOrVec<V, IS_VEC> {
  fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
  where
    D: serde::Deserializer<'de>,
  {
    use serde::Deserializer;
    let content = serde::__private::de::Content::deserialize(deserializer)?;
    let de: serde::__private::de::ContentRefDeserializer<D::Error> =
      serde::__private::de::ContentRefDeserializer::new(&content);

    // Try to deserialize as a sequence first.
    let mut was_seq = false;
    let res = de.deserialize_seq(SeqVisitor {
      was_seq: &mut was_seq,
      phantom: PhantomData,
    });

    if was_seq {
      // Allow fallback if we know the value is also a list (e.g. selector).
      if res.is_ok() || !IS_VEC {
        return res.map(ValueOrVec::Vec);
      }
    }

    // If it wasn't a sequence, try a value.
    let de = serde::__private::de::ContentRefDeserializer::new(&content);
    return V::deserialize(de).map(ValueOrVec::Value);

    struct SeqVisitor<'a, V> {
      was_seq: &'a mut bool,
      phantom: PhantomData<V>,
    }

    impl<'a, 'de, V: serde::Deserialize<'de>> serde::de::Visitor<'de> for SeqVisitor<'a, V> {
      type Value = Vec<V>;

      fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
        formatter.write_str("a sequence")
      }

      fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
      where
        A: serde::de::SeqAccess<'de>,
      {
        *self.was_seq = true;
        let mut vec = Vec::with_capacity(seq.size_hint().unwrap_or(1));
        while let Some(v) = seq.next_element()? {
          vec.push(v);
        }
        Ok(vec)
      }
    }
  }
}

struct TokensOrRaw<'i>(ValueOrVec<TokenOrValue<'i>>);

impl<'i, 'de: 'i> serde::Deserialize<'de> for TokensOrRaw<'i> {
  fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
  where
    D: serde::Deserializer<'de>,
  {
    use serde::__private::de::ContentRefDeserializer;

    #[derive(serde::Deserialize)]
    struct Raw<'i> {
      #[serde(borrow)]
      raw: CowArcStr<'i>,
    }

    let content = serde::__private::de::Content::deserialize(deserializer)?;
    let de: ContentRefDeserializer<D::Error> = ContentRefDeserializer::new(&content);

    if let Ok(res) = Raw::deserialize(de) {
      let res = TokenList::parse_string_with_options(res.raw.as_ref(), ParserOptions::default())
        .map_err(|_| serde::de::Error::custom("Could not parse value"))?;
      return Ok(TokensOrRaw(ValueOrVec::Vec(res.into_owned().0)));
    }

    let de = ContentRefDeserializer::new(&content);
    Ok(TokensOrRaw(ValueOrVec::deserialize(de)?))
  }
}

trait List<V>: Index<usize, Output = V> + IndexMut<usize, Output = V> {
  fn len(&self) -> usize;
  fn remove(&mut self, i: usize);
  fn replace(&mut self, i: usize, items: Vec<V>);
}

impl<V> List<V> for Vec<V> {
  fn len(&self) -> usize {
    Vec::len(self)
  }

  fn remove(&mut self, i: usize) {
    Vec::remove(self, i);
  }

  fn replace(&mut self, i: usize, items: Vec<V>) {
    self.splice(i..i + 1, items);
  }
}

impl<V, T: smallvec::Array<Item = V>> List<V> for SmallVec<T> {
  fn len(&self) -> usize {
    SmallVec::len(self)
  }

  fn remove(&mut self, i: usize) {
    SmallVec::remove(self, i);
  }

  fn replace(&mut self, i: usize, items: Vec<V>) {
    let len = items.len();
    let mut iter = items.into_iter();
    self[i] = iter.next().unwrap();
    if len > 1 {
      self.insert_many(i + 1, iter);
    }
  }
}

impl<'i, R> List<CssRule<'i, R>> for CssRuleList<'i, R> {
  fn len(&self) -> usize {
    self.0.len()
  }

  fn remove(&mut self, i: usize) {
    self[i] = CssRule::Ignored;
  }

  fn replace(&mut self, i: usize, items: Vec<CssRule<'i, R>>) {
    self.0.replace(i, items)
  }
}