servo/components/style/custom_properties.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/. */

//! Support for [custom properties for cascading variables][custom].
//!
//! [custom]: https://drafts.csswg.org/css-variables/

use crate::hash::map::Entry;
use crate::properties::{CSSWideKeyword, CustomDeclarationValue};
use crate::selector_map::{PrecomputedHashMap, PrecomputedHashSet};
use crate::Atom;
use cssparser::{Delimiter, Parser, ParserInput, SourcePosition, Token, TokenSerializationType};
use precomputed_hash::PrecomputedHash;
use selectors::parser::SelectorParseErrorKind;
use servo_arc::Arc;
use smallvec::SmallVec;
use std::borrow::{Borrow, Cow};
use std::cmp;
use std::fmt::{self, Write};
use std::hash::Hash;
use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};

/// The environment from which to get `env` function values.
///
/// TODO(emilio): If this becomes a bit more complex we should probably move it
/// to the `media_queries` module, or something.
#[derive(Debug, MallocSizeOf)]
pub struct CssEnvironment;

struct EnvironmentVariable {
    name: Atom,
    value: VariableValue,
}

macro_rules! make_variable {
    ($name:expr, $value:expr) => {{
        EnvironmentVariable {
            name: $name,
            value: {
                // TODO(emilio): We could make this be more efficient (though a
                // bit less convenient).
                let mut input = ParserInput::new($value);
                let mut input = Parser::new(&mut input);

                let (first_token_type, css, last_token_type) =
                    parse_self_contained_declaration_value(&mut input, None).unwrap();

                VariableValue {
                    css: css.into_owned(),
                    first_token_type,
                    last_token_type,
                    references: Default::default(),
                    references_environment: false,
                }
            },
        }
    }};
}

lazy_static! {
    static ref ENVIRONMENT_VARIABLES: [EnvironmentVariable; 4] = [
        make_variable!(atom!("safe-area-inset-top"), "0px"),
        make_variable!(atom!("safe-area-inset-bottom"), "0px"),
        make_variable!(atom!("safe-area-inset-left"), "0px"),
        make_variable!(atom!("safe-area-inset-right"), "0px"),
    ];
}

impl CssEnvironment {
    #[inline]
    fn get(&self, name: &Atom) -> Option<&VariableValue> {
        let var = ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name)?;
        Some(&var.value)
    }
}

/// A custom property name is just an `Atom`.
///
/// Note that this does not include the `--` prefix
pub type Name = Atom;

/// Parse a custom property name.
///
/// <https://drafts.csswg.org/css-variables/#typedef-custom-property-name>
pub fn parse_name(s: &str) -> Result<&str, ()> {
    if s.starts_with("--") {
        Ok(&s[2..])
    } else {
        Err(())
    }
}

/// A value for a custom property is just a set of tokens.
///
/// We preserve the original CSS for serialization, and also the variable
/// references to other custom property names.
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub struct VariableValue {
    css: String,

    first_token_type: TokenSerializationType,
    last_token_type: TokenSerializationType,

    /// Whether a variable value has a reference to an environment variable.
    ///
    /// If this is the case, we need to perform variable substitution on the
    /// value.
    references_environment: bool,

    /// Custom property names in var() functions.
    references: PrecomputedHashSet<Name>,
}

impl ToCss for SpecifiedValue {
    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
    where
        W: Write,
    {
        dest.write_str(&self.css)
    }
}

/// A map from CSS variable names to CSS variable computed values, used for
/// resolving.
///
/// A consistent ordering is required for CSSDeclaration objects in the
/// DOM. CSSDeclarations expose property names as indexed properties, which
/// need to be stable. So we keep an array of property names which order is
/// determined on the order that they are added to the name-value map.
///
/// The variable values are guaranteed to not have references to other
/// properties.
pub type CustomPropertiesMap = OrderedMap<Name, Arc<VariableValue>>;

/// Both specified and computed values are VariableValues, the difference is
/// whether var() functions are expanded.
pub type SpecifiedValue = VariableValue;
/// Both specified and computed values are VariableValues, the difference is
/// whether var() functions are expanded.
pub type ComputedValue = VariableValue;

/// A map that preserves order for the keys, and that is easily indexable.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct OrderedMap<K, V>
where
    K: PrecomputedHash + Hash + Eq + Clone,
{
    /// Key index.
    index: Vec<K>,
    /// Key-value map.
    values: PrecomputedHashMap<K, V>,
}

impl<K, V> OrderedMap<K, V>
where
    K: Eq + PrecomputedHash + Hash + Clone,
{
    /// Creates a new ordered map.
    pub fn new() -> Self {
        OrderedMap {
            index: Vec::new(),
            values: PrecomputedHashMap::default(),
        }
    }

    /// Insert a new key-value pair.
    ///
    /// TODO(emilio): Remove unused_mut when Gecko and Servo agree in whether
    /// it's necessary.
    #[allow(unused_mut)]
    pub fn insert(&mut self, key: K, value: V) {
        let OrderedMap {
            ref mut index,
            ref mut values,
        } = *self;
        match values.entry(key) {
            Entry::Vacant(mut entry) => {
                index.push(entry.key().clone());
                entry.insert(value);
            },
            Entry::Occupied(mut entry) => {
                entry.insert(value);
            },
        }
    }

    /// Get a value given its key.
    pub fn get(&self, key: &K) -> Option<&V> {
        let value = self.values.get(key);
        debug_assert_eq!(value.is_some(), self.index.contains(key));
        value
    }

    /// Get whether there's a value on the map for `key`.
    pub fn contains_key(&self, key: &K) -> bool {
        self.values.contains_key(key)
    }

    /// Get the key located at the given index.
    pub fn get_key_at(&self, index: u32) -> Option<&K> {
        self.index.get(index as usize)
    }

    /// Get an ordered map iterator.
    pub fn iter<'a>(&'a self) -> OrderedMapIterator<'a, K, V> {
        OrderedMapIterator {
            inner: self,
            pos: 0,
        }
    }

    /// Get the count of items in the map.
    pub fn len(&self) -> usize {
        debug_assert_eq!(self.values.len(), self.index.len());
        self.values.len()
    }

    /// Returns whether this map is empty.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Remove an item given its key.
    fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: PrecomputedHash + Hash + Eq,
    {
        let index = self.index.iter().position(|k| k.borrow() == key)?;
        self.index.remove(index);
        self.values.remove(key)
    }
}

/// An iterator for OrderedMap.
///
/// The iteration order is determined by the order that the values are
/// added to the key-value map.
pub struct OrderedMapIterator<'a, K, V>
where
    K: 'a + Eq + PrecomputedHash + Hash + Clone,
    V: 'a,
{
    /// The OrderedMap itself.
    inner: &'a OrderedMap<K, V>,
    /// The position of the iterator.
    pos: usize,
}

impl<'a, K, V> Iterator for OrderedMapIterator<'a, K, V>
where
    K: Eq + PrecomputedHash + Hash + Clone,
{
    type Item = (&'a K, &'a V);

    fn next(&mut self) -> Option<Self::Item> {
        let key = self.inner.index.get(self.pos)?;

        self.pos += 1;
        let value = &self.inner.values[key];

        Some((key, value))
    }
}

/// A struct holding information about the external references to that a custom
/// property value may have.
#[derive(Default)]
struct VarOrEnvReferences {
    custom_property_references: PrecomputedHashSet<Name>,
    references_environment: bool,
}

impl VariableValue {
    fn empty() -> Self {
        Self {
            css: String::new(),
            last_token_type: TokenSerializationType::nothing(),
            first_token_type: TokenSerializationType::nothing(),
            references: PrecomputedHashSet::default(),
            references_environment: false,
        }
    }

    fn push(
        &mut self,
        css: &str,
        css_first_token_type: TokenSerializationType,
        css_last_token_type: TokenSerializationType,
    ) {
        // This happens e.g. between two subsequent var() functions:
        // `var(--a)var(--b)`.
        //
        // In that case, css_*_token_type is nonsensical.
        if css.is_empty() {
            return;
        }

        self.first_token_type.set_if_nothing(css_first_token_type);
        // If self.first_token_type was nothing,
        // self.last_token_type is also nothing and this will be false:
        if self
            .last_token_type
            .needs_separator_when_before(css_first_token_type)
        {
            self.css.push_str("/**/")
        }
        self.css.push_str(css);
        self.last_token_type = css_last_token_type
    }

    fn push_from(
        &mut self,
        position: (SourcePosition, TokenSerializationType),
        input: &Parser,
        last_token_type: TokenSerializationType,
    ) {
        self.push(input.slice_from(position.0), position.1, last_token_type)
    }

    fn push_variable(&mut self, variable: &ComputedValue) {
        debug_assert!(variable.references.is_empty());
        self.push(
            &variable.css,
            variable.first_token_type,
            variable.last_token_type,
        )
    }

    /// Parse a custom property value.
    pub fn parse<'i, 't>(input: &mut Parser<'i, 't>) -> Result<Arc<Self>, ParseError<'i>> {
        let mut references = VarOrEnvReferences::default();

        let (first_token_type, css, last_token_type) =
            parse_self_contained_declaration_value(input, Some(&mut references))?;

        Ok(Arc::new(VariableValue {
            css: css.into_owned(),
            first_token_type,
            last_token_type,
            references: references.custom_property_references,
            references_environment: references.references_environment,
        }))
    }
}

/// Parse the value of a non-custom property that contains `var()` references.
pub fn parse_non_custom_with_var<'i, 't>(
    input: &mut Parser<'i, 't>,
) -> Result<(TokenSerializationType, Cow<'i, str>), ParseError<'i>> {
    let (first_token_type, css, _) = parse_self_contained_declaration_value(input, None)?;
    Ok((first_token_type, css))
}

fn parse_self_contained_declaration_value<'i, 't>(
    input: &mut Parser<'i, 't>,
    references: Option<&mut VarOrEnvReferences>,
) -> Result<(TokenSerializationType, Cow<'i, str>, TokenSerializationType), ParseError<'i>> {
    let start_position = input.position();
    let mut missing_closing_characters = String::new();
    let (first, last) =
        parse_declaration_value(input, references, &mut missing_closing_characters)?;
    let mut css: Cow<str> = input.slice_from(start_position).into();
    if !missing_closing_characters.is_empty() {
        // Unescaped backslash at EOF in a quoted string is ignored.
        if css.ends_with("\\") && matches!(missing_closing_characters.as_bytes()[0], b'"' | b'\'') {
            css.to_mut().pop();
        }
        css.to_mut().push_str(&missing_closing_characters);
    }
    Ok((first, css, last))
}

/// <https://drafts.csswg.org/css-syntax-3/#typedef-declaration-value>
fn parse_declaration_value<'i, 't>(
    input: &mut Parser<'i, 't>,
    references: Option<&mut VarOrEnvReferences>,
    missing_closing_characters: &mut String,
) -> Result<(TokenSerializationType, TokenSerializationType), ParseError<'i>> {
    input.parse_until_before(Delimiter::Bang | Delimiter::Semicolon, |input| {
        // Need at least one token
        let start = input.state();
        input.next_including_whitespace()?;
        input.reset(&start);

        parse_declaration_value_block(input, references, missing_closing_characters)
    })
}

/// Like parse_declaration_value, but accept `!` and `;` since they are only
/// invalid at the top level
fn parse_declaration_value_block<'i, 't>(
    input: &mut Parser<'i, 't>,
    mut references: Option<&mut VarOrEnvReferences>,
    missing_closing_characters: &mut String,
) -> Result<(TokenSerializationType, TokenSerializationType), ParseError<'i>> {
    let mut token_start = input.position();
    let mut token = match input.next_including_whitespace_and_comments() {
        // FIXME: remove clone() when borrows are non-lexical
        Ok(token) => token.clone(),
        Err(_) => {
            return Ok((
                TokenSerializationType::nothing(),
                TokenSerializationType::nothing(),
            ))
        },
    };
    let first_token_type = token.serialization_type();
    loop {
        macro_rules! nested {
            () => {
                input.parse_nested_block(|input| {
                    parse_declaration_value_block(
                        input,
                        references.as_mut().map(|r| &mut **r),
                        missing_closing_characters,
                    )
                })?
            };
        }
        macro_rules! check_closed {
            ($closing:expr) => {
                if !input.slice_from(token_start).ends_with($closing) {
                    missing_closing_characters.push_str($closing)
                }
            };
        }
        let last_token_type = match token {
            Token::Comment(_) => {
                let token_slice = input.slice_from(token_start);
                if !token_slice.ends_with("*/") {
                    missing_closing_characters.push_str(if token_slice.ends_with('*') {
                        "/"
                    } else {
                        "*/"
                    })
                }
                token.serialization_type()
            },
            Token::BadUrl(u) => {
                let e = StyleParseErrorKind::BadUrlInDeclarationValueBlock(u);
                return Err(input.new_custom_error(e));
            },
            Token::BadString(s) => {
                let e = StyleParseErrorKind::BadStringInDeclarationValueBlock(s);
                return Err(input.new_custom_error(e));
            },
            Token::CloseParenthesis => {
                let e = StyleParseErrorKind::UnbalancedCloseParenthesisInDeclarationValueBlock;
                return Err(input.new_custom_error(e));
            },
            Token::CloseSquareBracket => {
                let e = StyleParseErrorKind::UnbalancedCloseSquareBracketInDeclarationValueBlock;
                return Err(input.new_custom_error(e));
            },
            Token::CloseCurlyBracket => {
                let e = StyleParseErrorKind::UnbalancedCloseCurlyBracketInDeclarationValueBlock;
                return Err(input.new_custom_error(e));
            },
            Token::Function(ref name) => {
                if name.eq_ignore_ascii_case("var") {
                    let args_start = input.state();
                    input.parse_nested_block(|input| {
                        parse_var_function(input, references.as_mut().map(|r| &mut **r))
                    })?;
                    input.reset(&args_start);
                } else if name.eq_ignore_ascii_case("env") {
                    let args_start = input.state();
                    input.parse_nested_block(|input| {
                        parse_env_function(input, references.as_mut().map(|r| &mut **r))
                    })?;
                    input.reset(&args_start);
                }
                nested!();
                check_closed!(")");
                Token::CloseParenthesis.serialization_type()
            },
            Token::ParenthesisBlock => {
                nested!();
                check_closed!(")");
                Token::CloseParenthesis.serialization_type()
            },
            Token::CurlyBracketBlock => {
                nested!();
                check_closed!("}");
                Token::CloseCurlyBracket.serialization_type()
            },
            Token::SquareBracketBlock => {
                nested!();
                check_closed!("]");
                Token::CloseSquareBracket.serialization_type()
            },
            Token::QuotedString(_) => {
                let token_slice = input.slice_from(token_start);
                let quote = &token_slice[..1];
                debug_assert!(matches!(quote, "\"" | "'"));
                if !(token_slice.ends_with(quote) && token_slice.len() > 1) {
                    missing_closing_characters.push_str(quote)
                }
                token.serialization_type()
            },
            Token::Ident(ref value) |
            Token::AtKeyword(ref value) |
            Token::Hash(ref value) |
            Token::IDHash(ref value) |
            Token::UnquotedUrl(ref value) |
            Token::Dimension {
                unit: ref value, ..
            } => {
                if value.ends_with("<EFBFBD>") && input.slice_from(token_start).ends_with("\\") {
                    // Unescaped backslash at EOF in these contexts is interpreted as U+FFFD
                    // Check the value in case the final backslash was itself escaped.
                    // Serialize as escaped U+FFFD, which is also interpreted as U+FFFD.
                    // (Unescaped U+FFFD would also work, but removing the backslash is annoying.)
                    missing_closing_characters.push_str("<EFBFBD>")
                }
                if matches!(token, Token::UnquotedUrl(_)) {
                    check_closed!(")");
                }
                token.serialization_type()
            },
            _ => token.serialization_type(),
        };

        token_start = input.position();
        token = match input.next_including_whitespace_and_comments() {
            // FIXME: remove clone() when borrows are non-lexical
            Ok(token) => token.clone(),
            Err(..) => return Ok((first_token_type, last_token_type)),
        };
    }
}

fn parse_fallback<'i, 't>(input: &mut Parser<'i, 't>) -> Result<(), ParseError<'i>> {
    // Exclude `!` and `;` at the top level
    // https://drafts.csswg.org/css-syntax/#typedef-declaration-value
    input.parse_until_before(Delimiter::Bang | Delimiter::Semicolon, |input| {
        // At least one non-comment token.
        input.next_including_whitespace()?;
        // Skip until the end.
        while let Ok(_) = input.next_including_whitespace_and_comments() {}
        Ok(())
    })
}

// If the var function is valid, return Ok((custom_property_name, fallback))
fn parse_var_function<'i, 't>(
    input: &mut Parser<'i, 't>,
    references: Option<&mut VarOrEnvReferences>,
) -> Result<(), ParseError<'i>> {
    let name = input.expect_ident_cloned()?;
    let name = parse_name(&name).map_err(|()| {
        input.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(name.clone()))
    })?;
    if input.r#try(|input| input.expect_comma()).is_ok() {
        parse_fallback(input)?;
    }
    if let Some(refs) = references {
        refs.custom_property_references.insert(Atom::from(name));
    }
    Ok(())
}

fn parse_env_function<'i, 't>(
    input: &mut Parser<'i, 't>,
    references: Option<&mut VarOrEnvReferences>,
) -> Result<(), ParseError<'i>> {
    // TODO(emilio): This should be <custom-ident> per spec, but no other
    // browser does that, see https://github.com/w3c/csswg-drafts/issues/3262.
    input.expect_ident()?;
    if input.r#try(|input| input.expect_comma()).is_ok() {
        parse_fallback(input)?;
    }
    if let Some(references) = references {
        references.references_environment = true;
    }
    Ok(())
}

/// A struct that takes care of encapsulating the cascade process for custom
/// properties.
pub struct CustomPropertiesBuilder<'a> {
    seen: PrecomputedHashSet<&'a Name>,
    may_have_cycles: bool,
    custom_properties: Option<CustomPropertiesMap>,
    inherited: Option<&'a Arc<CustomPropertiesMap>>,
    environment: &'a CssEnvironment,
}

impl<'a> CustomPropertiesBuilder<'a> {
    /// Create a new builder, inheriting from a given custom properties map.
    pub fn new(
        inherited: Option<&'a Arc<CustomPropertiesMap>>,
        environment: &'a CssEnvironment,
    ) -> Self {
        Self {
            seen: PrecomputedHashSet::default(),
            may_have_cycles: false,
            custom_properties: None,
            inherited,
            environment,
        }
    }

    /// Cascade a given custom property declaration.
    pub fn cascade(&mut self, name: &'a Name, specified_value: &CustomDeclarationValue) {
        let was_already_present = !self.seen.insert(name);
        if was_already_present {
            return;
        }

        if !self.value_may_affect_style(name, specified_value) {
            return;
        }

        if self.custom_properties.is_none() {
            self.custom_properties = Some(match self.inherited {
                Some(inherited) => (**inherited).clone(),
                None => CustomPropertiesMap::new(),
            });
        }

        let map = self.custom_properties.as_mut().unwrap();
        match *specified_value {
            CustomDeclarationValue::Value(ref unparsed_value) => {
                let has_references = !unparsed_value.references.is_empty();
                self.may_have_cycles |= has_references;

                // If the variable value has no references and it has an
                // environment variable here, perform substitution here instead
                // of forcing a full traversal in `substitute_all` afterwards.
                let value = if !has_references && unparsed_value.references_environment {
                    let result =
                        substitute_references_in_value(unparsed_value, &map, &self.environment);
                    match result {
                        Ok(new_value) => Arc::new(new_value),
                        Err(..) => {
                            map.remove(name);
                            return;
                        },
                    }
                } else {
                    (*unparsed_value).clone()
                };
                map.insert(name.clone(), value);
            },
            CustomDeclarationValue::CSSWideKeyword(keyword) => match keyword {
                CSSWideKeyword::Initial => {
                    map.remove(name);
                },
                // handled in value_may_affect_style
                CSSWideKeyword::Unset | CSSWideKeyword::Inherit => unreachable!(),
            },
        }
    }

    fn value_may_affect_style(&self, name: &Name, value: &CustomDeclarationValue) -> bool {
        match *value {
            CustomDeclarationValue::CSSWideKeyword(CSSWideKeyword::Unset) |
            CustomDeclarationValue::CSSWideKeyword(CSSWideKeyword::Inherit) => {
                // Custom properties are inherited by default. So
                // explicit 'inherit' or 'unset' means we can just use
                // any existing value in the inherited CustomPropertiesMap.
                return false;
            },
            _ => {},
        }

        let existing_value = self
            .custom_properties
            .as_ref()
            .and_then(|m| m.get(name))
            .or_else(|| self.inherited.and_then(|m| m.get(name)));

        match (existing_value, value) {
            (None, &CustomDeclarationValue::CSSWideKeyword(CSSWideKeyword::Initial)) => {
                // The initial value of a custom property is the same as it
                // not existing in the map.
                return false;
            },
            (Some(existing_value), &CustomDeclarationValue::Value(ref specified_value)) => {
                // Don't bother overwriting an existing inherited value with
                // the same specified value.
                if existing_value == specified_value {
                    return false;
                }
            },
            _ => {},
        }

        true
    }

    /// Returns the final map of applicable custom properties.
    ///
    /// If there was any specified property, we've created a new map and now we
    /// need to remove any potential cycles, and wrap it in an arc.
    ///
    /// Otherwise, just use the inherited custom properties map.
    pub fn build(mut self) -> Option<Arc<CustomPropertiesMap>> {
        let mut map = match self.custom_properties.take() {
            Some(m) => m,
            None => return self.inherited.cloned(),
        };
        if self.may_have_cycles {
            substitute_all(&mut map, self.environment);
        }
        Some(Arc::new(map))
    }
}

/// Resolve all custom properties to either substituted or invalid.
///
/// It does cycle dependencies removal at the same time as substitution.
fn substitute_all(custom_properties_map: &mut CustomPropertiesMap, environment: &CssEnvironment) {
    // The cycle dependencies removal in this function is a variant
    // of Tarjan's algorithm. It is mostly based on the pseudo-code
    // listed in
    // https://en.wikipedia.org/w/index.php?
    // title=Tarjan%27s_strongly_connected_components_algorithm&oldid=801728495

    /// Struct recording necessary information for each variable.
    struct VarInfo {
        /// The name of the variable. It will be taken to save addref
        /// when the corresponding variable is popped from the stack.
        /// This also serves as a mark for whether the variable is
        /// currently in the stack below.
        name: Option<Name>,
        /// If the variable is in a dependency cycle, lowlink represents
        /// a smaller index which corresponds to a variable in the same
        /// strong connected component, which is known to be accessible
        /// from this variable. It is not necessarily the root, though.
        lowlink: usize,
    }
    /// Context struct for traversing the variable graph, so that we can
    /// avoid referencing all the fields multiple times.
    struct Context<'a> {
        /// Number of variables visited. This is used as the order index
        /// when we visit a new unresolved variable.
        count: usize,
        /// The map from custom property name to its order index.
        index_map: PrecomputedHashMap<Name, usize>,
        /// Information of each variable indexed by the order index.
        var_info: SmallVec<[VarInfo; 5]>,
        /// The stack of order index of visited variables. It contains
        /// all unfinished strong connected components.
        stack: SmallVec<[usize; 5]>,
        map: &'a mut CustomPropertiesMap,
        /// The environment to substitute `env()` variables.
        environment: &'a CssEnvironment,
    }

    /// This function combines the traversal for cycle removal and value
    /// substitution. It returns either a signal None if this variable
    /// has been fully resolved (to either having no reference or being
    /// marked invalid), or the order index for the given name.
    ///
    /// When it returns, the variable corresponds to the name would be
    /// in one of the following states:
    /// * It is still in context.stack, which means it is part of an
    ///   potentially incomplete dependency circle.
    /// * It has been removed from the map.  It can be either that the
    ///   substitution failed, or it is inside a dependency circle.
    ///   When this function removes a variable from the map because
    ///   of dependency circle, it would put all variables in the same
    ///   strong connected component to the set together.
    /// * It doesn't have any reference, because either this variable
    ///   doesn't have reference at all in specified value, or it has
    ///   been completely resolved.
    /// * There is no such variable at all.
    fn traverse<'a>(name: Name, context: &mut Context<'a>) -> Option<usize> {
        // Some shortcut checks.
        let (name, value) = {
            let value = context.map.get(&name)?;

            // Nothing to resolve.
            if value.references.is_empty() {
                debug_assert!(
                    !value.references_environment,
                    "Should've been handled earlier"
                );
                return None;
            }

            // Whether this variable has been visited in this traversal.
            let key;
            match context.index_map.entry(name) {
                Entry::Occupied(entry) => {
                    return Some(*entry.get());
                },
                Entry::Vacant(entry) => {
                    key = entry.key().clone();
                    entry.insert(context.count);
                },
            }

            // Hold a strong reference to the value so that we don't
            // need to keep reference to context.map.
            (key, value.clone())
        };

        // Add new entry to the information table.
        let index = context.count;
        context.count += 1;
        debug_assert_eq!(index, context.var_info.len());
        context.var_info.push(VarInfo {
            name: Some(name),
            lowlink: index,
        });
        context.stack.push(index);

        let mut self_ref = false;
        let mut lowlink = index;
        for next in value.references.iter() {
            let next_index = match traverse(next.clone(), context) {
                Some(index) => index,
                // There is nothing to do if the next variable has been
                // fully resolved at this point.
                None => {
                    continue;
                },
            };
            let next_info = &context.var_info[next_index];
            if next_index > index {
                // The next variable has a larger index than us, so it
                // must be inserted in the recursive call above. We want
                // to get its lowlink.
                lowlink = cmp::min(lowlink, next_info.lowlink);
            } else if next_index == index {
                self_ref = true;
            } else if next_info.name.is_some() {
                // The next variable has a smaller order index and it is
                // in the stack, so we are at the same component.
                lowlink = cmp::min(lowlink, next_index);
            }
        }

        context.var_info[index].lowlink = lowlink;
        if lowlink != index {
            // This variable is in a loop, but it is not the root of
            // this strong connected component. We simply return for
            // now, and the root would remove it from the map.
            //
            // This cannot be removed from the map here, because
            // otherwise the shortcut check at the beginning of this
            // function would return the wrong value.
            return Some(index);
        }

        // This is the root of a strong-connected component.
        let mut in_loop = self_ref;
        let name;
        loop {
            let var_index = context
                .stack
                .pop()
                .expect("The current variable should still be in stack");
            let var_info = &mut context.var_info[var_index];
            // We should never visit the variable again, so it's safe
            // to take the name away, so that we don't do additional
            // reference count.
            let var_name = var_info
                .name
                .take()
                .expect("Variable should not be poped from stack twice");
            if var_index == index {
                name = var_name;
                break;
            }
            // Anything here is in a loop which can traverse to the
            // variable we are handling, so remove it from the map, it's invalid
            // at computed-value time.
            context.map.remove(&var_name);
            in_loop = true;
        }
        if in_loop {
            // This variable is in loop. Resolve to invalid.
            context.map.remove(&name);
            return None;
        }

        // Now we have shown that this variable is not in a loop, and
        // all of its dependencies should have been resolved. We can
        // start substitution now.
        let result = substitute_references_in_value(&value, &context.map, &context.environment);

        match result {
            Ok(computed_value) => {
                context.map.insert(name, Arc::new(computed_value));
            },
            Err(..) => {
                context.map.remove(&name);
            },
        }

        // All resolved, so return the signal value.
        None
    }

    // We have to clone the names so that we can mutably borrow the map
    // in the context we create for traversal.
    let names = custom_properties_map.index.clone();
    for name in names.into_iter() {
        let mut context = Context {
            count: 0,
            index_map: PrecomputedHashMap::default(),
            stack: SmallVec::new(),
            var_info: SmallVec::new(),
            map: custom_properties_map,
            environment,
        };
        traverse(name, &mut context);
    }
}

/// Replace `var()` and `env()` functions in a pre-existing variable value.
fn substitute_references_in_value<'i>(
    value: &'i VariableValue,
    custom_properties: &CustomPropertiesMap,
    environment: &CssEnvironment,
) -> Result<ComputedValue, ParseError<'i>> {
    debug_assert!(!value.references.is_empty() || value.references_environment);

    let mut input = ParserInput::new(&value.css);
    let mut input = Parser::new(&mut input);
    let mut position = (input.position(), value.first_token_type);
    let mut computed_value = ComputedValue::empty();

    let last_token_type = substitute_block(
        &mut input,
        &mut position,
        &mut computed_value,
        custom_properties,
        environment,
    )?;

    computed_value.push_from(position, &input, last_token_type);
    Ok(computed_value)
}

/// Replace `var()` functions in an arbitrary bit of input.
///
/// If the variable has its initial value, the callback should return `Err(())`
/// and leave `partial_computed_value` unchanged.
///
/// Otherwise, it should push the value of the variable (with its own `var()` functions replaced)
/// to `partial_computed_value` and return `Ok(last_token_type of what was pushed)`
///
/// Return `Err(())` if `input` is invalid at computed-value time.
/// or `Ok(last_token_type that was pushed to partial_computed_value)` otherwise.
fn substitute_block<'i, 't>(
    input: &mut Parser<'i, 't>,
    position: &mut (SourcePosition, TokenSerializationType),
    partial_computed_value: &mut ComputedValue,
    custom_properties: &CustomPropertiesMap,
    env: &CssEnvironment,
) -> Result<TokenSerializationType, ParseError<'i>> {
    let mut last_token_type = TokenSerializationType::nothing();
    let mut set_position_at_next_iteration = false;
    loop {
        let before_this_token = input.position();
        // FIXME: remove clone() when borrows are non-lexical
        let next = input
            .next_including_whitespace_and_comments()
            .map(|t| t.clone());
        if set_position_at_next_iteration {
            *position = (
                before_this_token,
                match next {
                    Ok(ref token) => token.serialization_type(),
                    Err(_) => TokenSerializationType::nothing(),
                },
            );
            set_position_at_next_iteration = false;
        }
        let token = match next {
            Ok(token) => token,
            Err(..) => break,
        };
        match token {
            Token::Function(ref name)
                if name.eq_ignore_ascii_case("var") || name.eq_ignore_ascii_case("env") =>
            {
                let is_env = name.eq_ignore_ascii_case("env");

                partial_computed_value.push(
                    input.slice(position.0..before_this_token),
                    position.1,
                    last_token_type,
                );
                input.parse_nested_block(|input| {
                    // parse_var_function() / parse_env_function() ensure neither .unwrap() will fail.
                    let name = {
                        let name = input.expect_ident().unwrap();
                        if is_env {
                            Atom::from(&**name)
                        } else {
                            Atom::from(parse_name(&name).unwrap())
                        }
                    };

                    let value = if is_env {
                        env.get(&name)
                    } else {
                        custom_properties.get(&name).map(|v| &**v)
                    };

                    if let Some(v) = value {
                        last_token_type = v.last_token_type;
                        partial_computed_value.push_variable(v);
                        // Skip over the fallback, as `parse_nested_block` would return `Err`
                        // if we don't consume all of `input`.
                        // FIXME: Add a specialized method to cssparser to do this with less work.
                        while input.next().is_ok() {}
                    } else {
                        input.expect_comma()?;
                        let after_comma = input.state();
                        let first_token_type = input
                            .next_including_whitespace_and_comments()
                            // parse_var_function() ensures that .unwrap() will not fail.
                            .unwrap()
                            .serialization_type();
                        input.reset(&after_comma);
                        let mut position = (after_comma.position(), first_token_type);
                        last_token_type = substitute_block(
                            input,
                            &mut position,
                            partial_computed_value,
                            custom_properties,
                            env,
                        )?;
                        partial_computed_value.push_from(position, input, last_token_type);
                    }
                    Ok(())
                })?;
                set_position_at_next_iteration = true
            }
            Token::Function(_) |
            Token::ParenthesisBlock |
            Token::CurlyBracketBlock |
            Token::SquareBracketBlock => {
                input.parse_nested_block(|input| {
                    substitute_block(
                        input,
                        position,
                        partial_computed_value,
                        custom_properties,
                        env,
                    )
                })?;
                // It's the same type for CloseCurlyBracket and CloseSquareBracket.
                last_token_type = Token::CloseParenthesis.serialization_type();
            },

            _ => last_token_type = token.serialization_type(),
        }
    }
    // FIXME: deal with things being implicitly closed at the end of the input. E.g.
    // ```html
    // <div style="--color: rgb(0,0,0">
    // <p style="background: var(--color) var(--image) top left; --image: url('a.png"></p>
    // </div>
    // ```
    Ok(last_token_type)
}

/// Replace `var()` and `env()` functions for a non-custom property.
///
/// Return `Err(())` for invalid at computed time.
pub fn substitute<'i>(
    input: &'i str,
    first_token_type: TokenSerializationType,
    computed_values_map: Option<&Arc<CustomPropertiesMap>>,
    env: &CssEnvironment,
) -> Result<String, ParseError<'i>> {
    let mut substituted = ComputedValue::empty();
    let mut input = ParserInput::new(input);
    let mut input = Parser::new(&mut input);
    let mut position = (input.position(), first_token_type);
    let empty_map = CustomPropertiesMap::new();
    let custom_properties = match computed_values_map {
        Some(m) => &**m,
        None => &empty_map,
    };
    let last_token_type = substitute_block(
        &mut input,
        &mut position,
        &mut substituted,
        &custom_properties,
        env,
    )?;
    substituted.push_from(position, &input, last_token_type);
    Ok(substituted.css)
}