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LibWeb/CSS: Reimplement var()/attr() as arbitrary substitution functions

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"Arbitrary substitution functions" are a family of functions that
includes var() and attr(). All of them resolve to an arbitrary set of
component values that are not known at parse-time, so they have to be
substituted at computed-value time.

Besides it being nice to follow the spec closely, this means we'll be
able to implement the others (such as `if()` and `inherit()`) more
easily.

The main omission here is the new "spread syntax", which can be
implemented in the future.
This commit is contained in:
Sam Atkins 2025-06-19 17:03:26 +01:00 • committed by Tim Ledbetter
parent b417d13a7b
commit b6032b0fcd
Notes: github-actions[bot] 2025-07-09 15:45:46 +00:00
11 changed files with 480 additions and 346 deletions
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Libraries/LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.cpp Normal file
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/*
* Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <LibWeb/CSS/Parser/ArbitrarySubstitutionFunctions.h>
#include <LibWeb/CSS/Parser/Parser.h>
#include <LibWeb/CSS/PropertyName.h>
#include <LibWeb/CSS/StyleComputer.h>
#include <LibWeb/CSS/StyleValues/UnresolvedStyleValue.h>
#include <LibWeb/DOM/Element.h>
namespace Web::CSS::Parser {
bool SubstitutionContext::operator==(SubstitutionContext const& other) const
{
return dependency_type == other.dependency_type
&& first == other.first
&& second == other.second;
}
String SubstitutionContext::to_string() const
{
StringView type_name = [this] {
switch (dependency_type) {
case DependencyType::Property:
return "Property"sv;
case DependencyType::Attribute:
return "Attribute"sv;
case DependencyType::Function:
return "Function"sv;
}
VERIFY_NOT_REACHED();
}();
return MUST(String::formatted("{} {} {}", type_name, first, second));
}
void GuardedSubstitutionContexts::guard(SubstitutionContext& context)
{
for (auto& existing_context : m_contexts) {
if (existing_context == context) {
existing_context.is_cyclic = true;
context.is_cyclic = true;
return;
}
}
m_contexts.append(context);
}
void GuardedSubstitutionContexts::unguard(SubstitutionContext const& context)
{
[[maybe_unused]] auto const was_removed = m_contexts.remove_first_matching([context](auto const& other) {
return context == other;
});
VERIFY(was_removed);
}
Optional<ArbitrarySubstitutionFunction> to_arbitrary_substitution_function(FlyString const& name)
{
if (name.equals_ignoring_ascii_case("attr"sv))
return ArbitrarySubstitutionFunction::Attr;
if (name.equals_ignoring_ascii_case("var"sv))
return ArbitrarySubstitutionFunction::Var;
return {};
}
bool contains_guaranteed_invalid_value(Vector<ComponentValue> const& values)
{
for (auto const& value : values) {
if (value.contains_guaranteed_invalid_value())
return true;
}
return false;
}
// https://drafts.csswg.org/css-values-5/#replace-an-attr-function
static Vector<ComponentValue> replace_an_attr_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunctionArguments const& arguments)
{
// 1. Let el be the element that the style containing the attr() function is being applied to.
// Let first arg be the first <declaration-value> in arguments.
// Let second arg be the <declaration-value>? passed after the comma, or null if there was no comma.
auto const& first_argument = arguments.first();
auto const second_argument = arguments.get(1);
FlyString attribute_name;
Optional<FlyString> maybe_syntax = {};
auto failure = [&] -> Vector<ComponentValue> {
// This is step 6, but defined here for convenience.
// 1. If second arg is null, and syntax was omitted, return an empty CSS <string>.
if (!second_argument.has_value() && !maybe_syntax.has_value())
return { Token::create_string({}) };
// 2. If second arg is null, return the guaranteed-invalid value.
if (!second_argument.has_value())
return { ComponentValue { GuaranteedInvalidValue {} } };
// 3. Substitute arbitrary substitution functions in second arg, and return the result.
return substitute_arbitrary_substitution_functions(element, guarded_contexts, second_argument.value());
};
// 2. Substitute arbitrary substitution functions in first arg, then parse it as <attr-name> <attr-type>?.
// If that returns failure, jump to the last step (labeled FAILURE).
// Otherwise, let attr name and syntax be the results of parsing (with syntax being null if <attr-type> was
// omitted), processed as specified in the definition of those arguments.
auto substituted = substitute_arbitrary_substitution_functions(element, guarded_contexts, first_argument);
TokenStream first_argument_tokens { substituted };
// <attr-name> = [ <ident-token>? '|' ]? <ident-token>
// FIXME: Support optional attribute namespace
if (!first_argument_tokens.next_token().is(Token::Type::Ident))
return failure();
attribute_name = first_argument_tokens.consume_a_token().token().ident();
first_argument_tokens.discard_whitespace();
// <attr-type> = type( <syntax> ) | raw-string | <attr-unit>
// FIXME: Support type(<syntax>)
bool is_dimension_unit = false;
if (first_argument_tokens.next_token().is(Token::Type::Ident)) {
auto const& syntax_ident = first_argument_tokens.next_token().token().ident();
if (syntax_ident.equals_ignoring_ascii_case("raw-string"sv)) {
maybe_syntax = first_argument_tokens.consume_a_token().token().ident();
} else {
is_dimension_unit = syntax_ident == "%"sv
|| Angle::unit_from_name(syntax_ident).has_value()
|| Flex::unit_from_name(syntax_ident).has_value()
|| Frequency::unit_from_name(syntax_ident).has_value()
|| Length::unit_from_name(syntax_ident).has_value()
|| Resolution::unit_from_name(syntax_ident).has_value()
|| Time::unit_from_name(syntax_ident).has_value();
if (is_dimension_unit)
maybe_syntax = first_argument_tokens.consume_a_token().token().ident();
}
}
first_argument_tokens.discard_whitespace();
if (first_argument_tokens.has_next_token())
return failure();
// 3. If attr name exists as an attribute on el, let attr value be its value; otherwise jump to the last step (labeled FAILURE).
// FIXME: Attribute namespaces
auto attribute_value = element.element().get_attribute(attribute_name);
if (!attribute_value.has_value())
return failure();
// 4. If syntax is null or the keyword raw-string, return a CSS <string> whose value is attr value.
// NOTE: No parsing or modification of any kind is performed on the value.
if (!maybe_syntax.has_value() || maybe_syntax->equals_ignoring_ascii_case("raw-string"sv))
return { Token::create_string(*attribute_value) };
auto syntax = maybe_syntax.release_value();
// 5. Substitute arbitrary substitution functions in attr value, with «"attribute", attr name» as the substitution
// context, then parse with a attr value, with syntax and el. If that succeeds, return the result; otherwise,
// jump to the last step (labeled FAILURE).
auto parser = Parser::create(ParsingParams { element.element().document() }, attribute_value.value());
auto unsubstituted_values = parser.parse_as_list_of_component_values();
auto substituted_values = substitute_arbitrary_substitution_functions(element, guarded_contexts, unsubstituted_values, SubstitutionContext { SubstitutionContext::DependencyType::Attribute, attribute_name.to_string() });
// FIXME: Parse using the syntax. For now we just handle `<attr-unit>` here.
TokenStream value_tokens { substituted_values };
value_tokens.discard_whitespace();
auto const& component_value = value_tokens.consume_a_token();
value_tokens.discard_whitespace();
if (value_tokens.has_next_token())
return failure();
if (component_value.is(Token::Type::Number) && is_dimension_unit)
return { Token::create_dimension(component_value.token().number_value(), syntax) };
return failure();
// 6. FAILURE:
// NB: Step 6 is a lambda defined at the top of the function.
}
// https://drafts.csswg.org/css-variables-1/#replace-a-var-function
static Vector<ComponentValue> replace_a_var_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunctionArguments const& arguments)
{
// 1. Let el be the element that the style containing the var() function is being applied to.
// Let first arg be the first <declaration-value> in arguments.
// Let second arg be the <declaration-value>? passed after the comma, or null if there was no comma.
auto const& first_argument = arguments.first();
auto const second_argument = arguments.get(1);
// 2. Substitute arbitrary substitution functions in first arg, then parse it as a <custom-property-name>.
// If parsing returned a <custom-property-name>, let result be the computed value of the corresponding custom
// property on el. Otherwise, let result be the guaranteed-invalid value.
auto substituted_first_argument = substitute_arbitrary_substitution_functions(element, guarded_contexts, first_argument);
TokenStream name_tokens { substituted_first_argument };
name_tokens.discard_whitespace();
auto& name_token = name_tokens.consume_a_token();
name_tokens.discard_whitespace();
Vector<ComponentValue> result;
if (name_tokens.has_next_token() || !name_token.is(Token::Type::Ident) || !is_a_custom_property_name_string(name_token.token().ident())) {
result = { ComponentValue { GuaranteedInvalidValue {} } };
} else {
// Look up the value of the custom property
auto& custom_property_name = name_token.token().ident();
auto custom_property_value = StyleComputer::compute_value_of_custom_property(element, custom_property_name, guarded_contexts);
if (custom_property_value->is_guaranteed_invalid()) {
result = { ComponentValue { GuaranteedInvalidValue {} } };
} else if (custom_property_value->is_unresolved()) {
result = custom_property_value->as_unresolved().values();
} else {
dbgln_if(CSS_PARSER_DEBUG, "Custom property `{}` is an unsupported type: {}", custom_property_name, to_underlying(custom_property_value->type()));
result = { ComponentValue { GuaranteedInvalidValue {} } };
}
}
// FIXME: 3. If the custom property named by the var()’s first argument is animation-tainted, and the var() is being used
// in a property that is not animatable, set result to the guaranteed-invalid value.
// 4. If result contains the guaranteed-invalid value, and second arg was provided, set result to the result of substitute arbitrary substitution functions on second arg.
if (contains_guaranteed_invalid_value(result) && second_argument.has_value())
result = substitute_arbitrary_substitution_functions(element, guarded_contexts, second_argument.value());
// 5. Return result.
return result;
}
static void substitute_arbitrary_substitution_functions_step_2(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, TokenStream<ComponentValue>& source, Vector<ComponentValue>& dest)
{
// Step 2 of https://drafts.csswg.org/css-values-5/#substitute-arbitrary-substitution-function
// 2. For each arbitrary substitution function func in values (ordered via a depth-first pre-order traversal) that
// is not nested in the contents of another arbitrary substitution function:
while (source.has_next_token()) {
auto const& value = source.consume_a_token();
if (value.is_function()) {
auto const& source_function = value.function();
if (auto maybe_function_id = to_arbitrary_substitution_function(source_function.name); maybe_function_id.has_value()) {
auto function_id = maybe_function_id.release_value();
// FIXME: 1. Substitute early-invoked functions in func’s contents, and let early result be the result.
auto const& early_result = source_function.value;
// 2. If early result contains the guaranteed-invalid value, replace func in values with the guaranteed-invalid
// value and continue.
if (contains_guaranteed_invalid_value(early_result)) {
dest.empend(GuaranteedInvalidValue {});
continue;
}
// 3. Parse early result according to func’s argument grammar. If this returns failure, replace func in values
// with the guaranteed-invalid value and continue; otherwise, let arguments be the result.
auto maybe_arguments = parse_according_to_argument_grammar(function_id, early_result);
if (!maybe_arguments.has_value()) {
dest.empend(GuaranteedInvalidValue {});
continue;
}
auto arguments = maybe_arguments.release_value();
// 4. Replace an arbitrary substitution function for func, given arguments, as defined by that function.
// Let result be the returned list of component values.
auto result = replace_an_arbitrary_substitution_function(element, guarded_contexts, function_id, arguments);
// 5. If result contains the guaranteed-invalid value, replace func in values with the guaranteed-invalid value.
// Otherwise, replace func in values with result.
if (contains_guaranteed_invalid_value(result)) {
dest.empend(GuaranteedInvalidValue {});
} else {
// NB: Because we're doing this in one pass recursively, we now need to substitute any ASFs in result.
TokenStream result_stream { result };
Vector<ComponentValue> result_after_processing;
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, result_stream, result_after_processing);
dest.extend(result_after_processing);
}
continue;
}
Vector<ComponentValue> function_values;
TokenStream source_function_contents { source_function.value };
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source_function_contents, function_values);
dest.empend(Function { source_function.name, move(function_values) });
continue;
}
if (value.is_block()) {
auto const& source_block = value.block();
TokenStream source_block_values { source_block.value };
Vector<ComponentValue> block_values;
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source_block_values, block_values);
dest.empend(SimpleBlock { source_block.token, move(block_values) });
continue;
}
dest.empend(value);
}
}
// https://drafts.csswg.org/css-values-5/#substitute-arbitrary-substitution-function
Vector<ComponentValue> substitute_arbitrary_substitution_functions(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, Vector<ComponentValue> const& values, Optional<SubstitutionContext> context)
{
// To substitute arbitrary substitution functions in a sequence of component values values, given an optional
// substitution context context:
// 1. Guard context for the remainder of this algorithm. If context is marked as a cyclic substitution context,
// return the guaranteed-invalid value.
if (context.has_value()) {
guarded_contexts.guard(context.value());
if (context->is_cyclic)
return { ComponentValue { GuaranteedInvalidValue {} } };
}
ScopeGuard const guard { [&] {
if (context.has_value())
guarded_contexts.unguard(context.value());
} };
// 2. For each arbitrary substitution function func in values (ordered via a depth-first pre-order traversal) that
// is not nested in the contents of another arbitrary substitution function:
Vector<ComponentValue> new_values;
TokenStream source { values };
substitute_arbitrary_substitution_functions_step_2(element, guarded_contexts, source, new_values);
// 3. If context is marked as a cyclic substitution context, return the guaranteed-invalid value.
// NOTE: Nested arbitrary substitution functions may have marked context as cyclic in step 2.
if (context.has_value() && context->is_cyclic)
return { ComponentValue { GuaranteedInvalidValue {} } };
// 4. Return values.
return new_values;
}
Optional<ArbitrarySubstitutionFunctionArguments> parse_according_to_argument_grammar(ArbitrarySubstitutionFunction function, Vector<ComponentValue> const& values)
{
// Equivalent to `<declaration-value> , <declaration-value>?`, used by multiple argument grammars.
auto parse_declaration_value_then_optional_declaration_value = [](Vector<ComponentValue> const& values) -> Optional<ArbitrarySubstitutionFunctionArguments> {
TokenStream tokens { values };
auto first_argument = Parser::parse_declaration_value(tokens, Parser::StopAtComma::Yes);
if (!first_argument.has_value())
return OptionalNone {};
if (!tokens.has_next_token())
return ArbitrarySubstitutionFunctionArguments { first_argument.release_value() };
VERIFY(tokens.next_token().is(Token::Type::Comma));
tokens.discard_a_token(); // ,
auto second_argument = Parser::parse_declaration_value(tokens, Parser::StopAtComma::No);
if (tokens.has_next_token())
return OptionalNone {};
return ArbitrarySubstitutionFunctionArguments { first_argument.release_value(), second_argument.value_or({}) };
};
switch (function) {
case ArbitrarySubstitutionFunction::Attr:
// https://drafts.csswg.org/css-values-5/#attr-notation
// <attr-args> = attr( <declaration-value> , <declaration-value>? )
return parse_declaration_value_then_optional_declaration_value(values);
case ArbitrarySubstitutionFunction::Var:
// https://drafts.csswg.org/css-variables/#funcdef-var
// <var-args> = var( <declaration-value> , <declaration-value>? )
return parse_declaration_value_then_optional_declaration_value(values);
}
VERIFY_NOT_REACHED();
}
// https://drafts.csswg.org/css-values-5/#replace-an-arbitrary-substitution-function
Vector<ComponentValue> replace_an_arbitrary_substitution_function(DOM::AbstractElement& element, GuardedSubstitutionContexts& guarded_contexts, ArbitrarySubstitutionFunction function, ArbitrarySubstitutionFunctionArguments const& arguments)
{
switch (function) {
case ArbitrarySubstitutionFunction::Attr:
return replace_an_attr_function(element, guarded_contexts, arguments);
case ArbitrarySubstitutionFunction::Var:
return replace_a_var_function(element, guarded_contexts, arguments);
}
VERIFY_NOT_REACHED();
}
}