LibWeb+LibGC: Import GC swift module into LibWeb and an initial user

Start work on a speculative HTML Parser in Swift. This component will
walk ahead of the normal HTML parser looking for fetch() requests to
make while the normal parser is blocked. This work exposed many holes in
the Swift C++ interop component, which have been reported upstream.

Libraries/LibWeb/CMakeLists.txt

@@ -999,7 +999,8 @@ if (ENABLE_SWIFT)
         HTML/Parser/HTMLToken.swift
         HTML/Parser/HTMLTokenizer.swift
         HTML/Parser/HTMLTokenizerHelpers.cpp
+        HTML/Parser/SpeculativeHTMLParser.swift
     )
     target_link_libraries(LibWeb PRIVATE AK Collections)
-    add_swift_target_properties(LibWeb LAGOM_LIBRARIES AK LibGfx)
+    add_swift_target_properties(LibWeb LAGOM_LIBRARIES AK LibGfx LibGC COMPILE_DEFINITIONS LIBGC_WORKAROUND_BOOL_BITFIELD)
 endif()

Libraries/LibWeb/HTML/Parser/HTMLParser.h

@@ -214,10 +214,14 @@ private:
 
     GC::Ptr<DOM::Text> m_character_insertion_node;
     StringBuilder m_character_insertion_builder;
-};
+} SWIFT_UNSAFE_REFERENCE;
 
 RefPtr<CSS::CSSStyleValue> parse_dimension_value(StringView);
 RefPtr<CSS::CSSStyleValue> parse_nonzero_dimension_value(StringView);
 Optional<Color> parse_legacy_color_value(StringView);
 
+// Swift interop
+using HTMLParserGCPtr = GC::Ptr<HTMLParser>;
+using HTMLParserGCRef = GC::Ref<HTMLParser>;
+
 }

Libraries/LibWeb/HTML/Parser/SpeculativeHTMLParser.swift

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2025, Andrew Kaster <andrew@ladybird.org>>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+import AK
+import Collections
+import Foundation
+import GC
+@_exported import WebCxx
+
+// Workaround for https://github.com/swiftlang/swift/issues/80231
+// If any line of this changes, the whole thing breaks though
+extension GC.Cell.Visitor {
+    public func visit(_ parser: Web.HTML.HTMLParserGCPtr) {
+        if let parser = parser.ptr() {
+            let cell: GC.Cell = cxxCast(parser)
+            visit(cell)
+        }
+    }
+}
+
+struct SpeculativeMockElement {
+    let name: Swift.String
+    let localName: Swift.String
+    let attributes: [HTMLToken.Attribute]
+    var children: [SpeculativeMockElement]
+
+    init(name: Swift.String, localName: Swift.String, attributes: [HTMLToken.Attribute]) {
+        self.name = name
+        self.localName = localName
+        self.attributes = attributes
+        self.children = []
+    }
+
+    mutating func appendChild(_ child: consuming SpeculativeMockElement) {
+        children.append(child)
+    }
+}
+
+public final class SpeculativeHTMLParser: HeapAllocatable {
+    var parser = Web.HTML.HTMLParserGCPtr()  // FIXME: Want HTMLParserGCRef here, but how to initialize it?
+
+    public init(cell: GC.Cell) {
+        self.cell = cell
+    }
+    public var cell: GC.Cell
+
+    public static func create(on heap: GC.Heap, `for` parser: Web.HTML.HTMLParserGCPtr) -> GC.Cell {
+        precondition(heap.is_gc_deferred())
+        let _self = allocate(on: heap)
+        _self.pointee.parser = parser
+        return _self.pointee.cell
+    }
+
+    public func visitEdges(_ visitor: GC.Cell.Visitor) {
+        visitor.visit(parser)
+    }
+}

Libraries/LibWeb/Loader/FileRequest.h

@@ -9,6 +9,7 @@
 #include <AK/ByteString.h>
 #include <AK/Error.h>
 #include <AK/Function.h>
+#include <AK/Swift.h>
 
 namespace Web {
 
@@ -22,6 +23,6 @@ public:
 
 private:
     ByteString m_path {};
-};
+} SWIFT_UNSAFE_REFERENCE; // FIXME: This type is actually move-only, not unsafe
 
 }

Tests/LibWeb/CMakeLists.txt

@@ -29,6 +29,6 @@ if (ENABLE_SWIFT)
     target_sources(TestLibWebSwift PRIVATE ../Resources/SwiftTestMain.swift)
 
     set_target_properties(TestLibWebSwift PROPERTIES SUFFIX .swift-testing)
-    target_link_libraries(TestLibWebSwift PRIVATE AK LibWeb SwiftTesting::SwiftTesting)
+    target_link_libraries(TestLibWebSwift PRIVATE AK LibWeb LibGC SwiftTesting::SwiftTesting)
     add_test(NAME TestLibWebSwift COMMAND TestLibWebSwift)
 endif()