fix: generate mutable self binding for @JS mutating struct methods

Plugins/BridgeJS/Sources/BridgeJSCore/SwiftToSkeleton.swift

@@ -1198,6 +1198,14 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
             className: classNameForABI
         )
 
+        if let mutatingModifier = node.modifiers.first(where: { $0.name.tokenKind == .keyword(.mutating) }) {
+            diagnose(
+                node: mutatingModifier,
+                message: "@JS does not support mutating struct methods: mutations to 'self' cannot be propagated back to JavaScript",
+                hint: "Remove the mutating keyword or redesign the API to return the updated value instead"
+            )
+            return nil
+        }
         guard let effects = collectEffects(signature: node.signature, isStatic: isStatic) else {
             return nil
         }
@@ -1522,7 +1530,7 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
         }
     }
 
-    /// Walks extension members under the matching type’s state, returning whether the type was found.
+    /// Walks extension members under the matching type's state, returning whether the type was found.
     ///
     /// Note: The lookup scans dictionaries keyed by `makeKey(name:namespace:)`, matching only by
     /// plain name. If two types share a name but differ by namespace, `.first(where:)` picks

Plugins/BridgeJS/Sources/BridgeJSSkeleton/BridgeJSSkeleton.swift

@@ -635,11 +635,35 @@ public struct Effects: Codable, Equatable, Sendable {
     public var isAsync: Bool
     public var isThrows: Bool
     public var isStatic: Bool
+    public var isMutating: Bool
 
-    public init(isAsync: Bool, isThrows: Bool, isStatic: Bool = false) {
+    public init(isAsync: Bool, isThrows: Bool, isStatic: Bool = false, isMutating: Bool = false) {
         self.isAsync = isAsync
         self.isThrows = isThrows
         self.isStatic = isStatic
+        self.isMutating = isMutating
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case isAsync, isThrows, isStatic, isMutating
+    }
+
+    public init(from decoder: any Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.isAsync = try container.decode(Bool.self, forKey: .isAsync)
+        self.isThrows = try container.decode(Bool.self, forKey: .isThrows)
+        self.isStatic = try container.decode(Bool.self, forKey: .isStatic)
+        self.isMutating = try container.decodeIfPresent(Bool.self, forKey: .isMutating) ?? false
+    }
+
+    public func encode(to encoder: any Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(isAsync, forKey: .isAsync)
+        try container.encode(isThrows, forKey: .isThrows)
+        try container.encode(isStatic, forKey: .isStatic)
+        if isMutating {
+            try container.encode(isMutating, forKey: .isMutating)
+        }
     }
 }
 

Plugins/BridgeJS/Tests/BridgeJSToolTests/Inputs/MacroSwift/MutatingStructMethod.swift

@@ -0,0 +1,12 @@
+@JS struct Counter {
+    var number: Int
+}
+
+extension Counter {
+    @JS public mutating func increment() {
+        number += 1
+    }
+    @JS public mutating func add(_ value: Int) {
+        number += value
+    }
+}