no_upcast

The no_upcast attribute disables automatic generation of Upcast trait implementations for an imported type.

By default, wasm-bindgen automatically generates the following Upcast implementations for imported types:

• Upcast<JsValue> - all types can upcast to JsValue
• Upcast<Self> - identity upcast for non-generic types
• Structural covariance for generic types (Type<T> → Type<U> when T: Upcast<U>)
• Upcast<SuperClass> for each type in the extends attribute

Use no_upcast when you need to provide custom Upcast implementations, for example when a type has special covariance rules.

use wasm_bindgen::convert::Upcast;

#[wasm_bindgen]
extern "C" {
    // Automatic Upcast implementations are generated
    #[wasm_bindgen(extends = Object)]
    type NormalType;

    // No automatic Upcast implementations - must be provided manually
    #[wasm_bindgen(extends = Object, no_upcast)]
    type CustomType<T>;
}

// Provide custom Upcast implementations for CustomType
impl<T> Upcast<JsValue> for CustomType<T> {}
impl<T> Upcast<Object> for CustomType<T> {}
// ... additional custom implementations

This is useful for types that have:

• Complex generic covariance rules (e.g., contravariant type parameters)
• Custom inheritance hierarchies
• Special relationship with other types that require manual Upcast implementations