Support undefined static imports with Option

[daxpedda]

extern "C" {
    type Crypto;
    #[wasm_bindgen(thread_local_v2, js_name = crypto)]
    static CRYPTO: Option<Crypto>;
}

This currently throws an error in JS if crypto is undefined.

This change would set CRYPTO to None instead.

[RunDevelopment]

The title of this PR doesn't make sense to me. If the global crypto variable was set to undefined, then wouldn't the imported value be JsValue::UNDEFINED (or whatever the constant is called)?

I think what you really mean is that it now supports undeclared variables, right? So for your example, crypto might not be defined by the browser/runtime, so just accessing it might give a ReferenceError at runtime. If so, then this is very different from the global variable being set to undefined. Please change the changelog entry if that's what you meant.

Also, please document this feature. Its behavior is not at all obvious. E.g. WBG doesn't support Option<JsValue> in functions at all, so it's surprising that it's allowed here. Globally defined variables set to undefined or null are also handled differently from just static FOO: JsValue.

[daxpedda]

The title of this PR doesn't make sense to me. If the global crypto variable was set to undefined, then wouldn't the imported value be JsValue::UNDEFINED (or whatever the constant is called)?

It would not, that's why Option<JsValue> isn't supported in the first place. I fixed OP.

I think what you really mean is that it now supports undeclared variables, right? So for your example, crypto might not be defined by the browser/runtime, so just accessing it might give a ReferenceError at runtime. If so, then this is very different from the global variable being set to undefined. Please change the changelog entry if that's what you meant.

Right, the distinction between undefined and undeclared wasn't quite clear to me. It should be noted that in either case the output would be None.

Also, please document this feature. Its behavior is not at all obvious. E.g. WBG doesn't support Option<JsValue> in functions at all, so it's surprising that it's allowed here. Globally defined variables set to undefined or null are also handled differently from just static FOO: JsValue.

I might consider expanding this feature to cover JsValue as well, that would make sense.

[daxpedda]

Also, please document this feature. Its behavior is not at all obvious. E.g. WBG doesn't support Option<JsValue> in functions at all, so it's surprising that it's allowed here. Globally defined variables set to undefined or null are also handled differently from just static FOO: JsValue.

I might consider expanding this feature to cover JsValue as well, that would make sense.

Or not actually? As you said, this feature is not about undefined, its about undeclared.

[daxpedda]

Adjusted and added documentation.

So to be clear: this PR does not add support for Option<JsValue>!

[RunDevelopment]

LGTM! Thank you @daxpedda!