the trait From<anyhow::Error> is not implemented for JsValue

[junglie85]

Summary

This code generates the following error when run with 0.2.81:

#[cfg_attr(target_arch = "wasm32", wasm_bindgen(start))]
pub fn run() -> anyhow::Result<()> {

error[E0277]: the trait bound `JsValue: From<anyhow::Error>` is not satisfied
  --> src\lib.rs:10:36
   |
10 | #[cfg_attr(target_arch = "wasm32", wasm_bindgen(start))]
   |                                    ^^^^^^^^^^^^^^^^^^^ the trait `From<anyhow::Error>` is not implemented for `JsValue`

From #1742, I was expecting to be able to do this.
How can I use anyhow here?

[ranile]

I suggest you look at gloo. gloo-utils::error::JsError can be created from JsValue and it implements std::error::Error so it can be used by anyhow

[Liamolucko]

You can't use anyhow::Result as the return type, but you can use wasm_bindgen::JsError, which should work almost as well. It implements From for anything that implements std::error::Error (including anyhow::Error), which is implicitly used by the ? operator to be able to return any well-behaved error type.

I suggest you look at gloo. gloo-utils::error::JsError can be created from JsValue and it implements std::error::Error so it can be used by anyhow

That's for converting from a JS error to a Rust error, which is the wrong way around here.

[stefnotch]

@Liamolucko I can't seem to convert an anyhow::Error to a wasm_bindgen::JsError.

e.g. I have a function that currently returns an anyhow error, and so I figured I'd try to manually convert it. That didn't work out.

error[E0277]: the trait bound `anyhow::Error: StdError` is not satisfied
   --> wasm\src\application.rs:182:57
    |
182 |         let mut app = CpuApplication::new().map_err(|e| JsError::from(e))?;
    |                                                         ^^^^^^^ the trait `StdError` is not implemented for `anyhow::Error`, which is required by `JsError: From<anyhow::Error>`
    |
    = help: the trait `From<E>` is implemented for `JsError`
    = note: required for `JsError` to implement `From<anyhow::Error>`

[Liamolucko]

@Liamolucko I can't seem to convert an anyhow::Error to a wasm_bindgen::JsError.

Oops, I was wrong: anyhow::Error doesn't implement std::error::Error. I'm guessing that's because it there would be a conflict between its From<Self> and From<E> where E: Error impls if it did?

It does implement Deref<Target = dyn Error> though, so you can still convert one to a JsError like so (since &dyn Error implements Error):

let mut app = CpuApplication::new().map_err(|e| JsError::from(&*e))?;

JsError::from just calls e.to_string(), so this doesn't run into any errors about e not living long enough.

[daxpedda]

See #[wasm_bindgen(main)] as well, which should be able to handle this.