Support anyhow::Error (#988)

[tyranron]

Resolves #988

Synopsis

See #988:

anyhow is a dominant error handling library for rust. Currently, there is no convenient way to use anyhow Errs in juniper code. For example,

struct Mutation;

#[juniper::graphql_object]
impl Mutation {
  async fn Foo(line_number: i32) -> FieldResult<bool> {
    anyhow::ensure!(line_number >= 0);
    Ok(true)
  }
}

doesn't work.

IntoFieldResult offers a way of translating custom error types into FieldResults, but unfortunately implementing IntoFieldResult for anyhow::Error is not possible due to https://doc.rust-lang.org/error-index.html#E0117.

Solution

Provide IntoFieldError implementation for anyhow::Error behind anyhow Cargo feature.

Backtrace

Write anyhow::Error::backtrace() into FieldError::extensions() as backtrace field, if anyhow library supports it. Controllable by backtrace Cargo feature in the same manner as anyhow crate does.