`IntoFieldResult` for `anyhow::Error`

[samuela]

Is your feature request related to a problem? Please describe.
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.

Side note: why is there IntoFieldResult instead of simply Into<FieldResult>?

Describe the solution you'd like
Not 100% clear what the exact solution ought to be, but I think just implementing IntoFieldResult for anyhow::Error would do the trick.

Describe alternatives you've considered
n/a

Additional context
n/a

[samuela]

My hacky workaround is to just use a macro:

macro_rules! juniper_ensure {
  // TODO why doesn't recursion work?
  ($cond:expr $(,)?) => {
    if !$cond {
      return Err(juniper::FieldError::new(
        concat!("Condition failed: `", stringify!($cond), "`"),
        juniper::Value::null(),
      ));
    }
  };
  ($cond:expr, $msg:literal $(,)?) => {
    if !$cond {
      return Err(juniper::FieldError::new($msg, juniper::Value::null()));
    }
  };
}