Feature: track caller

[har7an]

This PR tries to add caller tracking with a new feature, "track_caller". This will allow, if set, to track the caller of any call of context or with_context and print it in the errors output.

Current State

MWE:

use anyhow::Result;
use anyhow::Context;

fn main() -> Result<()> {
    None
        .context("A")
        .context("B")
        .with_context(|| format!("{}", "C"))?
}

Output:

Error: C
  at src/main.rs:8


Caused by:
    0: B
    1: A

TODO

The PR isn't complete yet. It's still missing the ability to access the stored caller Location information in the call to fmt::debug. However, I do not know how to turn a &dyn Error into a Ref<ImplError> for this purpose. Does anyone have advice for me, please?

Related to #139

[dtolnay]

Thanks for the PR!

I have already been working on this (65290ed). It doesn't look like there is anything in this PR that isn't already done there, and since it sounds like you are stuck on the debug implementation, I'll close this in favor of continuing my own commit.

[rikvdkleij]

@dtolnay Any idea when your location feature will be released?

[Equim-chan]

@dtolnay The location branch seems to have been deleted. Is this feature still in plan?