REQUEST: Add method to iterate entire context chain, not just errors

[stevenroose]

Currently, anyhow::Error::chain exposes a chain over all Error types in the context chain. Additionally, anyhow::Error::downcast[_ref] gives access to a single non-Error context in the chain. There is however no way to access all non-Error contexts in the chain.

Imagine if one wants to tag errors with a Tag enum type. And then later one wants to extract all the tags places on the context. That's currently impossible:

use anyhow::{bail, Context};

#[derive(Debug)]
pub enum Tag {
	One,
	Two,
}

impl std::fmt::Display for Tag {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
	    std::fmt::Debug::fmt(self, f)
	}
}

fn bails() -> anyhow::Result<()> {
	bail!("bad!");
}

fn add_one_tag() -> anyhow::Result<()> {
	bails().context(Tag::One)?;
	Ok(())
}

fn add_two_tag() -> anyhow::Result<()> {
	add_one_tag().context(Tag::Two)?;
	Ok(())
}

fn main() {
	match add_two_tag() {
		Ok(()) => {}, // all good
		Err(e) => {
			// some error, let's get the tags...
			if let Some(tag) = e.downcast_ref::<Tag>() {
				println!("found tag through downcast: {}", tag);
			}

			for _item in e.chain() {
				// can't do this because Tag doesn't implement Error
				// if let Some(tag) = item.downcast_ref::<Tag>() {
				// 	println!("found tag through chain: {}", tag);
				// }
			}
		}
	}
}

I would like to propose either

• a anyhow::Error::context_chain(&self) -> ContextChain that allows iterating through the entire chain as a dyn Any chain
• a anyhow::Error::downcast_chain<T>(&self) -> DowncastChain<T> that iterates over all the contexts that succesfully downcast to type T.