EDIT: Current Status Update

Building upon these prior issues:

• Tracking issue for RFC 2504, "Fix the Error trait" rust#53487
• Stabilize the backtrace feature. rust#72981

From this comment Backtrace stabilization is currently blocked on prototyping one or more of the suggested solutions for moving the std::error::Error trait into core

That comment suggests three possible solutions.

1. Remove `backtrace` from `Error` and instead provide it via some generic context mechanism (RFC 2895). 2. Define a `Backtrace` trait in core and have an implementation of that trait in libstd. `Error::backtrace` would then return a `&dyn Backtrace`. 3. Define `Backtrace` in core but have it dispatch via an internal vtable. This is effectively the same as 2 but the trait is hidden. The `capture` method will have to be moved to a free function in libstd. 

Option one is already known to work, and so it does not need to be prototyped. In addition to these backtrace related issues a prototype of the error trait in core will need to find a way to work around pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> since Box won't be available in core. The top level comment in @withoutboats's stabilization PR goes into more detail on this second issue and outlines how a new core::error::Error implementation could use hooks that are defined in std, similar to panic hooks, to work solve the issues around Box.

Boats has already raised issues about option two in this comment so I think the best way forward is to start working on a core::error::Error trait and core::backtrace::Backtrace type that both rely on hooks / manual vtables to maintain backwards compatibility with the existing error trait.