Debugger support for Rust async/await

[tmandry]

It would be very nice if a debugger could show things like

• The set of active async tasks, similar to the list of threads in a program

• 73524 A "backtrace" of a given async task

• The state of any OS objects (files, sockets, etc) a task is blocking on
• The origin of an async task (where it was spawned), with possible backtrace captured at that point

This is a "meta issue" to track features and ideas like this.

Prior art

• Zero-cost async stack traces in Javascript

[nikomatsakis]

Tagging as triaged for now.

[memoryruins]

For an update to prior art, tokio-console implements some of these ideas and others. There was an announcement post last year, and its readme mentions sharing a bit in the vision of pnkfelix's series of blog posts. For user interfaces, a CLI/TUI is provided by tokio-console, but its API can also be built on by others, such as a Grafana data source.