[WIP] Add discarded_future restriction lint

[Manishearth]

Rain mentioned during their RustConf talk that it is easy to accidentally discard a Future before awaiting it when it is wrapped in a Result that you wish to ignore.

let _ = futurelike();

It generally makes sense that there are many reasons to ignore a Result, but fewer to ignore a future; so enabling it as a restriction lint seems useful for codebases that are heavily async.

This lint isn't correct yet: I'm not sure if we should lint on Result<Future>, Future<Result>, both, or those types + options.

Open q on naming: discarded_futures or discarded_future?

• Followed lint naming conventions
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

changelog: [discarded_future]: new lint

Summary Notes

• Feature-freeze by github-actions[bot]

Managed by @rustbot—see help for details

[github-actions]

No changes for fbbaa0e

[Manishearth]

https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future