Automatic detection of compatibility

[jethrogb]

A lot of existing crates “just work” when using them on another platform or in
another environment. Others require minor changes. How does one find such
crates? Alternatively, given a crate, how does one know whether it's going to
work in their current setup?

This issue is about trying to come up with static analysis tools that can be
used to help answer such questions.

Ideas/prior work:

• RFC 1868
• Tracking issue for RFC 1868: A portability lint rust-lang/rust#41619
• Check dependencies for no_std crates rust-lang/rust#38509
• Automatic detection for no_std compatibility https://twitter.com/japaricious/status/968683708092960769
• @aturon suggested
  that once a system like this exists, it's no longer necessary to have a
  separate core and std. Instead, one could leverage this system to enforce
  portability.

[Ericson2314]

Per my comment in rust-lang/rust#38509 (comment), now that we have an improved [patch], all we need is some of 1133 if we do keep std and core separate.