Guidelines for implementation-specific rules

I think the authoring guide should have guidelines for how to handle implementation-specific rules. Should they ever be allowed? Where does the line get drawn?

Some current examples:

• Is the filesystem layout part of the language or part of the implementation?
• All the various notes we currently have regarding rustc and cargo.

In the past, there has been a sentiment from some in the project to never allow that. I largely agree with that. However, I think there will be gray areas that will be difficult to navigate with such a prohibition.

Unspecified is slightly different which is noted in https://github.com/rust-lang/reference/issues/2026. This is more related to the C-style "implementation-defined behavior — unspecified behavior where each implementation documents how the choice is made".

Note that there is another category of this that is "target-specific" behavior, though I think we have resolved that in https://github.com/rust-lang/spec/pull/58 which is now in the authoring guide.

Start by reading the authoring guide and the discussion in issue #2026 about unspecified behavior. Review the target-specific guidance from pull request #58 and compare it with the filesystem-layout, rustc, and cargo examples named here. Done means the authoring guide has an agreed boundary and guidance for implementation-specific rules, including gray areas.